C. Bordas

MQDT analysis of rovibrational interactions and autoionization in Na2 Rydberg states

Abstract The n d Rydberg series of the Na 2 molecule have been excited by optical-optical double resonance. The experimental spectra are analysed in the framework of the multichannel quantum defect theory including rotational and vibrational channels. The rovibrational interactions in the Rydberg series are calculated and good agreement with experimental energy levels is obtained. The dynamical aspect of the autoionization process is theoretically investigated and interference effects are discussed for Δν=2 autoionization. The autoionization lifetimes are experimentally measured. Agreement between theory and experiment is only qualitative because the autoionization lifetime of the Δν=2 process depends drastically on the variation of the quantum defects with the intemuclear distance. Moreover, the lifetimes are found to be very sensitive to an external electric field.

Electronic properties and geometric structures of Li4H and Li9H from optical absorption spectra

Optical absorption spectra of Li4H and Li9H clusters have been recorded by depletion spectroscopy in the visible range. From comparison with ab initio calculations, geometries of both clusters are identified. The hydrogen atom assumes a peripheral position bridging two and three Li atoms in the planar and three‐dimensional structures of Li4H and Li9H, respectively. Na4F and Na9F clusters are also theoretically studied and it is shown how the strong electronegativity of the F atom leads to different geometries than in lithium hydrids. Finally, the metallic character of these clusters is discussed and in both cases, the hydrogen or fluorine atom localizes one valence electron. However, the optical absorption spectra are much broader than in pure Lin and Nan clusters due to the lower symmetry.

Shell structure of small indium clusters below N≈200 atoms

Abstract Neutral indium cluster In N ( N ≤200) are laser photoionized near threshold and analyzed by time-of-flight mass spectrometry. Strong shell effects are observed in cluster mass spectra. For In N clusters below N =125, individual ionization potentials are deduced. They are compared with theoretical predictions calculated within the framework of the spherical-jellium model. The agreement is qualitatively quite good and the observed shell effects correspond to the successive major electronic shell closures. Deviations from electronic shell model predictions are much less important for indium clusters than for aluminum ones. A tentative explanation of this difference is given.

Electronic properties of mixed barium–oxygen clusters

Herein electronic structure as a function of cluster size has been probed through ionization potentials measurements in order to derive general trends in the evolution of the structure and stability of lithium oxide clusters LinOm. The structural evolution was investigated by varying progressively the oxygen rate in order to observe transitions from the metallic Lin to the ionic structure (Li2O)n. We have demonstrated that this structural transition splits into three specific ranges. This result, in contrast with the general behavior observed with other electronegative elements combined with alkaline or alkaline-earth metal clusters, but similar to what is known about BanOm and CsnOm, constitutes a signature of a specific role played by the oxygen atoms when included inside a metallic cluster.

Transfer-matrix-based method for an analytical description of velocity-map-imaging spectrometers

We propose a simple and general analytical model describing the operation of a velocity-map-imaging spectrometer. We show that such a spectrometer, possibly equipped with a magnifying lens, can be efficiently modeled by combining analytical expressions for the axial potential distributions along with a transfer matrix method. The model leads transparently to the prediction of the instruments operating conditions as well as to its resolution. A photoelectron velocity-map-imaging spectrometer with a magnifying lens, built and operated along the lines suggested by the model has been successfully employed for recording images at threshold photoionization of atomic lithium. The models reliability is demonstrated by the fairly good agreement between experimental results and calculations. Finally, the limitations of the analytical method along with possible generalizations, extensions, and potential applications are also discussed. The model may serve as a guide for users interested in building and operating such spectrometers as well as a tutorial tool.

Strontium clusters and ionization of the Sr2 dimer

Abstract Strontium clusters produced in a seeded molecular beam have been observed up to Sr 5 . These preliminary results demonstrate that spectroscopy of small Sr n clusters is achievable by means of depletion techniques. Investigation of the ionization region of the Sr 2 molecule was conducted by one- or two-photon ionization techniques which give slightly different estimations of the ionization potential that are discussed here. A value of 38200 ± 100 cm −1 is deduced for the ionization potential and simultaneously Rydberg series are partly analyzed.

Decay of C60 by delayed ionization and C2 emission: Experiment and statistical modeling of kinetic energy release

C(60) molecules highly excited in the nanosecond regime decay following ionization and dissociation by emitting a series of carbon dimers, as well as other small fragments if excitation is strong enough. The fragmentation mass spectrum and kinetic energy release of all charged fragments obtained in these experiments are interpreted within the framework of the Weisskopf theory, using a realistic Monte Carlo procedure in which the rates of all relevant decay channels are modeled using Arrhenius expressions. Comparison between the measurements and the simulated spectra allows the distribution of deposited energy to be accurately estimated. The dependence of the fragment kinetic energies on the laser fluence, found in the simulation but not observed in the experimental results, indicates that the small fragments are not necessarily emitted from small fullerenes resulting from C(60) by sequential decay. Rather, direct multifragmentation of C(60) is invoked to interpret the observed patterns. The possible role of post-ionization of neutral emitted fragments is discussed.

Spectroscopy of the 1 2Πu state of Na+2

We report here the first experimental observation of bound–bound transitions between the ground X 2Σ+g and excited 1 2Πu states of Na+2. The basis of our experiment is to study doubly excited Rydberg states of Na2 by preparing a well‐defined nd 1Λg singly excited Rydberg state of Na2 and then, by exciting the Na+2 core with a tunable laser. In this paper, we show that the ionic transitions may be directly deduced from the doubly excited Rydberg states spectra [C. Bordas, J. L. Vialle, and M. Broyer (submitted)]. We demonstrate that this technique is one of the most powerful to study the excited states of diatomic ions which are not predissociated. A detailed spectroscopic analysis of the 1 2Πu state has been performed and the results are compared with the more recent ab initio and pseudo‐ (or model‐) potential calculations.

Shell effects in photoionization spectra of heavy trivalent metal clusters

Laser photoionization experiments have been performed on a large size range of indium and thallium clusters. The metal clusters were produced by laser vaporization technique and analyzed after laser ionization by standard time-of-flight mass spectrometry. For the indium clusters, individual ionization potentials (IP) are deduced for N≤132. Abrupt decreases in the IP values are observed, which correspond to the openings of new electronic shell as predicted by the spherical jellium model. For larger indium clusters, the unresolved mass spectra present small but reproducible oscillations. Interpretations in terms of either electronic shell structure or cluster geometry remain undecided for the moment. Our results on thallium clusters are less abundant and only qualitative. Nevertheless, they show that thallium behaves more like a monovalent element than like a trivalent one.

Depletion spectroscopy of the Sr2 B 1Πu←X 1Σ+g system

Abstract Spectroscopic studies have been performed on Sr 2 molecules produced in a seeded molecular beam. Two-photon excitation techniques were used to investigate excited states of Sr 2 : the A←X system has been observed using two-photon ionization while depletion spectroscopy allowed us to characterize a new state of Sr 2 , B 1 Π u , dissociating in (5s 2 ) 1 S+ (5s5p) 1 P. Preliminary spectroscopic constants are deduced and the dissociation mechanism allowing depletion is discussed.