Christian Alcaraz

SU5: a calibrated variable-polarization synchrotron radiation beam line in the vacuum-ultraviolet range.

SU5 is a high-resolution variable-polarization synchrotron radiation (SR) beam line with which linear and circular dichroism experiments are performed in the vacuum ultraviolet (VUV) range (5-40eV), based on an electromagnetic crossed undulator called the Onduleur Plan/Helicoidal du Lure à Induction Electromagnétique (OPHELIE). To get precise knowledge of the polarization state of the emitted SR and to take into account the polarization transformations induced by reflection on the various optics, we set up an in situ VUV polarimeter to provide a precise and complete polarization analysis of the SR atthe sample location. The overall measured polarization performances were highly satisfactory, with measured linear polarization rates of more than 98% (83%) in the vertical (horizontal) linear polarization mode and an average 92.1% (95.2%) circular polarization rate for the right- (left)-handed circular polarization mode, which, to our knowledge, are the highest reported values in the VUV range. Despite some uneven photon energy efficiency, the OPHELIE crossed undulator behaves as expected in terms of polarization, permitting full control of the emitted polarization by manipulation of the vertical-to-horizontal magnetic field ratio (rho(und)) and the relative longitudinal phase (phi(und)).

Very high spectral resolution obtained with SU5 : A vacuum ultraviolet undulator-based beamline at Super-ACO

In this article we wish to report on the commissioning of the SU5 undulator-based beamline, whose primary scientific goal deals with high-resolution spectroscopy and photon-induced dynamics in the vacuum ultraviolet range on dilute species. In order to achieve such a scientific program, we have conceived an original optical design, centered around a 6.65 m off-plane Eagle monochromator equipped with two gratings (2400 and 4300 l/mm) illuminated by an astigmatic prefocusing optical system. The different components of the actual beamline, such as the optical elements, their holders/manipulators and the monochromator are described, with a special emphasis on critical aspects such as the mechanical resolution and stability, the vibrations limitations, and the thermal stability. Then, a spectral calibration procedure of the monochromator is described, followed by the presentation of the measured performances of the beamline in terms of ultimate resolution and flux. Owing to a gas phase ion yield experiment on rare gases, we have been able to measure a raw linewidth of 184 µeV at 21.61 eV (18s line of Ne) corresponding to a resolving power of 117 000 and a raw (respectively, lifetime-deconvoluted) linewidth of 119 µeV (respectively, 76 µeV) at 15.82 eV (13s line of Ar) corresponding to a raw resolving power of 133 000 (respectively, 208 000). The ultimate targeted 105 resolving power is therefore observed on most of the VUV range, which corresponds to an unprecedented performance. The measured photon flux in a 1/50 000 bandwidth is in reasonable agreement with the expected values lying in the 1010–109 photon/s range over the VUV range. These very satisfactory performances, with a slit-limited resolution down to about 15 µm slits aperture, are due to the special care which has been paid at both the conception and construction stages concerning critical issues such as the optical and mechanical design, the vibrations, the driving/encoding system, and the slope errors of the gratings.

CRITICAL REVIEW OF N, N+, N-2(+), N++, And N-2(++) MAIN PRODUCTION PROCESSES AND REACTIONS OF RELEVANCE TO TITAN'S ATMOSPHERE

This paper is a detailed critical review of the production processes and reactions of N, N+, N+ 2, N++, and N++ 2 of relevance to Titans atmosphere. The review includes neutral, ion-molecule, and recombination reactions. The review covers all possible active nitrogen species under Titans atmospheric conditions, specifically N2 (A3Σ+ u), N (4 S), N (2 D), N (2 P), N+ 2, N+ (3 P), N+ (1 D), N++ 2, and N++ species, and includes a critical survey of the reactions of N, N+, N+ 2, N++, and N++ 2 with N2, H2, D2, CH4, C2H2, C2H4, C2H6, C3H8 and the deuterated hydrocarbon analogs, as well as the recombination reactions of N+ 2, N+, N++ 2, and N++. Production processes, lifetimes, and quenching by collisions with N2 of all reactant species are reviewed. The N (4 S) state is reactive with radicals and its reactions with CH2, CH3, C2H3, and C2H5 are reviewed. Metastable states N2(A3Σ+u), N (2 D), and N (2 P) are either reactive or quenched by collisions with the target molecules reviewed. The reactions of N+ (1 D) have similar rate constants as N+ (3 P), but the product branching ratios differ significantly. Temperature effects and the role of the kinetic energy content of reactants are investigated. In all cases, experimental uncertainties of laboratory data are reported or estimated. Recommended values with uncertainties, or estimated values when no data are available, are given for rate constants and product branching ratios at 300 K and at the atmospheric temperature range of Titan (150-200 K for neutral reactions and 150 K for ion reactions).

Threshold photoelectron spectroscopy of the methyl radical isotopomers, CH3, CH2D, CHD2 and CD3: synergy between VUV synchrotron radiation experiments and explicitly correlated coupled cluster calculations.

Threshold photoelectron spectra (TPES) of the isotopomers of the methyl radical (CH(3), CH(2)D, CHD(2), and CD(3)) have been recorded in the 9.5-10.5 eV VUV photon energy range using third generation synchrotron radiation to investigate the vibrational spectroscopy of the corresponding cations at a 7-11 meV resolution. A threshold photoelectron-photoion coincidence (TPEPICO) spectrometer based on velocity map imaging and Wiley-McLaren time-of-flight has been used to simultaneously record the TPES of several radical species produced in a Ar-seeded beam by dc flash-pyrolysis of nitromethane (CH(x)D(y)NO(2), x + y = 3). Vibrational bands belonging to the symmetric stretching and out-of-plane bending modes have been observed and P, Q, and R branches have been identified in the analysis of the rotational profiles. Vibrational configuration interaction (VCI), in conjunction with near-equilibrium potential energy surfaces calculated by the explicitly correlated coupled cluster method CCSD(T*)-F12a, is used to calculate vibrational frequencies for the four radical isotopomers and the corresponding cations. Agreement with data from high-resolution IR spectroscopy is very good and a large number of predictions is made. In particular, the calculated wavenumbers for the out-of-plane bending vibrations, nu(2)(CH(3)(+)) = 1404 cm(-1), nu(4)(CH(2)D(+)) = 1308 cm(-1), nu(4)(CHD(2)(+)) = 1205 cm(-1), and nu(2)(CD(3)(+)) = 1090 cm(-1), should be accurate to ca. 2 cm(-1). Additionally, computed Franck-Condon factors are used to estimate the importance of autoionization relative to direct ionization. The chosen models globally account for the observed transitions, but in contrast to PES spectroscopy, evidence for rotational and vibrational autoionization is found. It is shown that state-selected methyl cations can be produced by TPEPICO spectroscopy for ion-molecule reaction studies, which are very important for the understanding of the planetary ionosphere chemistry.

Growth of Larger Hydrocarbons in the Ionosphere of Titan

Among the many fascinating results of the Cassini–Huygens mission, the mass spectrum of the ionosphere of Titan has attracted considerable attention. In brief, the ionosphere was found to be surprisingly complex, consisting of hydrocarbon ions CmHn + as well as nitrogen-containing ions CnHnNo + with mass-to-charge ratios up to the probe#s limit of m/z 100; even much heavier components have been proposed. While the formation of CmHn compounds with m 7 is reasonably well understood, routes to larger hydrocarbons are less obvious. Moreover, most of the present models rely on condensation reactions of CmHn + ions with unsaturated precursors such as acetylene, whereas methane, as the major hydrocarbon in the atmosphere of Titan, only plays a minor role in the subsequent growth processes. Here, we report carbon carbon (C C) coupling reactions of methane with medium-sized CmHn 2+ dications leading to larger hydrocarbon molecules. Despite low steady-state concentrations of the dicationic intermediates, kinetic modeling allows predictions about the larger hydrocarbon species present in the ionosphere of Titan, thereby rationalizing the results from the Cassini–Huygens mission which consideration of monocations only cannot explain. The activation of methane poses a particular challenge and usually involves energetic conditions or metal catalysis. Under the conditions of the Titan atmosphere (low temperatures and pressures), small hydrocarbon ions can indeed react with methane, but the rate constants decrease with size, and so far reaction 1 involves the largest CmHn + ion reacting with methane under thermal conditions.

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.

Prediction of a N2++ layer in the upper atmosphere of Titan

Calculations of dication N ++ 2 density in the atmosphere of Titan is performed for the first time. The metastable lifetime of this species is 3 seconds in its ground state. The density of the N ++ 2 layer centred around 1100-1200 km altitude can reach 10 4 m -3 . The ions are produced by both the double photoionisation and the photoelectron impact of N 2 . They are lost by dissociative recombination with the thermal electrons and chemical reactions with N 2 and CH 4 . The most recent chemical reaction rate constants, given by laboratory experiments, were used in this study. Finally, we explore the possible detection of this ion layer by a spectrophotometer and show that the UVIS instrument onboard CASSINI could be capable to observe it.

Determination of the absolute photoionization cross sections of CH3 and I produced from a pyrolysis source, by combined synchrotron and vacuum ultraviolet laser studies.

A pyrolysis source coupled to a supersonic expansion has been used to produce the CH3 radical from two precursors, iodomethane CH3I and nitromethane CH3NO2. The relative ionization yield of CH3 has been recorded at the SOLEIL Synchrotron Radiation source in the range 9.0-11.6 eV, and its ionization threshold has been modeled by taking into account the vibrational and rotational temperature of the radical in the molecular beam. The relative photoionization yield has been normalized to an absolute cross section scale at a fixed wavelength (118.2 nm, sigma(i)(CH3) = 6.7(-1.8)(+2.4) Mb, 95% confidence interval) in an independent laboratory experiment using the same pyrolysis source, a vacuum ultraviolet (VUV) laser, and a carefully calibrated detection chain. The resulting absolute cross section curve is in good agreement with the recently published measurements by Taatjes et al., although with an improved signal-to-noise ratio. The absolute photoionization cross section of CH3I at 118.2 nm has also been measured to be sigma(i)(CH3I) = (48.2 +/- 7.9) Mb, in good agreement with previous electron impact measurements. Finally, the photoionization yield of the iodine atom in its ground state 2P(3/2) has been recorded using the synchrotron source and calibrated for the first time on an absolute cross section scale from our fixed 118.2 nm laser measurement, sigma(i)(I2P(3/2)) = 74(-23)(+33) Mb (95% confidence interval). The ionization curve of atomic iodine is in good agreement, although with slight variations, with the earlier relative ionization yield measured by Berkowitz et al. and is also compared to an earlier calculation of the iodine cross section by Robicheaux and Greene. It is demonstrated that, in the range of pyrolysis temperature used in this work, all the ionization cross sections are temperature-independent. Systematic care has been taken to include all uncertainty sources contributing to the final confidence intervals for the reported results.

The vacuum ultraviolet photochemistry of the allyl radical investigated using synchrotron radiation

We report a study on the spectroscopy and the photochemistry of the allyl radical, C3H5, in the vacuum ultraviolet energy range using synchrotron radiation as the light source and a flash pyrolysis source to generate the radical. Mass spectra recorded over a wide energy range show the appearance of C3H3+ as a fragment in the dissociative photoionization of allyl above 10 eV. The threshold photoelectron spectrum of the allyl radical shows a progression of around 52 meV, corresponding to the CCC bending vibration.

A versatile electromagnetic planar/helical crossed undulator optimized for the SU5 low energy/high resolution beamline at Super-ACO

Abstract We present here a planar/helical crossed overlapped undulator, composed of two identical 10 (25-cm long) periods crossed undulators whose magnetic fields are produced by electromagnets. Therefore the gaps are fixed, the tuning range being set by varying the field excitation current. Such a design is quite simple, with only one mechanical translational motion, and versatile in terms of polarization settings and switching. After the optimization of the period length for minimizing the thermal load on the optics, the general concept of the crossed undulator is introduced, followed by a description of its actual magnetic design imposed by tolerances on the stored positron beam. Finally, the expected performances in terms of flux and polarization capabilities are presented. It appears that any state of polarization can be produced, at least on the first harmonic, with a large total polarization rate and a potentially high polarization switching frequency.