Vacuum ultraviolet circular dichroism spectroscopy using an ac-modulated polarizing undulator

Abstract

To obtain vacuum ultraviolet (VUV) circular dichroism (CD) measurements, an Onuki-type crossed undulator in an electron storage ring (TERAS) at the National Institute of Advanced Industrial Science and Technology (AIST) was used as a light source and a polarization modulator was used instead of a photoelastic modulator (PEM). Polarization modification through the beam-line optics was theoretically considered and experimentally examined by polarization analysis. The modulation frequency of 2Hz was chosen to optimize the signal-to-noise ratio. On this basis, VUV-CD spectra of alanine films were measured and calibrated. The resulting CD spectra were consistent with other data obtained that was measured by a conventional CD spectropolarimeter (JASCO, J720WI) at a wavelength range of 190–210nm. The spectra of L- and D-alanine films showed an obvious symmetry with respect to the baseline at a wavelength range of 120–210nm. These results proved that the true CD of the samples was measured. The present technique successfully extended the wavelength coverage of CD measurements to a shorter range than the practical limit of PEMs.