ChemInform Abstract: HIGH RESOLUTION FLUORESCENCE EXCITATION AND QUANTUM BEAT SPECTROSCOPY OF THE FIRST ELECTRONICALLY ALLOWED TRANSITION OF PERDEUTEROMETHYLGLYOXAL

Abstract

Laser induced fluorescence excitation spectra of supersonic nozzle beam cooled methylglyoxal‐d4 reveal that a deformation occurs during the radiative electronic transition that destroys the planarity of the carbonyls. Consistent with the expectation that deuteration results in an increased number of low frequency vibrations, the number of observed vibronic lines increased and the energy of the breakoff of sharp vibronic structure above the O–O band decreased compared to the protonated molecule. As for the protonated molecule, the observed vibronic structure can be almost completely assigned on the basis of two fundamentals and a repeating pattern of lines that involves the methyl internal rotation and the cis–trans internal rotation of the carbonyls. Systematic anharmonicities involving the pattern and only one of the two fundamentals suggests which molecular motions correspond to the observed fundamentals. Quantum beat and Zeeman effect experiments unambiguously show the triplet states that are responsible for the radiationless dynamics that are known to occur in these molecules. All quantum beat data were analyzable using our previously published perturbation theory method and yield triplet densities which are intermediate compared to methylglyoxal‐h4 and biacetyl‐h6 and in good agreement with an estimate obtained by direct state coupling. The density of effectively coupled triplet states increases with the energy of the initially prepared singlet state and the intramolecular coupling is 2–15 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. Zeeman experiments indicate extensive coupling of all molecular angular momenta which initially decouple at only ∼10 G. In this low field regime complicated splittings are observed which correspond roughly to g values of about 67% that of a free electron. At larger fields of about 50–60 G, the microsecond fluorescence is nearly completely quenched, and although we cannot completely explain this effect at present, we suspect both intra‐ and intermolecular processes are possible.Laser induced fluorescence excitation spectra of supersonic nozzle beam cooled methylglyoxal‐d4 reveal that a deformation occurs during the radiative electronic transition that destroys the planarity of the carbonyls. Consistent with the expectation that deuteration results in an increased number of low frequency vibrations, the number of observed vibronic lines increased and the energy of the breakoff of sharp vibronic structure above the O–O band decreased compared to the protonated molecule. As for the protonated molecule, the observed vibronic structure can be almost completely assigned on the basis of two fundamentals and a repeating pattern of lines that involves the methyl internal rotation and the cis–trans internal rotation of the carbonyls. Systematic anharmonicities involving the pattern and only one of the two fundamentals suggests which molecular motions correspond to the observed fundamentals. Quantum beat and Zeeman effect experiments unambiguously show the triplet states that are responsib...