Search

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]

Abstract: Triple ionisation of methane by decay of the singly charged ion with a 1s vacancy produces a trication spectrum starting near 70eV binding energy. Vibrational excitation in the initial hole state broadens and shifts the triple ionisation bands. Ionisation through core-valence doubly ionised states gives lower triple ionisation onsets and changes the spectral intensity pattern in accordance with retention of the initial valence holes in course of the double Auger effect. The double Auger effects occur both directly and as cascades, the different pathways producing different electron distributions and final state spectra. [Copyright &y& Elsevier]

Abstract: High level electronic structure calculations on the OCS2+ dication made by the spdfg cc-pV5Z/CASSCF/MRCI method are presented. They enable a detailed interpretation of the newly recorded spectra of this ion made by the time-of-flight photoelectron photoelectron coincidence (TOF-PEPECO) method, and provide a reliable basis for analysis of its dissociation pathways. The major dissociation to CO+ +S+ appears to occur not on the ground state surface, but from an excited electronic state. Possible involvement of the CSO2+ isomer in dissociation is discussed. [Copyright &y& Elsevier]

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]