| This dissertation is divided into three parts. In part I, the condensed phase two-photon states of benzene lying 6 to 7 eV above the ground state are probed by polarized fluorescence excitation and multiphoton ionization techniques. Fluorescence excitation reveals how the broad, featureless two-photon absorption in pure liquid benzene gives way to a well defined band as the pure liquid is diluted in alkane solvents. The polarization behavior of this new feature as well as its extreme temperature and solvent sensitivity are interpreted in terms of the promotion of an electron from the e(,1g) HOMO into the 3s Rydberg orbital of benzene in the condensed phase. Benzene, dissolved in a saturated hydrocarbon, appears to provide an unusually favorable situation for detecting the condensed phase 3s Rydberg orbital. The 3s (<---) e(,1g) transition is the only two-photon allowed band below 7 eV. The three well known valence states E(,1u), B(,1u) and B(,2u) are all two-photon forbidden.; In pure liquid benzene, the Rydberg transition cannot be identified. It is proposed that underlying benzene-benzene charge transfer states mix with the 3s Rydberg orbital. The reduced symmetry of the charge transfer dimers can rationalize mixed polarization ratios seen only in the neat liquid. High temperature two-photon spectra can also be understood in terms of this charge transfer assignment.; The multiphoton ionization of liquid benzene is an overall three-photon process at incident wavelengths longer than 360 nm. The initially excited states collapse to the lowest excited singlet excimer before absorption of a third photon produces an ionizable species. The observation of both electric field and thermally enhanced ionization yields implies that geminate pairs precede free ion formation.; Part II describes, briefly, two-photon studies of N,N'-tetramethylphenylenediamine (TMPD), pyrimidine and cyclohexane. An accurate method of locating two-photon ionization thresholds when three-photon ionizations interfere is illustrated by work on TMPD. For pyrimidine, polarized two-photon investigations show that, in contrast to the one-photon case, meta nitrogen substitution of the benzene carbon skeleton fails to destroy the two-photon forbiddeness of the lowest (pi) (--->) (pi)* state. The feasibility of detecting two-photon states lying as high as 9.32 eV above the ground state is demonstrated by thermal lensing studies of cyclohexane.; Electric field modulation of the absorption and reflection spectra of Al/tetracene/nesatron photovoltaic sandwich cells is discussed in part III. The spontaneous electrification of the cell structure can be explored by this technique. Electroabsorption measures an intrinsic 2 volt potential between the electrodes, while electroreflection indicates that surface fields of 2-3 x 10('5) V/cm are present at each electrode-tetracene contact. |
