A new instrument for kinetics and branching ratio studies of gas phase collisional processes at very low temperatures

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 (<100 K) including a photoionization spectrum of 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.

High-temperature hypersonic Laval nozzle for non-LTE cavity ringdown spectroscopy

A small dimension Laval nozzle connected to a compact high enthalpy source equipped with cavity ringdown spectroscopy (CRDS) is used to produce vibrationally hot and rotationally cold high-resolution infrared spectra of polyatomic molecules in the 1.67 µm region. The Laval nozzle was machined in isostatic graphite, which is capable of withstanding high stagnation temperatures. It is characterized by a throat diameter of 2 mm and an exit diameter of 24 mm. It was designed to operate with argon heated up to 2000 K and to produce a quasi-unidirectional flow to reduce the Doppler effect responsible for line broadening. The hypersonic flow was characterized using computational fluid dynamics simulations, Pitot measurements, and CRDS. A Mach number evolving from 10 at the nozzle exit up to 18.3 before the occurrence of a first oblique shock wave was measured. Two different gases, carbon monoxide (CO) and methane (CH₄), were used as test molecules. Vibrational (T_vib) and rotational (T_rot) temperatures were extracted from the recorded infrared spectrum, leading to T_vib = 1346 ± 52 K and T_rot = 12 ± 1 K for CO. A rotational temperature of 30 ± 3 K was measured for CH₄, while two vibrational temperatures were necessary to reproduce the observed intensities. The population distribution between vibrational polyads was correctly described with T_vib ^I=894±47 K, while the population distribution within a given polyad (namely, the dyad or the pentad) was modeled correctly by T_vib ^II=54±4 K, testifying to a more rapid vibrational relaxation between the vibrational energy levels constituting a polyad.

High enthalpy source dedicated to quantitative infrared emission spectroscopy of gas flows at elevated temperatures

The High Enthalpy Source (HES) is a novel high temperature source developed to measure infrared line-by-line integrated absorption cross sections of flowing gases up to 2000 K. The HES relies on a porous graphite furnace designed to uniformly heat a constant flow of gas. The flow compensates thermal dissociation by renewing continuously the gas sample and eliminating dissociation products. The flowing characteristics have been investigated using computational fluid dynamics simulation confirming good temperature uniformity. The HES has been coupled to a high-resolution Fourier transform spectrometer to record emission spectra of methane at temperatures ranging between 700 and 1400 K. A radiative model has been developed to extract absolute line intensities from the recorded spectra.

Low Temperature Kinetics of the First Steps of Water Cluster Formation

We present a combined experimental and theoretical low temperature kinetic study of water cluster formation. Water cluster growth takes place in low temperature (23-69 K) supersonic flows. The observed kinetics of formation of water clusters are reproduced with a kinetic model based on theoretical predictions for the first steps of clusterization. The temperature- and pressure-dependent association and dissociation rate coefficients are predicted with an ab initio transition state theory based master equation approach over a wide range of temperatures (20-100 K) and pressures (10^{-6}-10  bar).

Infrared Spectra of a Species of Astrochemical Interest: Aminoacrylonitrile (3-Amino-2-propenenitrile)

Ammonia easily reacts on cyanoacetylene in the gas phase or in a solvent to form the Z- and E-isomers of aminoacrylonitrile (3-amino-2-propenenitrile, 2). This kinetically stable enamine presents interest for its possible presence in the interstellar medium, the comets, the atmospheres of Planets including the Primitive Earth, and from a theoretical point of view. B3LYP/6-311+G(3df,2p) and G2 calculations indicate that the imine isomer is significantly less stable than the enamine 2. DFT and G2 calculations indicate that the Z-isomer of compound 2 lies ca. 8.0 kJ mol(-1) lower in energy than the E-isomer. The infrared spectra of the aminoacrylonitrile, in both the gas and condensed phases were recorded in the range 500-4000 cm(-1). Consistent with the theoretical calculations, the imine and the E-isomer of the enamine have never been detected in the infrared spectrum of a gaseous sample and only the Z-isomer has been observed. With a neat sample in the condensed phase, IR spectra of a 1:1 and 20:1/Z:E mixtures were recorded. The comparison of these data with the spectrum of the Z-isomer in the gas phase allowed us to deduce the IR spectrum of the E-isomer. The E-Z isomerization takes place through a torsion around the C=C bond. A possible mechanism involving a previous enamine-imine tautomerism must be discarded because it implies a much larger barrier than the direct isomerization process. Consistently, the presence of a deuterium atom has not been observed on the sp2 carbon of the products of distillation of a 1:1/E:Z mixture of the NCCH=CHND2.

Structural and conformational properties of 2-propenylgermane (allylgermane) studied by microwave and infrared spectroscopy and quantum chemical calculations

The structural and conformational properties of allylgermane have been investigated using Stark and Fourier transform microwave spectroscopies, infrared spectroscopy, and high-level quantum chemical calculations. The parent species H2C=CHCH2GeH3 was investigated by microwave spectroscopy and infrared spectroscopy, while three deuterated species, namely, H2C=CDCH2GeH3, H2C=CHCHDGeH3, and H2C=CHCH2GeD3, were studied only by infrared spectroscopy. The microwave spectra of the ground vibrational state as well as of the first excited state of the torsion vibration around the sp2-sp3 carbon-carbon bond were assigned for the 70Ge, 72Ge, and 74Ge isotopomers of one conformer. This rotamer has an anticlinal arrangement for the C=C-C-Ge chain of atoms. The infrared spectrum of the gas in the 500-4000 cm(-1) range has been assigned. No evidence of additional rotameric forms other than anticlinal was seen in the microwave and infrared spectra. Several different high-level ab initio and density functional theory calculations have been performed. These calculations indicate that a less stable form, having a synperiplanar conformation of the C=C-C-Ge link of atoms, may coexist with the anticlinal form. The energy differences between the synperiplanar and anticlinal forms were calculated to be 5.6-9.2 kJ/mol depending on the computational procedure. The best approximation of the equilibrium structure of the anticlinal rotamer was found in the MP2/aug-cc-pVTZ calculations. The barrier to internal rotation of the germyl group was found to be 6.561(17) kJ/mol, from measurements of the splitting of microwave transitions caused by tunneling of the germyl group through its threefold barrier.

Numerical investigation of the discharge characteristics of the pulsed discharge nozzle

The characteristics of the plasma generated by a pulsed discharge slit nozzle (PDN) are investigated. The PDN source is designed to produce and cool molecular ions creating an astrophysically relevant environment in the laboratory. A discharge model is applied to this system to provide a qualitative as well as a quantitative picture of the plasma. We find that the plasma's properties and behavior are characteristic of those of a glow discharge. We model the electron density and energy, as well as the argon ion and metastable atom number density. The results reveal a high abundance of metastable argon atoms in the expansion region, which is more than one order of magnitude higher than the abundance of electrons and ions. These findings confirm experimental observations, which concluded that large molecular ions are dominantly formed through Penning ionization of the neutral molecular precursors seeded in the supersonic expansion of argon gas. The simulations presented here will help optimize the yield of formation of molecular ions and radicals in the PDN source; they will also provide key physical insight into the characteristics of interstellar molecules and ions analogs in laboratory experiments.

Vibrational Spectra, DFT Calculations, and Assignments of the syn and the gauche Forms of Vinylphosphine

Infrared spectra (3500-600 cm(-1)) of vinylphosphine and its P-dideuterated derivative in the gas phase were recorded at 1 cm(-1) resolution. Both the infrared absorption bands of the syn and gauche conformers of the vinylphosphine were observed and assigned. The assignment was based on density functional theory calculations performed at the B3LYP/6-311G(d) level. The agreement between calculated and observed frequencies for both CH(2)&dbond;CHPH(2) and CH(2)&dbond;CHPD(2) was fairly good. The integrated intensities of isolated and overlapping vibrational bands were determined experimentally. Copyright 2001 Academic Press.

Uniform Supersonic Expansion for FTIR Absorption Spectroscopy: The nu(5) Band of (NO)(2) at 26 K

A high-resolution Fourier transform interferometer (Bruker IFS 120 HR) was combined with a uniform supersonic expansion produced by means of axisymmetric Laval nozzles. The geometry profile of the nozzle enabled us to work under precise thermodynamic and kinetic conditions. The effect of the cooling rate of different nozzles on cluster nucleation is illustrated. The experimental sensitivity was tested by recording the nu(5) band of (NO)(2) at 26 K. Copyright 2000 Academic Press.