A 1+1 ' resonance-enhanced multiphoton ionization scheme for rotationally state-selective detection of formaldehyde via the (A)over-tilde (1)A(2) <- (X)over-tilde (1)A(1) 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 (A) over tilde (1)A(2) <- (X) over tilde (1)A(1) transition. Molecules are then directly ionized from the (A) over tilde state by one photon of 157 nm. The results indicate that the ionization cross section from the 41 vibrational level of the (A) over tilde 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 (A) over tilde <- (X) over tilde absorption intensities and can be used for quantitative measurement of (X) over tilde -state population distributions.