VUV photoionization of the CH2NC radical: adiabatic ionization energy and cationic vibrational mode wavenumber determinations

Abstract

The photoelectron spectroscopy of CH₂NC (isocyanomethyl) radical species is investigated for the first time between 9.3 and 11.2 eV in the vicinity of the first photoionizing transition X⁺¹A₁← X ²B₁. The experiment combines a microwave discharge flow-tube reactor to produce the radicals through the CH₃NC + F → CH₂NC + HF reaction, a VUV synchrotron radiation excitation, and a double imaging electron/ion coincidence spectrometer which allows the recording of mass-selected threshold photoelectron spectra. Assignment of the observed vibrational structure of the CH₂NC⁺ cation is guided by ab initio calculations and Franck-Condon simulations. From the experimental spectrum, the first adiabatic ionization energy of the CH₂NC radical is measured as 9.439(6) eV. Fundamental wavenumbers are determined for several vibrational modes of the cation: [small nu, Greek, tilde]₁⁺(CH₂ symmetric stretch) = 2999(80) cm^-1, [small nu, Greek, tilde]₂⁺(NC stretch) = 1925(40) cm^-1, [small nu, Greek, tilde]₄⁺(H₂C-N stretch) = 1193(40) cm^-1, [small nu, Greek, tilde]₆⁺(CNC out-of-plane bend) = 237(50) cm^-1, and [small nu, Greek, tilde]₈⁺(CH₂ rock) = 1185(60) cm^-1.