S. D. Le Picard

A KINETIC DATABASE FOR ASTROCHEMISTRY (KIDA)

We present a novel chemical database for gas-phase astrochemistry. Named the KInetic Database for Astrochemistry (KIDA), this database consists of gas-phase reactions with rate coefficients and uncertainties that will be vetted to the greatest extent possible. Submissions of measured and calculated rate coefficients are welcome, and will be studied by experts before inclusion into the database. Besides providing kinetic information for the interstellar medium, KIDA is planned to contain such data for planetary atmospheres and for circumstellar envelopes. Each year, a subset of the reactions in the database (kida.uva) will be provided as a network for the simulation of the chemistry of dense interstellar clouds with temperatures between 10 K and 300 K. We also provide a code, named Nahoon, to study the time-dependent gas-phase chemistry of zero-dimensional and one-dimensional interstellar sources.

The 2014 KIDA network for interstellar chemistry

Chemical models used to study the chemical composition of the gas and the ices in the interstellar medium are based on a network of chemical reactions and associated rate coefficients. These reacti ...

How Measurements of Rate Coefficients at Low Temperature Increase the Predictivity of Photochemical Models of Titan’s Atmosphere†

The predictivity of photochemical models of Titans atmosphere depends strongly on the precision and accuracy of reaction rates. For many reactions, large uncertainty results from the extrapolation of rate laws to low temperatures. A few reactions have been measured directly at temperatures relevant to Titans atmosphere. In the present study, we observed the consequences of the reduced uncertainty attributed to these reactions. The global predictivity of the model was improved, i.e., most species are predicted with lower uncertainty factors. Nevertheless, high uncertainty factors are still observed, and a new list of key reactions has been established.

The Si(

The rate coecient of the reaction Si( 3 PJ )+O 2! SiO +Oh as been measured at temperatures down to 15 Ku sing aC RESU (Cinetique de R eaction en Ecoulement Supersonique Uniforme) apparatus coupled with the PLP-LIF (Pulsed Laser Photolysis { Laser Induced Fluorescence) technique. The temperature dependence of the rate coecient is well tted using the expression: 1:7210 10 (T /300 K) 0:53 exp( 17 K=T )c m 3 molecule 1 s 1 in the temperature range 15{300 K. The silicon chemistry in interstellar clouds is reviewed and possible consequences of our study are stressed.

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Rate coefficients for the reaction of ground-state silicon atoms Si(3PJ) with acetylene and ethylene have been measured at temperatures down to 15 K. The experiments have been performed in a continuous flow CRESU (Cinetique de Reaction en Ecoulement Supersonique Uniforme) apparatus using pulsed laser photolysis of Si(CH3)4 to generate Si(3PJ) atoms and laser-induced fluorescence to observe the kinetic decay of the atoms and hence determine the rate coefficients. Both reactions are found to be fast, and the reaction rates show a very mild dependence on temperature. The rate coefficients match the expressions k(Si+C2H2)=(2.6±0.6)10−10(T/300)−(0.71±0.24) exp(−(29±10)/T) cm3 molecule−1 s−1 and k(Si+C2H4)=(3.7±0.3)10−10(T/300)−(0.34±0.10) exp(−(16±4)/T) cm3 molecule−1 s−1 in the temperature range 15–300 K. The nature of the products and the similarities of the carbon and silicon chemistry are discussed.

PJ) + O

We present both an experimental and a theoretical study of the reactions Si(3PJ) + O2 and Si(3PJ) + NO. The CRESU (Cinetique de Reaction en Ecoulement Supersonique Uniforme) experimental technique is used to measure the rate constants in the temperature range 15–300 K. The laws of variation of the rate constants as a function of temperature are found to exhibit a maximum around 30 K for both reactions and are experimentally described using the following expressions: kSi+O2(T) = (1.72 ± 0.17) × 10−10 (T/300 K)−(0.53±0.10)exp−(17±4)K/T cm3 molecule−1 s−1 and kSi+NO(T) = (0.90 ± 0.10) × 10−10(T/300 K)−(0.96±0.10)exp−(28±3)K/T cm3 molecule−1 s−1. Adiabatic capture calculations are also performed and the temperature dependences obtained for the rate constants present a maximum in good agreement with the experimental results. A perturbative approach is eventually used in the limit of zero temperature to differentiate the spin–orbit reactivity of the silicon atom. These results are also compared with an earlier similar study concerning the reactivity of carbon atoms in the ground state, C(3PJ), with the same molecules.

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Studies of electron attachment in HBr and HCl gases at low temperatures have indicated that attachment to clusters of these gases can become efficient even though attachment to the monomers is endothermic and exhibits very low attachment rates. A complementary measurement of the reaction of OH radicals with HBr has enabled us to establish a lower limit for the rate of electron attachment to HBr clusters of approximately 10−8 cm3 s−1.

reaction: A fast source of SiO at very low temperature; CRESU measurements and interstellar consequences

We present a combined theoretical and experimental study of intramultiplet transitions in collisions of C(3P) and Si(3P) with He. Relaxation rate constants have been measured using the CRESU (Cinetique de Reaction en Ecoulement Supersonique Uniforme) technique at 15 K for C(3P)+He and over the 15–49 K temperature range for Si(3P)+He. Three sets of interaction potentials have been generated for the 3∑− and 3∏ electronic states of CHe and SiHe systems. Quantum-mechanical calculations have been performed on these potential curves for the spin–orbit relaxation and excitation. Cross sections and rate constants are very sensitive to the interaction potential. For the Si(3P)+He system, an overall good agreement between the theoretical and experimental rate constants is found with the best quality interaction potential, while for the C(3P)+He system experimental rate constants are much smaller than the theoretical ones.

Rate coefficients for the reactions of Si(3PJ) with C2H2 and C2H4: Experimental results down to 15 K

This paper reports the first measurements at low temperatures (44–137 K) of rate coefficients for collision induced spin-orbit transitions, using a swarm experiment consisting of a uniform supersonic flow coupled with pulsed laser photolysis and laser-induced fluorescence techniques. It has been stated that both excitation and relaxation rates have a positive temperature dependence. The excitation (respectively relaxation) rate constant is 0.15×10−12 cm3 s−1 (respectively 1.63×10−12) at 53 K and 2.76×10−12 cm3 s−1 (respectively 4.48×10−12) at 137 K. A theoretical modelization of the Al–Ar system has been developed in the 30–300 K range, that reproduces the temperature dependence.

A comparative study of the reactivity of the silicon atom Si(3PJ) towards O2 and NO molecules at very low temperature

To interpret the concentrations of the products measured in Titan’s atmosphere and to better understand the associated chemistry, many theoretical models have been developed so far. Unfortunately, large discrepancies are still found between theoretical and observational data. A critical examination of the chemical scheme included in these models points out some problems regarding the reliability of the description of critical reaction pathways as well as the accuracy of kinetic parameters. Laboratory experiments can be used to reduce these two sources of uncertainty. It can be: i) experimental simulations: in our laboratory (LISA), representative Titan’s simulation experiments are planned to be carried out in a reactor where the initial gas mixture will be exposed, for the first time, to both electrons and photons. Thus, the chemistry between N atoms and CH3 , CH2 , CH fragments, issued from electron dissociation of N2 and photo-dissociation of CH4 respectively, will be initiated. Thank to a time resolved technique, we will be able to analyse “in situ”, qualitatively and quantitatively, the stable species as well as the short life intermediates. Then, the implied chemistry will be determined precisely, and consequently, its description will be refined in theoretical models. The current status of this program will be given. ii) specific experiments: they are devoted, for example, to determine kinetic rate constants and low temperature VUV spectra that will be used to feed models and to interpret observational data. Such experiments performed in LISA and in Rennes’ laboratory concern polyynes and cyanopolyynes as these compounds could link the gaseous and the solid phase in planetary atmosphere. Results concerning C4H hydrocarbons kinetic rate constants and VUV cross section of HC3N and HC5N will be detailed.