Photoionization of Organic Molecules in Dielectric Liquids.

The photoionization threshold of organic molecules in dielectric liquids has been determined and the effect of the solvent on the molecular properties has been studied utilizing two different techniques, the integrated fluorescence technique and the two-photon ionization technique. With the integrated fluorescence technique the first ionization threshold, I(,L)('1), of a fluorescing molecule was determined from the decline of the integrated fluorescence intensity, measured as a function of the excitation wavelength, caused by the presence of an electron attaching molecule A. The I(,L)('1) of pyrene, N,N,N,',N'-tetramethyl-p-phenylenediamine (TMPD) and fluoranthene in liquid media comprising a dielectric liquid--n-pentane, n-hexane, n-heptane, isopentane, cyclopentane, neohexane, cyclohexane, tetramethylsilane--and an electron attaching compound--perfluoro-normal-hexane(PFn-H), perfluoro -dimethylcyclobutane, perfluoro-methyl-cyclohexane, perfluoro -dimethyl-cyclohexane--at rather high concentrations were measured with this technique. The I(,L)('1) of pyrene and fluoranthene were found to be independent of the dielectric liquids and the concentrations of A used. The average value of the I(,L)('1) of pyrene and fluoranthene for all cyclic A's was 5.48 and 5.57 eV respectively and for the linear A(PFn-H) was 5.88 and 5.97 eV respectively. For both pyrene and fluoranthene the I(,L)('1) as determined with the cyclic A's was (TURN)0.40 eV lower than the value for the linear A. The I(,L)('1) of TMPD in n-hexane was found to be 5.02 eV using PFn-H as A. Laser two-photon ionization (TPI) spectroscopy was applied to determine the ionization threshold of TMPD and fluoranthene in n-pentane. The I(,L)('1) was found to be 3.88 (+OR-) 0.05 and 4.50 (+OR-) 0.05 eV respectively, i.e., 2.32 and 3.22 eV lower than the respective gas phase ionization potentials. The I(,L)('1) of TMPD in n-pentane was also measured by one-photon induced ionization and found to be 3.83 (+OR-) 0.02 eV in agreement with the two -photon I(,L)('1) value. For both molecules the TPI spectra showed distinct structure due to autoionization. For fluoranthene the TPI spectrum suggests that the molecule dissociates upon excitation in the vibrational progression of the first excited singlet state. The absorption spectra of benzene, naphthalene, azulene, TMPD, pyrene and fluoranthene in PFn-H and n-pentane were measured in the VUV spectral region. The effect of the medium on the properties of the bound molecular states is discussed.