Laser photolysis studies of hydrazine vapor: 193 and 222‐nm H‐atom primary quantum yields at 296 K, and the kinetics of H + N2H4 reaction over the temperature range 222–657 K

Abstract

AbstractThe primary quantum yield of H‐atom production in the pulsed‐laser photolysis of hydrazine vapor, N2H4 + hν → H + N2H3, was measured to be (1.01 ± 0.12) at 193 nm relative to HBr photolysis, and (1.06 ± 0.16) at 222 nm relative to 248‐nm N2H4 photolysis, in excess He buffer gas at 296 K. The H‐atoms were directly monitored in the photolysis by cw‐resonance fluorescence detection of H(2S) at 121.6 nm. The high H‐atom yield observed in the photolysis is consistent with the continuous ultraviolet absorption spectrum of N2H4 involving unit dissociation of the diamine from repulsive excited singlet state(s). The laser photodissociation of N2H4 was thus used as a ‘clean’ source of H‐atoms in excess N2H4 and He buffer gas to study the gas‐phase reaction, H + N2H4 → products; (k1), in a thermostated photolysis reactor made of quartz or Pyrex. The pseudo‐first‐order temporal profiles of [H] decay immediately after photolysis were determined for a range of different hydrazine concentrations employed in the experiments to calculate the absolute second‐order reaction rate coefficient, k1. The Arrhenius expression was determined to be k1 = (11.7 ± 0.7) × 10−12 exp[−(1260 ± 20)/T] cm3 molec−1 s−1 in the temperature range 222–657 K. The rate coefficient at room temperature was, within experimental errors, independent of the He buffer gas pressure in the range 24.5–603 torr. The above temperature dependence of k1 is in excellent agreement to that we determine in our discharge flow‐tube apparatus in the temperature range 372–252 K and in 9.5 torr of He pressure. The Arrhenius parameters we report are consistent with a metathesis reaction mechanism involving the abstraction of hydrogen from N2H4 by the H‐atom. © 1995 John Wiley & Sons, Inc.