methanol photodissociation studies using millimeter and submillimeter spectroscopy jacob c. laas...
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![Page 1: Methanol Photodissociation Studies using Millimeter and Submillimeter Spectroscopy Jacob C. Laas & Susanna L. Widicus Weaver Department of Chemistry, Emory](https://reader030.vdocuments.site/reader030/viewer/2022032607/56649ed35503460f94be2b4e/html5/thumbnails/1.jpg)
Methanol Photodissociation
Studies using Millimeter and Submillimeter Spectroscopy
Jacob C. Laas & Susanna L. Widicus WeaverDepartment of Chemistry, Emory University, Atlanta, GA
30322
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Interstellar chemical complexity arises from condensed-phase processes:1) Photodissociation2) Radical-radical addition reactions
Advances in observations: pushing limits of understanding
There is a strong need for quantitative reaction rate information
Motivation
Herschel (ESA)
ALMA (ESO/NAOJ/NRAO), photo credit: J. Guarda (ALMA)
SOFIA (NASA/DLR)
CSO (Caltech/NSF),
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H
H2
CO
HCO+
H2O
H2
H2H2
H2
H2
N2H+
H2
H
H2
H2
H2
CH3CN
H2CO
COHCO+
H2O
CH3OH
H2
NH3
H2H2CO
H2
H2
O
H2
H2
NH2CHO
CH3NH2
CH3OCHO
CH3CH2OH
CH3COCH3
HCOOH
Dust grain
Ice mantle
H2O, CH3OH, CO, NH3 , H2CO, etc…
hν
Why Methanol?
• Methanol photodissociation is a major source of organic radicals
CH3OH CH2OH + HCH3O + HCH3 + OHH2CO + H2
hν• Methanol is a highly abundant interstellar organic molecule in both gases and ices
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These photodissociation products may then go on to form complex organics
Astrochemical modeling has confirmedthe importance of methanol photodissociation reactions.
Branching ratios are yet to bequantitatively measured
HCOHCOCH2OHHCOOCH3
CH3CHO
-H
+OHCH3COOH
CH2OHCH3OCH3
Laas, Garrod, Herbst, & Widicus Weaver 2011, ApJ, 728, 71
Why Methanol?
Sgr B2(N-LMH)CH3 @ 90%CH3O @ 90%CH2OH @ 90%
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Accurately determine gas-phase methanol photodissociation branching ratios
Objective
Coincidental mass of primary(?) radicals
Products are highly reactive
Wavelength-dependent UV absorption bands
Challenges
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• Seeded supersonic expansion
• Direct absorption rotational spectroscopy
• UV photodissociation on expansion
Experimental Design
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Sample seeded in carrier gas- Ar/He/Ne
Pulsed general valve (Parker Hannifin)- stabilization + reaction-free environment
Collimating expansion source- enhances sensitivity
Experimental Design:Supersonic Expansion
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High-resolution mm/submm spectroscopy- provides unique spectral fingerprints
Virginia Diodes, Inc. (VDI) multiplier chain- 50-1200 GHz
Double-modulation lock-in detection
Multipass optical cell- Herriott-type (Kaur et al. 1990)
L-He cooled detector (InSb) (QMC Instruments Ltd.)
Experimental Design:Rotational Spectroscopy
FC04
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CH3OH UV absorption is well-characterized
High-flux lamps across VUV spectral range (Opthos Instruments, Inc.)
- Ly α (121.6 nm), Ar/Xe/Kr cont. (115-210 nm), Hg lines (184.9 & 253.7 nm)
Experimental Design:UV Photodissociation
Cheng, Bahou, Chen, Yui, Lee, & Lee 2002, JCP, 117(4), 1633
Wavelength (nm)Wavelength (nm)
Lyman-α
Ar
Kr
Xe Hg
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Confirmed detections of H2CO & CH3O from CH3OH- agrees with Melnik et al. 2011
- characterization via multi-line detection
Product Abundancew.r.t. CH3OH Trot (K)*
CH3O ~0.41% < 50
H2CO ~0.079% ~12.0
*methanol was observed at ~14.3 K
Testing Product Detectability: HV Discharge
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Non-detections of H2CO & CH3O via photodissociation- λ ≈ 150-200 (Xe cont.)
1-10% photodissociation efficiency is expected
Current Results:Photodissociation detection limits
Product Detection Limit
CH3O ≤ 0.05%
H2CO ≤ 0.008%
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Continued deep searches for photodissociation products- expect primary products to be detectable (CH3O/H2CO/CH2OH)
- minor products may be beyond reach of sensitivity (CH3 + OH)
Rotational spectrum of hydroxymethyl (CH2OH)
Incorporate results into astrochemical models
Identify photodissociation products in ISM
Ongoing and Future Work
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Acknowledgements
Widicus Weaver Group (Emory)
Michael Heaven & Joel Bowman (Emory)
Eric Herbst (UVA)
Thomas Orlando (GA Tech)
Funding:NASA APRA (NNX11AI07G)NSF CAREER (CHE-1150492)