Methanol Photodissociation Branching Ratios and Their Influence on Interstellar Organic Chemistry...
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Transcript of Methanol Photodissociation Branching Ratios and Their Influence on Interstellar Organic Chemistry...
Methanol Photodissociation Branching Ratios and Their Influence
on Interstellar Organic Chemistry
Jacob Laas1, Susanna Widicus Weaver1, and Robin Garrod2
1Department of Chemistry, Emory University2Department of Astronomy, Cornell University
H
H2 CO
HCO+
H2OH2
H2
H2
H2
H2H2
H2
H2
H2
H2H2
CH3CN
H2CO
COHCO+
H2O
CH3OHCH3OH
H2
NH3
H2 H2CO
H2
H2O
H2
H2
NH2CHOCH3NH2
CH3OCHO
CH3CH2OH
CH3COCH3
CH3COOH
Dust grain
Ice mantle
H2O, CH3OH,CO, NH3 ,
H2CO
hn
Methanol photodissociation studies are tied together via gas/grain astrochemical modeling of hot cores
• 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
Importance of Methanol
·CHOHCOCH2OHHCOOCH3
HCOCH3
-H+OH
CH3COOH
CH3OH ·CH2OH + HCH3O· + H·CH3 + ·OHH2CO + H2
hν
Past Photolysis Studies• 70 years of previous studies in literature
• Most gas-phase studies involve indirect measurements of BRs
• Most comprehensive lab study indicates:
Hagege et al. 1968, Trans. Faraday Soc., 64, 3288
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
Proposed Technique• Quantitative submm spectroscopy
• Supersonic expansion
• Variety of arc lamps available forwavelength-dependent study
Laboratory Spectroscopy• Reproducible depletion of methanol lines achieved
– 10 ± 3% photolysis efficiency
• Current focus:– Removal of signal contribution
from background gas enablingfull quantitative analysis
– Search for photolysis products
Astrochemical ModelingMethodTest varying sets of BRs at different warm-up timescales
Branching RatiosCH3:CH2OH:CH3O (%)
Label
60:20:20 Standard1
12:73:15 Öberg2
90:5:5 Methyl
5:90:5 Hydroxymethyl
5:5:90 Methoxy
Fast Intermediate Slow
5·104 yr 2·105 yr 1·106 yr
1 Garrod et al. 2008, ApJ, 682, 2832 Öberg et al. 2009, A&A, 504, 891
Astrochemical ModelingResults• Some sets of BRs improved the agreement between predicted
abundances and observationsSgr B2(N-LMH)1
Standard90% Methoxy
1 Garrod et al. 2008, ApJ, 682, 283
Astrochemical ModelingResults (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
Astrochemical ModelingImplications• 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
Acknowledgements• Widicus Weaver Group, Emory• Eric Herbst, OSU• Thom Orlando & Greg Grieves,
GA Tech• NSF Center for Chemistry of the
Universe, UVa