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ENERGY TRANSFER IN HBr + HBr AND HBr + He COLLISIONS

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Presentation on theme: "ENERGY TRANSFER IN HBr + HBr AND HBr + He COLLISIONS"— Presentation transcript:

1 ENERGY TRANSFER IN HBr + HBr AND HBr + He COLLISIONS
M. H. Kabir, I. O. Antonov, J. M. Merritt, and M. C. Heaven Emory University Department of Chemistry Atlanta, GA 30322 62nd Ohio State University International Symposium on Molecular Spectroscopy June , 2006

2 MOTIVATION  The output powers and temporal features of optically pumped gas lasers (OPGL) are critically dependent upon collisional processes.  Collisional energy transfer (CET) processes determine the conditions under which a population inversion can be created and maintained.  Collisional energy transfer can increase the efficiency of an optically pumped molecular gas lasers by decreasing saturation of the pump transition.

3 Concept - Optically Pumped Gas Lasers
molecular gas Pump 1 w1 w2 v =0 v =1 v =2 molecule high quantum efficiency pump 2

4 WHY ARE GAS LASERS INTERESTING?
The development of high-power lasers using Fiber (~ multiple Watt) and Diodes (~kW) are currently limited by material damage and heat dissipation. Many applications in the military and commercial sector call for even higher power than the existing Fiber & Diode lasers offer.  Specific features and possibilities offered by a gas such as large absorption cross sections, gas cycling, large selection of active media, CW output powers (~ MW) attracts as a superb chemical entities for high-power lasers.

5 Atmospheric Transmission
emission of molecular gases that have shown promise for OPGL’s 100 80 60 40 20 Transmittance (%) 1.0 4.5 2.0 2.5 3.0 1.5 3.5 4.0 0.5 Wavelength ( m m) 100 100 80 80 60 60 Transmittance (%) Transmittance (%) HBr HBr 40 40 Rb Cs 20 20 CO CO CO 1.0 4.5 2.0 2.5 3.0 1.5 3.5 4.0 0.5 Wavelength ( m m) 0.5 0.5 1.0 1.0 1.5 1.5 2.0 2.0 2.5 2.5 3.0 3.0 3.5 3.5 4.0 4.0 4.5 4.5 Wavelength ( Wavelength ( m m m) m)

6 Studies of HBr Collisional Energy Transfer
Rotational energy transfer HBr(v, J) + HBr  HBr(v, J+DJ) + HBr Rotation-vibration energy transfer HBr(v, J) + HBr  HBr(v+Dv, J+DJ) + HBr Energy exchange between isotopes. e.g., H79Br(v, J) + H81Br  H81Br(v+Dv, J+DJ) + H79Br 6

7 Scheme for HBr RET measurements
Optical double resonance spectroscopy Ionization level HBr(E, v’, J’) + hn’  HBr+ + e- Probe (3rd photon ionizes) E 1+ HBr(X, v,J) + 2hn’ (248.4 nm)  HBr(E, v’, J’) v’ = 0 Rotational energy transfer HBr(v=2, J) + M  HBr(v=2, J+DJ) + M  HBr(v=1, J+DJ’) + M v = 2 HBr (v=0) + hn (1.98 mm)  HBr(v=2, J) Pump J X 1+ v = 0 t IR-UV delay 7

8 HBr kinetics studied using pulsed laser
pump-probe technique Nd:YAG laser 532 nm Seeded Nd:YAG laser DFG HBr Cell Delay Generator Boxcar Osc. Computer HBr abs. cell PD 690 nm 1064 nm Dye laser 355 nm SHG Lens 8

9 (2 + 1) REMPI spectra of HBr
Q-branch of the E-X(0-0)

10 IR-UV DR spectra for HBr
Pump: 2-0, R(3) Probe: E-X, 0-2, Q-branch lines Total removal rate constants (10-10 cm3 s-1) 10

11 Typical RET decay curve for initially excited level
J´=2 v = 2 J=2 P(3) v = 0 X 1+ t IR-UV delay

12 Derivation of RET rate constants of HBr-He from ab initio Calculations
• RET measurements yields direct determinations of the state-state rate constants. • Rate constants needed to model of OPGL. • Parameterized expressions for the rate constants will be developed using the guidance by theoretical model.

13 RET rate constants of HBr-He from ab initio Calculations
PES calculation H Br He R q r MOLPRO: Cs symmetry RCCSD(T)/Aug-cc-pVQZ + {33211} Br=Aug-cc-pVQZPP {33211} at midpoint between HBr C.O.M and He R = angstrom =0 –180 : 20 degrees Interaction energy: BSSE corrected Fit potential to:

14 Scattering Calculations of HBr-He
MOLSCAT : scattering code Hamiltonians: Schrödinger equation: State-to-state cross section: State-to-state rate coefficient:

15 Theoretical Rate Constants for HBr + He
Jf Ji 1 2 3 4 5 6 7 9.94E-10 9.04E-11 1.03E-10 1.82E-11 1.28E-11 9.41E-12 3.79E-12 9.26E-13 3.27E-11 1.04E-09 6.57E-11 5.99E-11 1.51E-11 6.59E-12 3.14E-12 1.18E-12 2.62E-11 4.64E-11 6.02E-11 4.08E-11 7.91E-12 2.71E-12 1.29E-12 4.24E-12 3.86E-11 5.49E-11 5.19E-11 2.84E-11 5.39E-12 1.47E-12 3.23E-12 1.05E-11 4.02E-11 5.60E-11 4.80E-11 2.10E-11 3.89E-12 2.92E-12 5.65E-12 9.59E-12 3.78E-11 5.92E-11 4.44E-11 1.58E-11 1.62E-12 3.71E-12 4.54E-12 9.90E-12 3.58E-11 6.13E-11 4.06E-11 6.09E-13 2.15E-12 3.31E-12 4.16E-12 1.02E-11 3.35E-11 6.23E-11 1.05E-09 rate constants in units of cm3 molecule-1s-1

16 Dependence of Rate Constants on J
Rate constants show no preference for transitions with even or odd J

17 SUMMARY & FUTURE WORK Summary:  Time-resolved pump-probe measurements were used to examine HBr + HBr RET within the HBr v =2 rotational manifold. State-to-state rate constant matrix for HBr-He collisions was generated using ab initio calculations. Future work:  RET rate constants for the HBr-He collisions will be measured for comparision with ab initio theoretical calculations.  State-to-state rate constant matrix for HBr-HBr collisions will be generated using ab initio calculations for comparision with our observed results.


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