Laser-assisted (1+1′)-photon ionization-detected absorption spectrum of the 3pπ 2Π state of HCO and DCO

We re-examine the ionization-detected ultraviolet absorption spectrum of the 3pπ 2Π←X 2A′ transition in HCO and DCO using a high-power visible laser to enhance the observation of first-photon resonant features. This technique, which we term here, assisted REMPI, significantly improves the signal-to-noise ratio of the spectrum, making many weak vibronic sub-bands visible for the first time. A comprehensive fit to the structure evident in a progression of bending levels from (000) to (040) refines the assignment of Song and Cool [X. M. Song and T. A. Cool, J. Chem. Phys. 96, 8664 (1992)] to yield a set of rotational constants that vary with K in relation to v2, together with a higher-order contribution to the Renner–Teller splitting in HCO, which is mirrored in DCO for all levels but (040). The (040) band falls at a frequency that is commensurate with that of CD stretch, and Fermi resonance between 3pπ 2Π(1000)Π and the higher-energy (040) K=1(Π) component gives rise to an added splitting that increases the energy of this (040) component and causes an apparent increase in the Renner parameter.