Effects of dimerization on the photoelectron angular distribution parameters from chiral camphor enantiomers obtained with circularly polarized vacuum-ultraviolet radiation

Abstract

As an intermediate state of matter between the free monomeric gas phase and the solid state, clusters may exhibit a specific electronic structure and photoionization dynamics that can be unraveled by different types of electron spectroscopies. From mass-selected ion yield scans measured for photoionization of ($R$)-camphor, the ionization potentials (IPs) of the monomer ($8.66\ifmmode\pm\else\textpm\fi{}0.01$ eV), and of the homochiral dimer ($\ensuremath{\leqslant}8.37\ifmmode\pm\else\textpm\fi{}0.01$ eV) and trimer ($\ensuremath{\leqslant}8.30\ifmmode\pm\else\textpm\fi{}0.01$ eV) were obtained. These spectra, combined with threshold photoelectron spectroscopy and velocity map ion imaging, allow us to show that the camphor monomer and dimer photoionization channels are decoupled, i.e., that the highest occupied molecular orbital (HOMO) of the dimer does not undergo a dissociative ionization process that would lead to a spurious contribution to the monomer ion channel. Therefore mass selection, as achieved in our imaging photoelectron-photoion coincidence experiments, leads to size selection of the nascent monomer or dimer species. Since both the monomer and dimer are chiral, their photoelectron angular distribution (PAD) not only involves the usual $\ensuremath{\beta}$ anisotropy parameter but also a chiral asymmetry parameter $b$${}_{1}$ that can generate a forward-backward asymmetry in the PAD. This has been investigated using circularly polarized light (CPL) to record the photoelectron circular dichroism (PECD) in the near-threshold vacuum-ultraviolet (VUV) photoionization region. Analysis of size-selected electron images recorded with left- and right-handed CPL shows that over the first $1.5$ eV above the HOMO orbital ionization potentials (IPs), the $\ensuremath{\beta}$ parameter is not affected by the dimerization process, while the chiral $b$${}_{1}$ parameter shows clear differences between the monomer and the dimer, confirming that PECD is a subtle long-range probe of the molecular potential.