Methanol Photodissociation Studies using Millimeter and Submillimeter Spectroscopy Jacob C. Laas &...
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Transcript of Methanol Photodissociation Studies using Millimeter and Submillimeter Spectroscopy Jacob C. Laas &...
Methanol Photodissociation
Studies using Millimeter and Submillimeter Spectroscopy
Jacob C. Laas & Susanna L. Widicus WeaverDepartment of Chemistry, Emory University, Atlanta, GA
30322
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),
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
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%
Accurately determine gas-phase methanol photodissociation branching ratios
Objective
Coincidental mass of primary(?) radicals
Products are highly reactive
Wavelength-dependent UV absorption bands
Challenges
• Seeded supersonic expansion
• Direct absorption rotational spectroscopy
• UV photodissociation on expansion
Experimental Design
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
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
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
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
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%
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
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)