Comparison of the Na(4p)+H-2 and Na(3p)+H-2 reactive/quenching systems studied with CARS, resonance-enhanced CARS, and DFWM

Three different nonlinear optical techniques, CARS (coherent anti-Stokes Raman scattering), resonance-enhanced CARS, and DFWM (degenerate four-wave mixing), were used to compare the reactive and quenching behavior of the two different electronically excited sodium atoms, Na(3p) and Na(4p), in a collision with H-2. In the chemical reaction channel both excited sodium states produce NaH molecules, but in the case of Na(3p) it is shown that the reaction is not a direct formation process and involves more than one step. Both sodium states induce a population of the vibrational levels v'' = 0-3 of NaH. For the Na(3p) excitation the population of the NaH molecules is concentrated at v''=0, whereas far the excitation of the Na(4p) state NaH has its maximum population in the v''=1 level. The differences are attributed to different potential energy surfaces. The quenching investigations were focused on the behavior of the Na(4p) state and the results were compared with the well-known behavior of the Na(3p) state. Because no quenching products were detected in the CARS spectra with Na(4p) excitation, the upper limit for the integrated quenching cross section was estimated to be less than 5X10(-17) cm(2), as based on the detection sensitivity of the CARS apparatus. (C) 1997 American Institute of Physics.