Electronic Energies and Electronic Structures of the Fluoromethanes

Abstract

The high-resolution He i and He ii photoelectron spectra of all fluoromethanes in the series CH4 to CF4 and their deuterated analogs have been recorded and are compared with the Koopmans' theorem results of near-Hartree–Fock calculations performed in a Gaussian basis. The agreement is very good in general and offers an unambiguous assignment of almost all of the bands observed. In particular, repeated correlations are demonstrated between the compositions of the orbitals from which the electrons are ejected and the characters of the resulting photoelectron bands. Identifiable trends throughout the series are stressed and an anomalous feature in the CF4 spectrum is noted. Jahn–Teller effects in CH4 and CH3F are clearly evident, but as expected, they are not observed in CHF3 and CF4. Comparison of the photoelectron spectra excited with He i and He ii radiation shows wide variations in the relative intensities of various bands in certain of the more symmetric molecules, suggesting that relative intensities can be a poor measure of relative orbital degeneracies. Mass-spectrometric appearance-potential data are briefly discussed in the light of the photoelectron results. The carbon and fluorine 1s binding energies as measured with 1254-eV x rays are shown to be electronically adiabatic. The accurate determination of the lower ionization potentials of these molecules leads readily to the assignment of several of their electronic transitions as lower members of ns and np Rydberg series.

Dissociative autoionization as a mechanism for the proton formation from methane and methane-d4 by low energy electron impact

“…7 shows that the'se protons have a constant appearance energy for retarding potentials of 0-3.5 eV. The measured appearance energy lies close to the second adiabatic ionization energy of CH + 4 at 22.39 eV [12] for which the vibrational structure extends up to 24 eV. The formation of the protons at 22.17 eV by populating the 2 A 2 state of CH + 4 in its successive vibrational levels which subsequently predissociate could not account for the present experimental observations.…”

“…3. This onset lies in the energy range where no ionization cross section is observed in the photo-electron spectrum of CH 4 [12]. The two first adiabatic ionization energies of CH 4 in the X 2 T 2 and à 2 A 1 states of CH + 4 are observed at 12.615 eV [11] and 22.39 eV [12].…”

“…This onset lies in the energy range where no ionization cross section is observed in the photo-electron spectrum of CH 4 [12]. The two first adiabatic ionization energies of CH 4 in the X 2 T 2 and à 2 A 1 states of CH + 4 are observed at 12.615 eV [11] and 22.39 eV [12]. At this energy the protons have to be formed by dissociative autoionization through the process An indication for the existence of such an auto-ionizing state in the energy range of 21.3 eV could be found in a recent work published by Marmet et al [16] who studied the electroionization efficiency curves of CH + 4 , CH + 3 and CH + 2 with monoenergetic electrons.…”

“…The same reference is used when the ionization efficiency curves are recorded at different values of the retarding potential. The first adiabatic ionization potential of CH 4 at 12.61 eV [11] (and 12.88 for CD 4 [12]) measured by linear extrapolation [10] of the first differentiated ionization efficiency curve of CH + 4 , is used for the electron energy scale calibration.…”

Vacuum–ultraviolet absorption and fluorescence spectroscopy of CF2H2, CF2Cl2, and CF2Br2 in the range 8–22 eV

“…Many groups have measured He I and He II photoelectron spectra. [20][21][22][23][24][25][26][27][28] Selected values are listed in Table I. The energies of the key dissociation channels of CF 2 X 2 (X= H,Cl,Br) are also listed in Table I. The ground state energies were generally calculated from enthalpies of formation at 0 K given in the JANAF tables.…”