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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 (4σ)−1 inner-valence ionization is induced by left-handed circularly polarized synchrotron radiation at hν=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.

The origin of biomolecular asymmetry could be found in asymmetric photochemistry in interstellar regions. It is assumed that the amino acids were formed in these regions. The irradiation of solid d,lleucine probes with right circularly polarized synchrotron radiation at 182 nm produced an enantiomeric excess of the d enantiomer of 2.6 %. For more details see the Communication by Meierhenrich, Nahon et al. on the following pages. The image was taken in June 1998 on the space shuttle Discovery.

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.

[1] The calculation of the dication CO 2 2+ density in the atmosphere of Mars is performed for the first time. The metastable lifetime of these species reaches 4 seconds. The density of a layer centered around 155 -160 km altitude can reach from 3 to and 5 10 6 m À3 . The ions are produced by CO 2 photoionization and photoelectron impact on CO 2 . They are lost by dissociative recombination with the thermal electrons and chemical reaction with CO 2 . This work is based upon relevant chemical reaction coefficient rates that have been measured in laboratory very recently. We suggest that this ion layer can be detectable by a mass spectrometer onboard the orbiter of the CNES PREMIER 07 mission. This work opens a series of promising studies on double ionization processes in the Mars ionosphere, such as their implications for the production of energetic electrons or fast fragmentation ion products which could participate in atmospheric escape.

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)).

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.

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