Electron-ion Coincidence Technique Research Articles

Valence photoionization and the following fragmentation pathways inSb4clusters

The valence photoelectron spectrum of Sb{sub 4} clusters and the following fragmentation patterns have been studied using synchrotron radiation and electron-ion coincidence technique. An experimental photoelectron spectrum of the 5t{sub 2} ionization of Sb{sub 4} is presented. Theoretical molecular calculations, together with the existing data on the noble gas xenon, were used to describe the experimental results. The bonding properties of the molecular orbitals involved are used to qualitatively describe the dissociation process.

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.

Molecular Frame Photoemission: Probe of the Photoionization Dynamics for Molecules in the Gas Phase

Molecular frame photoemission is a very sensitive probe of the photoionization (PI) dynamics of molecules. This paper reports a comparative study of non-resonant and resonant photoionization of D2 induced by VUV circularly polarized synchrotron radiation at SOLEIL at the level of the molecular frame photoelectron angular distributions (MFPADs). We use the vector correlation method which combines imaging and time-of-flight resolved electron-ion coincidence techniques, and a generalized formalism for the expression of the I(, e, e) MFPADs, where is the orientation of the molecular axis with respect to the light quantization axis and (e, e) the electron emission direction in the molecular frame. Selected MFPADs for a molecule aligned parallel or perpendicular to linearly polarized light, or perpendicular to the propagation axis of circularly polarized light, are presented for dissociative photoionization (DPI) of D2 at two photon excitation energies, h = 19 eV, where direct PI is the only channel opened, and h = 32.5 eV, i.e. in the region involving resonant excitation of Q1 and Q2 doubly excited state series. We discuss in particular the properties of the circular dichroism characterizing photoemission in the molecular frame for direct and resonant PI. In the latter case, a remarkable behavior is observed which may be attributed to the interference occurring between undistinguishable autoionization decay channels.

EICO measurements of inner-core excited mixed rare-gas clusters

The spectra of deep inner-core excited mixed rare-gas clusters were recorded by using electron ion coincidence (EICO) and multi-hit momentum imaging (MHMI) techniques. The EICO spectra for Ar99Kr1 clusters reveal that singly charged ions are emitted from the inner-core excited clusters in addition to the multiple charged ions. The dependence of the EICO spectra on photon energy and cluster size suggests that the holes created through vacancy cascade on the krypton atoms are transferred to the surrounding atoms, and that the singly charged ions are the primary product of the krypton photoabsorption. Charge localization is suggested for the inner-core excited mixed rare-gas clusters from the analysis of the EICO peak width. The MHMI measurements give us direct evidence for the strong charge migration from X-ray absorbing atoms to surrounding atoms. The photon energy dependence of the PSD image for fragment ions suggests that the momentum of the fragment ions depends on the number of charges generated by the vacancy cascade.

Photoemission in the molecular frame induced by soft X-ray elliptically polarized light

A new step for complete experiments in photoionization (PI) of linear molecules is achieved when the electron-ion vector correlation method, based on imaging and time-of-flight resolved electron-ion coincidence techniques, is combined with the use of elliptically polarized light. The molecular frame photoelectron angular distributions (MFPADs), as well as the s 1 and s 2 Stokes parameters characterizing the polarization state of the light, can be derived from a single experiment performed with undefined elliptically polarized light. Using this method, we report and discuss MFPADs for site selective K-shell ionization of the NO and N 2O linear molecules in the region of the N 1s and O 1s shape resonances. A large molecular frame circular dichroism is observed, well predicted by the multi-channel Schwinger configuration interaction calculations reported for PI of NO. We also present recent experimental results for PI of non-linear molecules like CH 3Cl measured above the Cl 2p and C 1s ionization thresholds: the recoil frame photoelectron angular distributions (RFPADs) obtained by the vector correlation method may be influenced both by the molecular frame photoemission pattern and the fragmentation process.

Electron-ion coincidence studies of epichlorohydrin molecule following core-level excitation

Electron-ion coincidence technique (PEPICO) and synchrotron radiation have been employed in the study of the ionic fragmentation of epichlorohydrin (C 3H 5ClO) molecule. Total ion yield and mass spectra have been recorded for the first time for this molecule in the vicinity of the C 1s, O 1s and Cl 1s core excitation edges. We have found evidences of site selective fragmentation upon core excitation, as well as indication of molecular quasi-alignment in the photoexcitation and dissociation at the Cl 1s resonance.

Phase-sensitive wave-packet dynamics caused by a breakdown of the rotating-wave approximation

The present thesis is devoted to theoretical studies of pulse propagation of light through linear and nonlinear media, and of light-induced nuclear dynamics. The first part of the thesis addresses propagation of light pulses in linear periodical media - photonic crystals. The main accent was put on studies of the angular properties of two qualitatively different types of photonic crystals: holographic photonic crystals, and impurity band based photonic crystals. The anisotropy of band structure, group velocity and pulse delay with respect to the light polarization are analyzed. In the second part of the thesis a strict theory of nonlinear propagation of a few strong interacting light beams is presented. The key idea of this approach is a self-consistent solution of the nonlinear wave equation and the density matrix equations of the material. This technique is applied to studies of dynamics of cavityless lasing generated by ultra-fast multi-photon excitation. It is shown that interaction of co- and counter-propagating pulses of amplified spontaneous emission (ASE) affects the dynamics and efficiency of nonlinear conversion. Our dynamical theory allows to explain the asymmetric spectral properties of the forward and backward ASE pulses, which were observed in recent experiment with different dye molecules. It is shown that the ASE spectral profile changes drastically when the pump intensity approaches the threshold level. The effect of the temporal self-pulsation of ASE is studied in detail. The third part of the thesis is devoted to light-induced nuclear dynamics. Time- and frequency-resolved X-ray spectroscopy of molecules driven by strong and coherent infrared (IR) pulses shows that the phase of the IR field strongly influences the trajectory of the nuclear wave packet, and hence, the X-ray spectrum. Such a dependence arises due to the interference of one (X-ray) and two-photon (X-ray + IR) excitation channels. The phase of the light influences the dynamics also when the Rabi frequency approaches the vibrational frequency, breaking down the rotating-wave approximation. The probe X-ray spectra are also sensitive to the delay time, the duration, and the shape of the pulses. The evolution of the nuclear wave packets in the dissociative core-excited state affects the dynamics of resonant Auger scattering from fixed-in-space molecules. One of the important dynamical effects is the atomic-like resonance which experiences electronic Doppler shift. We predict that the scattering of the Auger electrons by nearby atoms leads to new Doppler shifted resonances. These extra resonances show sharp maxima in the bond directions, which makes them very promising as probes for local molecular structure using energy and angular resolved electron-ion coincidence techniques. Our theory provides prediction of several new effects, but also results that are in good agreement with the available experimental data.

Photoemission in the molecular frame using the vector correlation approach: from valence to inner-valence shell ionization

The ( V A + , V e, e ˆ ) 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 O 2 via the O 2 +(3 2Π u) ionic state, performed with a single circular polarization of the light at a photon excitation energy h ν = 24.4 eV. 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 CO 2 and N 2O isoelectronic molecules selecting two specific satellite lines using light with h ν ≃ 35 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.

Multiphoton double ionization via field-independent resonant excitation.

The double ionization of xenon in the multiphoton regime has been studied at two wavelengths (0.77 and 0.79 microm) using an electron-ion coincidence technique and an intensity binned ion ratio method. Sharp resonant structures in the electron energy distribution correlated with the doubly charged ion, as well as a wavelength dependence of the Xe(2+)/Xe(+) ratio provides new insights. A mechanism involving the shelving of population in Rydberg states followed by excitation of a core electron is proposed.

Relaxation dynamics of SF 6 studied by energy-resolved electron ion coincidence technique

The mass spectra from the SF 6 molecule acquired by energy-resolved electron ion coincidence (EREICO) technique using (1t 1g + [5t 1u,1t 2u]), (3e g + 1t 2g), 4t 1u, and 5a 1g electrons after both valence and S 2p −1 6a 1g core excitation reveal a strong selectivity in the dissociation from these states. From the comparison of the obtained spectra, the influence of the core-excited state in the relaxation pathway of the molecule is studied. The S 2p → 6a 1g core excitation does not affect the relaxation dynamics after participator Auger decay to the (1t 1g + [5t 1u,1t 2u]) −1 and (4t 1u) −1 states, but it alters the relaxation dynamics after participator Auger decay to the (3e g + 1t 2g) −1 and (5a 1g) −1 states with respect to the decay processes after direct excitation of the same orbitals. These observations point to core excitation induced dissociation in the SF 6 molecule.

Core-excitation-induced dissociation inCD4after participator Auger decay

The fragmentation of the ${\mathrm{CD}}_{4}$ molecule after selective ionization of the ${1t}_{2}$ and ${2a}_{1}$ electrons with photons from 70 to 290 eV has been studied with the energy-resolved electron-ion coincidence technique. The mass spectra acquired in coincidence with ${1t}_{2}$ electrons reveal ${\mathrm{CD}}_{4}^{+},$ ${\mathrm{CD}}_{3}^{+},$ and ${\mathrm{CD}}_{2}^{+}$ fragments, depending on the excitation energy used. The production of ${\mathrm{CD}}_{3}^{+}$ is strongly enhanced after C $1s$ excitation to different core excited states, with respect to the production observed after direct ionization of the ${1t}_{2}$ orbital. This enhancement is correlated with the changes of the molecular geometry when it relaxes from the core-excited state.

Observation of a transition in the dynamics of strong-field double ionization.

The double ionization of argon and xenon in an intense laser field has been studied in detail using an electron-ion coincidence technique. The observed double ionization electron spectra in xenon show resonancelike structures here resolved for the first time. In argon, the featureless spectra are consistent with rescattering. This represents a clear transition in the dynamics of strong-field double ionization, analogous to the well-known transition between the tunneling and multiphoton regimes in single ionization.

A multi-coincidence study of the double, triple photoionization and fragmentation of the SiF 4 molecule around the Si 2p edge

Multiple ionization of the SiF 4 molecule has been studied using synchrotron radiation and time-of-flight mass spectrometry in a multi-coincidence mode, in the photon energy range of 70–109.4 eV, which encompasses the Si 2p edge. Photoionization branching ratios have been measured under efficient ion extraction and the results are compared to previously published data. Two electron–ion coincidence techniques (PE2PICO, and PE3PICO) have been used in the elucidation of the fragmentation mechanisms of highly excited molecular ions, formed following the absorption of a high-energy photon. It is shown that the unstable doubly charged parent molecule, SiF 4 2+, fragments preferentially (90%) via the asymetric mechanism m 2+→ m 1 2++ m 2.

Electron energy spectra from intense laser double ionization of helium.

The double ionization of helium in the strong-field limit has been studied using an electron-ion coincidence technique. The observed double ionization electron energy spectra differ significantly from the single ionization distributions. This gives new support to the rescattering model of double ionization and explicitly reveals the role of backward electron emission following the e-2e ionizing collision.

Strong-field double ionization of rare gases

We have studied the double ionization of helium and other rare gases using an electron-ion coincidence technique. With this scheme, the electron energy spectrum correlated to the creation of a doubly charged ion may be compiled. In all cases, the observed double ionization electron distributions are similar and enhanced at high energies, while the single ionization spectra exhibit distinct differences.

4d−1multiplet structure of rare-earth atoms studied by photoelectron-ion coincidence spectroscopy

The $4d$ photoelectron spectra of atomic lanthanides Ce, Pr, Nd, Sm, and Eu have been measured using synchrotron radiation. The multiplet structure of the ${4d}^{\ensuremath{-}1}$ hole states has been investigated by applying the electron-ion coincidence technique in the form of final ion-charge resolving electron (FIRE) spectroscopy. FIRE spectroscopy is a spin-sensitive method for the study of ${4d}^{\ensuremath{-}1}$ decay processes. Photoelectron spectra have been calculated in the one-configuration approximation for the rare-earth atoms from Ce to Gd and FIRE spectra have been calculated for Ce, Pr, Nd, Sm, and Eu. Comparing theoretical and experimental results, we discuss the trend of the development of the ${4d}^{\ensuremath{-}1}$ multiplet structure from Ce to Gd.

Doppler effects in resonant x-ray Raman scattering

Theory for Doppler effects in resonant x-ray Raman scattering (RXS) is presented. It is shown that the ``electron'' Doppler effect is important in nonradiative RXS for decay transitions between continuum nuclear states lying above the dissociation threshold, and that the averaging of the RXS cross section over molecular orientations can lead to strong non-Lorentzian broadenings of the atomiclike resonances. The Doppler effect is found to give a unique possibility to distinguish dissociating identical atoms, because different peaks correspond to atoms with opposite Doppler shifts. Spectral features of the atomiclike profile are predicted and analyzed. Strong oscillations of the RXS cross section will occur as a consequence of the interference of the Auger electrons. Due to the Doppler effect and the interference, the atomiclike profile can be associated with supernarrow spectral features, the width of which goes below the lifetime broadening and is practically independent of the spectral distribution of the incident radiation. As another consequence of the oscillations and strong anisotropy caused by the interference, we predict parity selection rules for Auger decay transitions in both bound and dissociative systems. The corresponding experiments can be realized by measurements of resonant Auger of surface adsorbed molecules and for molecules by the electron-ion coincidence technique.

L23−MMAuger spectrum ofK-ionized argon: Decomposition by electron-ion and electron-electron coincidence techniques

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Determination of ion multiplier sensitivities as a function of ionic mass for use in quantitative mass spectrometry

An experimental method has been developed for the determination of ion multiplier sensitivity to ions of different mass to charge ratio using the electron-ion coincidence technique of dipole (e,e+ion) spectroscopy. Time-of-flight mass spectra of each of the noble gas atoms He, Ne, Ar, Kr, and Xe are collected using the ion multiplier under investigation. In the present work we have determined relative efficiencies, at 7.5–8 keV ion impact energy, over the mass range 1–140 amu for two different large active area multipliers, a Johnston focused mesh multiplier and a microchannel-plate detector, both operated in the saturated pulse counting mode. Significant variations in detector sensitivity occur at low m/e values for both multipliers. The microchannel-plate detector shows a level response function above m/e=∼30, while the Johnston multiplier exhibits a slow decrease in sensitivity in the same mass range. The applicability of the measured response functions to the correction of molecular mass spectra and experimental photoion branching ratio data has been investigated. It appears that the detection efficiencies for atomic and polyatomic ions of the same m/e may be different.

Dissociative Photoionisation of Methane and its Deuterated Compounds in the A State Region

Dissociative photoionisation of methane is studied in a wide range of internal energy (12.6–28 eV photon energy) by a threshold electron–ion coincidence technique. Whereas the CH4+ X ground state is statistical, the A (2a1)-1 state (22.4–25 eV) and the doubly excited states (26–32 eV) show direct dissociation mechanisms and inefficient internal conversion to the CH4+ ground state. In the Franck–Condon gap between the X and A states, we observe a 1.7 eV shift in the sequential dissociation of CH3+ into CH2+ + H. This is interpreted as the result of dissociation of Rydberg states into neutrals followed by resonant autoionisation of the CH3** fragment into a stable excited electronic state of CH3+.