Methanol Photodissociation Studies using Millimeter and Submillimeter Spectroscopy Jacob C. Laas & Susanna L. Widicus Weaver Department of Chemistry, Emory University, Atlanta, GA 30322

Interstellar chemical complexity arises from condensed-phase processes: 1) Photodissociation 2) 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 H2H2 CO HCO + H2OH2O H2H2 H2H2 H2H2 H2H2 H2H2 N2H+N2H+ H2H2 H H2H2 H2H2 H2H2 CH 3 CN H 2 CO CO HCO + H2OH2O CH 3 OH H2H2 NH 3 H2H2 H 2 CO H2H2 H2OH2O H2H2 H2H2 NH 2 CHO CH 3 NH 2 CH 3 OCHO CH 3 CH 2 OH CH 3 COCH 3 HCOOH Dust grain Ice mantle H 2 O, CH 3 OH, CO, NH 3, H 2 CO, etc… hν Why Methanol? Methanol photodissociation is a major source of organic radicals CH 3 OH CH 2 OH + H CH 3 O + H CH 3 + OH H 2 CO + H 2 hνhν 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 confirmed the importance of methanol photodissociation reactions. Branching ratios are yet to be quantitatively measured HCO HCOCH 2 OH HCOOCH 3 CH 3 CHO -H +OH CH 3 COOH CH 2 OH CH 3 O CH 3 Laas, Garrod, Herbst, & Widicus Weaver 2011, ApJ, 728, 71 Why Methanol? Sgr B2(N-LMH) CH 90% CH 3 90% CH 2 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 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

CH 3 OH UV absorption is well-characterized High-flux lamps across VUV spectral range (Opthos Instruments, Inc.) - Ly α (121.6 nm), Ar/Xe/Kr cont. ( nm), Hg lines (184.9 & nm) Experimental Design: UV Photodissociation Cheng, Bahou, Chen, Yui, Lee, & Lee 2002, JCP, 117(4), 1633 Wavelength (nm) Lyman-α Ar Kr Xe Hg

Confirmed detections of H 2 CO & CH 3 O from CH 3 OH - agrees with Melnik et al characterization via multi-line detection Product Abundance w.r.t. CH 3 OH T rot (K)* CH 3 O ~ 0.41%< 50 H 2 CO ~ 0.079% ~ 12.0 *methanol was observed at ~14.3 K Testing Product Detectability: HV Discharge

Non-detections of H 2 CO & CH 3 O via photodissociation - λ ≈ (Xe cont.) 1-10% photodissociation efficiency is expected Current Results: Photodissociation detection limits Product Detection Limit CH 3 O ≤ 0.05% H 2 CO ≤ 0.008%

Continued deep searches for photodissociation products - expect primary products to be detectable (CH 3 O/H 2 CO/CH 2 OH) - minor products may be beyond reach of sensitivity (CH 3 + OH) Rotational spectrum of hydroxymethyl (CH 2 OH) 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 )