In photoionization of free, unoriented chiral molecules with circularly polarized radiation, a significant circular dichroism, i.e., an asymmetry in the forward-backward electron emission, has been observed in the photoelectron angular distribution. This leads also to an asymmetry in the momentum transfer to the photoions. The spectra for the left-and right-handed enantiomers of bromocamphor exhibit asymmetries up to several percent which vary as a function of orbital binding energy. This enantioselective effect can similarly occur for biomolecules with handedness, like amino acids, and may thus be a contributing factor related to the origin of the terrestrial biomolecular homochirality. DOI: 10.1103/PhysRevLett.86.1187 Nature is dissymmetric on the macroscopic and microscopic scale. Processes with chiral molecules exhibiting strong enantiomeric selectivity and the unresolved origin of the homochirality of terrestrial life, i.e., the predominance of one handedness for all biomolecules, have received special attention [1]. Asymmetries in the interaction of polarized light with chiral molecules have been studied since Pasteur's pioneering experiments on optical activity [2]. While optical techniques with visible light are routinely used in this context, only a few studies of chirality were performed with ionizing radiation. Very recently, it was found that aerosol particles of chiral molecules exhibit in the total photoelectron yield a circular dichroism which increases with particle size and crystallite order [3]. However, up to now, no experimental observations were reported on the intrinsic effects of the molecular chirality in photoelectron spectroscopy. Here we show that ionization with circularly polarized radiation leads to a significant circular dichroism in the photoelectron angular distribution of free, unoriented ͑1R͒-͑1͒ bromocamphor ͑C 10 H 15 BrO͒ molecules. Our findings confirm general predictions [4,5] for the angular dependence and demonstrate that sizable dichroic effects exist for isolated, randomly oriented gas-phase molecules with definite handedness.Because of its "handed" structure, a chiral molecule has no symmetry plane and also no center of inversion. The two enantiomers of a chiral molecule are mirror images of each other and cannot be superimposed; however, on the basis of electromagnetic interaction theory they have identical electronic structure. As was first pointed out by Ritchie [4] and later discussed in detail by Cherepkov [5,6], this lack of symmetry should lead to an optical activity, i.e., helicity-dependent effects, in photoemission, even if the molecules are randomly oriented. In contrast to the circular dichroism (CD) of the total (angle-integrated) photoelectron yield, the circular dichroism parameter of the angular distribution (CDAD) should be larger since it occurs already in pure electric dipole approximation. Very recently, Powis has obtained pronounced effects in numerical CDAD calculations for chiral molecules [7]. For atoms and unoriented (nonchiral) molecules the...

No abstract

Xenon spectra involving emission from ion species of Xe7+ to Xe12+ were analyzed for a plasma focus discharge developed for extreme ultraviolet lithography. Low and higher resolution spectra were recorded in the 8–21 nm wavelength region for different operating conditions and different He–Xe gas mixtures. The spectra have been compared with Hartree–Fock calculations. The modeling included the distribution of the various xenon ion levels at a given electron equilibrium temperature and plasma opacity effects. Spectral analysis showed that the 4d–5p transition arrays are fairly well separated in wavelength for the ions Xe8+ to Xe11+. Good agreement between experiment and calculations was obtained for line positions and intensities, in particular, for the wavelength region at around 13.5 nm.

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.