The vacuum-ultraviolet photoelectron spectra of CH2F2 and CH2Cl2 revisited

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The threshold photoelectron spectrum (TPES) of difluoromethane and dichloromethane has been recorded at the Swiss Light Source with a resolution of 2 meV or 16 cm−1. Electronic and vibronic transitions are simulated and assigned with the help of Franck–Condon (FC) calculations based on coupled cluster electronic structure calculations for the equilibrium geometries and harmonic vibrational frequencies of the neutrals, and of the ground and excited electronic states of the cations. Notwithstanding a high-resolution pulsed-field ionisation study on CH2F2 (Forysinski et al., 2010) in which a number of transitions to the  state have been recorded with unprecedented accuracy, we report the first complete vibrationally resolved overview of the low-lying electronic states of CH2X2+, X = F or Cl. Hydrogen atom loss from CH2F2+ occurs at low energy, making the ground state rather anharmonic and interpretation of the  band challenging in the harmonic approximation. By Franck–Condon fits, the adiabatic ionisation energies to the 2B22A2 and 2B2 states have been determined as 14.3 ± 0.1, 15.57 ± 0.01 and 18.0 ± 0.1 eV, respectively. The first band in the CH2Cl2 TPES is complex for a different reason, as it is the result of two overlapping ionic states, 2B2 and 2B1, with derived ionisation energies of 11.0 ± 0.2 and 11.317 ± 0.006 eV, and dominated by an extended progression in the CCl2 bend (in ) and a short progression in the CCl2 symmetric stretch (in ), respectively. Furthermore, even though Koopmans’ approximation holds for the vertical ionisations, the  state of CH2Cl2+ is stabilized by geometry relaxation and corresponds to ionisation from the (HOMO−1) orbital. That is, the first two vertical ionisation energies are in the same order as the negative of the orbital energies of the highest occupied orbitals, but the adiabatic ionisation energy corresponding to electron removal from the (HOMO−1) is lower than the adiabatic ionisation energy corresponding to electron removal from the HOMO. The second band in the spectrum could be analysed to identify the vibrational progressions and determine adiabatic ionisation energies of 12.15 and 12.25 eV for the 2A1 and 2A2 states. A comparison of the assignment of electronic states with the literature is made difficult by the fact that the B1 and B2irreducible representations in C2v symmetry depend on the principal plane, i.e. whether the CX2 moiety is in the xz or the yz plane, which is often undefined in older papers.


Original languageEnglish
Pages (from-to)172–183
JournalJournal of Molecular Spectroscopy
Early online date3 Mar 2015
Publication statusPublished - 1 Sep 2015


  • Threshold photoelectron spectroscopy, Synchrotron radiation, Methylene fluoride, Methylene chloride, Ionization energy, Vibrational modes, iPEPICO, EOM-IP-CCSD