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

The primary quantum yield of H-atom production in the pulsed-laser photolysis of hydrazine vapor, N2H4 + h v + H + NzH3, was measured to be (1.01 2 0.12) at 193 nm relative to HBr photolysis, and (1.06 2 0.16) at 222 nm relative to 248-nm NzH4 photolysis, in excess He buffer gas a t 296 K. The H-atoms were directly monitored in the photolysis by cw-resonance fluorescence detection of H('S) at 121.6 nm.The high H-atom yield observed in the photolysis is consistent with the continuous ultraviolet absorption spectrum of NzH4 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 NzH4 and He buffer gas to study the gas-phase reaction, H + NzH4 -products; (kl), in a thermostated photolysis reactor made of quartz or Pyrex. The pseudo-first-order temporal profiles of [HI 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, k l . The Arrhenius expression was determined to be k l = (11.7 -t 0.7) X exp[-(1260 2 20)/T] cm3 molec-l s-l in the temperature range 222-657 K.The rate coefficient a t 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 NzH4 by the H-atom.

Discharge flow-tube studies of O(3P)+N2H4 reaction: The rate coefficient values over the temperature range 252–423 K and the OH(X 2Π) product yield at 298 K

“…13,40,48 Our experiments have shown that the removal of a single H atom is a minor process, and as might be expected we see excitation into the new bond of the reaction product in the form of vЉϭ1 and 2 OH vibrations. Of the total initial OH yield measured, we estimate that ϳ50% is distributed in the ground vibrational state.…”

“…The flow-tube rate coefficients, k 3 , measured in the temperature range 252-372 K were in excellent agreement with those measured in our pulsed-photolysis reactor. 40 The fact that we obtain similar rate coefficient values in two different apparatuses suggest that there are no significant systematic errors in determining the various flow-tube parameters which are required to deduce the second-order rate coefficients from the measured pseudo-first-order ͓O͑ 3 P͔͒ decays. The precision of the individually measured second-order rate coefficient, k 1 , may be derived from the standard propagation of random error analysis of the various flow-tube parameters that determine k 1 .…”

“…This is very unlikely in this work from our previous measured and known behavior of N 2 H 4 flow in Teflon lines and over Pyrex surfaces. 40 To avoid any possible complications to the kinetic measurements from heterogeneous decomposition of N 2 H 4 , we have limited our work to below 423 K. Also, the halocarbon wax above this temperature exerted a sufficient amount of vapor pressure that its condensation on the relatively cooler optical windows of the apparatus degraded the signal-to-noise ratio to such an extent that reliable kinetic measurements could not be made in our experiments.…”

“…40,41 O atom S 0 is the signal that is extrapolated to time zero in the exponential O atom S profile that is concurrently monitored with the OH S profile. Provided that the relative detection efficiency ratio, O atom f / OH f , is known in the experiment, then a plot of the right-hand side ͑rhs͒ of the above expression vs ͓N 2 H 4 ͔ will yield a value for k OH from the linear slope.…”

Experimental and Theoretical Investigations of the Radical–Radical Reaction: N2H3 + NO2

“…To minimize the interference effect from secondary radical chemistry in our N 2 H 3 + NO 2 rate coefficient measurements, the initial radical photochemistry was carefully controlled. Equations , , , , , and – below show the important chemical reactions relevant to the present analysis. The first-order wall loss rates for eqs – are expected to be typically in the 1–20 s –1 range for the present flow-tube setup . Photolysis through eq provides the primary source for the N 2 H 3 radicals in the experiment, though secondary production is possible via eqs and .…”