Photoionization-Efficiency Spectrum and Ionization Energy of the Cyanomethyl Radical CH2CN and Products of the N(4S) + C2H3 Reaction

Photoionization efficiency (PIE) spectra of the CH 2 CN radical were measured over the wavelength range λ = 115−130 nm using a discharge-flow−photoionization mass spectrometer coupled to a dispersed synchrotron-radiation source. The cyanomethyl radical was produced by the reaction F + CH 3 CN → CH 2 CN + HF, and the PIE spectrum displayed steplike behavior near threshold. From the half-rise point of the initial step, a value of 10.28 0 ± 0.01 0 eV was obtained for the adiabatic ionization energy (IE) of CH 2 CN based on five independent determinations. From a single measurement of the PIE spectrum and threshold for CD 2 CN, we obtain IE(CD 2 CN) = 10.24 eV. The experimental result for CH 2 CN is compared with previous measurements, estimates, and calculations. The present PIMS study of the CH 2 CN radical provides experimental measurements of the adiabatic ionization energy that are simultaneously the most direct and the most precise available. For the reaction N( 4 S) + C 2 H 3, the C 2 H 2 N radical product exhibits a PIE spectrum that may include CH 2 CN along with another species that has a gradual threshold that is at a considerably longer wavelength than the steplike threshold of CH 2 CN (derived from F + CH 3 CN). A possible source of this difference is the contribution from higher-energy C 2 H 2 N isomers and/or from excited CH 2 CN. In sharp contrast to the results for the N( 4 S) + C 2 H 3 reaction, no signal attributable to an isomer of the C 2 D 2 N radical was observed from the N( 4 S) + C 2 D 3 reaction. The C 2 H 3 N/C 2 D 3 N adducts from the N( 4 S) + C 2 H 3 /C 2 D 3 reactions were also studied. The adduct was observed to be solely CH 3 CN for the N( 4 S) + C 2 H 3 reaction, while for N( 4 S) + C 2 D 3, the PIE spectrum appears to include significant contributions from both the lowest-energy isomer CD 3 CN and one or more higher-energy isomers.