DETERMINATION OF THE REACTION DYNAMICS OF SODIUM HYDRIDE IN A HYDROGEN ATMOSPHERE WITH DEGENERATE 4-WAVE-MIXING

Degenerate four-wave mixing was successfully applied to the determinat ion of the reactive and diffusive loss mechanisms of photochemically p roduced sodium hydride in Na + H-2 and Na + H-2 + Ar atmospheres and i n a supplement argon filling. Sodium hydride is formed on the microsec ond time-scale in a quenching-reaction sequence after the 3s-3p excita tion of sodium in the excitation volume. The concentration of NaH decr eases with time depending on the buffer gas pressure. For small pressu res (<20 hPa) diffusion out of the detection volume is the predominant process. The binary diffusion constant of sodium hydride in hydrogen was determined to be 0.66 +/- 0.07 cm(2) s(-1) at 1013 hPa and 273 K. Moreover, experimental data show an additional reactive loss mechanism present at high gas pressures. A second-order reaction with a thermal rate constant of k(2) = 3.8 x 10(-7) cm(-3) s(-1) (sigma = 5.6 x 10(- 12) cm(2)) at 520 K was observed. The thermal rate constant is indepen dent of the hydrogen partial pressure, indicating that the reaction Na H + NaH --> Na-2 + H-2 is most probable. The activation energy of the reaction was estimated to be of the order of 0.9 eV. (C) 1998 John Wil ey & Sons, Ltd.