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State-specific radiative lifetimes and electronic quenching cross sections are reported for four different vibrational modes of CH2CHO (B̃2A‘‘). Ground-state vinoxy radicals are produced by 193-nm excimer laser photolysis of CH3OCHCH2, and pumped to the B̃ state by a tunable dye laser operating between 332 and 348 nm. Fluorescence decay rates are determined in the presence of 12 collision partners:  He, Ar, N2, O2, CO, H2, HCl, N2O, CO2, C2H4, CH3OCHCH2, and SF6. The measured electronic quenching cross sections vary from 0.01 to 66.5 Å2. Zero pressure radiative lifetimes are found to vary from 98 ± 10 to 154 ± 18 ns, depending on the vibrational mode. Possible mechanisms for lifetime shortening and collisional quenching are discussed.

By making the atomic effective charge self-consistent with the molecular charge density, it is shown that the PPP method can accurately predict the ionization potential and electron affinity computed as an energy difference of separately minimized ground and ionized states. The calculation is applied to a large variety of conjugated organic molecules, including heterocyclic systems and a-electron-contributing substituent groups. I. BackgroundIt is well known that calculation of the ionization potential (IP) from Koopman's theorem applied to ground state wave functions obtained with the Pariser-Paar-Pople method [2] can yield differences of 1-1.5 eV from the experimental value. In the years 1960-1962 developed an open-shell Hoyland and Goodman [4] analyzed the reasons for this failure and proposed four essential modifications to be included in the PPP method for successful IP and EA calculations. These were: (1) calculate the IP or EA as an energy difference of a closed-shell singlet state (usually the ground state) and an open-shell doublet state (usually the ion); (2) adjust the atomic valence state ionization potential in the ion; (3) correct the one-center Coulomb integral in the ion; and (4) allow for sigma electron polarization during the ionization process. Note that even for calculating the vertical IP, which corresponds to a "fast" measurement where the nuclei retain the ground state configuration in the ion, the rearrangement of all the electrons must be accounted for. For example, Hoyland and Goodman show that the contribution of the u term (4) is -0.8 eV for a polyacene.The PPP method so modified retains enough physics to become a powerful analytical tool; for example, in 1970 Nelson and Selsby [4] applied this theory to probe the interaction of photosensitizing cyanine dyes adsorbed on semiconducting substrates.In 1977, Selsby et al. [6] proposed an automatic method of incorporating the Hoyland and Goodman corrections into the PPP method. The procedure is based on making the local atomic effective nuclear charge, Z,, a function of the molecular 7~ charge density. All atomic semiempirical parameters are expressed in terms of Z , and

Vibronic state specific radiative lifetimes and and rate constants for electronic quenching are reported for four different vibrational modes ofCHCHO (B2A"). Vinoxy radicals are produced in the ground state by 193mn excimer laser photolysis ofmethyl vinyl ether, and irradiated between 332 and 348 nm to generate the second excited state. Bimolecular quenching rate constants are determined in the presence oftwelve collision partners: He, Ar, N2, 02, CO. H2, HC1, N20, C02, C2H4, CH3OCH-CH2 and SF6. Radiative lifetimes are found to vaiy from 98±10 to 154±18 ns, depending on the vibronic level. Some possible mechanisms for lifetime shortening and collisional quenching are discussed.