Vibrational deactivation of HE in the H2-F2chain reaction

The population of the excited vibrational states
\upsilon = 1
through
\upsilon = 6
of hydrogen fluoride has been determined by spectroscopic measurements on a chain reacting mixture of hydrogen and fluorine dilute in helium. The reaction is initiated by partially dissociating the hydrogen molecules with an electric discharge prior to mixing with the fluorine. Mixing is accomplished with a specially designed laminar flow nozzle to provide an easily modeled reaction region. Typical experimental conditions are a mixture of 1:1:20 (H
2
:F
2
:He) at a total pressure of 5 torr and a flow velocity of 100 m/s. Information on deactivation rates is obtained by making population measurements at various distances along the flow, which is equivalent to following the time development of the populations. Because there are a number of time dependent phenomena occurring simultaneously (e.g., reaction, diffusion,
\upsilon-\upsilon
and
\upsilon-t
processes) it is not possible to determine deactivation rates directly from the experimental data. Instead, the experimental data are compared with the predictions of a computer model of the reaction which treats all the significant processes and includes the best currently available reaction and deativation rates. The calculated populations for vibrational levels
\upsilon = 4-6
are consistently larger than the experimentally measured populations, particularly for
\upsilon = 6
where the error approaches a factor of 10. The low populations imply a deactivation rate for the higher vibrational levels of HF which is significantly larger than the currently accepted values. A straightforward analysis based upon the known pumping rates and the known concentrations of deactivating species strongly suggests that deactivation by hydrogen atoms is responsible. The data do not provide sufficient information to choose between the two hydrogen atom reactions 1) H + HF* → HF + H 2) H + HF* → H
2
+ F as the cause of the deactivation. Computer calculations using reactions 1 and 2 separately produce better agreement, however, for the reverse "cold" reaction 2. The rate constant required to explain the low population of
\upsilon = 6
is approximately
2 \times 10^{-10}
cm
³
/molecule.s or an effective collision cross section 0.4 times the gas kinetic cross section.