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A new apparatus has been developed to detect and measure angular correlations between energy-selected photoelectrons and coincident mass-analyzed fragment ions from photoionization at selected wavelengths. It achieves velocity imaging for electrons and ions simultaneously and has high collection efficiency for both particles, with moderate mass and energy resolution. Angular and energy correlations between the two particles are measured, as are the angular distributions of each particle independently relative to the light polarization direction. Fixed-molecule electron angular distributions are deduced in cases of pure axial recoil. Examples of angular distributions from photoionization of diatomic molecules are reported. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract: The present work combines experimental and theoretical investigations of HCN2+ to investigate the spectroscopy and dissociation dynamics of this dication. Its spectrum is a complex blend of overlapping features, consistent with theoretical calculations showing that the low-lying electronic states of HCN2+ exhibit Renner–Teller and vibronic interactions. The double ionisation spectra are dominated by C–N stretching excitations for all the electronic states investigated. Long-lived HCN2+ ions are observed and are believed to be in low vibrational levels of the X3Σ− and/or 1Δ and/or 1Σ+ electronic states; some have metastable lifetimes of about 25 ns. The formation of H++CN+ products is attributed to predissociation by the lowest 3A″ component resulting from a conical intersection between the X3Σ− and 3Π states. The overall predissociation processes involve vibronic, Renner–Teller and spin–orbit couplings between several electronic states of HCN2+. [Copyright &y& Elsevier]

Photoelectron-photoion coincidence (PEPICO) spectra at 73.6 nm reveal an interesting fact; from hexafluorobenzene cation the C5F3+ fragment is most abundant whilst from the benzene cation C4H4+ is far more abundant than C5H3+. These contrasting results are connected, as shown by the PEPICO spectra of other fluorinated benzenes, by a regular change of the dominant fragment from C5F3+ to C4H4+ as the number of fluorine atoms decreases. This regularity is consistent with the appearance energies, at QCISD[T] or CBS-QB3 levels, of C5FmH3−m+ in the acetylene-cyclopropene structure and C4FnH4−n+ in the methylene-cyclopropene and/or the vinyl-acetylene structures. The energy-selected PEPICO data further support the theoretical results. [Copyright &y& Elsevier]

Abstract: Double photoionisation spectra of ethyne and ethyne-d2 have been measured by the TOF–PEPECO technique and by the TPEsCO method with vibrational resolution. With the help of new calculations of the dication structure and Franck–Condon factors we deduce adiabatic double ionisation energies for the first three states of the normal hydrogen isotopomer as 31.74eV (X3Σg-), 32.42 (a1Δg) and 33.07 (b1Σg+). The two techniques give spectra with very different vibrational structure and reasons for the difference are discussed. [Copyright &y& Elsevier]