Photoelectron Imaging Spectroscopy of S1(1B2u π,π*) Benzene via 611n (n = 0−3) Levels

A "circularisation" method to repair deformations and determine the centre of velocity map images

A problem besetting the analysis of velocity map images, particularly those of photoelectrons, is the presence of distortions that cause the features in the image to deviate from circularity, leading to a loss of resolution in the spectrum extracted. A method is presented to repair such distortions based on fitting the angular behaviour of each of the ring structures to a trigonometric expansion. The repair function allows the intensity at any value of radius and angle to be mapped to a new position that removes the distortion and returns the features to circular. While the method relies on the analysis of the structure in an image, it could also be applied to determine the "repair function" using a calibration image (or series of images) for the experiment. Once the image has been circularised it can be processed by any of the approaches that have been developed for that purpose. The analysis also enables the image centre to be determined with high accuracy. The fitting method utilises an inverse Abel transformation of the image in polar coordinates as a means to reshape the image into a series of spectral features in order to determine the radial position of features at each angle. Although the velocity distribution is not in general spherically symmetric and so this is not a mathematically correct means to extract the velocity distribution, the feature positions are accurately reproduced in the resulting spectrum while the intensity and anisotropy parameters can be remarkably close to those obtained using the proper inverse Abel transformation of the image.

Vacuum upgrade and enhanced performances of the double imaging electron/ion coincidence end-station at the vacuum ultraviolet beamline DESIRS

We report here the recent upgrade of the SAPHIRS permanent photoionization end-station at the DESIRS vacuum ultraviolet beamline of synchrotron SOLEIL, whose performances have been enhanced by installing an additional double-skimmer differential chamber. The smaller molecular beam profile obtained at the interaction region has increased the mass resolution of the double imaging photoelectron photoion coincidence (i(2)PEPICO) spectrometer, DELICIOUS III, installed in the photoionization chamber of the SAPHIRS endstation, by a factor of two, to M/ΔM ∼ 1700 (FWHM). The electron kinetic energy resolution offered by the velocity map imaging (VMI) part of the spectrometer has been improved down to 2.8% (ΔE/E) as we show on the N2 photoionization case in the double skimmer configuration. As a representative example of the overall state-of-the-art i(2)PEPICO performances, experimental results of the dissociation of state-selected O2(+)(B(2)∑(g)(-), v(+) = 0-6) molecular ions performed at the fixed photon energy of hν = 21.1 eV are presented.

Intramolecular Processes Revealed Using UV-Laser-Induced IR-Fluorescence: A New Perspective on the "Channel Three" of Benzene

Radiative relaxation in the infrared (IR) is common following nonradiative electronic relaxation processes, but it is rarely measured. We present ultraviolet laser-induced infrared fluorescence (UV-LIIRF) excitation spectroscopy and dispersed UV-LIIRF spectroscopy of gas phase benzene vibronically excited around the onset of channel 3, using a homemade spectrometer. We found that the vibrational IR fluorescence yield is clearly higher when benzene is excited above the onset than when it is excited below. Significant changes in dispersed IR emission profiles resulting from excitations below and above the onset of channel 3 were also observed. These results suggest that isomerization of benzene toward fulvene occurs efficiently below the opening of channel 3 and confirm that channel 3 involves a photophysical relaxation pathway that efficiently competes with isomerization.

Time-resolved photoelectron imaging of S2 → S1 internal conversion in benzene and toluene

Ultrafast internal conversion of benzene and toluene from the S(2) states was studied by time-resolved photoelectron imaging with a time resolution of 22 fs. Time-energy maps of the photoelectron intensity and the angular anisotropy were generated from a series of photoelectron images. The photoelectron kinetic energy distribution exhibits a rapid energy shift and intensity revival, which indicates nuclear motion on the S(2) adiabatic surface, while the ultrafast evolution of the angular anisotropy revealed a change in the electronic character of the S(2) adiabatic surface. From their decay profiles of the total photoelectron intensity, the time constants of 48 ± 4 and 62 ± 4 fs were determined for the population decay from the S(2) states in benzene and toluene, respectively.