The vacuum-ultraviolet photoelectron spectra of CH2F2 and CH2Cl2 revisited

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 (PC) 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 (X) over tilde (+) 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 (X) over tilde (+) band challenging in the harmonic approximation. By Franck-Condon fits, the adiabatic ionisation energies to the (A) over tilde+ B-2(2), (C) over tilde (+) (2)A(2) and (D) over tilde (+) B-2(2) 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, (X) over tilde (+) B-2(2) and (A) over tilde (+) B-2(1), 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 (X) over tilde (+)) and a short progression in the CCl2 symmetric stretch (in (A) over tilde (+)), respectively. Furthermore, even though Koopmans' approximation holds for the vertical ionisations, the (X) over tilde (+) 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 (B) over tilde (+) (2)A(1) and (C) over tilde (+) (2)A(2) states. A comparison of the assignment of electronic states with the literature is made difficult by the fact that the B-1 and B-2 irreducible representations in C-2v 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. (C) 2015 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).