Jie Wei, Boris Karpichev, Hanna Reisler Department of Chemistry, University of Southern California, Los Angeles, CA Unimolecular processes in CH 2 OH studied by O-H stretch overtone excitation OSU, 06/20/2005

Motivation [1] S. Saebo, L. Radom, and H. F. Schaefer III, J. Chem. Phys. 78, 845 (1983) We have excited OH stretch (ν 1 ) overtones to  =4, focusing on IVR rates in different energy region and H-atom fission mechanism (only with 4ν 1 ). A feature of this work is that the reaction coordinate is directly pumped. Besides roles in atmospheric and combustion chemistries, CH 2 OH is an intriguing system for unimolecular reaction study. D 0 (OH) ~ 9700 cm -1 ; Theoretical barrier heights are uncertain.

Experimental Setup Detection schemes (Double-Resonance-Ionization or H-atom yield ) Photolysis (355 nm) Pulsed valve Cl 2 +h  2Cl CH 3 OH+Cl  CH 2 OH+HCl …… 2x10 -6 Torr Probe Pump MCP T(R)~13 K 2x10 -7 Torr 3p z I.P. H + CH 2 O H+H+ Pump laser bandwidths: 2 1 : 0.4 cm -1 3&4 1 : 0.1 cm -1 CH 2 OH ( ) 4321043210

DRI spectra of 2ν 1 and 3ν 1 [2] L. Feng, J. Wei, and H. Reisler, J. Phys. Chem. A, 108: 7903 (2004) [3] R. H. Judge and D. J. Clouthier, Comput. Phys.Commun. 135: 293 (2001).  (CH 2 OH*) > 60 ns NO H-atom fragment Pump-on Pump-off were alternated  =0.8 cm  =0.4 cm -1 Simulation 3  =0.8 cm -1

4ν 1 : H-atom yield spectra & KER analyses 1-photon dissociation leads to the H-atom yields dependence on frequencies! 2 arrows: 1-photon dissociation limits CD 2 OH CH 2 OH

Birge-Sponer plot OH stretch nature of the overtones: CH 3 OH (3769.6, 86.1), NH 2 OH (3743, 90.6), HOOH (3701, 90.5). [5] D. Rueda, O. Boyarkin, T. Rizzo, A. Chirokolava and D. Perry, J. Chem. Phys., 122(4): [6] J. Scott, D. Luckhaus, S. Brown and F. F. Crim, J. Chem. Phys. 102(2):675 (1995) Dissociation barrier > 4 ν 1 : The barrier doesn’t affect the energies. However, 4 ν 1 feels the barrier, and possesses unique characters (dissociation, rotational constants).

Two-tiered model for IVR 7 І: low-order coupling, strong; ІІ: coupling to bath dark states. 2ν 1, 0.8 cm -1 : is due to coupling І, key states are unresolved. 3ν 1, 0.4 cm -1 : Unimolecular reactions are ruled out. It may be attributed to coupling ІІ. Discussion: Line broadening and H-atom fission mechanism І ІІ [7] P. R. Stannard and W. M. Gelbart, J. Phys. Chem., 85: 3592 (1981).

4ν 1, 1.3 cm -1 : it is difficult to distinguish between IVR and dissociation. Some estimation: (1) low order coupling? Not likely,  (CH 2 OH) ~  ( CD 2 OH); (2) coupling to dark bath states? Not likely, comparing to : υ NH 2 OH 6 (cm -1 ) ~7 CH 3 OH 8 (cm -1 ) (3) Isomerization is not likely because of no D-atom from CD 2 OH; (4) Tunneling of H(O) atoms through the dissociation barrier? Most probably, it dominates the line width, as a subsequence of directly pumping the reaction coordinate. [8] O. Boyarkin, T. R. Rizzo, and D. S. Perry, J. Chem. Phys., 110(23): (1999). Discussion: Line broadening and H-atom fission mechanism

Ongoing work: double resonance overtone excitation  4ν 1, (ν 1 +3ν 1 ): identifying possible low-order resonance states with higher rotational selectivity;  5ν 1, (2ν 1 +3ν 1 ): passing the reaction barrier with higher pump efficiency. $$$ DOE 3 1

Thank you for your attention !

Change of rotational constant with vibrational levels V A (cm -1 ) R(Å) R(Å )A(cm -1 ) B(cm -1 ) C(cm -1 )