Julien Lecointre

Toward real-time charged-particle image reconstruction using polar onion-peeling.

A method to reconstruct full three-dimensional photofragment distributions from their two-dimensional (2D) projection onto a detection plane is presented, for processes in which the expanding Newton sphere has cylindrical symmetry around an axis parallel to the projection plane. The method is based on: (1) onion-peeling in polar coordinates [Zhao et al., Rev. Sci. Instrum. 73, 3044 (2002)] in which the contribution to the 2D projection from events outside the plane bisecting the Newton sphere are subtracted in polar coordinates at incrementally decreasing radii; and (2) ideas borrowed from the basis set expansion (pBASEX) method in polar coordinates [Garcia et al., Rev. Sci. Instrum. 75, 4989 (2004)], which we use to generate 2D projections at each incremental radius for the subtraction. Our method is as good as the pBASEX method in terms of accuracy, is devoid of centerline noise common to reconstruction methods employing Cartesian coordinates; and it is computationally cheap allowing images to be reconstructed as they are being acquired in a typical imaging experiment.

Ultrafast Relaxation Dynamics Observed Through Time-Resolved Photoelectron Angular Distributions†

Time-resolved photoelectron imaging of the 7,7,8,8-tetracyanoquinodimethane (TCNQ) radical anion is presented. Photoelectron angular distributions (PADs) are qualitatively analyzed in terms of the simple s-p model that is based on symmetry arguments. The internal conversion dynamics from the first excited state (1(2)B(3u)) to the ground state ((2)B(2g)) may be observed through temporal changes in the PADs of the spectrally overlapping photoelectron features arising from photodetachment of the ground state and the excited state. A formulism for extracting the population dynamics from the β(2) anisotropy parameter of overlapping spectroscopic features is presented. This is used to extract the lifetime of the first excited state, which is in good agreement with that observed in the time-resolved photoelectron spectra.

Velocity-map imaging at low extraction fields

We present a velocity-map imaging (VMI) setup for photoelectron imaging that utilizes low electric extraction fields. This avoids any complications that could arise from electrostatic interactions between the extraction field and the molecular properties that are probed and has a minimal effect on the trajectory of ions in ion beam experiments. By using an attractive potential supplied to the detector, and keeping the electrodes at ground (zero) potential, we show that fringe fields between the VMI arrangement and the vacuum chamber can be eliminated, which is important in experiments on ions.

Spectroscopy and dynamics of the 7,7,8,8-tetracyanoquinodimethane radical anion.

The photoelectron spectrum of the 7,7,8,8-tetracyanoquinodimethane (TCNQ) radical anion has been measured at 3.1 eV. Additionally, the ultrafast relaxation dynamics of the first excited state (1(2)B(3u)) of TCNQ(-) have been studied using time-resolved photoelectron spectroscopy, which reveals that it undergoes internal conversion back to the ground state ((2)B(2g)) with an associated lifetime of 650 fs and shows evidence of coherent nuclear motion.

Absolute cross sections and kinetic energy release distributions for electron impact ionization and dissociation of CD

Absolute cross sections for electron impact dissociative excitation and ionization of CD+ leading to the formation of ionic products ( D+, C+, C2+ and C3+) are reported in the energy range from their respective thresholds to 2.5 keV. Around the maximum, cross sections are found to be ( 10.5 +/- 1.0) x 10(-17) cm(2), ( 20.6 +/- 3.5) x 10(-17) cm(2), ( 1.20 +/- 0.11) x 10(-17) cm(2) and ( 8.2 +/- 1.5) x 10(-20) cm(2) for D+, C+, C2+ and C3+, respectively. In the very low- energy region, dissociative excitation leading to the C+ formation dominates over the D+ one. The cross section for dissociative ionization ( C+ + D+ formation) is found to be ( 6.9 +/- 1.3) x 10(-17) cm(2) around 105 eV and the corresponding threshold energy is ( 22.1 +/- 0.5) eV. The animated crossedbeams method is used and the analysis of ionic product velocity distributions allows the determination of the kinetic energy release distributions. They are seen to extend from 0 to 15 eV both for C+ and for D+, and up to 40 eV both for C2+ and for C3+. For singly charged products, the comparison of the present energy thresholds and kinetic energy release with published data allows the identification of states contributing to the observed processes. In particular, contributions from primary ions formed in the a(3)Pi metastable state are perceptible. At fixed electron energy, the cross sections for the various ionization channels are seen to reduce exponentially with the potential energy of each dissociated ion pair. Anisotropies are estimated to be in the range 8 +/- 2% for both C+ and D+. The total CD+ single ionization cross section calculated by application of the Deutsch - Mark formalism is found to be in good agreement with experimental results.

A crossed-beam experiment for electron impact ionization and dissociation of molecular ions: its application to CO+

A crossed electron-ion beam experimental set-up has been upgraded for the study of electron impact ionization and dissociation of molecular ions by means of ionic product detection. Both the experimental set-up and the data analysis procedures are described in detail for the estimation of ( i) absolute cross sections, ( ii) kinetic energy release distributions ( KERD) and ( iii) anisotropies of angular distributions. Absolute cross sections are obtained separately for dissociative excitation ( DE) and for dissociative ionization ( DI). A double focusing magnetic field analyser is used for the observation of product velocity distributions, in the laboratory frame, at selected electron energies. The KERD in the centre of mass frame is calculated from the measured velocity distribution as well as the anisotropy of the angular distribution with respect to the initial orientation of the molecular ions. Results are reported for dissociative ionization and dissociative excitation of CO+ to C+ and O+ fragments in the energy range from about 5 eV to 2.5 keV. Absolute cross sections for DE at maximum, i. e. for an electron energy around 35 eV, are found to be ( 9.69 +/- 2.08) x 10(-17) cm(2) and ( 6.24 +/- 1.33) x 10(-17) cm(2), for C+ and O+, respectively, and the corresponding threshold energies are found to be ( 8.5 +/- 0.5) eV and ( 14.8 +/- 0.5) eV. The DE process leading to C+ production is seen to dominate at low electron energies. For DI, the absolute cross section is found to be ( 12.56 +/- 2.38) x 10(-17) cm(2) around 125 eV and the corresponding threshold energy is ( 27.7 +/- 0.5) eV. KERDs, which extend from 0 to 24 eV both for C+ and O+, exhibit very different shapes at low electron energy but similar ones above 100 eV, confirming the role observed respectively for DE and DI. The groups of states contributing to the different processes are identified by comparing present energies thresholds values and the KERDs with theoretical values. Anisotropies are estimated to be in the range 3-6% for both C+ and O+.

Merged-beam study of mutual neutralization of H− and H+

Total and partial cross sections have been measured for the mutual neutralization of H− and H+ by means of a merged and inclined beam set-up. The low energy data between 10 meV and 5 eV contradict one previous set of measurements, while above 5 eV the data fall in excellent agreement with previously published results by two other groups.

Multiple ionization of C+, N+ and O+ ions by fast electron impact

Absolute cross sections for electron impact single and multiple ionization of C + , N + and O + ions leading to the formation of C q + (q = 2-4), N q + (q = 2-5) and O q + (q = 2-5) are reported. The animated crossed beam method is applied in the energy range extending from the respective thresholds up to 2.5 keV. The concerned target ions belong to the second row of the periodic table, with configurations 2s 2 2p x (x = 1-3). The maximum cross sections for the multiply charged products C q + are found to be in the range from 2.3× 10 -20 cm 2 (for C 4 + ) up to 6.3× 10 -17 cm 2 (for C 2 + ); for N q + they range from 3.0× 10 -22 cm 2 (for N 5 + ) up to 5.1× 10 -17 cm 2 (for N 2 + ) and, lastly, for O q + they range from 5.5× 10 -22 cm 2 (for O 5 + ) up to 5.2× 10 -17 cm 2 (for O 2 + ). The corresponding threshold energies are found to be in satisfactory agreement with spectroscopic values. The general feature of the measured cross sections is investigated. Their values for single ionization are reasonably explained by the calculations using the Coulomb-Born approximation with exchange, while those for multiple ionization are found to agree well with the semiempirical model for q = 3, but they appear to be notably overestimated by a semiempirical Bethe-Born-type formula when q > 3.

Screening effects and total cross sections of single, double and triple ionization of neon by fast electrons

The Bethe-Born theory is applied to estimate the effective charges experienced by the ejected electrons at single, double and triple ionization of neon by fast electrons. The expansion coefficients appearing in the asymptotic expressions for total cross sections sigma(n+) similar to[a(n+) In(E-i/In+) + b(n+)]/E-i are determined by fitting to these expressions the experimental data for single and multiple ionization of neon. Coefficients a(n+) and b(n+) are seen to decrease exponentially with increasing number of ejected electrons. Empirical formulae are found to describe this behaviour. Analytical expressions for coefficients a(n+) are derived within the Bethe-Born approximation for single, double and triple ionization of neon. The effective charges are determined by assuming that the derived analytical expressions for a(n+) can be reduced to the empirical formula. It is shown that, in order to obtain the reasonable values for the effective charges, coefficient b(n+) should be taken into consideration. It is also revealed that the inclusion of interaction between the ejected electrons in the final state in calculation improves the results.

Electron-impact dissociation and ionization of NH+: formation of N+ and N2+

Absolute cross sections for electron-impact dissociation and ionization of NH+ leading to the formation of N+ and N2+ products have been measured by applying the animated electron-ion beam method, in the energy range from the respective thresholds up to 2.5 keV. The maximum total cross sections are observed to be (15.7 +/- 0.7) x 10(-17) cm(2) and (11.1 +/- 0.2) x 10(-18) cm(2) for N+ and N2+, respectively. Absolute cross sections are determined separately for dissociative excitation and for dissociative ionization processes. The measurements for slow N+ ions show a noticeable contribution in the low incident electron energy range; these data are attributed to excitation processes. Dissociative excitation is assumed to play a significant role in the collision energy region close to the vertical excitation energies for the lowest electronic transitions in the Franck-Condon region. The yields of fast N+ ions have also been measured; these energetic dissociations are generally ascribed to ionization processes. Kinetic energy release distributions are seen to extend from 0 eV to 15 eV for the N+ fragments and up to 20 eV for the N2+ ones. The present energy thresholds and kinetic energy release results are compared with available published data, allowing in some cases identification of fragmentation patterns and of molecular states contributing to observed processes. The possibility of dissociative excitation of the molecular ion which could occur via a mechanism involving autoionizing resonances is discussed.