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[Music]

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and thanks Brandon for the delightful

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introductory um my name is Elsa cook I'm

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a graduate student here in

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Charlottesville at the Department of

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Chemistry and at the University of

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Virginia and today I want to talk to you

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about some work that we've been doing to

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measure the UV photo distraction of um

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astro chemical Isis of interstellar

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importance now this work is being done

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in collaboration with Karen Oberg and

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Edith Vail and at the

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harvard-smithsonian Center for

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Astrophysics and I'd also like to thank

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John Yates whose laboratory this work

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has been conducted in and as well as

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Eric herbs Rob Garrett and Matthew rash

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so I just wanted to begin by talking a

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little bit about how we tend to think

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about UV radiation in life and most of

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you will be more familiar with the

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left-hand side of this plot so how we

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think about UV radiation on earth and

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and we tend to think of it as a

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destructive and mechanism so for example

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in skin damage so sunburn and Sun tans

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and as well as DNA damage on the

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cellular level and we also use a UV

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radiation on earth as a sanitizer so for

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example in food storage and though we

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also know that on a UV radiation can be

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essential to life forming and so for

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example in vitamin D production which is

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important in calcium metabolism as well

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as producing our own ozone layer by the

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photo distraction of oxygen and in

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interstellar space we tend to think of

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UV radiation as a heat or energy source

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which can drive reactions that wouldn't

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occur at such low temperatures a UV

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radiation is able to dissolve molecules

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as well as destroying molecules and ions

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and creating reactive fragments they can

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then combine and form more complex

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species as these icy dust grains play a

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key role in producing the complex

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organic molecules that we observe in

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interstellar regions and so with

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infrared telescopes we can see the

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actual bands of several condensed phase

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molecules and I'm showing some of them

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are on the right and where the areas

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roughly correspond to their relative

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abundance and the Isis so we have water

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which is the most abundant ice species

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followed by co2 and co and then we have

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less abundant species like methanol

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ammonia and methane and these ices can

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be processed and not only by thermal

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heating but also by UV photons as well

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as other energetic particles like

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electrons cosmic rays and ions and this

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ice processing can result in reactive

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fragments that can then recombine and

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form potentially prebiotic organic

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molecules so to understand the

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importance of these processes we really

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need our accurate laboratory

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measurements of their cross-sections go

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back okay so what do I mean when I'm

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talking about a cross-section and so I

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just talk to you a little bit about how

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I'll refer to these cross-sections in

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this talk so for example when we're

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talking about UV photodissociation we

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have some molecule that has a rate of

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being destroyed which is d molecule by

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DT and then this is equal to the photon

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flux and this is a number of photons per

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some area per time and the concentration

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of the molecule in the ice and then we

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have this Sigma value which is

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essentially a probability that the

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photon will be absorbed and then destroy

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the ice molecule and these probability

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factors are especially important because

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once we've measured them in the lab we

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can then use them with the UV flux in

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interstellar space to calculate some

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rate coefficient K which can then be

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input into extra chemical models and so

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how do we go about creating the UV field

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in the laboratory

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well the UV field and interstellar space

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really depends on the physical

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properties of the environment so near

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stars the UV radiation tends to be

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dominated by blackbody emission but when

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we go deep into dense cloud cores or on

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the disk surfaces you can see we have

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this really strong emission at

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lyman-alpha which comes from the

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recombination of hydrogen with cosmic

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rays so in the laboratory and there's

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several different ways that we can

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create the

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radiation and so people have used

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discharge lamps lasers and synchrotrons

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in our laboratory we're using a hydrogen

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discharge lamp that's shown in this

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image here and this is a spectra that

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shows the emission from our hydrogen

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lamp so it's 10 percent hydrogen diluted

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an argon and you can see it also has

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this really strong emission at 121

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nanometers as this brings me to our

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current work where we're measuring the

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photo distraction of a range of astra

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chemically relevant ice species and

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they've been oh go back there been

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several studies of the photo destruction

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of interstellar ices and that most of

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them don't report a UV cross-section two

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main studies have been conducted which

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do report the cross section however

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their ices were really thick so only the

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about the top team bassoon of the ice

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and was able to absorb the photons a

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study of Isis and the optically thin

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regime hasn't been previously attempted

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and while some case studies do exist and

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there's no systematic study that

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measures the ice photo distraction for

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optically thin ices and so I'm going to

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talk to you a little bit today about the

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ices better shown in red so the co2

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methanol water and ammonia ices and we

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will study their photo destruction in

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optically thin ices and will also

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measure not only the pure ices but also

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their photo destruction in Astrophysical

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irrelevant matrices like water and Co

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and also in a noble gas matrices just to

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understand the physics a little bit

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better and we also measure the cross

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sections at a range of different

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temperatures which can tell us more

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about the chemistry okay so I just want

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to tell you a little bit about the

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experimental setup that we're using so

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we use a vacuum chamber so as Brandon

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was saying and in interstellar space the

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pressures are much lower so we need to

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pump away all the air so we use a vacuum

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chamber it's coupled to an infrared

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spectrometer which means it measures the

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condensed phase of the ice and the ices

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are deposited from the gas phase at the

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back of the chamber from using a gas

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phase dozer and then we have our photon

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source which is as I said before a

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hydrogen discharge lamp it's

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with a hydrogen argon mixture and then

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excited by a radiofrequency coil and

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then beneath the cell we have a mass

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spectrometer which is able to monitor

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the gas phase and so if I just cut away

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the walls of the chamber you can get a

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clearer image of the radiation

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configuration and so the ice is

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condensed here on a potassium bromide

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disk that's pressed into a tungsten grid

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and this is cooled by a closed cycle

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helium Croat and the ice is suspended at

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45 degrees to both the infrared beam and

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our UV photon source and so I just want

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to introduce a couple of the preliminary

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results from our experiments so the

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first ice species that we've studied in

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co2 ice so to co2 ices have only one

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major channel at 121 nanometers and

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that's the formation of carbon monoxide

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and oxygen atoms and then the gas phase

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thus precedes with efficiency that's

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near unity however in Isis the co can

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recombine of oxygen atoms to reform the

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co2 molecules and but this does have

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some barriers so we will need some

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excess energy we want that process to

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happen the oxygen atoms can also combine

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or co2 to form this carbon trioxide

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species or also with other oxygen atoms

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to form Oh to or ozone and so this these

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pictures show the radiation of co2 over

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time so the scale bar is just a measure

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of the time of the irradiation you can

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see the loss of this stretching mode of

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co2 over time and you can see the co in

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the carbon dioxide are growing in during

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the experiment and so in order to

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extract the cross-section from these

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measurements we fit a first-order decay

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to the data and so this has the form d

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co2 by DT and is the concentration of

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the co2 times some rate coefficient J

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and the integrated rate expression is an

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exponential so we plot the logarithm of

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the loss of the co2 over time and then

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the slope is that rate coefficient J and

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then from J we can extract our cross

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section using the knowing laboratory

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photon flux

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I see here some initial results for the

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co2 Isis and you can see we've ran these

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experiments at 25 40 and 55 Kelvin for

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co2 is shown in the black circles and

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you can see that the co2

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photodissociation cross-section doesn't

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really change over temperature and the

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photodissociation cross-section is low

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compared to the gas phase so only around

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one co2 per seven photons is destroyed

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whereas it's near unity and the gas

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phase we also add co2 the ice in the

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cross section is essentially the same

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and compared to adding water a water ice

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matrix where we can see the cross

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section is decreased and this is

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probably because the water ice when it's

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floated associated forms these hydroxyl

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radicals which can react with the photo

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produced Co and reform the co2 we also

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looked at methanol Isis so it's branding

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kind of mentioned methanol is really

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important to us because for us it's a

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complex molecule and it can be a

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precursor to potentially prebiotic

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molecules like Geico aldehyde and so

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these spectra show the loss of the

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methanol band over time and you can see

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some of the photo products that are

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growing in so we have co2 and co and

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formaldehyde and also methane and there

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are some other smaller signatures of

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more complex species as well so here are

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the results of the measurements of the

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cross sections for methanol and so in

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contrary to co2 you can see that the

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cross section does increase with

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temperature actually it's almost

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exponential I mean there are a couple of

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explanations for why this might happen

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so as we increase the temperature the

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photo fragments and the methanol ice can

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diffuse away from each other more easily

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so we'll have less recombinations to

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reform the methanol and the photo

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fragments also as temperature is

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increased can dissolve more easily so we

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also would have this reformation of

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methanol and we can see again that the

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cross section is lower compared to the

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gas phase so only about one in 25

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photons and resulted in it dissociation

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of methanol at 25 Kelvin and this

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increases to about one and

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even at 100 Kelvin we can also see

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contrary to co2 adding a water ice

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matrix doesn't really change the

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cross-section indicating that the photo

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fragment diffusion in the water s matrix

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isn't really changed by having a water

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ace and some of these results can be

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explained in part by the cage effect in

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Isis so we saw that the gas phase cross

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section tended to be much higher than

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the solid phase cross section and this

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is because the ice matrix so these red

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ice species and hinders the diffusion of

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the photo fragments so then we just get

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more recombination events as we increase

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the temperature the photo fragment

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diffusion becomes more easy and then we

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will see that the cross section would

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increase with temperature and we see

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this result for our methanol ices and so

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the next steps in our survey are to

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continue these measurements for other

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ices of interstellar importance so we've

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made just a couple of preliminary

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measurements for water and ammonia races

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and so ammonia at around 70 Kelvin and

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water at 120 Kelvin and you can see

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actually the cross sections are very low

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even compared to our co2 in methanol

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ices and this is probably because the

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recombination it happens really fast in

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the water ice or ammonia matrix because

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it's most likely due to hydrogen

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recombination and so just in summary so

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we're conducting the survey of photo

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destruction cross sections for optically

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thin ices of interstellar importance and

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the photodissociation rates will help us

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provide some information about the

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chemistry occurring in the ices and the

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radical recombination rates and the

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temperature dependencies and relative

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rates can and in part be explained by

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the cage effect and these quantitative

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measurements will be especially

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important for understanding ice

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chemistry that could potentially form

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prebiotic molecules and interstellar

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questions just I didn't get the part

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00:13:24,490 --> 00:13:21,380
that you had to excite your sample with

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00:13:26,199 --> 00:13:24,500
radio frequency before doing so it's not

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00:13:28,749 --> 00:13:26,209
the sample the radio frequency is just

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to provide the radiation source so we

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00:13:33,160 --> 00:13:31,069
need some we're just providing a

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hydrogen discharge lamp to create the UV

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radiation the radio frequency source

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isn't involved in the sample dye sample

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okay so it has been shown in prebiotic

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00:13:51,400 --> 00:13:47,120
chemistry Studies on early Earth that

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hydrogen cyanide has a potential clue

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00:13:58,749 --> 00:13:55,910
role in combination with UV light so I

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was wondering if first you have done

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some studies on hydrogen cyanide and

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then if you have done some studies on

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combination of molecules so if you can

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00:14:12,610 --> 00:14:09,050
detect something different when you mix

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00:14:14,379 --> 00:14:12,620
kind different kind of molecules sure so

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00:14:16,600 --> 00:14:14,389
we haven't done hydrogen cyanide it's a

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00:14:19,269 --> 00:14:16,610
possibility and it's very dangerous so

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we tend to stick with somewhat dangerous

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species last weekend and that people

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00:14:25,660 --> 00:14:24,199
have studied the photodissociation of

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hydrogen cyanide as well

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and we haven't done it yet and as for

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the ice mixtures this is also something

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we'd like to do but we start with the

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simplest I supposed and then work our

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00:14:37,509 --> 00:14:35,839
way up to the more complicated species

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just so we can really understand what's

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happening in the ice anybody else so

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00:14:49,269 --> 00:14:47,600
this is really cool I was curious you

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00:14:51,160 --> 00:14:49,279
mentioned that you have Co plus o

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recombination and from a photochemical

351
00:14:55,210 --> 00:14:53,029
perspective that's often treated as

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being spin forbidden so what do you

353
00:14:59,019 --> 00:14:56,990
think there's any intermediate steps or

354
00:15:01,600 --> 00:14:59,029
do you have any impurities that to drive

355
00:15:03,129 --> 00:15:01,610
that catalysis I would say that we don't

356
00:15:05,019 --> 00:15:03,139
know that we have Co plus our

357
00:15:06,819 --> 00:15:05,029
recombination is just a possibility and

358
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the reason why I just showed it is

359
00:15:11,170 --> 00:15:09,500
because we may have some excited CO and

360
00:15:13,360 --> 00:15:11,180
oxygen atoms that this process could

361
00:15:14,050 --> 00:15:13,370
occur so when we radiate the ice some of

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00:15:16,510 --> 00:15:14,060
them will stay in

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00:15:18,760 --> 00:15:16,520
excited state maybe allowing some synth

364
00:15:20,230 --> 00:15:18,770
ridden process to heaven but there's no

365
00:15:21,700 --> 00:15:20,240
way that we can possibly tell that

366
00:15:23,320 --> 00:15:21,710
that's happening and actually I would

367
00:15:25,210 --> 00:15:23,330
think it's not happening because we

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00:15:29,410 --> 00:15:25,220
don't have a temperature dependence with

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00:15:31,510 --> 00:15:29,420
a co2 irradiation so something we can't

370
00:15:32,980 --> 00:15:31,520
really distinguish but is differently

371
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forbidden and I think the barrier is

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00:15:37,390 --> 00:15:34,940
pretty high it's maybe like a thousand

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00:15:44,470 --> 00:15:37,400
Kelvin or something so maybe it doesn't

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00:15:47,230 --> 00:15:44,480
happen high house I enjoyed your talk

375
00:15:49,210 --> 00:15:47,240
quick question I understand that the

376
00:15:51,820 --> 00:15:49,220
more complex organic molecules don't

377
00:15:53,680 --> 00:15:51,830
show up in the actual samples what I'm

378
00:15:55,150 --> 00:15:53,690
wondering is has there been any work in

379
00:16:03,270 --> 00:15:55,160
terms of modeling them numerically that

380
00:16:07,060 --> 00:16:03,280
would show necessarily if it's not so

381
00:16:08,980 --> 00:16:07,070
the gong the I was wondering if the if

382
00:16:10,900 --> 00:16:08,990
the molecules more complex molecules

383
00:16:13,210 --> 00:16:10,910
have been modeled to show what the rate

384
00:16:16,240 --> 00:16:13,220
of dissociation or probability of more

385
00:16:18,010 --> 00:16:16,250
complex yeah those would be um I don't

386
00:16:20,020 --> 00:16:18,020
as far as I know I don't think that

387
00:16:23,980 --> 00:16:20,030
there are any cross-section measurements

388
00:16:25,230 --> 00:16:23,990
for really complex I species I'm sorry

389
00:16:27,700 --> 00:16:25,240
go ahead

390
00:16:29,110 --> 00:16:27,710
yeah and the modeling why are you going

391
00:16:31,810 --> 00:16:29,120
to hear about some modeling in the next

392
00:16:33,820 --> 00:16:31,820
door and but I think modeling for

393
00:16:35,980 --> 00:16:33,830
complex Isis gets really complicated

394
00:16:38,260 --> 00:16:35,990
really fast so Chris will talk about the

395
00:16:41,230 --> 00:16:38,270
most simplest ice system but even

396
00:16:43,840 --> 00:16:41,240
getting to water or just a methanol ice

397
00:16:46,510 --> 00:16:43,850
it gets so complicated so I think it's

398
00:16:49,900 --> 00:16:46,520
something that the field should do but

399
00:16:51,610 --> 00:16:49,910
it's going to take a lot of time um but

400
00:16:53,620 --> 00:16:51,620
as for the experiments which is what I

401
00:16:55,750 --> 00:16:53,630
know a little bit more about there are

402
00:16:57,820 --> 00:16:55,760
experiments where people do see complex

403
00:16:59,800 --> 00:16:57,830
molecules and Isis if you want to look

404
00:17:01,780 --> 00:16:59,810
at really complicated species you need

405
00:17:04,450 --> 00:17:01,790
to do something like GCMs so you would

406
00:17:06,400 --> 00:17:04,460
collect the ice sample and then look at

407
00:17:07,780 --> 00:17:06,410
the mass spectrometer and figure out

408
00:17:10,360 --> 00:17:07,790
what's in there and people have seen

409
00:17:12,770 --> 00:17:10,370
things like amino acids and other things

410
00:17:16,600 --> 00:17:12,780
well I say they have