Laboratory Submillimeter Spectroscopy as a Probe of Methanol Photodissociation Jacob C. Laas & Susanna L. Widicus Weaver Department 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 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 yields three organic radicals Importance of Methanol CH 3 OH CH 2 OH + H CH 3 O + H CH 3 + OH H 2 CO + H 2 hνhν

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 HCO HCOCH 2 OH HCOOCH 3 CH 3 CHO -H +OH CH 3 COOH CH 3 OH CH 2 OH + H CH 3 O + H CH 3 + OH H 2 CO + H 2 hνhν

Recent modeling (Laas et al. 2011) has shown that relative abundance of C 2 H 4 O 2 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 scheme and kinetics model to extract photodissociation branching ratios Öberg et al. (2009) estimates BRs CH 2 OH:CH 3 O:CH 3 to be 5 : 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), CH 3 O+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 CH 3 O 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)

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 H 2, from Gredel et al Example FUV Spectra of two T Tauri stars, from Bergin et al Discharge lamps are available for each UV region from Opthos Instruments, Inc Lyman-α Ar Kr Xe Hg λ (nm)

Initial Experimental Design Photolysis via UV discharge lamps Quantitative submm spectroscopy Supersonic expansion

Updated Design Improved 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 Design Improved submm optical setup Blah Initial (June 2010) Current (June 2011)

Updated Design Improved UV coupling

Other sources of depletion? Scan of v t = 1 line are not observed with lamp on no vibrational excitation no temperature change

Current Results No detection of CH 3 O with nm UV lamp Suggests an upper limit of <10% CH 3 O may be vibrationally excited Not enough sensitivity for OH with current setup; CH 3 is planar CH 3 +OH channel cannot be probed

Ongoing and Future Work Investigation of methoxy non-detection Inadequate cooling? Collision-induced conversion to CH 2 OH? Search for CH 2 OH lines Quantify methanol photodissociation BRs at many λ Ly α, Ar/Xe/Kr UV continuum, nm Hg line Complete spectral coverage (90 GHz – 1 THz) for CH 3 O & CH 2 OH 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