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

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 ($8.66\pm 0.01$ eV), and of the homochiral dimer ($⩽8.37\pm 0.01$ eV) and trimer ($⩽8.30\pm 0.01$ 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 $\beta$ anisotropy parameter but also a chiral asymmetry parameter $b$${}_1$ that can generate a forward-backward asymmetry in the PAD. This has been investigated using circularly polarized light (CPL) to record the photoelectron circular dichroism (PECD) in the near-threshold vacuum-ultraviolet (VUV) photoionization region. Analysis of size-selected electron images recorded with left- and right-handed CPL shows that over the first $1.5$ eV above the HOMO orbital ionization potentials (IPs), the $\beta$ parameter is not affected by the dimerization process, while the chiral $b$${}_1$ parameter shows clear differences between the monomer and the dimer, confirming that PECD is a subtle long-range probe of the molecular potential.

Figure 1

Photoion velocity map images of a camphor beam seeded in He recorded at $h\nu =$ 8.8 eV for 0.5 bars (a) and 3 bars (b) of He backing pressure. Along the bottom of each image is shown the projection of ion distribution along the molecular beam direction, corresponding to the beam velocity distribution. This shows more clearly the diffuse, central, isotropic thermal background signal. At high backing pressure the lower temperature in the molecular beam favors the formation of clusters, as seen in (b), where the monomer, dimer. and trimer all appear, dispersed along the beam direction, in the image. The fine structure observed results from the 90% transmission mesh across the front detector face.

Figure 2

PEPICO measurements recorded at 8.80 eV with right-handed CPL on camphor seeded in He (3 bars). (a) Mass spectrum; (b) raw total electron image; (c)–(e) Mass-filtered raw electron images corresponding, respectively, to the monomer, dimer, and trimer. The size of the mass-selected images (c)–(e) increases due to the progressive lowering of the cluster IPs (see text for details).

Figure 3

Ion yield curves for monomer, dimer, and trimer ions, recorded in a cold, supersonic beam (10 K, 3 bars He backing pressure). The ion counts are normalized to photon flux and scaled as shown. The two insets provide expanded views of the monomer and cluster threshold regions. The latter compares monomer, dimer, and trimer yields on the same scale and includes a fitted straight line extrapolation of the higher-energy region of the dimer curve to the baseline (see text). The former includes a comparison of the monomer threshold behavior observed in a warm, nonclustering beam (300 K, 0.6 bars He backing pressure).

Figure 4

TPES of the HOMO band of the camphor monomer. The lower spectrum was recorded under cold, cluster beam conditions, while the upper spectrum was recorded for a warm, nonclustering beam. A smoothed curve is added through the lower spectrum to guide the eye. Simulated spectra (see text) calculated at, respectively, 10 and 300 K are overlaid. The lower panel also includes a stick spectrum showing positions and relative intensities of the more intense calculated excitations. This is omitted for the upper panel due to congestion from the many additional hot band transitions, but a lower-resolution PES recorded in a 350 K gas cell is included for comparison (from Ref. [36]).

Figure 5

Experimental dimer TPES compared with that of the monomer (as in Fig. 4) recorded in the same 10 K beam.

Figure 6

Optimised structure for camphor dimer, showing anti-parallel stacking of the $\mathrm{C}=$O dipoles.

Figure 7

PES (solid curves) and chiral $b$${}_1$ values (points) for the HOMO orbital of the monomer (a) and dimer (b) of $R$-camphor obtained by PEPICO mass-selected velocity map electron imaging, recorded at 10.3 eV photon energy.

Figure 8

Mean HOMO band angular parameters: (a) $b_2^{\{\pm 1\}}\hspace{0.5em}$ [$=$$-(1/2)\beta ]$; (b) $b_1^{\{+1\}}$ recorded for the $R$-($+$) camphor monomer and $\mathit{RR}$ homodimer, presented as a function of electron kinetic energy.