Selective vibrational excitation of the ethylene–fluorine reaction in a nitrogen matrix. I

Abstract

Selective vibrational excitation of the cryogenic reactions between fluorine and C₂H₄, CH₂CD₂, trans‐CHDCHD, and cis‐CHDCHD have been studied in solid nitrogen at 12 K. For the C₂H₄⋅F₂ reaction, quantum yields showed a general increase over five orders of magnitude as photon energy increased; ν (cm⁻¹), φ_ν: 953, −⁶; 1896, 6.6×10⁻⁴, 2989, 2.3×10⁻²; 3076, 7.0×10⁻²; 3105, 4.3×10⁻²; 4209, 3.1×10⁻¹. Deviations from this general trend give evidence of mode selectivity. Similar deviations indicative of mode selectivity are found for trans‐CHDCHD. At comparable photon energy, the quantum yields of the dideuteroethylenes decrease markedly in the sequence φ(t‐CHDCHD)≳φ(c‐CHDCHD) ≊φ(CH₂CD₂). These differences are attributable to a g, u selection rule in the phonon‐relaxation mechanism. All of the results are consistent with a competition between chemical reaction of the vibrationally excited ethylene and phonon‐assisted relaxation. The resulting cascading process accounts for the general rise in φ as ν increases and deviations from this general rise are attributable, at least in part, to mode selectivity.