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|Title:||Quantum Beat Spectroscopy and Radiationless Transitions|
|Department / Program:||Chemistry|
|Degree Granting Institution:||University of Illinois at Urbana-Champaign|
|Abstract:||Laser induced fluorescence excitation spectra of the series glyoxal-h(,2), methylglyoxal-h(,4), methylglyoxal-d(,4), biacetyl-h(,6), and 2,3-pentanedione-h(,8) at high resolution show that except for glyoxal these molecules undergo a characteristic structural deformation during the electronic transition. Our nearly complete assignment suggests that the methyl internal rotation and carbonyl skeletal motions (probably out of plane) are involved. The position of the O-O bands apparently shift to the blue as an alkyl chain is added to glyoxal but not as the chains are lengthened. Using the excitation spectra, we have excited single rovibraonic transitions and observed quantum beats in the reversible intersystem crossing of these simple (alpha)-dicarbonyls. We have analyzed the beats by two methods to obtain information concerning the density of interacting states and the average intramolecular coupling energies. The simpler beat patterns can be analyzed using a perturbation theory approach and the more complex beat patterns of biacetyl were analyzed using a method based on the properties of random matrices. The density of vibrationally hot triplet states which interact with excited singlet states that are connected to the ground electronic state via optical excitation ((TURN) 22,000 cm('-1)) 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 states increases. We find satisfactory agreement between the density obtained from the quantum beats and that calculated using well known analytical formulae and direct state counting. The density increases with the rotational quantum number of the initially excited singlet state. The average value of the spin-orbit interaction is about 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. In field free space, the biacetyl beats are collisionally quenched with helium at the same rate as the overall fluorescence with a cross section of about 300 (+OR-) 200(ANGSTROM)('2). Zeeman experiments with methylglyoxal indicate extensive coupling of all molecular angular moments with decouple at only (TURN) 10 Gauss and in the low field regime the complicated splittings are consistent with g-values of about 67% that of a free electron. At larger fields of about 50-60 Gauss, the microsecond fluorescence is nearly completely quenched and although we cannot fully explain this effect at present, we suspect both intra and inter-molecular processes are possible.|
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1982.
|Date Available in IDEALS:||2014-12-15|