Transcript Laas - OSU MSS 65.pptx

Methanol Photodissociation
Branching Ratios and Their Influence
on Interstellar Organic Chemistry
Jacob Laas1, Susanna Widicus Weaver1, and Robin Garrod2
1Department of Chemistry, Emory University
2Department of Astronomy, Cornell University
H2
Methanol photodissociation
studies are tied together via
gas/grain astrochemical
modeling of hot cores
CO
H2 O
H2
H
H2
HCO+
H2
H2
H2
H2
H2
H2
hn
H2
H2
H2
Ice mantle
Dust grain
H2CO
H2
HCO+ NH3
H2O, CH3OH,
CO, NH3 ,
H2CO
H2
CH3CH2OH
H2O
CO
H2
CH3OH
H2
H2CO
CH3CN
NH2CHO
CH3NH2
CH3OH
CH3OCHO
CH3COOH
H2O
CH3COCH3
H2
Importance of Methanol
• Methanol is highly abundant in both gas and ice
• Methanol photodissociation yields three organic radicals;
branching ratios (BRs) are not known
CH3OH
hν
·CHO
HCOCH
·CH2OH
+ H 2OH
CH3O·HCOOCH
+H
3
·CH3 +HCOCH
·OH 3
H2CO + H2
-H
+OH
CH3COOH
• 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
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 for
wavelength-dependent study
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
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
Label
CH3:CH2OH:CH3O (%)
60:20:20
Standard1
12:73:15
Öberg2
90:5:5
Methyl
5:90:5
Hydroxymethyl
5:5:90
Methoxy
1 Garrod
2 Öberg
et al. 2008, ApJ, 682, 283
et al. 2009, A&A, 504, 891
Astrochemical Modeling
Results
• Some sets of BRs improved the agreement between predicted
abundances and observations
Sgr B2(N-LMH)1
Standard
90% Methoxy
1 Garrod
et al. 2008, ApJ, 682, 283
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
Predicted peak abundances using methoxy BRs
• A combination of BRs favoring CH3O channel and slow
warm-up timescale give the best match to Sgr observations
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
Acknowledgements
•
•
•
•
Widicus Weaver Group, Emory
Eric Herbst, OSU
Thom Orlando & Greg Grieves,
GA Tech
NSF Center for Chemistry of the
Universe, UVa