Ivan Powis

Two-dimensional charged particle image inversion using a polar basis function expansion

We present an inversion method called pBasex aimed at reconstructing the original Newton sphere of expanding charged particles from its two-dimensional projection by fitting a set of basis functions with a known inverse Abel integral. The basis functions have been adapted to the polar symmetry of the photoionization process to optimize the energy and angular resolution while minimizing the CPU time and the response to the cartesian noise that could be given by the detection system. The method presented here only applies to systems with a unique axis of symmetry although it can be adapted to overcome this restriction. It has been tested on both simulated and experimental noisy images and compared to the Fourier-Hankel algorithm and the original Cartesian basis set used by [Dribinski et al.Rev. Sci. Instrum. 73, 2634 (2002)], and appears to give a better performance where odd Legendre polynomials are involved, while in the images where only even terms are present the method has been shown to be faster and simpler without compromising its accuracy.

Photofragment investigations of the 280 nm photodissociation of methyl iodide using rempii atom detection

Abstract The A-band photodissociation of CH 3 I has been investigated around 280 nm, to the red of the absorption maximum. Iodine atom photofragments are state-selectively detected by multiphoton ionization and a re-examination of the pertinent region of the atomic iodine MPI spectrum is presented. From a time-of-flight photofragment analysis β anisotropy parameters are deduced for the I* and I dissociation channels. The reduction in β for the latter is discussed in terms of the surface crossing in this channel. ν 2 umbrella mode methyl radical excitation distributions are also derived and compared to previous measurements at 248 and 266 nm, and to the predictions of a multisurface photodissociation model.

Rotational structure and predissociation dynamics of the methyl 4pz(v=0) Rydberg state investigated by resonance enhanced multiphoton ionization spectroscopy

The vibrationless bands of the methyl 4p 2A‘2 ←X 2AA‘2 0–0 Rydberg transition have been recorded by mass selected (2+1) REMPI spectroscopy using photolytically generated CH3 and CD3 radicals. A least‐squares analysis of the spectra determines values for the band origin and the rotational constants B’, D’N, DNK, and (C’−C‘). Linewidths in the CH3 spectrum show a pronounced rotational level dependence and by means of a spectral simulation a heterogeneous predissociation is identified and shown to be induced by a perpendicular Coriolis coupling mechanism.

Circular dichroism in the photoelectron angular distribution from randomly oriented enantiomers of camphor

Circular dichroism in the angular distribution of valence photoelectrons emitted from randomly oriented chiral molecules has been observed in experiments that use circularly polarized VUV synchrotron radiation. Photoionization of the outermost carbonyl oxygen lone pair electrons from pure enantiomers of the prototype chiral molecule camphor is shown to have an asymmetry in the forward–backward scattering of photoelectrons (relative to the propagation direction of the light beam) of magnitude approaching 3% at 9.2 eV photon energy. The asymmetry reverses on exchange of either the helicity of the radiation or of the molecular enantiomer, confirming theoretical predictions of an effect that arises in the pure electric-dipole approximation.

Fragmentation of valence electronic states of CF+4 and SF+6 studied by threshold photoelectron-photoion coincidence spectroscopy

Abstract Threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy has been used to measure, state selectively, the decay pathways of all the valence states of CF+4 and SF+6 in the range 15–28 eV. Radiation in the vacuum UV from a synchrotron radiation source ionises the parent molecule, and the electrons and ions are detected by threshold electron analysis and time-of-flight mass spectroscopy, respectively. TPEPICO spectra are recorded continuously as a function of photon energy, allowing both threshold photoelectron spectra and yields of all the fragment ions to be obtained. Kinetic energy releases are measured at fixed photon energies with good time resolution. The results orf the X 2T1, A 2T2 and B 2E states of CF+4 (all below the energy of He I radiation) confirm earlier measurements; results for the C 2T2 and D 2A1 states at 21.7 and 25.1 eV, respectively, are new. They confirm previous measurements that radiative decay from both states is an important process, and we have measured their state-selected fluorescence quantum yields. For SF+6, fluorescence does not occur from any of the excited valence states, and we have measured the fragmentation channels and branching ratios for all of the valence states.

Determination of chiral asymmetries in the valence photoionization of camphor enantiomers by photoelectron imaging using tunable circularly polarized light

An electron imaging technique has been used to study the full angular distribution of valence photoelectrons produced from enantiomerically pure molecular beams of camphor when these are photoionized with circularly polarized light. In addition to the familiar beta parameter, this provides a new chiral term, taking the form of an additional cosine function in the angular distribution which consequently displays a forward-backward electron ejection asymmetry. Several ionization channels have been studied using synchrotron radiation in the 8.85-26 eV photon energy range. With alternating left and right circularly polarized radiations the photoelectron circular dichroism (PECD) in the angular distribution can be measured and shows some strong dynamical variations with the photon energy, depending in sign and intensity on the ionized orbital. For all orbitals the measured PECD has a quite perfect antisymmetry when switching between R and S enantiomers, as expected from theory. In the HOMO(-1) channel the PECD chiral asymmetry curves show a double maxima reaching nearly 10% close to threshold, and peaking again at approximately 20% some 11 eV above threshold. This is attributed to a resonance that is also visible in the beta parameter curve. Newly optimized CMS-Xalpha photoionization dynamics calculations are also presented. They are in reasonably good agreement with the experimental data, including in the very challenging threshold regions. These calculations show that PECD in such randomly oriented samples can be understood in the electric dipole approximation and that, unlike the case pertaining in core-shell ionization-where a highly localized achiral initial orbital means that the dichroism arises purely as a final state scattering effect-in valence shell ionization there is a significant additional influence contributed by the initial orbital density.

Photoelectron circular dichroism of the randomly oriented chiral molecules glyceraldehyde and lactic acid

The differing interaction of left and right circularly polarized light with chiral molecules is shown to lead to different angular distributions of the photoelectrons created by photoionization of a given enantiomer, even when the target molecules are randomly oriented. Numerical calculations are presented to demonstrate the magnitude of this effect for the C3H6O3 structural isomers lactic acid and glyceraldehyde, including two different conformations of the latter. Circular dichroism in the angular distributions (CDAD) of the valence electrons of these biomolecules is most pronounced close to threshold, but tends to vanish as the electron kinetic energy approaches 20 eV and above. CDAD signals are predicted to range, typically, from 10% to 40% and sometimes to more than 60% of the differential cross section.

A refocusing modified velocity map imaging electron/ion spectrometer adapted to synchrotron radiation studies

We present a modified velocity map imaging (VMI) spectrometer to be used in angle-resolved molecular photoionization studies in the gas phase with synchrotron radiation (SR) in the VUV/soft x-ray range. The main modifications as compared to the original design of Eppink and Parker [A. T. J. B. Eppink and D. H. Parker, Rev. Sci. Instrum. 68, 3477 (1997)] are an open repeller which allows the VMI spectrometer to be coupled to an independent dispersive electrostatic analyzer for combined operation in coincidence mode experiments, and the introduction of a coupled double Einzel lens in the flight tube in order to collect the full 4π solid angle for higher kinetic energy particles. The length and position of the lenses have been optimized by a genetic algorithm to obtain the maximum kinetic energy possible without compromising the energy resolution. Ray-tracing simulations and SR experiments show that the lenses can increase the kinetic energy bandwidth by a factor of up to 2.5. Furthermore, a remarkable impro...

Vibrationally induced inversion of photoelectron forward-backward asymmetry in chiral molecule photoionization by circularly polarized light

Electron–nuclei coupling accompanying excitation and relaxation processes is a fascinating phenomenon in molecular dynamics. A striking and unexpected example of such coupling is presented here in the context of photoelectron circular dichroism measurements on randomly oriented, chiral methyloxirane molecules, unaffected by any continuum resonance. Here, we report that the forward-backward asymmetry in the electron angular distribution, with respect to the photon axis, which is associated with photoelectron circular dichroism can surprisingly reverse direction according to the ion vibrational mode excited. This vibrational dependence represents a clear breakdown of the usual Franck–Condon assumption, ascribed to the enhanced sensitivity of photoelectron circular dichroism (compared with other observables like cross-sections or the conventional anisotropy parameter-β) to the scattering phase off the chiral molecular potential, inducing a dependence on the nuclear geometry sampled in the photoionization process. Important consequences for the interpretation of such dichroism measurements within analytical contexts are discussed.

Photoelectron circular dichroism in core level ionization of randomly oriented pure enantiomers of the chiral molecule camphor

The inner-shell photoionization of unoriented camphor molecules by circularly polarized light has been investigated from threshold to a photoelectron kinetic energy of approximately 65 eV. Photoelectron spectra of the carbonyl C 1s orbital, recorded at the magic angle of 54.7 degrees with respect to the light propagation direction, show an asymmetry of up to 6% on change of either the photon helicity or molecular enantiomer. These observations reveal a circular dichroism in the angle resolved emission with an asymmetry between forward and backward scattering (i.e., 0 degrees and 180 degrees to the light beam) which can exceed 12%. Since the initial state is an atomiclike spherically symmetric orbital, this strongly suggests that the asymmetry is caused by final-state effects dependent on the chiral geometry of the molecule. These findings are confirmed by electron multiple scattering calculations of the photoionization dynamics in the electric-dipole approximation.