Circular dichroism in valence photoelectron spectroscopy of free unoriented chiral molecules: Camphor and bromocamphor

Abstract

The circular dichroism in the photoelectron angular distribution was investigated for valence photoionization of randomly oriented pure enantiomers of camphor and bromocamphor molecules using circularly polarized light in the vacuum ultraviolet. The forward-backward electron emission spectra were recorded simultaneously with two spectrometers at several opposite angles relative to the propagation direction of the photon beam and compared for each of the two substances. Measurements were also carried out for reversed light helicity and opposite molecular handedness. For the left-and right-handed enantiomers of both molecules we observed asymmetries of comparable magnitude up to several percent. The measured asymmetry parameters vary strongly for different orbital binding energies and also for the selected photon energies in the valence region. The results for both molecules are compared. They suggest a strong influence of the final states on the asymmetry, depending on the chiral geometry of the molecular electronic structure, as well as a significant dependence on the initial states involved. They also confirm theoretical predictions describing the effect in pure electric-dipole approximation.

Conformational Effects on Circular Dichroism in the Photoelectron Angular Distribution

“…This observation is in agreement with previous studies on the CDAD effect. [6,11,12] In particular, the comparison of the D profile relative to the HOMO and HOMO-1 orbitals with the experimental profile relative to bromine lone pair in bromocamphor [6] does not present any similarity. The D profile in bromocamphor is close to zero at almost all experimental points, while in both the HOMO and HOMO-1 states of I, significant D values have been calculated for all considered conformations.…”

A Valence Photoelectron Imaging Investigation of Chiral Asymmetry in the Photoionization of Fenchone and Camphor

“…Here the lower panel contains camphor results obtained in the course of a number of different experiments conducted at different SR beamlines. [8][9][10] In particular those camphor measurements made at the SU5 beamline (LURE, Orsay) extend to lower photon energies than were available for the present measurements on fenchone since, following the closure of LURE, these latter measurements could be made only at Elettra. The upper panel, containing the fenchone results includes an additional variant of the [6,2,2] potential calculation discussed above that uses an alternative criterion (Norman radii [28] with a 0.88 scaling factor) for choosing the atomic sphere sizes for the model fenchone potential.…”

Circular Dichroism in Ion Yields of Femtosecond‐Laser Mass Spectrometry

“…However, molecular chirality also causes CD effects in fluorescence [4] and photoelectron emission. [5] Today a variety of measurement techniques exists. The quest for stereo-selective synthesis has stimulated considerable interest in attempts to either separate or transform enantiomers by means of laser excitation employing different kinds of circular polarization.…”