Photoelectron spectroscopy of (C6F6)n- and (C6F6)I- clusters in a novel instrument

Abstract

The capabilities of a new photoelectron spectrometer are characterised in the study of (C6F6)n– and (C6F6)I– clusters. The photoelectron spectrometer consists of a series of vacuum chambers that facilitate a molecular beam of gas-phase anions. These anions and clusters are generated at the point of intersection between an electron beam and the supersonic expansion produced by a high temperature Even-Lavie pulsed valve. The anions are extracted orthogonally and mass-separated in a Wiley-McLaren time-of-flight mass spectrometer. The analyte is addressed by a laser pulse produced by either a tuneable nanosecond OPO for one-photon frequency-resolved measurements or a femtosecond pump-probe regime for two-photon time-resolved measurements. The kinetic energy of the resulting photoelectrons are measured in a velocity-map imaging spectrometer.

C6F6- is of interest due to the prediction that it ought to host a meta-stable binding mode of the excess electron known as a correlation-bound state (CBS). Similar to other non-valence binding modes, such as the dipole-bound state, this CBS is thought to act as a doorway state in the mechanism of low energy electron capture.

The CBS is characterised by a large and diffuse orbital in which the excess electron is bound primarily through charge:induced-dipole interactions with the molecule’s valence orbital system. In the CBS, C6F6- is predicted to adopt a planar geometry like that of the neutral species, in contrast to the buckled geometry adopted when the excess electron occupies a valence orbital.

Frequency- and time-resolved measurements of the anion were made in an effort to generate and observe the CBS. Initially, frequency resolved measurements of (C6F6)n– revealed a vertical detachment energy of 1.60 ± 0.07 eV for n = 1, increasing by 200 meV per additional cluster unit up to n = 5. The broad shape of the direct detachment peak confirms the disparity in geometry between the anion and neutral species. However, no evidence of the CBS was evident in these data.

In subsequent explorations, an in-situ electron donor was employed to mimic the electron impact process. An I– ion was introduced to the neutral C6F6 molecule to produce clusters of (C6F6)I–. Frequency-resolved measurements of this cluster revealed a mechanism for electron loss below the threshold for the single-photon direct detachment process. This suggested the presence of a charge-transfer channel centred around hv = 3.3 eV.

Time-resolved measurements of (C6F6)I– confirmed the presence of the charge-transfer state and revealed an instantaneous and coherent transfer of the electron onwards into the valence orbital system of the C6F6 molecule. This transfer of charge causes the molecule to vibrate as its geometry changes from planar to buckled. The oscillations observed in the photoelectron spectra are coherent and sinusoidal and have a frequency of 121 ± 2 cm−1. This compares very favourably to the frequency of 122 cm−1 calculated by time-dependent density functional theory for the bending mode of C6F6 along the coordinate between the planar and buckled geometries. I argue that this charge-transfer state exhibits the predicted characteristics of a CBS.