Determination of the reaction dynamics of sodium hydride in a hydrogen atmosphere with degenerate four-wave mixing

Abstract

Degenerate four-wave mixing was successfully applied to the determination of the reactive and diffusive loss mechanisms of photochemically produced sodium hydride in Na+H2 and Na+H2+Ar atmospheres and in a supplement argon filling. Sodium hydride is formed on the microsecond time-scale in a quenching–reaction sequence after the 3s–3p excitation of sodium in the excitation volume. The concentration of NaH decreases with time depending on the buffer gas pressure. For small pressures (<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 cm2 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 k2=3.8×10-7 cm-3 s-1 (σ=5.6×10-12 cm2) at 520 K was observed. The thermal rate constant is independent of the hydrogen partial pressure, indicating that the reaction NaH+NaH→Na2+H2 is most probable. The activation energy of the reaction was estimated to be of the order of 0.9 eV. © 1998 John Wiley & Sons, Ltd.