A 1+1′ Resonance-Enhanced Multiphoton Ionization Scheme for Rotationally State-Selective Detection of Formaldehyde via the A 1a2←x 1a1 Transition

An axis-specific rotational rainbow in the direct scatter of formaldehyde from Au(111) and its influence on trapping probability

“…In ref. 32 we characterized our REMPI scheme and found no significant discrepancies between our measured relative intensities and the relative intensities in the calculated absorption spectrum, to within an uncertainty of B20%. Although the spectra analyzed in that work were obtained at rotational temperatures of 300 K or less, and did not include significant intensity from transitions involving states with K a 00 Z 9, our results allow us to rule out the possibility that the observed intensity at higher K a 00 arises from artifacts such as K a -dependent ionization cross sections or K a -dependent dissociative lifetimes of the intermediate à state levels.…”

“…The rotational distribution in the vibronic ground state was monitored in the incoming and scattered molecular beam via a 1 + 1 0 REMPI scheme, which we recently reported. 32 Briefly, the frequency-doubled output of a pulsed dye laser (Sirah Cobra Stretch SL, operating with LDS 698 or 722 dye), pumped at 10 Hz by a Nd:YAG laser (Continuum Powerlite 8010), was scanned across the rotationally resolved à 1 A 2 ' X 1 A 1 (4 1 0 ) transition at 353 nm. (We use the notation 4 v 4 0 v 4 00 to denote the number of quanta of out-of-plane wagging vibration n 4 in the upper (v 4 0 ) and lower (v 4 00 ) electronic state, respectively.)…”

“…The experiment is operated in a regime where the signal is linear with respect to both laser pulses, and the relative integrated ion intensities are directly proportional to the à ' X absorption cross section to within B20% uncertainty. 32 Molecular cations were collected by a set of ion optics and detected on a 2-stage microchannel plate assembly (Hamamatsu F1552) in chevron configuration. At E i = 0.1 eV, the scattering is dominated by a trapping/ desorption mechanism in which molecules physisorb to the surface before undergoing thermal desorption.…”

“…There are other features that appear with high relative intensity in the Boltzmann fit, but are not observed in the spectrum. Among the most prominent of these features are the lines arising from p P 1 (32)(33)(34)(35)(36)(37)(38), which are labeled in Fig. 3.…”