Persistent spectral hole burning of molecular clusters in a supersonic jet

Abstract

by reaction 2 with a rate constant value5 of k2 -1.3 X 10"m cm3 molecule"1 s"1. The rate constants for the pertaining C2H2-HC-COand CH2('A,) reactions were taken from the current literature;1"9 k values for the less important singlet CH2 removal processes by O and H atoms were estimated to be 1.3 X 10"m and 2.5 X 10"10 cm3 molecule"1 s"1, respectively. The model predicts [CH2('A1)] 4.3 X 109 molecules cm"3 at its t = 2 ms maximum, i.e., nearly equal to the (approximate) measurement.An important additional observation is that at short reaction times (<2 ms) the CH2(1A1) signals vanish upon removing hydrogen from the reaction mixture; this lends support to the proposed singlet CH2 formation reaction 2 and rules out direct formation by the primary C2H2 + O reaction.As anticipated above, the likely mechanism of reaction 2 is an HCCO + H combination process leading to vibrationally excited ketene in the singlet ground state which then rapidly fragments into CH2('A,) + CO. Thus, the mechanism is quite similar to that for ketene photodissociation at 308 nm,18 where CH2 is formed solely in the singlet state.16 There is, however, a difference in the vibrational energy content of the CH^O^'A,) molecule: in the 308-nm photolysis process it is 93 kcal/mol whereas in the chemical activation reaction 2 it is about 105 kcal/mol, using A//f(HCCO) = 42 kcal/mol19 and Ai/f(CH2CO) = -11 kcal/ mol.20 Thus in the latter case the lifetime for fragmentation will be even shorter and hence singlet -triplet intersystem crossing even less probable. Also, the excess energy available to the fragments is only 7 kcal/mol in the 308-nm photolysis,16,17 whereas it is 19 kcal/mol for reaction 2; as a consequence, in the latter process a smaller fraction of the singlet CH2 will arise in the (0,0,0) vibrational ground state.

Molecular and supramolecular chirality: R2PI spectroscopy as a tool for the gas‐phase recognition of chiral systems of biological interest

“…Any transition that shares its initial level with that used in the probe step appears as a decrease in the signal from the probe laser. 20,21 2cR2PI: Ionization Thresholds, Photodissociation, and Photochemical Reactions…”

IR double resonance experiments with size selected clusters for identification of isomers

“…First steps in this direction were made by Colson and coworkers [12] who did not find any isomers for phenol-ethanol complexes and Topp and coworkers [13] who successfully separated low frequency bands of different isomers of perylene complexes. In both cases the cluster size was identified by their spectral features and not by mass spectrometric detection.…”

Spectroscopic studies of enantiomeric discrimination in jet‐cooled chiral complexes

“…9 They involve a pump-probe excitation scheme, with two counterpropagating lasers delayed in time. The wavelength of the first intense laser (the pump) is scanned through the region of interest: when its energy matches that of the transition arising from a given ground-state level it induces the depopulation of the corresponding species.…”