1. Methanol Photodissociation Branching Ratios and Their Influence on Interstellar Organic Chemistry
Thank you Susanna. So I’ve been combining both laboratory work and computer modeling in order to gain a better understanding of methanol and its role in interstellar chemistry.
Jacob Laas1, Susanna Widicus Weaver1, and Robin Garrod2
1Department of Chemistry, Emory University
2Department of Astronomy, Cornell University

2. H H2 CO
HCO+
H2O
Methanol photodissociation studies are tied together via gas/grain astrochemical modeling of hot cores
Dust grain
Ice mantle
H2O, CH3OH, CO, NH3 , H2CO hn
CH3CN
H2CO CO
HCO+
H2O
CH3OH H2
NH3
NH2CHO
CH3NH2
CH3OCHO
CH3CH2OH
CH3COCH3
CH3COOH
Before I go into detail about my studies, let me first illustrate the emerging picture of the formation of complex organic molecules in interstellar hot cores. If you look in dense, cold clouds, you’ll find mostly simple molecules and dust grains. Because the temperatures are so low, these dust grains can get coated with an ice mantle. Complex organics can then form through processing in these ices and eventual warm-up and evaporation will return the matter to the gas phase, with the gas containing more complex organics than before. Of course, this evolution is much more dynamical but it should put things into perspective.

3. Importance of Methanol
Methanol is highly abundant in both gas and ice
Methanol photodissociation yields three organic radicals; branching ratios (BRs) are not known
Photolysis products may significantly contribute to the structural isomerism of complex organic molecules
May play a role in the formation of methyl formate and its structural isomers acetic acid and glycolaldehyde
CH3OH
·CH2OH + H
CH3O· + H
·CH3 + ·OH
H2CO + H2 hν
·CHO
HCOCH2OH
HCOOCH3
HCOCH3 -H
+OH
CH3COOH
There are several reasons for why I’m focusing on methanol. For one, it is highly abundant in both the gas phase (at ~10^-5 per hydrogen atom) and in interstellar ices (where the literature quotes concentrations in the range of 1-30% of the total ice). Also, its photolysis yields three organic radicals. The combination of these two facts leads us to believe that methanol photolysis is THE primary source for these species. However, the branching ratios for these photolysis products are not known.
Despite the increasing number of interstellar detections of complex organic molecules, our understanding of their spatial structure and relative abundances of the organic isomerism is still quite lacking. A good example of this is reconciling the relative abundances of methyl formate and its structural isomers acetic acid and glycolaldehyde. Gas/grain modeling has improved this outlook, but the observed and predicted relative abundances still have not been able to match up very well. What I hope to accomplish in my current studies is to shed some light upon this discrepancy since the photolysis products may be important predecessors for these species.
Let me first discuss my ongoing laboratory work.

4. Past Photolysis Studies
70 years of previous studies in literature
Most gas-phase studies involve indirect measurements of BRs
Most comprehensive lab study indicates:
If one scours the literature, you’ll find that there have been MANY previous studies of methanol photolysis conducted in the past. However most of the laboratory studies involved indirect measurements for branching ratios, for example kinetics models that incorporated chemical scavengers or measurements of system pressures that were then related to hydrogen gas. The most comprehensive study, which made use of a kinetics model and chemical scavengers, was reported in 1968 and yielded the following estimates. One should take note that what is believed to be the dominant channel actually comprises TWO channels, illustrating one of the primary experimental challenges…
Hagege et al. 1968, Trans. Faraday Soc., 64, 3288

5. Laboratory Challenges
Some branching channels are difficult to differentiate
CH3O and CH2OH have the same mass, thus mass-spec does not work well
Photolysis products are highly reactive
Must use direct detection methods and/or prevent side reactions
Must determine wavelength-dependence of photolysis for astrochemical models
…notably that two of the radical products have the same mass, making mass-spectrometry techniques much more complicated. Also, most of the photolysis products are highly reactive, owing to the fact that they are radical species. Another major challenge is the fact that most photolysis studies involve small-bandwidth lasers. This may not seem like a challenge to a physical chemist because they might think, sure give me a nice powerful laser and I’ll photolyze anything for you! However, lasers present a challenge to models, which ideally should come to terms with the wavelength-dependence of photodissociation processes, particularly in the case of methanol which has several distinct absorption bands in the VUV region which are each thought to primarily affect different photolysis channels.

6. Proposed Technique Quantitative submm spectroscopy
Supersonic expansion
Variety of arc lamps available for wavelength-dependent study
To this end, my current experimental setup makes use of submm spectroscopy for directly studying each photolysis channel. The molecular sample is prepared via a cooled supersonic expansion into a vacuum chamber. As is shown in the figure to the right, a UV beam is focused at the throat of the expansion and the submm spectroscopy is directed through a down-wind cross section, just before the mach disk. A variety of arc lamps are available across a broad UV spectrum in the range of about nm, where methanol begins to strongly absorb and its absorption cross-section has been well-characterized. Spectral lines are necessary for at least one product of each photolysis channel. Spectra for one channel’s species, however, is unknown, and this will first be predicted and then characterized in the laboratory.

7. Laboratory Spectroscopy
Reproducible depletion of methanol lines achieved
10 ± 3% photolysis efficiency
Current focus:
Removal of signal contribution
from background gas enabling
full quantitative analysis
Search for photolysis products
At this point, the basic experimental setup has been built and I’ve been able to reproducibly deplete source lines at an efficiency of about 10%. The most recent challenge has been dealing with signal contribution from background gas, which has prevented a much more rigorous quantitative analysis. This effect essentially results in a consistent drop in line intensity of the “cool” lines that are thought to be associated with the jet expansion but the background gas has been observed to contribute too significantly for a more complete analysis of the sample. So my intention is to use purge gas to help diminish this effect. And then I’ll go on to search for photolysis products.
That’s the status of the lab work, so let me move on the modeling effort…

8. Astrochemical Modeling
Method Test varying sets of BRs at different warm-up timescales
Fast
Intermediate
Slow
5·104 yr
2·105 yr
1·106 yr
Branching Ratios
CH3:CH2OH:CH3O (%)
Label
60:20:20
Standard1
12:73:15
Öberg2
90:5:5
Methyl
5:90:5
Hydroxymethyl
5:5:90
Methoxy
The modeling process has been fairly simple: run different sets of branching ratios at different warm-up timescales using a gas/grain code that was developed here at OSU in the Herbst group. The timescales were chosen to reflect three different classes of “hot core” molecular clouds distinguished by their different masses. That is, under this model, high-mass hot cores are associated with a warm-up timescale of approximately 50,000 years and the lowest-mass hot cores are associated with a warm-up time on the order of one million years.
The specific branching ratios that were tested are shown here to the right. The standard value set is what was used in the 2008 paper that introduced the current chemical network that I am working with. The other sets were tested for either only with the grain-surface channels, only in the gas-phase or with both cases. Also, for clarification, the values labeled as Oberg are those values that were derived from Karin Oberg’s experiments involving methanol ices while she was in the Leiden group.
1 Garrod et al. 2008, ApJ, 682, 283
2 Öberg et al. 2009, A&A, 504, 891

9. Astrochemical Modeling
Results
Some sets of BRs improved the agreement between predicted abundances and observations
Sgr B2(N-LMH)1
Standard
90% Methoxy
So one of the major effects that was seen after all these different trials were ran was that there WAS a noticeable difference in the predicted abundances for various molecules, particularly among the more complex organics. And this makes sense because these radical products are thought to have a greater affect on more complex chemistry. In fact, there were a few branching ratio sets that considerably improved upon the agreement between predicted abundances and those observed in Sgr B2, one of the more notable ones is with the methoxy channel dominating the photolysis.
1 Garrod et al. 2008, ApJ, 682, 283

10. Astrochemical Modeling
Results (cont’d)
Qualitative agreement found for relative abundances of methyl formate and structural isomers
Warm-up timescale also significantly influences the relative abundances of complex molecules
A combination of BRs favoring CH3O channel and slow warm-up timescale give the best match to Sgr observations
Predicted peak abundances using methoxy BRs
Regarding the relative abundances of methyl formate and its structural isomers, qualitative agreement is reached using the methoxy branching ratios at slow warmup timescales. What we didn’t expect is the significance of the warmup timescale, which suggests that the physical model affects the results much more significantly than was anticipated. So it is only with a combination of BRs favoring the methoxy channel and the slow warm-up timescale that gives the best match to Sgr observations.
For Sgr B2
n ~ 3000 cm^-3 (~ 30 times more dense than typical cloud)
width ~ 45 pc (1 pc ~ 2E13 mi)
mass ~ 3E6 solar masses (1 solar mass ~ 2E30 kg)

11. Astrochemical Modeling
Implications
Methanol photolysis branching ratios, warm-up timescales greatly influence the relative abundances of complex organic molecules in interstellar clouds
Physics of Sgr B2 is likely more complicated than model
Observations of more sources are needed for comparison
Important formation and destruction routes are likely lacking in the reaction network
Barrierless gas-phase ion-molecule channels leading to trans-methyl formate have been found through ab initio calculations (Pate Group)
Laboratory measurements are required to determine branching ratios quantitatively
In summary, it was found that the hypothesis which predicted the importance of methanol photolysis was confirmed. However, it was observed that probably there’s more to the story than just the branching ratios: that is, the timescales played a much more important role than anticipated, and suggests that the physics is likely more complicated than what the model considers. But before we jump to conclusions about the physical model, other sources with well-characterized physical parameters are needed for comparison.
Another point to make is the fact that qualitative agreement of structural isomer abundances is not necessarily the end of the story… the actual predicted abundances were still off by up to an order of magnitude, suggesting that formation/destruction routes of relevant chemistry are likely still lacking.
And finally, now that the effects of methanol photolysis have been tested, lab work must ultimately be done to define the extent of these effects.

12. Acknowledgements Widicus Weaver Group, Emory Eric Herbst, OSU
Thom Orlando & Greg Grieves, GA Tech
NSF Center for Chemistry of the Universe, UVa
And with that, I must thank…
the rest of my research group at Emory, because without any of their help, the laboratory work wouldn’t have been possible from such a new lab.
I must thank Eric Herbst for his help with the modeling work,
the Orlando group at GA Tech,
the Center for Chemistry of the Universe, which is a NSF Center for Chemical Innovation based out of UVA,
and you for your attention!