Ford Lecture Series (Sept. 28, 2023)
“Measuring Electron Flow: From Small Molecules to Solutions”
Hans Jakob Wörner, Cancer Researcher & Professor, West Virginia University
The Dwain L Ford Lecture Series is sponsored by the Department of Chemistry & Biochemistry in honor of former Chair and Professor Dwain L Ford who started this seminar program in 1965. The Series provides students, faculty and the wider community an opportunity to hear guest speakers from academia, industry and government present topics of current interest and importance in chemistry.
Our co-host for this lecture is Ms. Chloe Gaban, Andrews University Biochemistry Major.
Guest Speaker Bio:
Hans Jakob Wörner is a Professor at ETH Zürich since 2010. He obtained his PhD from ETH Zürich in 2007, after which he worked as a postdoctoral researched at the Laboratoire Aimé-Cotton in France and the National Research Council of Canada in Ottawa. His research area is attosecond spectroscopy in the gas and liquid phases. His research group holds the current world record of the shortest pulse of light ever measured (43 as; 1 attosecond(as) = 10^-18 seconds).
Abstract:
This lecture will discuss the development of table-top soft-X-ray spectroscopy, which led to the current world record of the shortest laser pulse (43 as) and its application to observing the rearrangement of unoccupied molecular states during chemical reactions. X-ray spectroscopy offers an attractive approach to investigate attosecond time-resolved measurements of complex systems, such as large molecules, molecular aggregates or nanoparticles in solution. Turning from the gas phase to the liquid phase, the lecture will discuss recent results on the observation of femtosecond proton transfer in ionized urea dimers in aqueous solution. These results demonstrate the potential of attosecond soft-X-ray spectroscopy for studying electronic dynamics of chemically relevant systems under ambient conditions.
All right so we want to welcome everyone in the room in person as well as those online to our chemistry seminar and I believe this is the the Third um yes the third the third episode of Kim Sam so today um we have a very distinguished uh
Professor that’s going to tell us how fast electrons flow yes how fast electrons flow to introduce him we uh all know uh Chloe garban uh she’s a senior biochemistry major here at Andrews University uh she was born in Sabah Malaysia which is part of the island of Borneo
She hopes to attend uh pharmacy school after she graduates um her interests include reading historical fiction traveling and spending time with her family so Chloe it’s your turn to introduce our speaker today hi thank you Dr Murray for the introduction it is my honor to be sent out guest speaker Professor Hans
Jacob Warner he is a professor at eth Zurich since 2010 in 2007 he obtained his THD from eth Zurich after which he worked as a postdoctoral I’m a cotton in France and the national research Council of Canada in Ottawa his research area consists of at the second spectroscopy in gas and liquid phases
Currently his research group holds the world record of the shortest pulse of light ever measured which is 43 aroseconds so just so you know one out of seconds is equal to 10 to the negative to the negative 18 seconds with it which is ultra fast so without further Ado please welcome Professor Verner
Thank you very much for the kind introduction and good afternoon everyone it’s a great honor to be invited to present this lecture to you which will be about measuring electron flow in small molecules in the gas space where we can do very accurate measurements and then trying to extend these two larger
Systems especially molecules in solution where most chemistry and biochemistry takes place so today I’m speaking to you from Switzerland Switzerland is this small red country here in the middle of Europe um and more specifically I’m located in Zurich which is here in the northern part of Switzerland in the
German-speaking part of Switzerland and these institutions you can see here are the federal research institutions there is eth Zurich there is epfl which are the two universities and the others are Research Institute institutions similar to the National Labs in the US but of course smaller in size proportional to the size of the
Country at PSI we have a synchrotron and a free electron laser so this is where we also like to do some research I will not speak about today though so briefly I was asked to introduce myself so some of this has been said already but I studied chemistry in lausanne and
In Zurich and then I got my PhD in high resolution molecular spectroscopy so this is frequency domain spectroscopy and then I did a postdoc in theoretical spectroscopy but then I I got more and more interested in time domain spectroscopy and this is why I did a postdoc in atosecond science which which
Are the the shortest time scales that we can access nowadays in direct time result measurements and this then became my research field also in 2010 I returned to Zurich as an assistant professor and in 2013 I got my current tenure position in physical chemistry at eth Zurich so since 10 years now I’ve
Been there this is what our campus look looks like it’s located just outside of the city of Zurich that you can see here the there’s a nice lake Lake Zurich and in in the far away you can see the mountains we covered with snow um and so this is where we work our
Laboratories are located here in this building that hosts the Department of Chemistry and applied biosciences which is a pharmacology and Pharmacy essentially and also Material Science they’re all in the same in the same building so we do our work here this is what our group looks like as of this
Summer it’s a very International group students and postdocs from China Russia USA India Germany pretty much most Moses most larger countries are represented and what we like to do in in winter is we like to go skiing and this is a recent picture from our group on a ski trip
What what we do in the lab can be seen in these pictures we build large machines we build large experimental setups um these are all vacuum Chambers we need to work on the high vacuum because atosecond pulses they would be neatly absorbed by air they are created with short wavelengths in the extreme
Ultraviolet or in the soft x-ray domain and so we need to work on the vacuum so we build this relatively large machines and this is this is another vacuum setup where we work with liquids uh so liquids and vacuum don’t go well together but there are techniques one can use to
Inject liquids into vacuum and then perform atosecond measurements on liquids which I will talk a bit about in the second part of the talk we use intense lasers so we have a lot of cool looking lasers in our lab and we we use those to make these very short pulses
And I won’t go into very much detail about how we do the pulses because I want to focus on the applications but feel free to ask questions about this so what we are interested in are these very short time scales and this is the the shorter end of the Dynamics of uh in
Molecules so these Dynamics they cover many orders of magnitude um you are certainly familiar with protein motions protein folding which takes place on millisecond and slower time scales but there is also proton transfer for example in liquid water proton transfer that takes place on nanosecond time scale and it’s a kinetic
Process but then when one goes uh it looks into more fundamental emotions of molecules there are rotations that take place on picosecond time scales and vibrations of the molecules that take place on femtosecond time scales and ultimately the electron motion this takes place on auto second time scale
And as was nicely introduced One auto second is 10 to minus 18 seconds and in the classical or picture of the boa model the electron takes 150 out of seconds to go around the the proton once but of course one needs quantum mechanics to describe electrons and so
We will not be talking much about classical motion but instead about quantum mechanical relations that relate the period of the quantum Dynamics T in femto seconds to the energy interval Delta e for example in electron volts and then the relation factor is four so if we have an energy interval of one
Electron voltage which is typical for valence transitions in molecule we have a period of about four of them to seconds and most energy intervals are on this order of magnitude so electron motion is typically uh one or few femtoseconds or just below one 22 seconds in the valence shell of molecules
And so the nice thing about atosecond spectroscopy is when we measure on these time scales only the electrons move and the nuclei I don’t have time to move and so we can really in a way simplify the problem the spectroscopic problem we don’t have to deal with all the types of
Motion at the same time but only the electrons move and this is in in some sense a simplification of the dynamical problem there have been a few Nobel prizes in what is called Ultra fast science but what is defined as ultrafast has evolved over time so about 70 years ago there
Was a nobler price in chemistry to manage Ronald Norris and George Porter for their studies of extremely fast chemical reactions affected by disturbing the equilibrium with very short pulses of energy what they discovered are ways to visualize proton transfer and other very fast Dynamics nanosecond Dynamics which
Was the limit of capability 60 years ago you know then 30 years ago there was an oil price to Ahmed sewal for his studies of the transition states of chemical reactions this was the era of pentosecond spectroscopy and just five years ago there was another Nobel Prize
In this field which is for the method of generating high intensity Ultra short Optical pulses and this this is is an important development in Laser Technology and this development has led to Auto second signs thanks to these technique developed by these people we now have other second pulses and what
They did is I was already 20 years before so after second science started based on laser development and this is now an overview of how these atoseconds or generally laser pulses became shorter and shorter over time soon after the invention of the laser people already had tens of picosecond pulses and they
Become shorter and shorter until the mid 80s and this enabled them to chemistry so here with the availability of femtosecond pulses people were able to observe bonds being broken and and bonds being formed and this is known as femtochemistry so the motion of nuclei following the motion of nuclei and how
That gives rise to chemistry but then around 2000 the first sub femtosecond or other second passes were made and this led to another quick decrease in the pulse durations until 2006 and then a few years ago our group generated this 43 at a second pulse which is still the shortest at the
Second pulse that has been made um and I will not as I said I will not speak in detail about how we make these pulses and how we measure them because that’s a separate lecture that I will really focus on the application what can we do with these Auto second pulses
Looking at molecules and so this is the outline of the talk I will start speaking about how fast charge can move across molecules and explain what charge migration is then how charge migration is influenced by nuclear motion and finally how we can extend such measurements from the gas phase to the
Solution phase and look at proton transfer and and the interaction with nuclear motion so in chemistry the concept of charge transfer is mostly related to Marcus Theory because this is an extremely successful Theory how one can describe charge transfer in chemical systems and the basic idea is that one the system
Starts in a state where charge is on the donor and then evolves over transition state in a state where the charge is on the acceptor and for this process one can define a chemical rate law as given here with an activation energy that contains the Gibbs free energy and the
Reorganization energy Lambda and this is an extremely successful Theory one can describe many aspects of charge transfer with this but in all of these cases charge transfer is driven by a rearrangement of the nuclei so the nuclei have to move in order for charge to move across molecules and this
Is indeed the case in many system for example in here in photosynthesis in photovolta type so in molecular Electronics these are all relatively recent experiments where indeed charge transfer is driven by nuclear motion but this fundamentally limits the time scale because on in all these examples charge can only move as fast as
The nuclei move so in other words in femtoseconds tens of femtoseconds and not faster and so this gives rise to the question whether one can potentially find a way to transport charge on time scale which are actually faster than nuclear Motion in principle it should be possible because electrons are light
Enough they can travel faster but is it really possible so about uh 20 25 years ago there were some experiments without time resolution and but they were interpreted as indicating that there has to be an electronic charge migration so emotion of the charge faster than nuclei and this was indeed suggested by simulations
Done in 2006 which are shown here so if you ionize this tetropeptide consisting of tryptophan leucine timer then you create an electron hole at one one end of the molecule and this hole is indicated here in terms of the electron density by these red clouds but this
Hole is not staying on the tryptophan group it’s moving across the molecule and it finds itself at the other end in less than one frame per second so clearly here there is motion of charge that takes much less than tens of femtoseconds for nuclear motion this is really purely electronic motion but this
Was a theoretical prediction and this uh so in order to get this kind of electron motion a few um conditions need to be fulfilled first we need at least two electronic States and these two states they need to be coherently populated so they need to have a well-defined phase relationship a
Quantum mechanical phase relationship to each other and in addition the electronic wave functions of these two states they need to overlap spatially and the the simplest example I can give of this is the H2 plus molecule so you all you’re all familiar with H2 plus the ground state wave function
Looks like this and the first exercise State wave function looks like that it has an old plane and now if we make a coherent superposition States it means this is as schroding as cat State we make a superposition of these two states now this becomes time dependent
And the simplest version I wrote down here so you you add the ground state wave function and then you add the excited state wave function with this has a phase factor that evolves so it’s time t and now the electron densities are no longer time independent but if we look
At the electron density of this so-called superposition State this evolves in time as you can see here because if we calculate the magnitude square of this sum of two complex number we have two modulus Square which are time independent this is just the density of the two states but we have an overlap
Term multiplied with a cosine and this gives rise to electron density that now moves from the left to the right in 174 atoseconds and then keeps oscillating back and forth and so this is charge migration now the electron moves without the nuclei moving and it moves on the purely electronic time scale
Thank you now does this really occur in nature so there has been a lot of theoretical work since that time and people have found that indeed um charge migration is a quite Universal response to ionization because whenever you ionize a molecule a neutral molecule and you do this on a very short time
Scale then you create several states of the cation and you coherently populate them which gives rise to the electron flowing from one end to the other in on a few femtosec no other second time skills so how can we now um distinguish the charge transfer and charge migration charge transfer as
Described by macro Theory and studied in femtochemistry is when charge moves from the donor to the acceptor as a result of nuclear motion this is the reaction coordinate Q in this case and in contrast to this if we have a superposition state so two coherently related electronic States then we have
Charge migration but now nuclei can still move of course so these two nuclear wave packets they can still evolve and this gives rise to the interesting question what happens when they meet here that’s the relative phase play a role at what happens at this intersection this is called the conical
Intersection between two states and that is the coherence preserved or does it disappear and this is this is the field of atochemistry which is just emerging now where one makes use of electronic coherences to influence chemical reactions and hopefully one day to steer chemical reactivity so one of the first experiments that
Um accessed charge migration experimentally was this work that we published in 2015 where we studied a relatively simple molecule iodor acetylene so icch and we were able to reconstruct from experimental data the electron hole density as defined here which we found flows from the iodine end of the molecule to
The acetylene and in less than a femtosecond and comes back and depending on on how we oriented the molecule with respect to the laser field we found that the electron hole can be created on the iodine side or it can be created on the acetylene side so we had also control
Over where the additional hole started from and under conditions where we align the molecule parallel to the laser field we found that a lot of population got transferred between electronic States and the corresponding relative phase showed jumps so we have we have ways to reconstruct these electron densities
Observe how the charge flows and reconstruct this in a quantum mechanical way in both amplitude and phase but soon after that uh people started to wonder so if we have charge migration these three lies on electronic coherence and once coherence um um and once the nuclei moved these coherence can quickly disappear because
These are now theoretical predictions again if we look at the paroxylene molecule shown here then if we fix the nuclei then we see that electron can indeed flow around the aromatic unit here in 5.2 femtosecond and this gives rise to a very nice very strong charge
Migration with a period of 5 frames per second but if we allow the nuclide to move then this oscillation is damped and we see here that within a few oscillations this electronic coherence is lost or this is showing you the coherence drops to zero which means the electronic motion actually stops and
Similar predictions have been made for water cation with a very fast defacing or phenylanylene cation with also a two femtosecond e-phasing of the electronic coherence and so this suggested that in some cases the electronic coherence might be lost but we wanted to have experimental access to this question and also see
What in general can happen with electronic coherence and so this is the um this was the goal of this first experiment where we looked at the influence of nuclear motion on the other second charge migration in a molecule we chose again a quite simple molecule here
The cylane molecule sih4 and this is uh the schematic setup so we use a very short pump pulse in the infrared visible domain so very short in time means very very Broad in Spectrum so it covers part of the visible and part of the IR spectrum it’s very intense so we we
Excite the sample with several photons at the time and then after some delay delta T we probe with an isolated atosecond pulse and then we disperse the atosecond pulse with the grating onto a camera and we measure the Spectrum as a function of this time delay in delta T
So This is called transient absorption spectroscopy it’s not much more difficult to understand than traditional apps option spectroscopy is called transient because we have a pump pulse and so the the Spectrum changes as a function of time and it tells us about what the electrons in the nuclei do in
The molecule so what these pump pulse does in our case is it takes an electron from the highest occupied molecular orbital and it excites it into a valence state but at the same time it also populates a high align with perk State then this weekly line here indicates electronic coherence so we
Simultaneously prepare a valence State and the reitburg state and they are coherently related to each other this is the prerequisite to see charge migration and we probe this with an isolated atosecond pulse this is now shorter than 200 Auto seconds it excites electrons from the inner shell the 2p shell of the silicon
Atom into these same final States and it allows us to probe this coherence and how it evolves as a function of time and these are the experimental data so on top here you can see the absorption spectrum of the silane molecule in the ground state side molecule this broad
Band here these are the two valence States and these sharper Peaks here these are the high align with work states and on the bottom here I’m showing the change of this spectrum how much this spectrum changes as a function of the delay between the pump pulse and the pro
Pass this is this delta T quantity and what you can see here in the boxes is that these absorption lines they are oscillating intensity very fast with a period of about 1.3 frame per second and so in the next few slides I will show you why we know that this is now the
Direct experimental signature of this electron flow in the molecule the charge migration and you can see that this oscillation disappears over time and then after about 45 frames per second it comes back so here we see electron flow in the molecule how it disappears and this is
Caused by nuclear motion I will show you why we know this and then how it comes back after about 57 seconds so first what we have to do to assign such a spectrum is we have to Fourier transform so we have to take a Fourier transformation along the time
Delay axis so what was time delay now becomes frequency but on the horizontal axis we still have Photon energy so we can now based on this spectrum here of which we knew the assignment from previous work we were able to assign which states are being populated and basically this looks
Relatively complicated but in reality there’s just uh two frequencies namely this group of frequency here corresponds to a coherence between the valence excited state and the reitburg state and this higher frequency here corresponds to a lower lying valence State and the same upper read perk state where these Wiggles still represent electronic
Coherence and now we know the electronic configuration of these states and we can compare this to a valence absorption spectrum and we can now identify this lower frequency coherence is between B and C States and the higher frequency coherence this is between the a state
And the C state but the a state is dark it’s not directly dipole allowed it’s um it’s a dark state but yet we see that it gets populated and why this gets populated is actually interesting in itself but let’s first focus on this lower frequency coherence the dominant one
Here you can see the experimental signal it oscillates with this 1.3 frame per second period which is nicely reproduced by calculations done by our colleagues in in Heidelberg and these oscillations they correspond to this electron flow in the radial direction of the molecule so the whole charge density of the molecule is breathing
Um it takes only 600 atoseconds for the the charge distribution to inflate and then another 600 atosecond to compress again and this gives rise to these oscillations that we see in the experimental data so we directly see there is this oscillation when the oscillations Decay and then after some
Time they revive and then they Decay again and this this is caused by the vibration of the molecule so by the nuclear motion and this is nicely shown by these calculations again done by our colleagues at the at time zero this is the moment when we excite the molecule
These the nuclear wave packets overlap but then when time evolves they oscillate with different frequencies and this gives rise to the fact that they lose overlap and when they lose overlap also the coherence goes away so the electronic coherence is dependent on having an overlap of the nuclear wave packet
And I will I will play this once more so that you can see where the Revival occurs so in Time Zero the overlap here they lose overlap but after about 50 frames per second they come back and here they overlap again and this gives rise to the Revival
That we observe in the electronic coherence so the separation of these wave packets uh suppresses the charge oscillation um and then when they overlap again it comes back but this initial coherence that was created by by the pulse this is what we expected because the B state is bright
It can be directly excited but what we did not expect is that the a state can also appear in our measurements and this is the the even more interesting result from these measurements is we found that an initial coherence created between B and C which causes this radial motion of
The charge density this can give rise to another coherence between the a state and the C State because the a state gets populated in just seven frames per second on this very short time skip we have population flowing to the a state and this is the experimental evidence so
The BC coherence appears at Time Zero but the AC coherence take some time to build up and then decays and now we can ask ourselves what does this AC coherence correspond to in terms of electron density so this is the one I discussed it’s this radial breathing of
The charge density that we can see in the molecule and the AC coherence the one that gets created with some delay this corresponds to radial motion of the electron density that you can see in these plots and now we can put this together so the Reconstruction from the
Experimental data with the help of Theory shows us that initially we have this radial breathing of the charge density um and then at intermediate times we have little change in the density this is the coherence of the charge migration but then after around 40 frame per second we initially see the angular motion
Starting and then around 15 times a second we have this radial motion again starting and so this um is more or less a complete picture of what the electron flow in this molecule does Over The View uh or a few tens of femtoseconds and what we found here is that initially we
Have radial charge migration and this can be converted to angular charge Migration by the transfer of population to the lower lying state so take home messages from this part is that we have been able to measure the flow of electron density which takes only about 600 atoseconds in neutral
Siding we have observed the decoherence and Revival of charge migration and we have seen that electronic coherence can survive for more than 100 femtoseconds even when there are many electronic states that are strongly coupled to each other and what we saw in this last part is that is that electronic coherence can
Be transferred through conical intersections which can which can switch the mode of charge migration from radial to angular so how am I doing on time okay um we have about 10 more minutes okay fantastic 10 minutes yes I will finish within 10 minutes so this was all gas phase measurements
And now I would like to tell you how we uh are now transferring these methods to the to the liquid phase and uh to more complex systems so what I discussed was an experiment on silane at 100 electron volts but if we go to higher energies
Here shown in this graph we have access to the 1s binding energies of carbon and nitrogen and oxygen and um this allows us to perform this kind of spectroscopy on more complex systems for example this this is a literature spectrum of a polymer a rather complicated system but you can nicely
Identify the different elements in it thanks to the characteristic binding energies and in addition to this um there are site specific binding energy so if you look at Carbon 1s and you look at different sites in the molecule they have different carbon one as binding energies and these
Chemical shifts there they can be related to NMR chemical shifts for example so you have a site sensitivity uh in these uh carbon 1s or nitrogen 1s binding energies and finally if you go in these high Photon energy range water becomes transparent so the absorption lengths of water is between 2 and 10
Micron in this so-called water window which allows one to realize measurements in liquid water and so this was the motivation for us a few years ago to start and develop a source of atosecond pulses at these high Photon energies so very briefly we go from the the normal
800 nanometer titanium Sapphire laser to a longer wavelengths 1800 nanometer and we comprise this to down to less than two Optical cycles and when we generate atosecond Pulses from these short midi IR pulses then we see indeed Spectra that cover the entire water window so they cover the
Carbon one the the one is binding energies of carbon nitrogen and oxygen with sufficient flux to do tiny dependent measurements and now instead of using a gas jet we use a so-called liquid flatjet so in the interaction region instead of putting a gas Target we put a liquid Jet
And this is a liquid jet that is formed by colliding two cylindrical Jets and this makes a thin sheet a flowing liquid sheet that is less than one micrometer in thickness and this is now thin enough that we can transmit the atosecond pulses through the liquid and in this
Way realize measurement in the liquid phase so this is the absorption Spectrum we measure when we put methanol in pure methanol in the liquid jet we see the the absorption Edge from the carbon the oxygen absorption Edge and what we did with this type of source is we now studied
Urea at 10 molar very concentrated accurate solution of urea this is the urea molecule so it consists of carbon oxygen and nitrogen and at the carbon Edge we see and a characteristic absorption Spectrum at the nitrogen Edge as well and the small difference between the orange and
The blue Spectrum here this is the change in absorption induced by ionization so now we ionize the urea solution with an intense 400 nanometer pulse and we look at the change in absorption induced by this ionization event why did we choose urea and concentrated urea it’s because concentrated urea
Solution are thought to have possibly played a role in the formation of biomolecules on the primordial Earth and this comes from the the famous Mila Yuri experiment where it was found that under the conditions of the primordial Earth urea is formed from this charge into gas mixtures of water methane ammonia and H2
After refluxing this for a few days or weeks it was found that amino acids appear but urea also appears in large concentrations and other Studies have since shown that when urea is exposed to ionizing radiation it can form malonic acid and urea together with malonic acid forms nuclear bases and therefore
It is assumed that urea is likely Prebiotic precursor of the nuclear bases and so this was the basic motivation for this study we wanted to look at what happens to concentrated urea solution exposed to ionizing radiation what what are the first steps and and how does this reaction sequence here initiate so
Here’s what we see we look at the change in the absorbance as a function of time delay at the carbon Edge so here we’re looking at the carbon Edge region and what we see is a band in the pre-edged region that shifts in position and rises in
Intensity and this band is only there at very high concentrations 10 molar urea it’s absent in five molar urea is also absent in lower concentrations so this is visibly a process that only takes place at very high urea concentrations in water so to interpret these data we collaborated with our colleagues in
Hamburg and the group of Robin santra and they performed so-called qmm so quantum mechanics molecular mechanics calculations what they do is they take urea a small group of urea molecules that they describe Quantum mechanically and then they embed this into a molecular mechanics Ensemble of the other urea molecules and water molecules
And in this way they can simulate the Dynamics of this system and what they found is that when they ionize a urea diameter this transfers a proton and this gives rise to this band that shifts and rises in intensity but there are other dimers that did not react this did
Not give rise to any change in the Spectrum and also ionized urea does not transfer a proton to water and ionized water does not transfer a proton to urea so the only reaction that takes place in this system is the proton transfer within the urea dimer and so this is
Indeed uh how we can understand this experimental data so this experimental signature agrees very well with this calculated feature and this allows us to conclude that indeed when we ionize urea solution the only reaction that takes place on very short time scales is that an ionized urea molecule transfers a
Proton to the neighbor and this takes less than 250 seconds so it’s a very fast type of proton transfer much faster than in neutral water or neutral Solutions in general and this is the consequence of ionization induced proton transfer what is interesting as a final note in
This experiment is that we found that the intensity of the absorption feature Rises on a slightly slower time scale than the position of the absorption resonance there’s a notable time scale difference between the two and this is because the absorption strengths this traces the amplitude of the electron
Hole on the carbon atom which initially is very small there is practically no electron hole amplitude on the carbon but eventually in the urea radical which is the final product of the reaction there is a large amplitude of the electron hole on the central atom and this gives rise to this rise in
Intensity but the the faster shift of the position this is this is directly tracing the motion of the proton that moves away from the ionized molecule to the neighbor um and then gives rise to the protonated uh urea molecule so take home messages from the second
Part is we that we have developed soft x-ray tabletop absorption spectroscopy we’ve applied this to Accurate Solutions and we’ve used this to identify femto second proton transfer in ionized urea dimers and we found in this experiment that time resolved x-ray absorption spectroscopy can distinguish the electronic rearrangement of the electron
Hole in this case from the charge transfer namely the proton moving away from the molecule and so this is a movie that shows that there is also very fast electron Dynamics and this is what we’re going to measure next namely within the first few frame to Second this electron
Hole oscillates back and forth between the two urea molecule eventually it localizes in this case on the upper molecule and that electron hole controls where the proton comes from which eventually gives rise to the ionized the protonated urea molecule and the urea radical and so I think this is the time
To stop here I would like to thank all the people who did the work also our collaborators who contributed the calculations and I would like to thank you very much for your attention and I look forward to your questions all right thank you so very much for excellent uh presentation um Chloe
Speaker thank you yes yes so do you want to start Chloe with any comments or um question well I do have a comment actually I actually lived in Switzerland for a while but I I lived in alve in the French part of Switzerland and I believe you mentioned something about
Um Luzon I think the University of Amazon I believe I went there for like like a school trip so we did like something along the lines of PCR actually there I mean that’s my comment and my question is this might be a general question but how did you become interested in
In the audience such this one Yes thank you um I I was I was interested initially I was interested in in organic chemistry but then soon I realized that I was more interested in in um doing uh physical chemistry and doing doing some calculations and combining quantum mechanics with chemistry and then this is what brought
Me to spectroscopy and I enjoyed very much the high resolution spectroscopy and learning in detail how how to understand molecules but then I was missing a bit the the the chemistry side so we’re studying the motion of molecule studying studying what molecules do and so this is how I came from doing
Spectroscopy in the frequency domain to doing a Time domain spectroscopy thank you that’s right um other questions let me check the chat and see uh we have Sylvan Sylvan you want to Let’s let’s see if you could if we could hear you speak louder let’s see if we could
Actually hear you oh yeah I was just wondering if like the weather had any effect technique um or okay did you did you hear the question unfortunately I did not understand the question no okay so he’s uh asking about the effect of weather on um how well your laser systems work and
He understands that you know it’s vacuum sealed and all of that but is there any sort of um external effect based on whether heat cold you know that sort of thing yes indeed our our laser systems are very very sensitive to changes in the environment temperature and to
Vibrations and so on so what we do is we we place the laser system in a separate room and no one is allowed to enter the room when we run the experiments and in that room we stabilize the temperature to 0.1 Kelvin so so we stabilize it extremely well and
We have a controlled airflow that keeps the the keeps the dust out and uh we stabilize the moisture as well the humidity and and this is this is important because uh otherwise the stability is not sufficient to run these kind of experiments right right um that uh is that Dr Randall’s question
With with the Silicon hydride work you shared were you describing something akin to vibrational normal modes but with electrons yes indeed indeed that’s that’s a very good point so let me go back to um doesn’t doesn’t allow me to jump surprisingly indeed so I was mentioning uh modes of
Charge migration at the end here so I said that the the radial charge migration that is there initially this is converted into an angular charge migration and one can indeed think about these these modes of charge migration in a way similar to normal modes because these modes of charge migration they
Depend on the relative symmetry of the states so what gives us radial charge migration here is the fact that we have a superposition of two totally symmetric States the 4a1 and 5a1 states in spectroscopic notation so both are totally symmetric and because they are totally symmetric this gives a radial
Wave packet but here we have two different symmetries here we have the three T2 and the 5a1 state so in an atomic picture this would be a p State and an S State and this P and S superposed this superimposed this gives an angular charge migration
And so one can indeed think of those as uh pretty much like normal modes good all right other questions speak loud let’s hear this may be a bit of a stupid question but I I recently learned in one of my classes whether or not the Moon sky can affect us
Things like really super senses like um concern I think it was a Dimensions that debate the face the moon or whether it’s like the tides would affect things like that is is this system complicated enough that uh the tides affected did phases affect ties so okay
I I think I I heard this question well so um as far as we know as far as we know no not so our lasers are not sensitive to these smaller changes in gravity uh luckily so we can run measurements in all moon phases at least we think right we never know because
Um I’ve been in the lab today and some things just didn’t work um this this happens and sometimes we don’t know I mean these experiments are complicated enough that we don’t have control over every detail which as much as we would and so I would not
Be able to exclude it with certainty but we we don’t think they’re sensitive to gravity to that level okay I have several questions um let me see if I uh the first one would be okay so does both um femtochemistry and atochemistry look at both intra molecular and intermolecular phenomena
Or is are they selective for one or the other I I would say they are interesting inter and intramolecular phenomena on both femtosec and up to second time scales so in femto second times case definitely because you can have unimolecular decomposition or you can have a
Bimolecular reaction or you can have a a charge transfer within two molecules all taking place on femtosecond time scale on the auto second time this time scale so far most people are thinking of in intramolecular processes yes but as as the the movie I showed at
The very end suggests yeah you can have electron hopping back and forth between two urea molecule and that would be an example of an in of an internal molecular up to Second process we haven’t seen it yet but uh it’s very likely to be there and we are working on
This at the moment so so we are working very hard to get this liquid phase uh urea experiment done with auto second resolution which we have only done with them to Second resolution so far but we have the technology to do it and and then we will most likely learn
Something interesting namely for how long this electron can really hop back and forth this this is unlikely to go on forever because this necessitates electronic coherence and once the coherence is lost this electron hopping will stop and and how fast that Deco hears is is an interesting question that we want to
Answer yeah so I’m glad to hear that you started off in organic chemistry because a number of our students here have already taken organic chemistry so they know about sn2 the sn2 mechanism so my question is which of these femto or ATO and it may
Already have been done I may not just be up on the literature um have uh sn2 reaction been studied that’s one part and we know that as well we assume that sn2 is a two electron process is it ah this this is indeed a very interesting question so first I would
Say in in sn2 you need nuclear motion right you need your nucleophile to approach but the the the the moment or or the transition state certainly has some atosecond Dynamics okay because when the nucleophile attacks uh then you have an you have new a new Bond forming you have
A the old bond breaking and this all involves electron motion and some of it will be very fast mm-hmm and both both has to take place obviously right the nuclear motion and electron right and in in many cases one would assume this is nuclear motion driven so it if
It proceeds adiabatically if it really follows the lowest like State all the time then it will be a pure FM to Second process but this is unlikely to be the case most such reactions are actually non-adiabatic and you have more than one state populated and when whenever you
Have more than one state populated you can have electronic coherence and then you can create these charge migration you can create this electron flow on the other second time scale and that may actually affect the outcome of the of the reaction and do you have a sense it’s really the
Classical two electron or is it you know from the donor this this I I can tell you because we haven’t done experiments on this and I and and I know no one has done Auto second experiments on sn2 type reactions but this is definitely something very interesting that one should look at the
Challenge with with these kind of reactions is is not so clear how one can initiate them on the other second time scale right it’s via how they’re initiated on the fem to second time scale by the Collision but one would need a trigger with atosecond resolution in order to access the purely electronic
Motion and this is a bit The Challenge there are some ideas around but it’s not completely clear how to do that yeah okay uh we could take a couple yeah hi this is a uh it’s maybe a simple-minded question but dealing with these extremely short time frames
You know is it better for your for the theory and practice of these experiments to think of elected electrons as waves as opposed to particles at what point does the usefulness of the particle idea versus The Wave idea come and interact with these very short time frames
Yeah that’s an excellent question so in in most in most representations um for example the ones here we use the wave picture because we use uh the orbitals because it’s easy it’s easiest to think in terms of the orbitals because we know how to calculate such
Curves and how to get the orbitals from them and so on but indeed in certain cases um this is not the most intuitive representation so one one example is um the what I showed at the very beginning I have to script this course so namely when we start thinking about where we
Creating a hole in when we ionize a molecule and then it’s easier to think of the particle picture so for example to understand why we create the hole on this side or on that side of the molecule date becomes indeed easier to think of the electron as a particle it
Gives us the intuitive picture where the hole is created which is not as by far not as intuitive in the wave picture because in the way you picture the answer is oh yeah the phase is pi or the phase is pi half but this doesn’t tell us anything intuitive what tells us
Something intuitive is is the electron leaves from the iodine atom because it’s the most polarizable it creates the hole here and so this is where the particle picture comes in it gives us an intuitive understanding of the electron of the ionization Dynamics when the electron is removed So again okay another question I had you had um water plus H2O plus are we looking at just the ionization the loss of one electron and is that electron coming from like the non-bonding pairs yes exactly so um when we when we look at the water plus the the electron came
From the homo which is uh the lone pair on the oxygen okay and and when we ionize urea it comes from um from a pie from the pi orbital yeah the pi Bond would be more accessible okay okay all right very good other questions see anything in the chat well thank you
So very much for a very good presentation I know I learned a lot from it um but uh continued success in uh your research thank you very much for the invitation it was a great pleasure to speak to you bye-bye yeah yeah these guys wanted we want to talk to Zurich sometime yeah
There there yeah and you made a diff at least a difficult subject for me yes uh somewhat understandable well done oh I’m very glad to hear that thank you thank you thank you very much