Two-color Resonant Four-wave Mixing Spectroscopy of Highly Predissociated Levels in the à 2 A 1 State of CH 3 S Ching-Ping Liu, a Scott A. Reid, b and Yuan-Pern Lee a,c a Department of Chemistry, National Tsing Hua University, Hsinchu, Taiwan b Department of Chemistry, Marquette University, Milwaukee, WI c Department of Applied Chemistry, National Chiao Tung University, Hsinchu, Taiwan and Institute for Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan 60 th International Symposium on Molecular Spectroscopy

Schematic PES along the C-S coordinate JCP 110, 805 (1999) Why CH 3 S?  Important atmospheric species, produced in oxidation of sulfur compounds  Model system for studies of predissociation  Previous studies of à state photochemistry have shown mode specificity, with C-S stretch a promoting mode (Lee, Miller, Neumark) Ambiguity remains regarding assignments.

LIF & DFWM spectra C.-P. Liu, Y. Matsuda, and Y.-P. Lee, J. Chem. Phys. 119, (2003). TC-RFWM

Spectral Assignments 3 n :2 1 3 n : n : n : 0 2

A 2A1A 2A1 X 2EX 2E TC-RFWM diagram of CH 3 S: Hole-burning scheme  4  1  3  2 YYYY YYXX YXXY YXYX Polarization Geometries

TC-RFWM setup: Dye Laser #2 G-L PR Iris PMT Excimer laser P MC 532 nm Dye Laser #1

Spectral Simplification via TC-RFWM A B DFWM (3 3 ) Wave number / cm -1 TC-RFWM Spectra (Probe B): (J=9/2,7/2; K =0)

TC-RFWM Spectra: Probing Line B in Wave number / cm  All progressions show an increase in linewidth with increasing quanta of 3  The TC-RFWM spectra confirm the n progression and provide a precise value for 1  A new progression n is identified

Illustration of mode specificity  The C-S stretch ( 3 ) is a clear promoting mode for dissociation

 L =4.6(2) cm -1 Fitting procedure  We expect a Lorentzian squared lineshape for homogenously broadened transitions  To account for the laser linewidth, we fit the profiles to the square of a Voigt profile, with the Gaussian component fixed at the laser linewidth (~ 0.2 cm -1 )

Assignment Lifetimes (in ns, except as noted) Chiang & LeeMillerThis work (70)1090(55) (30)870(40) (20)300(30) (30)460(30) (30)  (15)60(20)         (3) ps1.55(14) ps     (1) ps0.48(10)ps (2) ps1.09(14) ps (1) ps0.53(13)ps   (1) ps0.35(6) ps (2) ps (5) ps (3) ps (3) ps (10)ps CH 3 S Lifetimes  Our data agrees well with FD measurements of Miller group  Linewidths correspond to lifetimes as short as ~ 100 fs, limited by S/N  At higher energies bias favors observation of longer lived states (fewer quanta of 3 )

Plot of lifetime vs. energy 3n3n 213n213n 113n113n 11213n11213n  The C-S stretch ( 3 ) is a clear promoting mode for dissociation

à 2A1à 2A1 modeExp. Calc (Scaled) 1 (a 1 ) (2978) 2 (a 1 ) (1159) 3 (a 1 ) (410) 4 (e) 3367 (3147) 5 (e) 1488 (1391) 6 (e) 746 (697) Ref.1, this work2 Vibrational Frequencies: Experiment vs. Theory 1 C.-P. Liu, Y. Matsuda, and Y.-P. Lee, J. Chem. Phys. 119, (2003). 2 Q. Cui and K. Morokuma, Chem. Phys. Lett. 263, 54 (1996).

Summary We demonstrate the advantages of two-color resonant four-wave mixing in investigating highly predissociative levels of the à 2 A 1 state of CH 3 S. The spectral simplification afforded by the two-color method allowed accurate determination of band positions and homogeneous linewidths. The TC-RFWM spectra show pronounced mode specificity, with 3 a clear promoting mode for dissociation. The vibrational progressions v and v are assigned, and à 2 A 1 state vibrational frequencies 1, 2, and 3 determined

Acknowledgements National Science Council (Taiwan) ACS/PRF (SAR)