Photodynamics of Small Aliphatic Ketones in a Supersonic Jet Expansion

Abstract

The photodynamics of acetone and some other simple aliphatic ketones in the mt* state are discussed in the light of recent results obtained in seeded supersonic beams. The Born-Oppenheimer approximation is used to describe the sequence of events beginning with light absorption and ending in molecular fragmentation. Intersystem crossing is shown to be an important process even for the isolated molecule. Its rate increases with the internal energy and may be related to the density of states in the triplet state. For all molecules studied so far, the results can be rationalized by assuming that dissociation takes place on the triplet surface, the SI surface being stable in the energy range pertinent to these experiments (up to 35,000 ern-I above the ground state). Fluorescence spectroscopy may be used also to detect small clusters. In the case of acetone, their presence is indicated by the decrease in the intensity of some spectral lines as a function of cooling efficiency. The results are used to estimate an upper limit for the van der Waals binding energies. posal in the fragments of dissociating molecules, including vector correlations. The first aspect has been extensively discussed, experimentally and theoretically, in the case of pyrazine.v" and also of carbonyl compounds, such as methylglyoxal and biacetyl,? propynal," butynal,? and acetaldehyde,'? which are more closely related to the topic of this paper. Dissociation dynamics were mostly studied for three-or four-atom molecules: ICN, 15 H 20,16 HONO,17 and H 202.18,19 Some larger molecules were also studied, notably methylnitrite" and tertbutyl nitrite."Both aspects were extensively studied in the case of formaldehyde, for which the energy disposal into the different fragments was carefully measured." In the case of formaldehyde, spectroscopic assignments are reasonably well established, and perturbations can be analyzed to yield interstate interactions. Since photochemistry often involves curve-crossing (in the Born-Oppenheimer approximation), this molecule appears at present to provide the best link between spectroscopic and dynamic (time-resolved) studies.

Study of Acetone Photodissociation over the Wavelength Range 248−330 nm: Evidence of a Mechanism Involving Both the Singlet and Triplet Excited States

“…A sub 10 ns emission was observed by Haas and coworkers 4,[35][36][37]42 and assigned to dephasing of the wave packet out of the Franck-Condon region on the S 1 surface. This dephasing process has recently been measured in various femtosecond pump probe experiments 15,16,32 and is observed to occur in less than 200 fs.…”

“…This implies that information on the triplet can be observed only at low pressures, <25 Torr. Therefore, only direct emission studies ,− are able to be compared with our measurements. In the study by Copeland and Crosley, an excitation-wavelength-independent quenching efficiency for the phosphorescence T 1 emission was observed over the wavelength range of 305−325 nm.…”

Photochemical α-cleavage of ketones: revisiting acetone

“…The interpretation offered for these data 23,61 was that near the origin, non-radiative transitions to S 0 and to T 1 dominate the decay of S 1 . The isotope effect was assigned primarily to coupling to S 0 rather than to T 1 : the larger quantum of the CH oscillator (compared to the CD one) makes the transition more effective, in agreement with the energy gap law.…”