1. Laboratory Submillimeter Spectroscopy as a Probe of Methanol Photodissociation Jacob C. Laas & Susanna L. Widicus Weaver Department of Chemistry, Emory University, Atlanta, GA 30322

2. ~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).

3. 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

4. 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

5. 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

6. 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

7. 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)

8. 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

9. Cosmic-ray induced UV spectrum of H 2, 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. 80 100 120 140 160 180 Lyman-α Ar Kr Xe Hg 100 120 140 160 180 λ (nm)

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

11. 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

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

13. Updated Design Improved UV coupling

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

15. Current Results No detection of CH 3 O with 184.9 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

16. 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, 184.9 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

17. 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