Ian R. Sims’s research while affiliated with French National Centre for Scientific Research and other places

Cyanopolyynes are among the largest and most commonly observed interstellar Complex Organic Molecules in star-forming regions. They are believed to form primarily in the gas-phase, but their formation routes are not well understood. We present a comprehensive study of the gas-phase formation network of cyanobutadiyne, HC$_5$N, based on new theoretical calculations, kinetics experiments, astronomical observations, and astrochemical modeling. We performed new quantum mechanics calculations for six neutral-neutral reactions in order to derive reliable rate coefficients and product branching fractions. We also present new CRESU data on the rate coefficients of three of these reactions (C$_3$N + C$_2$H$_2$, C$_2$H + HC$_3$N, CN + C$_4$H$_2$) obtained at temperatures as low as 24 K. In practice, six out of nine reactions currently used in astrochemical models have been updated in our reviewed network. We also report the tentative detection of the $^{13}$C isotopologues of HC$_5$N in the L1544 prestellar core. We derived a lower limit of $^{12}$C/$^{13}$C > 75 for the HC$_5$N isotopologues, which does not allow to bring new constraints to the HC$_5$N chemistry. Finally, we verified the impact of the revised reactions by running the GRETOBAPE astrochemical model. We found good agreement between the HC$_5$N predicted and observed abundances in cold ($\sim$10 K) objects, demonstrating that HC$_5$N is mainly formed by neutral-neutral reactions in these environments. In warm molecular shocks, instead, the predicted abundances are a factor of ten lower with respect to observed ones. In this environment possessing an higher gas ionization fraction, we speculate that the contribution of ion-neutral reactions could be significant.

Cyanopolyynes are among the largest and most commonly observed interstellar Complex Organic Molecules in star-forming regions. They are believed to form primarily in the gas-phase, but their formation routes are not well understood. We present a comprehensive study of the gas-phase formation network of cyanobutadiyne, HC5N, based on new theoretical calculations, kinetics experiments, astronomical observations, and astrochemical modeling. We performed new quantum mechanics calculations for six neutral-neutral reactions in order to derive reliable rate coefficients and product branching fractions. We also present new CRESU data on the rate coefficients of three of these reactions (C3N + C2H2, C2H + HC3N, CN + C4H2) obtained at temperatures as low as 24 K. In practice, six out of nine reactions currently used in astrochemical models have been updated in our reviewed network. We also report the tentative detection of the 13C isotopologues of HC5N in the L1544 prestellar core. We derived a lower limit of 12C/13C > 75 for the HC5N isotopologues, which does not allow to bring new constraints to the HC5N chemistry. Finally, we verified the impact of the revised reactions by running the GRETOBAPE astrochemical model. We found good agreement between the HC5N predicted and observed abundances in cold (∼10 K) objects, demonstrating that HC5N is mainly formed by neutral-neutral reactions in these environments. In warm molecular shocks, instead, the predicted abundances are a factor of ten lower with respect to observed ones. In this environment possessing an higher gas ionization fraction, we speculate that the contribution of ion-neutral reactions could be significant.

Experimental studies of the products of elementary gas-phase chemical reactions occurring at low temperatures (<50 K) are very scarce, but of importance for fundamental studies of reaction dynamics, comparisons with high-level quantum dynamical calculations, and, in particular, for providing data for the modeling of cold astrophysical environments, such as dense interstellar clouds, the atmospheres of the outer planets, and cometary comae. This study describes the construction and testing of a new apparatus designed to measure product branching fractions of elementary bimolecular gas-phase reactions at low temperatures. It combines chirped-pulse Fourier transform millimeter wave spectroscopy with continuous uniform supersonic flows and high repetition rate laser photolysis. After a comprehensive description of the apparatus, the experimental procedures and data processing protocols used for signal recovery, the capabilities of the instrument are explored by the study of the photodissociation of acrylonitrile and the detection of two of its photoproducts, HC3N and HCN. A description is then given of a study of the reactions of the CN radical with C2H2 at 30 K, detecting the HC3N product, and with C2H6 at 10 K, detecting the HCN product. A calibration of these two products is finally attempted using the photodissociation of acrylonitrile as a reference process. The limitations and possible improvements in the instrument are discussed in conclusion.

HCN and its unstable isomer HNC are widely observed throughout the interstellar medium, with the HNC/HCN abundance ratio correlating strongly with temperature. In very cold environments HNC can even appear more abundant than HCN. Here we use a chirped pulse Fourier transform spectrometer to measure the pressure broadening of HCN and HNC, simultaneously formed in situ by laser photolysis and cooled to low temperatures in uniform supersonic flows of helium. Despite the apparent similarity of these systems, we find the HNC–He cross section to be more than twice as big as the HCN–He cross section at 10 K, confirming earlier quantum calculations. Our experimental results are supported by high-level scattering calculations and are also expected to apply with para-H2, demonstrating that HCN and HNC have different collisional excitation properties that strongly influence the derived interstellar abundances. HCN and its isomer HNC are both observed in the interstellar medium and inelastic collisions with helium and other species strongly influence their derived abundances. Now it has been shown experimentally and theoretically that HNC is much more strongly excited than HCN in collisions with helium at the low temperatures of interstellar space.