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 (1)A2 ← X[combining tilde] (1)A1 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 4(1) 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 ← X[combining tilde] absorption intensities and can be used for quantitative measurement of X[combining tilde]-state population distributions.
Highlights
Resonance-enhanced multiphoton ionization (REMPI) spectroscopy is a valuable tool for state-selective detection of atoms and molecules
The high sensitivity of the technique permits its use in a wide variety of dynamics experiments, including ultrahigh vacuum (UHV) molecular beam scattering studies, in which the scattered molecules may be widely distributed in space, time, and quantum state
Formaldehyde has already served as an important model system for the understanding of quantum-state specific photodissociation dynamics,[8] and was the prototype system for the elucidation of the roaming mechanism.[9]
Summary
Resonance-enhanced multiphoton ionization (REMPI) spectroscopy is a valuable tool for state-selective detection of atoms and molecules. We are aware of one previous study by Andreyev et al.[19] in which formaldehyde was ionized directly from the Astate These authors excite the A ’ Xtransition using a nontunable N2 laser at 337.1 nm and measure the lifetime of the excited state by ionizing with one photon of 160 nm radiation from an H2 laser. In order to make quantitative use of our REMPI intensities, it is important to understand the dependence of the signal on both excitation steps For some systems, such as ionization from the A 2S+ state of NO or the B 1S+ state of CO, the ionization cross section is found to be essentially independent of the rotational state used as intermediate.[20,21] in other cases, moderate to strong dependence of the cross section. Apart from isolated instances in which specific rotational levels of the A 41 intermediate exhibit anomalously short predissociation lifetimes, the intensities in the 1 + 10 REMPI spectrum are—to within the uncertainty of our experiment— directly proportional to the A ’ Xabsorption cross sections
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