Photoionization of organic molecules in dielectric liquids

The photoionization threshold of organic molecules in dielectric liquids has been determined and the effect of the medium (solvent) on the ionization threshold and the molecular properties has been studied. Two different techniques, the integrated fluorescence technique and the laser two-photon ionization technique, were developed and utilized to determine the ionization threshold of molecules embedded in liquids. With the integrated fluorescence technique the ionization threshold 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. With the two-photon ionization technique the threshold was established from the onset of the conductivity current, measured as a function of the wavelength of a tunable-dye laser, in a solution of the molecule under study. Absorption proper-ties of molecules in solution in the VUV region and the effect of the medium on them have also been studied utilizing an especially developed double-beam technique.

The first ionization thresholds, I_L¹, of pyrene, N,N,N,N'-tetra-1 L' methyl-p-phenylenediamine (TMPD) and fluoranthene in liquid media comprising a dielectric liquid-- n-pentane (n-P), n-hexane (n-H), n-heptane (n-Hp), isopentane (i-P), cyclopentane (c-P), neohexane (neo-H), cyclohexane (c-H), tetramethylsilane (TMS1)-- and an electron attaching compound (A)--perfluoro-normal-hexane (PFn-H), perfluoro-dimethyl-cyclo-butane (c-C₆F₁₂), perfluoro-methyl-cyclo-hexane (c-C₇F₁₄), perfluoro-dimethyl-cyclo-hexane (c-C₈F₁₆)--at rather high concentrations were measured with the integrated fluorescence technique. The I_L¹ of pyrene and fluoranthene were found to be independent of the dielectric hydrocarbon liquids and the concentration of A used. The average value of the I_L¹ of pyrene and fluoranthene for all cyclic A's (c-C₇F₁₄, c-C₆F₁₂, c-C₈F₁₆) 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_L¹ as determined with the cyclic A's was ~0.40 eV lower compared to its value for the linear A. The I_L¹ 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_L¹ was found to be 3.88 ± 0.05 and 4.50 ± 0.05 eV 1 L respectively, i.e., 2.32 and 3.22 eV lower than the respective gas phase ionization potentials. The I_L¹ of TMPD in n-pentane was also measured by one-photon induced ionization and found to be 1 L For both 3.83 ± 0.02 eV in agreement with the two-photon I_L¹ 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 molecular states is discussed.