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