Ian Richard Sims

Context. Non-local-thermodynamic-equilibrium (non-LTE) conditions in the interstellar medium require collisional rate coefficients to model astronomical observations; these are usually determined from theoretical scattering calculations. Aims. The aim of this study is to experimentally measure low-temperature pressure-broadening cross sections for the H ₂ CO–He system in order to validate the theoretical methodology involved in determining new collisional rate coefficients. Methods. The experiments employed the chirped-pulse in uniform supersonic flow method, and H ₂ CO was generated in situ by 193 nm excimer laser photolysis of tetrahydrofuran in cold He flows. State-of-the-art calculations were performed by computing a new potential energy surface for the H ₂ CO–He system, which was subsequently implemented in scattering calculations using the close-coupling method to derive pressure broadening cross sections and collisional rate coefficients. Results. Excellent agreement between theory and experiment is obtained, with the calculated values falling within the 95% confidence intervals of the experimental measurements. Such agreement validates the high accuracy of the theoretical data. Conclusions. Helium constitutes about 20% relative to H ₂ in the interstellar medium. The inclusion of collisional rate coefficients for H ₂ CO with He in radiative-transfer modeling leads to variations in the excitation temperature of frequently detected rotational lines of up to 12% in warm regions such as protostars.

This work presents calculations of state-to-state collisional rate coefficients for the rotational excitation of the deuterated formaldehyde isotopologs, HDCO and D ₂ CO, induced by collisions with molecular hydrogen, coupled with experimental verification by pressurebroadening measurements. Time-independent scattering calculations were performed using the close-coupling method to compute inelastic cross sections for collisions with both para- H ₂ ( j ₂ = 0) and ortho- H ₂ ( j ₂ = 1). These calculations are based on the H ₂ CO–H ₂ potential energy surface previously determined at the CCSD(T) level of theory and transformed to account for the isotopic substitution. The resulting rate coefficients cover the lowest 100 rotational states of HDCO and 60 states of ortho- and para- D ₂ CO with internal energies less than 200 cm ⁻¹ . Collisional data were obtained for temperatures up to 100 K for HDCO and D ₂ CO and extended up to 300 K using the coupled-states approximation. The computed rate coefficients show systematic differences between ortho- H ₂ and para- H ₂ collisions, with the former generally yielding larger values by a factor of 2–3. Rate coefficients for HDCO–H ₂ are found to be lower by a factor of 2 to 3 compared to those of H ₂ CO–H ₂ and D ₂ CO–H ₂ . Pressure-broadening coefficients were also calculated using the random-phase approximation and compared with new laboratory measurements, yielding agreement by about 5–30%.

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.

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.

Nucleation is a key process in cloud and dust particle formation in different systems including planetary atmospheres and circumstellar shells. It consists of a succession of unions of small gas-phase molecules leading to the formation and growth of droplets and particles. Under the appropriate conditions, low temperatures and/or high degrees of supersaturation, nucleation can become a barrierless process, and dimerization, the complexation of the two smallest molecular species involved, becomes its rate-limiting step. The kinetics of formation of homodimers have been investigated in a few previous studies, using the CRESU (reaction kinetics in uniform supersonic flow) technique mainly coupled with mass spectrometric detection. Here we have employed a completely new detection scheme, chirped-pulse Fourier transform mm-wave spectroscopy, to study for the first time the kinetics of formation of heterodimers. This innovative technique combines the ability to generate continuous cold uniform supersonic flows with the high selectivity and general applicability of rotational spectroscopy, allowing us to follow both reactant and product concentrations simultaneously. Furthermore, the high sensitivity achieved has allowed us to employ pseudo-first-order conditions to obtain absolute rate constants. We will report the first measurements of rate constants for the formation of a set of heterodimers, including formic acid, CO2 and water among others, at temperatures between 35 and 150 K.

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.

A new instrument dedicated to the kinetic study of low-temperature gas phase neutral–neutral reactions, including clustering processes, is presented. It combines a supersonic flow reactor with vacuum ultra-violet synchrotron photoionization time-of-flight mass spectrometry. A photoion–photoelectron coincidence detection scheme has been adopted to optimize the particle counting efficiency. The characteristics of the instrument are detailed along with its capabilities illustrated through a few results obtained at low temperatures (n-butane, the detection of formic acid dimer formation, and the observation of diacetylene molecules formed by the reaction between the C₂H radical and C₂H₂.

CN is known for its fast reactions with hydrocarbons at low temperatures, but relatively few studies have focused on the reactions between CN and aromatic molecules. The recent detection of benzonitrile in the interstellar medium, believed to be produced by the reaction of CN and benzene, has ignited interest in studying these reactions. Here, we report rate constants of the CN + toluene (C7H8) reaction between 15 and 294 K using a CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme; reaction kinetics in uniform supersonic flow) apparatus coupled with the pulsed laser photolysis – laser induced fluorescence (PLP-LIF) technique. We also present the stationary points on the potential energy surface of this reaction to study the available reaction pathways. We find the rate constant does not change over this temperature range, with an average value of (4.1 ± 0.2) × 10^-10 cm³ s^-1, which is notably faster than the only previous measurement at 105 K. While the reason for this disagreement is unknown, we discuss the possibility that it is related to enhanced multiphoton effects in the previous work.