Electron attachment on HI and DI in a uniform supersonic flow: Thermalization of the electrons

Toward a quantitative analysis of the temperature dependence of electron attachment to SF6

New flowing afterglow/Langmuir probe investigations of electronic attachment to SF₆ are described. Thermal attachment rate constants are found to increase from 1.5 × 10^-7 cm³ s^-1 at 200 K to 2.3 × 10^-7 cm³ s^-1 at 300 K. Attachment rate constants over the range of 200-700 K (from the present work and the literature), together with earlier measurements of attachment cross sections, are analyzed with respect to electronic and nuclear contributions. The latter suggest that only a small nuclear barrier (of the order of 20 meV) on the way from SF₆ to SF₆ ^- has to be overcome. The analysis shows that not only s-waves but also higher partial waves have to be taken into account. Likewise, finite-size effects of the neutral target contribute in a non-negligible manner.

A uniform flow-cavity ring-down spectrometer (UF-CRDS): A new setup for spectroscopy and kinetics at low temperature

The UF-CRDS (Uniform Flow-Cavity Ring Down Spectrometer) is a new setup coupling for the first time a pulsed uniform (Laval) flow with a continuous wave CRDS in the near infrared for spectroscopy and kinetics at low temperature. This high resolution and sensitive absorption spectrometer opens a new window into the phenomena occurring within UFs. The approach extends the detection range to new electronic and rovibrational transitions within Laval flows and offers the possibility to probe numerous species which have not been investigated yet. This new tool has been designed to probe radicals and reaction intermediates but also to follow the chemistry of hydrocarbon chains and PAHs which play a crucial role in the evolution of astrophysical environments. For kinetics measurements, the UF-CRDS combines the CRESU technique (French acronym meaning reaction kinetics in uniform supersonic flows) with the SKaR (Simultaneous Kinetics and Ring-Down) approach where, as indicated by its name, the entire reaction is monitored during each intensity decay within the high finesse cavity. The setup and the approach are demonstrated with the study of the reaction between CN (v = 1) and propene at low temperature. The recorded data are finally consistent with a previous study of the same reaction for CN (v = 0) relying on the CRESU technique with laser induced fluorescence detection.

On the Temperature Dependence of the Thermal Electron Attachment to SF 6 , SF 5 Cl, and POCl 3

A simplified model for electron attachment to neutral molecules, separating electron--phonon coupling (or intramolecular vibrational redistribution, IVR) factors into an electronic and a vibrational contribution, is tested by comparison with experimental results for the nondissociative electron attachments to SF6, SF5Cl, and POCl3. It is shown that the experimental results can approximately be reproduced by the fit of two parameters, one of them characterizing the dependence of the IVR factor on the electron energy and one accounting for an effective threshold energy for overlap of vibrational wave functions on anionic and neutral potential energy surfaces. After fitting these parameters, temperature dependences over wide ranges of conditions can be represented. Likewise, kinetic modeling of dissociative processes of the highly excited anions produced by attachment is facilitated.

Laboratory measurements of the recombination of PAH ions with electrons: implications for the PAH charge state in interstellar clouds

Laboratory measurements of the recombination of polycyclic aromatic hydrocarbon (PAH) ions with electrons are presented. Experimental data have been obtained at room temperature for azulene (C10H8) and acenaphthene (C12H10) cations by the Flowing Afterglow with PhotoIons method. The results confirm that the recombination of PAH ions is fast although well below the geometrical limit. The set of our recent and present measurements reveal a definite trend of increasing rate with the number of carbon atoms of the PAH. This behaviour that needs further characterization is potentially of great interest for charge state models as recombination is a dominant mechanism of PAH ion destruction in the interstellar medium. The design of experiments to measure the recombination of larger PAHs and their temperature dependence is discussed.