D. Dowek

Ion–electron velocity vector correlations in dissociative photoionization of simple molecules using electrostatic lenses

A new development of electrostatic lenses for ion and/or electron trajectory focusing has been implemented in a double velocity spectrometer which combines time of flight resolved coincidence and imaging techniques using fast position sensitive detectors. Ions and electrons produced by photoionization are extracted from the interaction region by a unique uniform electrostatic field, while the electrostatic lenses create a nonuniform field outside the extraction region. The space focusing reducing the effect of the finite dimensions of the interaction region on one side, and the global bending of the ion and/or electron trajectories preserving the time of flight information on the other side, enable a significant improvement of the resolution of the three measured velocity components for each particle. Such a device is particularly well adapted for the study of vector correlations in dissociative photoionization of simple molecules in the vacuum ultraviolet photon energy range. The gain achieved in the ene...

Complete description of linear molecule photoionization achieved by vector correlations using the light of a single circular polarization

In this paper we demonstrate that the vector correlation approach for the study of dissociative photoionization (DPI) of linear molecules enables us to achieve a complete description of molecular photoionization by performing a single experiment using only one state of circularly, or elliptically, polarized light. This is illustrated by the derivation of the complex dipole matrix elements for the benchmark DPI reaction of the NO molecule, where (4)–1 inner-valence ionization is induced by left-handed circularly polarized synchrotron radiation at h= 23.65 eV. The importance of electronic correlation for this process is emphasized by comparing the experimental results with multichannel Schwinger configuration interaction calculations. The energy dependence of the transition matrix elements and that of the electronic correlation in the 25–40 eV energy range are illustrated by the calculations and compared with the present results and recent experimental studies at 40.8 eV.

Vector correlations in dissociative photoionization of O2 in the 20–28 eV range. I. Electron-ion kinetic energy correlations

Imaging and time resolved coincidence techniques are combined to determine ion-electron (VO+,Ve,P) velocity vector correlations in dissociative photoionization (DPI) of the O2 molecule induced by linearly polarized synchrotron radiation (P). The ion-electron kinetic energy correlations identify each DPI process by its reaction pathway, intermediate molecular state and dissociation limit. The 4π collection of ions and electrons provides their branching ratios. Up to 12 DPI processes are identified in the 20–28 eV range. Photoionization into the O2+(B 2∑g−) in the Franck–Condon (FC) region populating the [O+(4S)+O(3P)] limit is the dominant process. In the 22.3–24 eV region excitation of the O2*(3 2∏u,nsσg) Rydberg series, followed by dissociation and atomic autoionization to the [O+(2D)+O(3P)] limit reaches about 10% of the DPI flux. A new DPI process is identified in the same energy range, which populates the [O+(4S)+O(1D)] limit. At higher energies the relative weight of the four distinct processes which...

Vector correlations in dissociative photoionization of O2 in the 20–28 eV range. II. Polar and azimuthal dependence of the molecular frame photoelectron angular distribution

A combined experimental and theoretical study of the polar and azimuthal dependence of the molecular frame photoelectron angular distributions (MFPADs) for inner-valence-shell photoionization of the O2 molecule into the O2+(B 2Σg−,3 2Πu,c 4Σu−) states is reported. The measured MFPADs, for each orientation of the molecular axis with respect to the linear polarization of the synchrotron radiation, are derived from the spatial analysis of the (VO+,Ve,P) vector correlation, where the nascent ion and electron velocity vectors VO+ and Ve are determined for each dissociative photoionization (DPI) event using imaging and time of flight resolved coincidence technique as described in the companion paper of this series [J. Chem. Phys. 114, 6605 (2001)]. Expressed in the general form of four FLN(θe) functions which contain all the dynamical information about the photoionization processes, they are compared with the MFPADs computed using the multichannel Schwinger configuration interaction method. A very satisfactory ...

Molecular frame photoelectron angular distributions in dissociative photoionization of H2 in the region of the Q1 and Q2 doubly excited states

Dissociative photoionization of H2 induced by VUV linearly polarized synchrotron radiation P has been studied using the (VH+,Ve,P) vector correlation method. The ion–electron kinetic energy correlation diagrams obtained for the three photon excitation energies hν = 20, 28.5 and 32.5 eV enable us to identify and select the dominant dissociative photoionization processes. The Iχ(θe,e) molecular frame photoelectron angular distributions for any orientation χ of the molecular axis with respect to the polarization are reported for direct photoionization of H2 into the H2+(2Σg+) ionic ground state at hν = 20 eV and for the dominant DPI processes involving autoionization of the H2(Q1 1Σu+(1)) and H2(Q2 1Πu(1)) doubly excited states into the H2+(2Σg+) and H2+(2Σu+) continua at hν = 28.5 and 32.5 eV. They show the dominant excitation of a p σu partial wave in autoionization of the Q1(1Σu+(1)) state into the H2+(1s σg) ionic state and that of a d πg partial wave in autoionization of the Q2(1Πu(1)) state into the H2+(2p σu) continuum. A molecular frame forward–backward electron emission anisotropy is observed when ionization takes place at large internuclear distance.

Molecular frame and recoil frame photoelectron angular distributions from dissociative photoionization of NO2

The authors report measured and computed molecular frame photoelectron angular distributions (MFPADs) and recoil frame photoelectron angular distributions (RFPADs) for the single photon ionization of the nonlinear molecule NO2 leading to the (1a2)-1 b 3A2 and (4a1)-1 3A1 states of NO2+. Experimentally, the RFPADs were obtained using the vector correlation approach applied to the dissociative photoionization (DPI) involving these molecular ionic states. The polar and azimuthal angle dependences of the photoelectron angular distributions are measured relative to the reference frame provided by the ion recoil axis and direction of polarization of the linearly polarized light. Experimental results are reported for the photon excitation energies hnu=14.4 and 22.0 eV. Theoretically the authors give expressions for both the MFPAD and the RFPAD. They show that the functional form in the recoil frame, where an average over the azimuthal dependence of the molecular fragments about the recoil direction is made, is identical to that they have earlier found for the DPI experiments performed on linear molecules. MFPADs were then computed using single-center expansion techniques within the fixed-nuclei frozen-core Hartree-Fock approximation. The computed cross sections for ionization to the (1a2)-1 b 3A2 state show a strong propensity for ionization with the polarization of the light perpendicular to the plane of the molecule, whereas the ionization to the (4a1)-1 3A1 state of the ion is of similar intensity for all orientations of the polarization of the light in the molecular frame. These qualitative features of the MFPAD are also evident in the RFPAD. The RFPAD for ionization leading to the (1a2)-1 b 3A2 state is strongly peaked in the perpendicular orientation, whereas the RFPAD for ionization leading to the (4a2)-1 3A1 state is much more nearly isotropic. Comparison between experimental and theoretical RFPADs indicates that the recoil angle for NO+ fragments is approximately 50 degrees relative to the symmetry axis of the initial C2v symmetry of the NO2 molecule in the ionization leading to the (1a2)-1 b 3A2 state and the recoil angle is approximately 120 degrees for the O+ fragment for ionization leading to the (4a1)-1 3A1 state.

Dissociative photoionization of N2O in the region of the N2O+(B 2Π) state studied by ion–electron velocity vector correlation

Dissociative direct photoionization of the N2O(X 1Sigma+) linear molecule via the N2O+(B 2Pi) ionic state induced by linearly polarized synchrotron radiation P in the 18-22 eV photon energy range is investigated using the (VA+,Ve,P) vector correlation method, where VA+ is the nascent velocity vector of the NO+, N2+, or O+ ionic fragment and Ve that of the photoelectron. The DPI processes are identified by the ion-electron kinetic energy correlation, and the IchiA+(thetae,phie) molecular frame photoelectron angular distributions (MFPADs) are reported for the dominant reaction leading to NO+ (X 1Sigma+,v) + N(2D)+ e. The measured MFPADs are found in satisfactory agreement with the reported multichannel Schwinger configuration interaction calculations, when bending of the N2O+(B 2Pi) molecular ion prior to dissociation is taken into account. A significant evolution of the electron scattering anisotropies is observed, in particular in the azimuthal dependence of the MFPADs, characteristic of a photoionization transition between a neutral state of Sigma symmetry and an ionic state of Pi symmetry. This interpretation is supported by a simple model describing the photoionization transition by the coherent superposition of two ssigma and ddelta partial waves and the associated Coulomb phases.

Collision spectroscopy with aligned and oriented atoms. III. Effects of initial orbital alignment on H+-Na(3p) charge transfer

For pt.II see ibid., vol.22, p.L585-9 (1989). The effect of initial orbital alignment has been studied in 0.5-2.0 keV H+-Na(3p) collisions. The alignment of the Na(3p) state was controlled through the polarization of the laser light used for excitation. Total cross sections for electron capture into the H(n) levels for principal quantum numbers n=2 and n>or=3 were measured, relative to the n=2 capture cross section for the H+-Na(3s) system. Significant variations of the alignment effects were found as a function of collision energy and n. Comparison with theoretical results of 49-state atomic basis calculations of Fritsch (1984,1987,1990) (n=2,3) and 19-state molecular basis calculations of Courbin et al. (1990) (n=2) shows a good overall agreement.

Double momentum spectrometer for ion-electron vector correlations in dissociative photoionization

We have developed a new momentum spectrometer dedicated to momentum vector correlations in the context of deep core photoionization of atomic and molecular species in the gas phase. In this article, we describe the design and operation of the experimental setup. The capabilities of the apparatus are illustrated with a set of measurements done on the sulphur core 1s photoionization of gas-phase CS2.

Circular dichroism in molecular frame photoemission

The paper reports on a comparative experimental and theoretical study of circular dichroism in electron angular distribution (CDAD) in the molecular frame (MF) of linear molecules photoionized by circularly or elliptically polarized light. The CDAD is derived from the analysis of the complete molecular frame angular distribution (MFPAD) I(χ,θe,φe ), where χ is the orientation of the molecule with respect to the light propagation axis and (θe,φe ) the electron emission direction in the MF, using the vector correlation method. The CDAD is quantified by the θe dependence of the left–right emission asymmetry maximum in the plane perpendicular to the light propagation axis k, for a space fixed molecule orthogonal to k. The experimental results for selected valence shell photoionization (PI) reactions in NO, O2, N2O compare very well with the multichannel Schwinger configuration interaction (MCSCI) ab initio calculations. Combined with a simple model of the ionization process, including the partial-wave composition of the initial state and phase shifts estimated from quantum defects for the various scattering partial-waves, these results provide the basis for a general discussion of the circular dichroism effect. This study enables one to disentangle the influence of the (spσ, pπ…) initial valence shell ionized orbital and that of the scattering dynamics on some fingerprint properties of the CDAD. On the other hand, the CDAD for sσ K-shell is purely assigned to a final state scattering effect. This analysis will be extended to PI of non-linear molecules where the circular dichroism characterizes recoil frame photoelectron angular distributions (RFPADs).