PUMPPROBE MEASUREMENTS OF ROTATIONAL ENERGY TRANSFER RATES IN

PUMP-PROBE MEASUREMENTS OF ROTATIONAL ENERGY TRANSFER RATES IN HBr + HBr COLLISIONS M. H. Kabir, I. O. Antonov, and M. C. Heaven Emory University Department of Chemistry Atlanta, GA 30322 63 rd Ohio State University International Symposium on Molecular Spectroscopy June 16 - 20, 2008

MOTIVATION HBr has been demonstrated to lase near 4 m region. W. Rudolph et al. IEEE J. Quant. Elec. 40, 1471 (2004). The development of high-power lasers using Fiber and Diodes are currently limited by material damage and heat dissipation. Detailed knowledge of collision-induced rotational energy transfer kinetics of HBr : quantum state populations for laser modeling. Provide test of fitting and scaling laws in modeling long range attractive potential in HBr-HBr.

PREVIOUS STUDY 1. Chen et al. CPL 17, 500 (1972): V-V transfer k =2. 97 x 10 -12 cm 3 s-1 2. Chen et al. JCP, 5551 (1971): V-T transfer k =1. 78 x 10 -14 cm 3 s-1 3. Leone et al. JCP 69, 5319 (1978): Isotopic resonant V-V transfer k =1. 5 x 10 -11 cm 3 s-1

Pump-probe Double Resonance Scheme HBr+ + e- g 3 -(0+) v = 0, J Probe: (2+1) REMPI p v’ =1, J’ 2559 Energy (cm-1) 75378 Ionization level v” = 0, J” 0 s HBr(v’ =1, J’)+ HBr (v” =0) HBr (v’ =1, J’+ J’)+ HBr (v” =0) HBr(v” = 0, J+ J) + HBr (v” =0) Collision-induced population evolution Pump: Stimulated Raman Scattering X 1 +

Experimental Setup 532 nm, 10 m. J Delay line HBr ~ 615 nm Nd: YAG laser 532 nm 4 m. J REMPI cell Dye laser Dichroic mirror + Filter Delay Generator P M T M C C HBr CARS Cell Oscilloscope Nd: YAG laser 355 nm Computer Dye laser SHG ~274 nm, ~1 m. J Pre-amp HV

CARS Spectrum of HBr: Q-branch (1 -0) transition s p 2 p- s = CARS p v=1 v=0 CARS energy scheme Isotopic abundance: H 79 Br (50. 5%) and H 81 Br (49. 5%)

2 + 1 REMPI Spectra & Line Strengths 3 1 + Q-branch of the g 3 -–X 1 + (0 -0) transition Q-branch of the g –X (0 -1) transition

Total Removal Rate Measurement

Total removal rate constants Hanson et al. : JMS 200, 138 (2000) Pressure broadening coefficient [P(2)]: 118. 3 x 10 -3 cm-1/atm [R(7)]: 87. 9 x 10 -3 cm-1/atm Our expt. (1, 2): 64. 7 x 10 -3 cm-1/atm (8, 7): 36. 9 x 10 -3 cm-1/atm

2+1 REMPI spectrum of the g 3 -–X 1 + 0 -1 band Single collision limit !! Neglecting multiple inelastic collisions Experimental (raw): 29 rate constants What this Figure tells us ? 1. Relative peak intensity tells: propensity for J = ± 1 > J = ± 2 > J=± 3. Multiquantum transitions: higher order multipole interactions 2. Population in the J = ± 2 and J=± 3 levels: direct population transfer and multiple J = +1 or – 1 steps.

Fitting Laws Only consider energy dependence of rate constants Exponential Energy Gap law (EGL): Statistical Power Gap Law (SPGL): Modified Exponential Gap Law (MEG):

Scaling Laws Rate constants dependence on transferred angular momentum Energy Corrected Sudden Power (ECS-P) law: Angular Momentum & Energy Corrected Sudden (AECS) law:

State-t 0 -State Rate Constants k x 10 -10 cm 3 s-1

3 D PLOTS OF MATRICES OF RATE CONSTANTS a) MEG d) ECS-P b) EG c) SPEG e) AECS

SIMULATION Master Equation approach: models the evolution of individual level populations Loss process: Diffusional loss out of the probe laser volume at the focal point

Kinetic Traces: Experiment & Simulation Ji = 3, Jf = 1 -6

Kinetic Traces: Experiment & Simulation Ji = 5, Jf = 2 -6

Spectral Traces: Experiment & Simulation

Comparison of State-to-State Rate Constants

Comparison of Rate Constants: Experiment & Fitting Laws

Contributions of J Transitions in Population Removal First order ( J = ± 1) transitions : dipole-dipole interactions Multiquantum transitions ( J = ± 2 , J=± 3 …. . . ): dipole-quadrupole interactions or quadrupole-quadrupole interactions.

Contributions of J Transitions: Other’s report D. Chandler et al. JCP 87, 5229(1987) G. D. Hager et al. JCP 21, 9281(2002)

SUMMARY Time-resolved pump-probe measurements were used to examine HBr + HBr RET within the HBr v =1 rotational manifold for the first time. State-to-state rate constants matrix for HBr + HBr collisions generated using fitting and scaling laws. Largest state-to-state rate constants were found for J = 1 transitions. Measured total rate coefficients were found pretty close to the self-collisional pressure broadening coefficients. ECS-P law provided a physically reasonable intermolecular interaction length, lc = 4. 0 0. 1 Å (Lennard-Jones diameter for HBr is 3. 35 Å) which is close to the value (4. 1 Å) of equilibrium intermolecular distance of (HBr)2. Flow of energy in HBr+HBr collisions is dominated by both the anisotropy of the long range intermolecular potential and the internal rotational level structure.