J. C. Houver

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. 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.

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).

Ion Pair Formation in Multiphoton Excitation of NO2 Using Linearly and Circularly Polarized Femtosecond Light Pulses: Kinetic Energy Distribution and Fragment Recoil Anisotropy†

The NO(2) ion pair photodissociation dynamics leading to NO(+)(X(1)Sigma(+),v) + O(-)((2)P(3/2) or (2)P(1/2)), induced by a 1 kHz femtosecond laser with wavelengths near 400 nm, has been characterized using the coincidence vector correlation method. The ion pair production after four-photon absorption reaches more than 15% of the primary ionization. The kinetic energy release of the fragments demonstrates a significant vibrational excitation of the NO(+)(X(1)Sigma(+),v) molecular fragment. Recoil ion fragment emission is strongly aligned along the polarization axis of linearly polarized light or preferentially emitted in the plane perpendicular to the propagation axis of circularly polarized light. The formalism describing the recoil anisotropy for bound-to-bound n-photon transition inducing prompt axial recoil dissociation of a nonlinear molecule has been developed to interpret the measured anisotropies in terms of excitation pathways via near-resonant intermediate states of specific symmetries. Possible reaction pathways are discussed that are consistent with the data and supported by calculations of potential energy surfaces and transition moments.

Recoil frame photoemission in multiphoton ionization of small polyatomic molecules: photodynamics of NO 2 probed by 400 nm fs pulses

We report a general method for the complete analysis of the recoil frame photoelectron angular distribution (RFPAD) in n-photon dissociative ionization of small polyatomic molecules, resulting from (n − 1) bound-to-bound transitions plus one-photon ionization of a neutral excited state of the target. This method relies on the decomposition of the RFPAD in terms of the R K (χ , θe) recoil frame azimuthal harmonics (RFAHs) which are the components of its Fourier expansion in φe, where χ and θe are the polar angles referring to the polarization axis P and the photoelectron momentum k relative to the ion fragment recoil direction, respectively, and φe is the azimuth of k relative to P. The RFAH expansion method is illustrated by a detailed experimental and theoretical study of one-colour multiphoton dissociative and non-dissociative ionization of the NO2 molecule of C2v symmetry induced by 400 nm fs laser pulses, which involve electronic and nuclear dynamics within the pulse duration of the order of 70 fs. The reaction mechanism proposed to account for five-photon dissociative ionization of NO2 involves the role of [R ∗ (6a1) −1 ] Rydberg states populated by three-photon absorption, subsequently ionized by a fourth photon into the NO2 + (X 1 � g + , v1,v2,v3) manifold involving autoionization of [R ∗ (4b2) −1 ] Rydberg states, and linear versus bent geometry selective dissociation of NO2 + (X 1 � g + , v1,v2,v3) by a fifth photon. The reported calculations provide a coherent picture of the experimental findings although all features are not yet well reproduced.

Valence and inner-valence shell dissociative photoionization of CO in the 26–33 eV range. I. Ion-electron kinetic energy correlation and laboratory frame photoemission

The (V(A+), V(e), ê) vector correlation method, combining imaging and time-of-flight resolved electron-ion coincidence techniques, is used to probe dissociative photoionization (DPI) of CO induced by vacuum ultra violet linearly or circularly polarized synchrotron radiation in the 26-33 eV photon excitation energy range. It provides original information about both the photoionization dynamics of the CO molecule and the dissociation dynamics of the CO(+) molecular ions. The explored region corresponds to valence and inner-valence CO(+) ionic states, which involve doubly or multiply excited electronic configurations. In this paper I we identify up to 17 DPI reaction pathways by the position of the intermediate CO(+) molecular states in the Franck-Condon region and the (C(+) + O) or (O(+) + C) dissociation limits to which they correlate. For these processes we report the laboratory frame beta(C+/O+) and beta(e) asymmetry parameters as well as the relative branching ratios in selected binding energy bands. The I(chi,theta(e),phi(e)) molecular frame photoelectron angular distributions for selected PI processes will be reported in a companion paper II and compared with multichannel Schwinger configuration interaction ab initio calculations of these observables.

Electron transfer from optically prepared states: He+ + Na(32P) → He(2 1,3P) + Na+ angular differential scattering

This paper reports experimental and theoretical scattering angle-dependent differential cross sections (DCS) for the electron transfer processes He+ + Na(3s or 3p) → He(2s or 2p) + Na+ at an impact energy of 1 keV. The He(2p) channels are the dominant contributors to electron transfer from the Na(3p) states. Beam experiments with optically prepared Na(3p) states are analysed using time-of-flight spectroscopy without resolution of the final He 21P and 23P states. The theoretical approach is based on a one-electron two-centre atomic orbital expansion of the electronic singlet and triplet scattering states, using the coupled-channel impact parameter method and the eikonal Bessel transformation to obtain quantal DCS predictions. The theoretical results show distinct differences between singlet and triplet channels. In spite of the smaller statistical weight, the singlet contribution about equals the triplet contribution due to the smaller singlet energy defect. For Na(3s) → He(2s) transfer DCS the theoretical results predict a Fraunhofer-type diffraction pattern as seen previously in Na(3s) → Li(2s) transfer, however, the rings are too narrow to be resolved in the present experiment. The spatial pattern of the scattered neutrals is strongly forwardly peaked with typical scattering angles of less than 0.05°. The results reveal pronounced spatial anisotropies, strongly dependent on the initial target preparation. In particular, when a Na(3p) orbital is prepared aligned along the direction of the initial ion beam, electron transfer is much more efficient than for the perpendicular orbital geometry. Furthermore, if the orbital is tilted by 45° with respect to the incident beam direction a strong left–right transfer scattering asymmetry is observed, with more neutrals being scattered to the side to which the p orbital is pointing than to the opposite side. The experimental observations compare well with the theoretical predictions for the spin-averaged electron transfer. Similarities and differences with earlier electron transfer studies for H+ and Li+ on Na(3p) at the same impact energy are pointed out.