[en] Vector correlation is a powerful method for the investigation of photofragmentation of molecules or clusters. It consists of measuring in coincidence the velocity vectors of the electrons and atomic fragments emitted from the same physical event, and provides detailed information about the spectroscopy and intramolecular dynamics of excited states. A new velocity spectrometer which combines imaging and time-resolved detection techniques has been developed and applied to the investigation of dissociative photoionization of diatomic molecules (NO, CO, O₂, H₂, etc.) induced by synchrotron radiation linearly polarized light (P)(AB+hν(P)→A⁺+B*+e in the VUV photon energy range (20 eV≤hν≤40 eV). The (V_A⁺,V_e) velocity vector correlation leads first to the correlation of the fragment ion and electron kinetic energies, which enables to identify and select each fragmentation process and to measure their branching ratios. The angular correlation of the ion and electron velocities for a selected process leads then to the angular distribution of the photoelectrons in the molecular frame, for each orientation of the molecular axis with respect to the linear polarization of the incident light. We illustrate the method by the recent data obtained for dissociative photoionization of NO and O₂

[en] Dissociative direct photoionization of the N₂O(X ¹Σ⁺) linear molecule via the N₂O⁺(B ²Π) ionic state induced by linearly polarized synchrotron radiation P in the 18-22 eV photon energy range is investigated using the (V_A+,V_e,P) vector correlation method, where V_A+ is the nascent velocity vector of the NO⁺, N₂⁺, or O⁺ ionic fragment and V_e that of the photoelectron. The DPI processes are identified by the ion-electron kinetic energy correlation, and the Iχ_A+(θ_e,φ_e) molecular frame photoelectron angular distributions (MFPADs) are reported for the dominant reaction leading to NO⁺(X ¹Σ⁺,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 ²Π) 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 Σ symmetry and an ionic state of Π symmetry. This interpretation is supported by a simple model describing the photoionization transition by the coherent superposition of two sσ and dδ partial waves and the associated Coulomb phases

[en] Dissociative photoionization of carbon monoxide with angle resolved photoelectron-photoion fragment detection has yielded recoil frame photoelectron angular distributions. Comparisons with accurate photoionization calculations have lead to the identification of the inner-valence ion states and estimates of their predissociative lifetimes.

[en] We report benchmark results for dissociative photoionization (DPI) spectroscopy and dynamics of the NO molecule in the region of the σ* shape resonance in the ionization leading to the NO⁺(c³π) ionic state. The experimental study combines well characterized extreme ultraviolet (XUV) circularly polarized synchrotron radiation, delivered at the DESIRS beamline (SOLEIL), with ion-electron coincidence 3D momentum spectroscopy. The measured (N⁺,e) kinetic energy correlation diagrams reported at four discrete photon energies in the extended 23-33 eV energy range allow for resolving the different active DPI reactions and underline the importance of spectrally resolved studies using synchrotron radiation in the context of time-resolved studies where photoionization is induced by broadband XUV attosecond pulses. In the dominant DPI reaction which leads to the NO⁺(c³π) ionic state, photoionization dynamics across the σ* shape resonance are probed by molecular frame photoelectron angular distributions where the parallel and perpendicular transitions are highlighted, as well as the circular dichroism CDAD(θ_e) in the molecular frame. The latter also constitute benchmark references for molecular polarimetry. The measured dynamical parameters are well described by multichannel Schwinger configuration interaction calculations. Similar results are obtained for the DPI spectroscopy of highly excited NO⁺ electronic states populated in the explored XUV photon energy range. (authors)

[en] We have studied the production of long-lived, highly excited, neutral hydrogen atom fragments following oxygen 1s inner-shell excitation/ionization of gas-phase water molecules using synchrotron radiation. Striking differences to fragment ion and low-n neutral H yields are observed close to threshold. To investigate the decay pathways involved, the neutral fragments were also detected in coincidence with H⁺, O²⁺, O⁺ and OH⁺ fragments.

[en] The (VA+, V_e, e-bar ) vector correlation method, combining imaging and time-of-flight resolved electron-ion coincidence techniques, is used to probe dissociative photoionization (DPI) of simple molecules induced by VUV polarized synchrotron radiation, to obtain the I(θ_e, φ_e) molecular frame photoelectron angular distributions (MFPADs). We focus on examples where DPI is induced by valence shell and inner-valence shell excitation. For valence shell ionisation, we report the results of a complete experiment for DPI of O₂ via the O₂⁺(3²Π_u) ionic state, performed with a single circular polarization of the light at a photon excitation energy hν = 24.4eV. A significant circular dichroism effect characterizes electron emission in the molecular frame. Inner-valence shell ionization induced by linearly polarized light is reported for the CO₂ and N₂O isoelectronic molecules selecting two specific satellite lines using light with hν ∼35-bar eV. For both valence and inner-valence ionization, the measured MFPADs are compared with multichannel configuration interaction calculations, as well as with the prediction of a simple theoretical model of photoionization dynamics that probe the role of the initial state molecular orbital and which is presented in this special issue

[en] This work reports new theoretical and experimental results for the circular dichroism in electron angular distribution (CDAD) in the molecular frame of D₂ ionized by circularly polarized light in the region involving the resonant excitation of Q₁ and Q₂ doubly excited states. At the 32.5 eV photon energy, dissociative ionization of D₂ into the ground state limit D⁺ + D(1s) leads to an ion kinetic energy distribution extending over 7 eV, with a continuous energy sharing between the electron and the fragments. On the experimental side we used the vector correlation method to characterize the evolution of the molecular frame photoelectron angular distributions (MFPADs) as a function of the E_D+ kinetic energy. The CDAD is quantified by the θ_e polar angle 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. A remarkable variation of the CDAD is observed along the E_D+ energy distribution. The results are compared with the prediction of full four body ab initio calculations

[en] The general form of the molecular frame photoelectron angular distributions (MFPADs) for linear ground-state molecules ionized by linearly polarized light (n) is reported. A comparison between computed and measured MFPADs as a function of the polar and azimuthal emission angles is presented for photoionization of NO leading to the c ³Π state of NO⁺. The importance of the azimuthal dependence of the MFPADs for the determination of the symmetry of the states involved in the excitation and of the underlying photoionization dipole matrix elements is demonstrated

[en] Dissociative photoionization of H₂ induced by VUV linearly polarized synchrotron radiation P has been studied using the (V_H⁺,V_e,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(θ_e,φ_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⁺(2^Σ+_g) ionic ground state ant hν = 20 eV and for the dominant DPI processes involving autoionization of the H₂(Q₁1^Σ_u⁺(1)) and H₂(Q₂1^Π_u(1)) doubly excited states into the H₂⁺(2^Σ_g⁺) and H₂⁺(2^Σ_u⁺) continua at hν = 28.5 and 32.5 eV. They show the dominant excitation of a pσ_u partial wave in autoionization of the Q₁(1^Σ_u⁺(1)) state into the H₂⁺(1sσ_g) ionic state and that of a dπ_g partial wave in autoionization of the Q₂(1^Π_u(1)) state into the H₂⁺(2pσ_u) continuum. A molecular frame forward-backward electron emission anisotropy is observed when ionization takes place at large internuclear distance