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

The photoelectron spectroscopy of CHNC (isocyanomethyl) radical species is investigated for the first time between 9.3 and 11.2 eV in the vicinity of the first photoionizing transition XA← X B. The experiment combines a microwave discharge flow-tube reactor to produce the radicals through the CHNC + F → CHNC + 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 CHNC cation is guided by ab initio calculations and Franck-Condon simulations. From the experimental spectrum, the first adiabatic ionization energy of the CHNC radical is measured as 9.439(6) eV. Fundamental wavenumbers are determined for several vibrational modes of the cation: [small nu, Greek, tilde](CH symmetric stretch) = 2999(80) cm, [small nu, Greek, tilde](NC stretch) = 1925(40) cm, [small nu, Greek, tilde](HC-N stretch) = 1193(40) cm, [small nu, Greek, tilde](CNC out-of-plane bend) = 237(50) cm, and [small nu, Greek, tilde](CH rock) = 1185(60) cm.