The vacuum-ultraviolet photoelectron spectra of CH2F2 and CH2Cl2 revisited

Richard Tuckett,Jonelle Harvey,Patrick Hemberger,Andras Bodi

doi:10.1016/j.jms.2015.02.012

Abstract

The threshold photoelectron spectrum (TPES) of difluoromethane and dichloromethane has been recorded at the Swiss Light Source with a resolution of 2meV or 16cm−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 X∼+ 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∼+ band challenging in the harmonic approximation. By Franck–Condon fits, the adiabatic ionisation energies to the A∼+2B2, C∼+2A2 and D∼+2B2 states have been determined as 14.3±0.1, 15.57±0.01 and 18.0±0.1eV, 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∼+2B2 and A∼+2B1, with derived ionisation energies of 11.0±0.2 and 11.317±0.006eV, and dominated by an extended progression in the CCl2 bend (in X∼+) and a short progression in the CCl2 symmetric stretch (in A∼+), respectively. Furthermore, even though Koopmans’ approximation holds for the vertical ionisations, the X∼+ 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 12.25eV for the B∼+2A1 and C∼+2A2 states. A comparison of the assignment of electronic states with the literature is made difficult by the fact that the B1 and B2 irreducible 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.

Highlights

Gas-phase vacuum ultraviolet photoelectron spectroscopy, established in the late 1960s, is a mature technique for determining ionisation energies of molecules and studying their electronic structure
We report the valence vacuum-UV threshold photoelectron spectrum of CH2F2 and CH2Cl2 recorded at the 3rd generation Swiss Synchrotron Light Source, SLS, from their ionisation onset, 11–12 eV, to 20 and 16 eV, respectively
The threshold photoelectron spectrum of CH2F2 between 12.7 and 20.5 eV is shown in Fig. 1, recorded at a resolution of 0.006 eV, along with calculated adiabatic ionisation energies at the equation-ofmotion for ionisation potential (EOM-IP-)coupled cluster theory with singles and doubles (CCSD)/cc-pVTZ level of theory

Summary

Introduction

Gas-phase vacuum ultraviolet photoelectron spectroscopy, established in the late 1960s, is a mature technique for determining ionisation energies of molecules and studying their electronic structure. In tandem with experimental improvements, transitions can often be assigned with the aid of ab initio calculations, in particular with Franck–Condon (FC) simulations of the vibrational progressions in each cationic electronic state These advances have led to surprises in the interpretation of the vacuum-UV photoelectron spectra of difluoromethane (CH2F2) and dichloromethane (CH2Cl2), especially in their ground-state bands, but for different reasons. It is, not surprising that the first photoelectron band corresponding to Xe þ CH2F2+ is complicated on spectroscopic grounds alone because Coriolis interactions are prevalent in the neutral, making formally-forbidden IR transitions, such as m5 (a2), observable. We analyse the threshold photoelectron spectra in the light of new ab initio calculations and Franck–Condon simulations, and reflect on how the interpretation of these apparently simple spectra has evolved over the years

Experimental and theoretical methods

Results and discussion

Higher energy peaks in the valence photoelectron spectra of CH2F2 and CH2Cl2

Conclusions