Predissociation of the A 2 S state of

Predissociation of the A 2 S+ state of the SH (SD) radical A. J. Orr-Ewing, R. A. Rose, C. -H. Yang, K. Vidma and D. H. Parker

Spectroscopic studies of SH(A 2 S+) • A 2 S+ (v, J) lifetimes for SH and SD from LIF (Ubachs et al. , Kawasaki et al. ) and CRDS (Wheeler et al. ) Lifetime SH A 2 S+ - X 2 P (1, 0) v' SH SD 0 3. 20 – 0. 95 ns 247 – 38 ns 1 5. 45 – 4. 61 ps 35 – 24 ns 2 ~ 1 ps 2. 31 ps M. D. Wheeler et al. J. Chem. Phys. 107, 7591 (1997)

Predissociation of the A 2 S+ state • Lifetimes modelled using Fermi Golden Rule calculations • Fitted A-state and ab initio repulsive potentials

Dissociation dynamics • Is dissociation adiabatic or non-adiabatic on the repulsive PE curves? • Asymptotic measurements of: • S(3 PJ) spin-orbit branching ratios • m-state populations • recoil velocity anisotropy (b parameters) • Velocity map imaging of S(3 PJ) photofragments • H-atom PTS (Jingsong Zhang, UC Riverside)

Velocity map imaging SH(X 2 P; v=0, j) SH(A 2 S+; v', j') S(3 PJ) + H(2 S) Probe S(3 PJ) by 2+1 REMPI KDP crystal Dye laser H Nd: YAG V 25% H 2 S (D 2 S)/ Xe Field free TOF MCPs Pulsed nozzle & discharge Ion optics P 47 CCD camera H V Photolysis laser KDP crystal Dye laser Nd: YAG SH(A 2 S+ v'=0, 1 & 2) via P 1(1. 5), Q 1(1. 5) and R 1(1. 5) + RQ 21(1. 5)

Information in velocity images Image intensity depends on the number of S(3 PJ) atom products r Radius is proportional to S atom speed Angular distributions give information on the recoil velocity direction and the alignment of angular momentum of S(3 P) atoms

S(3 PJ) branching – nonadiabatic dynamics S(3 P 0) S(3 P 1) SH(A, v’=1) v'=1 v'=2 A 2 S+ v' S(3 P 0) S(3 P 1) S(3 P 2) 0 0. 13(9) 0. 07(3) 0. 80(12) 2 H(2 S) + S(1 D) 1/2 v'=0 1 S(3 P 2) 0. 18(4) 0. 34(5) 0. 06(2) 0. 04(2) 0. 76(5) 0. 62(7) Repulsive PE curves SH 4 P 5/2 2 S- 1/2 2 S+ 1/2 3/2 1/2 X-state H(2 S) + S(3 P 0) H(2 S) + S(3 P 1) 4 S- A-state 1/2 3/2 2 P 3/2 H(2 S) + S(3 P 2)

Angular anisotropy in the images J = 0 so no alignment S(3 P 0) VV S(3 P 2) VH S(3 P 2)

Anisotropy parameters (bv) • For predissociative states, velocity anisotropy is reduced by rotation of molecule – depends on lifetime and interference between overlapping transitions. # Q 1(1. 5) SH A 2 S+ - X 2 P v' = 2 v'' = 0 P 1(1. 5) R 1(1. 5) # P. L. Houston et al. , J. Chem. Phys. 125, 133316 (2006)

Hyperfine depolarization • Nuclear spin coupling can depolarize bond alignment; • Hyperfine coupling timescales are a few ns; • Anisotropy parameters for SH/SD A 2 S+ v=0 reduced. Q 1(1. 5) Lifetime for SH(A, v=0, J=1. 5)

Angular momentum polarization • Analysis of probe laser polarization dependence of angular anisotropy of images; • Populations of m-states for S(3 P 2) and S(3 P 1). 3 P 1 3 P 2

m-state correlations • • 4 S - 1/2 S(3 P 2 m = 1) + H(2 S 1/2 m = ½) 2 P 3 + H(2 S 1/2 m = ½) 1/2 S( P 2 m = 0) • Non-adiabatic transitions between 4 S-1/2 and X 2 P 1/2 state at long range, mediated by spin-orbit coupling. • = 3/2 states correlate to S(3 P 2 m = 2); requires rotation-induced couplings.

Conclusions • Predissociation of the low v' levels of the A 2 S+ state of SH and SD is via coupling to the 4 S- state; • Lifetimes (measured by LIF and CRDS) affect the photofragment recoil velocity anisotropy; b values are successfully modelled by theory; • Hyperfine couplings can degrade the anisotropy; • S(3 PJ) branching ratios and m-state populations indicate non-adiabatic dissociation dynamics from the 4 S-1/2 state to the X 2 P 1/2 and 4 P 1/2 states.

Acknowledgements Rebecca Rose David Parker Chung-Hsin Yang Konstantin Vidma Gerrit Groenenboom