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Using a magnetic bottle time-of-flight electron spectrometer we have measured double photoionisation spectra of N 2 and CO 2 covering the range from threshold up to the triple ionisation energies. The experiments demonstrate the use of a new asynchronous chopper in eight-bunch mode of the synchrotron radiation source SOLEIL. For CO 2 2+ some broad bands in the Auger spectra are found to have multiple resolved counterparts in the photoionisation spectrum. All the bands in the Auger spectra have counterparts in the photoionisation spectra, where extra bands attributed to triplet states are present. In the spectrum of N 2 2+ we suggest reassignment of one band. [ABSTRACT FROM AUTHOR]

Dissociative double photoionisation of CO below the adiabatic double ionisation threshold has been studied by the time-of-flight photoelectron–photoelectron coincidence (TOF-PEPECO) and the velocity imaging photoionisation coincidence (VIPCO) methods. By combining the results from the two methods which give complementary information, the double photoionisation process is identified to CO + hv→ CO+* + e− → C+(2P) + [O+(2D/2P)]nl + e− → C+(2P) + O+(4S) + 2e−. The high energy resolution of the TOF-PEPECO spectrometer enables us to resolve individual fragment [O+(2D/2P)]nl states and also intermediate CO+* states. The angular correlations between electrons and ions, deduced from the VIPCO data, imply that the nuclear dissociation is faster or at least comparable in the time scale with molecular rotation, and that the second electron emission occurs after the C+ and O* atoms are far apart. A possible generic identification of the intermediate states is suggested. [Copyright &y& Elsevier]

Photoionization into the dissociation continuum of H2+(X2Σg+) has been studied using the velocity imaging photoionization coincidence method. Asymmetry parameters for the angular distributions of photoelectrons and fragment H+ ions have been measured at photon energies of 21.2, 23.1, 26.9 and 40.8 eV. Molecular frame photoelectron angular distributions (MF-PADs) have also been extracted for the parallel and perpendicular transitions at all the photon energies. All the MF-PADs show p-wave shape, which suggests a dominant p-wave contribution in the photoionization cross-sections at all energies. [Copyright &y& Elsevier]

A coincidence spectrometer combining time-focusing optics with velocity imaging has been developed to achieve high velocity resolution for both positive and negative particles. Positional broadening of particle images and temporal broadening of particle flight times caused by an extended source volume are effectively cancelled by the design of the electrostatic lenses. The performance of the new optics is demonstrated in photoionization of CO. Sample results are presented on dissociative photoionization from CO+(D2Π) and N2O+(C2Σ+), where molecular frame photoelectron angular distributions are obtained for the first time with vibrational resolution of both initial state and diatomic products. [Copyright &y& Elsevier]

Using the velocity imaging photoionization coincidence technique we have determined molecular-frame photoelectron angular distributions for ionization of H2 to the 2pσu, 2sσg and 2pπu states of H2+ from angular correlation between photoelectrons and the product protons. The results are analyzed in terms of electric dipole transitions populating symmetry-allowed low-l partial waves. In perpendicular ionization to 2pσu an almost pure dπg wave is observed, in accordance with simple symmetry predictions. [Copyright &y& Elsevier]

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]

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]

Breakdown of the ions created by photoionisation of C6F6 at 73.6, 58.4 and 30.4 nm has been examined by two separate photoelectron–photoion coincidence techniques and has been interpreted using density functional theory calculations. A full breakdown diagram, amplifying earlier metastable ion studies, is determined. The rate of the main dissociative rearrangement reaction forming C5F3+, measured as a function of the internal energy of the ion, has been fitted by RRKM calculations and a large kinetic shift is found. The structure of the C5F3+ ion and its 0 K threshold have been obtained from the experiments and also from theoretical calculations. Calculations of all the vibrational frequencies of the ground state hexafluorobenzene cation agree with existing experimental data. [Copyright &y& Elsevier]

Abstract: Simultaneous two-electron emissions from core-valence double photoionization in Ne and upon core-electron excitation in Ar have been studied using a very efficient multi-electron coincidence method. The former case demonstrates that the population ratio of singlet and triplet doubly-charged ion states reveals the double photoionization dynamics. The latter case shows that a weakly-bound electron can be used as a probe of the double Auger dynamics of a core-hole. In both cases, the inadequacy of the shake-off model is revealed by the experimental observations alone and the necessity to describe better electron correlation for two-electron emission, as done in the knock-out model. [ABSTRACT FROM AUTHOR]

Abstract: The double valence photoelectron spectrum of ICl has been measured at two photon energies, 303.78Å and 379.3Å, by the TOF-PEPECO technique. Relativistic molecular electronic structure calculations have been performed for electronic states connected to the three lowest groups of dissociation limits to support the interpretation, and vibrational constants have been calculated for the quasi-bound states. The two lowest double ionisation energies are found to be 27.45±0.1eV (vertical) and 27.69±0.1eV (vertical). They are associated with the electronic states of ICl2+, respectively, which are the only states predicted to be stable by the calculations. The two following states connected to the same configuration are in order a 1Δ2 and b . The double ionisation processes building up the spectrum are found to be mainly direct, and energies and widths of the bands are well reproduced by the calculations. [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]