Multiphoton ionization and photoelectron spectroscopy of formaldehyde via its 3p Rydberg states

The resonance-enhanced multiphoton ionization (REMPI) spectrum of formaldehyde, two photon resonant in the region of the A21(3px), A11(3py), and B21(3pz) states, is reported. The A21(3px) state spectrum is dominated by the ν3′ (CH2 scissors), ν4′ (CH2 out-of-plane bending), ν5′ (CH2 asymmetric stretching), and ν6′ (CH2 rock) modes, with weaker bands observed for excitation of the ν2′ (CO stretching) mode. Vibrational analysis of the spectrum provides many new frequencies for the A21(3px) state, not resolved or accessible in single photon spectroscopic measurements. Photoelectron spectroscopy is used to probe the nature of the vibronic levels associated with the A21(3px) intermediate state, to measure vibrational frequencies of the resulting cations, and to identify useful routes for preparing vibrational state-selected H2CO+. It is found that H2CO[1A2(3px)] is a well-behaved Rydberg state, generating cations in the same vibrational level that was populated in the intermediate. Cations with mode-selective excitation of up to 0.62 eV can be produced. Ab initio calculations are used to help assign the cation vibrations. In contrast to the well-behaved A21(3px) state, the A11(3py) and B21(3pz) states are strongly mixed with each other and with valence states.