State-resolved dissociation dynamics of glyoxal near the threshold for formation of fragment HCO

Abstract

Fluorescence excitation spectra for transition à 1 A u-X 1 A g of trans-glyoxal in a supersonic jet were recorded with laser excitation in a wavelength region near 395 nm. With resolution 0.04 cm Ϫ1 , most lines in these spectra are resolved and are assigned rotationally; of six bands assigned, three have c-type rotational structure, and another three have types a/b hybrid, a and b. Fluorescence decays with quantum beats, resulting from coherent excitation of S 1 and T 1 states, are observed for most rotational levels in this region. In spectra transformed to the frequency domain for these decays, the widths are larger than those obtained at low excitation energy. The widths from nonzero frequency lines, reflecting mostly the triplet character, indicate that the triplet state is dissociating. Fluorescence spectra of fragment HCO confirm that the dissociation channel correlated to the triplet surface of glyoxal involves formation of radical products. From an abrupt decrease of lifetime of the triplet state and onset of disappearance of fluorescence of glyoxal, the threshold for formation of fragment HCO from trans-glyoxal is determined to be 394.4 nm. With data for heat of formation this threshold yields an exit barrier 2.5Ϯ0.5 kcal/mol. Signal of HCO for wavelengths greater than the threshold position up to 400.5 nm is observed and is assigned to arise from the S 0 surface. Rotationally state-resolved appearance rates of HCO for a vibrational state near the dissociation threshold are measured. Relative to lifetimes obtained from eigenstates in the quantum-beat data, the appearance time of product HCO reflecting the overall depletion of glyoxal is on average longer. Some observed gateway states with enhanced yields of HCO are considered to result from strong singlet-triplet interaction.

Pressure and temperature dependent photolysis of glyoxal in the 355–414 nm region: evidence for dissociation from multiple states

“…Below this threshold they suggested that S 1 decays primarily by internal conversion (IC). In a further study by Chen and co-workers, 16 the HCO radical was directly monitored and from its formation threshold and appearance time, it was shown to be consistent with the onset of the emission lifetime shortening at 394.4 nm. However, there was still some HCO signal at 400.5 nm and this was assigned to arise from barrierless dissociation on ground-state glyoxal, S 0 , following internal conversion.…”

“…The most relevant work concerning the photo-dissociation mechanism for HCO formation is that by Chen and co-workers. 10,16,28 In their time-resolved emission study 28 on supersonic jet cooled glyoxal, excited over the wavelength region 393-417 nm, emission at the longer wavelengths was observed to be superimposed with quantum beats; from the splitting pattern in a magnetic field one of the states contributing to the beats phenomenon was observed to be a triplet. They ascribed the behaviour to coherent excitation of mixed S 1 and T 1 states.…”

“…Fig. 1 is a schematic diagram of the potential energy surfaces for dissociation to radicals, HCO, in glyoxal and is adapted from the paper by Chen et al 16 This diagram contains all the electronic surfaces that are required to explore the possible photo-dissociation mechanisms to describe the formation of HCO. The difference between the various mechanisms relates to how the electronic surfaces are coupled and which couplings are dominant.…”

“…In a collision free environment, molecular beam studies have established the existence of P1c and P1d [12][13][14][15] at wavelengths, B440 nm, beyond the thermodynamic limit for the radical channels. At shorter wavelengths, B395 nm, under collision free conditions, HCO radicals may be produced from two different states, 16 triplet (T 1 ) and singlet (S 0 ), which have been identified by the different HCO energy distributions. 10 Fig.…”

Photo‐Induced Hydrogen Exchange Reaction between Methanol and Glyoxal: Formation of Hydroxyketene

“…In a recent gas‐phase photolysis of glyoxal, Zhu et al6 detected production yield of HCO under bulk conditions at excitation wavelengths of 193, 248, 308 and 351 nm. The formation of 2HCO radicals from G results from photolysis at the excited triplet surface,11 a channel additional to the three given by Equations (1)–(3), that result from dissociation at the ground singlet surface.…”