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