31 May 2017 - UCI Media

AirUCI: Saswata Roy & Kara Kapnas

our next presentation is also a team

presentation by two graduate students students ashati Roy is a graduate student in the full Health Group studying theoretical chemistry and then Kara kapinas is a graduate student in the Craig Marie's group for doing experiments on kinetics and spectroscopy so we all know one thing about making a movie the director yells lights camera action but what you might not know is that this phrase can be applied to the subject of photochemistry so Rotom she played a critical role in the Nobel Prize won by sherry Rowland and Mario Molina when they discover that CFCs are being broken apart by radiation from the Sun and the photo products would then go on to participate in ozone destruction forming them the ozone hole we're interested in is this very first step when the sun's radiation interacts with the molecule more generally our core question is what happens to molecules under the sun's radiation we want to understand these gory details now even though this question might seem rather line move we are looking at answers like how many products are formed what are the different products formed how much of the energy is partitioned into each

of the fragments so from a theoretical viewpoint the actual quantum mechanical answer can be found only for one electrons and the exact solution grows exponentially as we increase the number of electrons so we have very limited hope of getting the exact answer to such a question now we would be using numerical methods to understand this and in particular we are limited by size so we would be looking at a model molecule in our case acetaldehyde and in particular we would be looking at one of the pathways in which the methyl and the four-mile radicals are formed and we would be looking at how much of the energy is partitioned into the jiggling or the vibrational load of the molecule or how much of it goes into the translational degrees of freedom because the internal degrees of freedom determines the further reactivity of these fragments so we use advanced experimental techniques coupled with high level of theoretical calculations to help us being a deeper understanding of fundamental photochemistry now I do experiments using lasers to simulate the Sun coupled with imaging techniques to

help us answer some of these questions so I use a technique known as velocity and mass ion imaging where this machine was home-built in our lab in the Murray group and so this technique allows me to be able to map the velocity distributions of photo products or the fragments of a molecule after it's been hit by UV light and it falls apart and then this data can be captured using a camera so we're going to show you then our movie called picturing photochemistry starring acetaldehyde to give you an idea of how this experiment works so White's camera action we're going to start zooming in onto the very initial point of our experiment so what we're going to see is we're going to supersonically expand our gas molecules which is going to cool them meaning that the molecules are going to have zero initial internal energies is going to be our starting point so our gas expands we have a molecular beam then what's going to happen is we're going to take a laser beam the waves are going to hit our molecules it's going to fragment then we're going to bring in a second laser beam that's going to ionize these fragments these ionized fragments are

going to travel through a stack of electrodes that have high voltages applied to them that's going to allow us to map then the velocity distributions of these fragments so we have our ion cloud here that's expanding is accelerating towards our position sensitive detector and bam it hits our detector we take a very narrow central slice of that ion cloud and what we see are these rings and so these rings represent the speed at which these fragments are traveling so the larger the Rings become means that our fragments are traveling faster and so we snap a picture of this data and this is what we get so we see as we scan our laser over multiple wavelengths corresponding to radiation from the Sun we see that our rings expand and this is telling us that as we put more energy into our system rings are getting larger and our fragments are traveling faster now all we see is what is at the very end of our detector what we'd like to see is a molecular view of this so I'm going to rewind our movie I'm going to rewind it back to the point where right where the laser is going to hit the molecules and

so we want to see what exactly up close this doing is this is where we're going to need theory so either from the molecular dynamics simulation but unlike at the previous group that presented we use ab initio molecular dynamics where the electronic energies are modeled by density functional theory and using just quantum mechanics and classical mechanics taking no information from my collaborator I sample a large number of these trajectories and my data also is another movie which looks like this in this particular case we can see that the methyl and the formal radicals break apart I do lo extra manipulation I just take statistical sampling and I keep collecting the velocity at which this methyl fragment flies away and this is what I see that with increasing amount of energy pumped into the system this is what I see the increasing amount of energy pumped into the system sorry about that we see that the metal fragment flies faster and faster the other degrees of freedom the internal rotational or the vibrational degrees of freedom do not increase in energy so much as the

translation so we want to be able to compare then his theoretical results of my experimental results and so just to remind you again of how we did our experiments remember we have our cooled gas or acid aldehyde molecules so we're going to hit with laser beams they're going to fragment and then we're going to detect these fragments on our detector here and we see these rings and so when the showed you then is the speed distributions so we have collected for the methyl fragment now I want to draw your attention to this dashed line right here and so this dashed line represents the maximum velocity that our methyl fragments could be traveling if all energy were to be put into translation and we see then that these distributions follow this line very nicely telling us that most of the energy is eat going into translation of these fragments and so another thing I need to mention is that we are detecting metal fragments but due to the conservation of energy we can also gain our insight into the other radical to formal radicals so as we see with our results they give us great confidence in the theoretical results telling us them that we can use

these theoretical methods to help project other photodissociation pathways that may be difficult for me as an experimentalist to detect more importantly we have an arsenal of tools both experimentally as well as theoretically to help us further ask more interesting questions about how Bhalla cules interact with the sun's radiation further telling us the fate of these molecules in the atmosphere so far further information or the director's cut of these movies contact us at these email id and thank you for your attention [Applause] [Music]

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