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)