A 1+1′ RESONANCE-ENHANCED MULTIPHOTON IONIZATION SCHEME FOR ROTATIONALLY STATE-SELECTIVE DETECTION OF FORMALDEHYDE VIA THE A 1A2←X 1A1 TRANSITION

The formaldehyde molecule is an important model system for understanding dynamical processes in small polyatomic molecules. However, prior to this work, there have been no reports of a resonance-enhanced multiphoton ionization (REMPI) detection scheme for formaldehyde suitable for rovibrationally state-selective detection in molecular beam scattering experiments. Previously reported tunable REMPI schemes are either non-rotationally resolved, involve multiple resonant steps, or involve many-photon ionization steps. In the current work, we present a new $1+1'$ REMPI scheme for formaldehyde. The first photon is tunable and provides rotational resolution via the vibronically allowed $mathrm{tilde{A}}$,$^1mathrm{A}_2leftarrowmathrm{tilde{X}}$,$^1mathrm{A}_1$ transition. Molecules are then directly ionized from the $mathrm{tilde{A}}$ state by one photon of 157~nm. The results indicate that the ionization cross section from the $4^1$ vibrational level of the $mathrm{tilde{A}}$ state is independent of the rotational level used as intermediate, to within experimental uncertainty. The $1+1'$ REMPI intensities are therefore directly proportional to the $mathrm{tilde{A}}leftarrowmathrm{tilde{X}}$ absorption intensities and can be used for quantitative measurement of $mathrm{tilde{X}}$-state population distributions. _x000d