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The disposal of excess energy above the dissociation threshold into internal excitation and translation in a photochemical decomposition of C2N2 has been determined by utilizing a technique by which measurements of the recoil velocity distribution for particular rotational states are possible. Simple phase space theory calculations appear to give a fairly good interpretation of the rotational energy distributions in a particular vibrational state, while the vibrational energy disposal is definitely nonstatistical with only about 63% of the total population predicted by statistical calculation in the first vibrational state and no excitation observed in the v=2 state. Our experimental results support the previous predictions of high translational energy disposal ( fT=75%) compared to a low or moderate product internal state excitations ( fR=18.6%, fv=6.4%). We also measure the dissociation energy of C2N2 to be 133±1 kcal/mol rather than the previously reported 128±1 kcal/mol.

Laser induced fluorescence excitation spectra of the series glyoxal, methylglyoxal, biacetyl, and 2,3-pentanedione at high resolution show that except for glyoxal these molecules undergo a characteristic structural deformation during the transition. Our incomplete assignment of the methylglyoxal spectrum suggests that the methyl internal rotation and carbonyl skeletal motions (probably out of plane) are involved. The positions of the 0–0 bands shift to the blue as a methyl group is added to glyoxal (22 000 cm−1) and methylglyoxal (22 125 cm−1) but not as the side groups are extended from biacetyl (22 336 cm−1) to 2,3-pentandedione (22 340 cm−1). We cannot precisely identify the interaction between the carbonyls and methyl groups that cause these effects. As the size of the carbon skeleton increases the number of observed low frequency vibronic excitations increases and the energy at which the spectra break off decreases. These observations are consistent with the known radiationless behavior of these molecules.

Magnetic field effects on the singlerovibroniclevel fluorescence of S 1(1 B 1) pyrimidine: Study of the singlet-triplet coupling by level anticrossings and quantum beats Laserinduced phosphorescence spectroscopy in supersonic jets. The lowest triplet states of glyoxal, methylglyoxal, and biacetylWe have observed quantum beats in the reversible intersystem crossing of simple a·dicarbonyls, and we have analyzed them to obtain information concerning the density of interacting states and the average intramolecular coupling energy. We have analyzed most of the biacetyl quantum beats using a method based on the properties of random matrices which is described elsewhere and we present the results here. We also present an analysis based on perturbation theory using the Fourier transforms of the quantum beats which is appropriate for understanding the methylglyoxal quantum beats. The density of vibrationally hot triplet states which interact with excited singlet states that are connected to the ground electronic state via optical excitation (-22 000 cm -I) is found to increase with the amount of vibrational excitation in the accessible singlet state at roughly the same rate as the overall density of triplet vibrational stress increases. We find satisfactory agreement between the density obtained from the quantum beats and that calculated using well known analytical formulas and direct state counting. The density of interacting states increases with the rotational quantum number of the initially excited singlet rovibronic state. The average value of the magnitude of the spin-orbit interaction is 1-10 MHz independent of the amount of vibrational-rotational excitation present. Radiationless transitions in these highly excited molecules are evidently not subject to any overriding selection rules other than spatial symmetry, and conservation of total energy, total angular momentum, and nuclear spin. The biacetyl quantum beats are collisionally quenched with helium at the same rate as the overall fluorescence. The cross section for this process is roughly 300 ± 200 A 2 • 106

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