Photoionization at threshold of N2, CO2, and BF3

Date

1977

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Abstract

The construction of an instrument capable of measuring the simultaneous photoion and threshold photoelectron spectra (TPES) is described in detail. The instrument incorporates a 1 meter vacuum UV monochromator to disperse the light from the Hopfield continuum produced by a Velonex 360 high power pulse generator. Electron energy discrimination is accomplished by using a collimated hole structure which operates on the principle of angular discrimination of electrons possessing more than thermal velocities. Results show a resolution (FWHM) of .014 eV for the TPES of the [raised 2]P[lowered 1/2] state when using a .42 A light bandwidth. Photoionization efficiency curves are collected either as total ionization efficiency or as separate mass analyzed curves by employing a monopole mass filter. The TPES and the total ion spectrum of the first three electronic states of N[lowered 2, raised +] are illustrated and discussed. Autoionization is prominent throughout the X[raised 2][sigma][lowered g, raised +] and A[raised 2][pi][lowered u] states. The V(0) level is the only member observed in the TPES for the X[raised 2][sigma][lowered g, raised +] vibrational progression. This is in contrast to other PES. Results from the TPES indicate that the reported enhancement of the V(l) through V(4) levels of the X[raised 2][sigma][lowered g, raised +] state in the NeI PES is due to autoionization. The vibrational intensity distributions in the TPES of the A[raised 2][pi][lowered u] and B[raised 2][sigma][lowered u, raised +] states compare more closely to the HeI spectrum. Detailed threshold electron and total ionization spectra are presented for three electronic states (X[raised 2][pi][lowered g], A[rasied 2][pi][lowered u], B[rasied 2][sigma][lowered g, raised +]) of the CO[lowered 2] ion. Autoionization is present in the vicinity of most of the X[raised 2][pi][lowered g] and A[raised 2][pi][lowered u] it states. Production of ions with vibrational excitation in the symmetric stretching mode is observed for each state with the longest progression occurring for the A[raised 2][pi][lowered u] state. The spin-orbit splitting of the X[rasied 2][pi][lowered g] state is readily observed. There is some indication for production of ions with vibrational excitation in the bending mode of X[rasied 2][pi][lowered g] state. The TPES, along with the total ionization efficiency curve obtained simultaneously, is presented for the first four electronic states of the BF[lowered 3, raised +] ion. Separate mass analyzed efficiency curves measured near the appearance energies for BF[lowered 3, raised +] and BF[lowered 2, raised +] are also presented and discussed in terms of the thermochemical data which can thereby be derived along with the problems of assigning the precise appearance potential. Results from the TPES indicate that the molecular geometry is distorted upon ionization, that production of BF[lowered 3, raised +] occurs with low probability at threshold, and that autoionization may be responsible for the contrast between photoelectron spectra and the TPES. The paucity of vibrational structure in the TPES is discussed in terms of various fragmentation processes.

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