Laboratory Submillimeter

Spectroscopy as a Probe of Methanol Photodissociation

Jacob C. Laas & Susanna L. Widicus WeaverDepartment of Chemistry, Emory University, Atlanta, GA

30322

~170 molecules have been detected in the ISM

Exact formation/destruction routes for many complex organic molecules (COMs) are unknown

Interstellar Chemistry

Adapted from http://www.astro.uni-koeln.de/cdms/molecules; ice species from Öberg et al., IAU Symposium 280 (2011).

Methanol is highly abundant in both interstellar gases and ices• gas: ~10-5 per hydrogen atom in molecular clouds• ices: ~1-30% of total

Methanol photodissociation yieldsthree organic radicals

Importance of Methanol

CH3OH CH2OH + HCH3O + HCH3 + OHH2CO + H2

hν

Methanol photodissociation branching ratios is predicted to affect the relative abundances of many COMs of prebiotic interest

Branching ratios are not known

Importance of Methanol

HCOHCOCH2OHHCOOCH3

CH3CHO-H

+OHCH3COOH

CH3OH CH2OH + HCH3O + HCH3 + OHH2CO + H2

hν

Recent modeling (Laas et al. 2011) has shown that relative abundance of C2H4O2 isomers are affected by branching ratios.

Astrochemical Modeling

Laas, Garrod, Herbst, & Widicus Weaver 2011, ApJ, 728, 71

Effects of Grain Surface BRs Effects of Gas-phase BRs

UV irradiation of methanol-rich ices yields other COMs (Öberg et al. 2009)• aldehydes• acids• alcohols• open shell species…

Must invoke reaction schemeand kinetics model to extractphotodissociation branchingratios

Öberg et al. (2009) estimates BRsCH2OH:CH3O:CH3 to be5 : 1 : < 1

Öberg, Garrod, van Dishoeck, & Linnartz 2009, A&A, 504, 891

Clues from the Past: Solid-state

At 185 & 193 nm in the laboratory (e.g. Hagege et al. 1968, Satyapal et al. 1989), CH3O+H is thought to be the dominant channel

How much? Quantitatively, branching ratios are unclear/imprecise

What about other wavelengths?

A theoretical study at 157 nm (Harich et al. 1999) suggests CH3O is still dominant but other channels may also be more active

Clues from the Past: Gas-phase

Cheng, Bahou, Chen, Yui, Lee, & Lee 2002, JCP, 117(4), 1633

Wavelength (nm)Wavelength (nm)

FUV Fields

Interstellar FUV fields are non-uniform

Particularly important for photochemistry in PDRs

Must study λ-dependence of photodissociation

Hollenbach & Tielens 1999, Rev. Mod. Phys.,71, 173

Cosmic-ray induced UV spectrum of H2, from Gredel et al. 1989.

Example FUV Spectra of two T Tauri stars, from Bergin et al. 2003.

Discharge lamps are available for each UV region from Opthos Instruments, Inc.

80 100 120 140 160

180Lyman-α

Ar

KrXe H

g

100 120 140 160

180

λ (nm)

Initial Experimental Design

• Photolysis via UV discharge lamps

• Quantitative submm spectroscopy

• Supersonic expansion

Updated DesignImproved submm optical setup

• higher spectral power• multipass system (≥7 passes)

(Herriott-type cell, Kaur et al. 1990)• improved SNR (>5x)

Improved UV coupling via focusing optics

~1” spectral interaction region;single transverse plane

Updated DesignImproved submm optical setup

Blah

Initial (June 2010) Current (June 2011)

Updated DesignImproved UV coupling

Other sources of depletion?

Scan of vt = 1 line are not observed with lamp on• no vibrational excitation• no temperature change

Current Results

No detection of CH3O with 184.9 nm UV lamp• Suggests an upper limit of <10%• CH3O may be vibrationally excited

Not enough sensitivity for OH with current setup; CH3 is planar• CH3+OH channel cannot be probed

Ongoing and Future Work

Investigation of methoxy non-detection• Inadequate cooling?• Collision-induced conversion to CH2OH?

Search for CH2OH lines

Quantify methanol photodissociation BRs at many λ• Ly α, Ar/Xe/Kr UV continuum, 184.9 nm Hg line

Complete spectral coverage (90 GHz – 1 THz) for CH3O & CH2OH• enables direct interstellar detection

Greater sensitivity is likely needed for minor products

Acknowledgements

Widicus Weaver Group (Emory)

Michael Heaven (Emory)

Eric Herbst (OSU)

Thomas Orlando (GA Tech)

Widicus Weaver Group (from left): Brian Hays, Le Zhong, Cate Levey, Susanna Widicus Weaver Jay Kroll, Max Farina, Jacob Laas, Brett McGuire, Mary Radhuber