The formaldehyde cation: Rovibrational energy level structure and Coriolis interaction near the adiabatic ionization threshold

Rotationally resolved pulsed-field-ionization zero-kinetic-energy photoelectron spectra of the 00, 61 and 41 vibrational levels of the X∼+2B2 ground electronic state of the formaldehyde cation were recorded using a resonant three-color three-photon excitation scheme. The first adiabatic ionization energy of CH2O (87793.33(1.30) cm−1) and the rigid-rotor rotational constants (A+ = 8.874(8) cm−1, B+ = 1.342(15) cm−1, C+ = 1.148(18) cm−1) of the vibronic ground state of CH2O+ were derived. A strong a-type Coriolis interaction between the 61 and 41 vibrational levels was observed. The Coriolis coupling parameter ξ4,6a and the deperturbed fundamental vibrational frequencies of the in-plane-rocking mode ν6 and the out-of-plane bending mode ν4 were determined to be 8.70(10) cm−1, 823.67(30) cm−1 and 1036.50(30) cm−1, respectively. The intensity distribution of the photoelectron spectra was analyzed in the realm of a simple photoionization model.