A 1 + 1′ resonance-enhanced multiphoton ionization scheme for rotationally state-selective detection of formaldehyde via the à 1A2 ← 1A1 transition

Abstract

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 à ¹A₂ ← ¹A₁ transition. Molecules are then directly ionized from the à state by one photon of 157 nm. The results indicate that the ionization cross section from the 4¹ vibrational level of the à 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 à ← absorption intensities and can be used for quantitative measurement of -state population distributions.