Guzmán Tejeda

The Raman spectrum of CO₂ from room temperature to 1800 K has been measured in a series of experiments. The differential Raman scattering cross-sections for the fundamental bands at 1285.41 cm⁻¹ and 1388.18 cm⁻¹ have been obtained from reference bands of H₂ and N₂ as intensity standards. The Raman cross-sections of CO₂ hot bands, involving vibrational energy levels up to 5000 cm⁻¹, were derived from those of the fundamental bands. The Raman cross-sections obtained this way were reduced to transition moments of the mean molecular polarizability, which make it possible to simulate the Raman spectrum of CO₂ up to 2000 K. This paves the way for local or remote diagnostics of CO₂ in hot environments using Raman based techniques.

The rotational relaxation of CO2 by inelastic collisions has been studied in three supersonic jets. The jets were probed by means of Raman spectroscopy with high spectral and spatial resolutions, measuring the rotational populations and the total number density. The time evolution of the rotational populations was analyzed by means of a kinetic master equation, with the help of the energy-corrected sudden power law to relate the numerous state-to-state rate (STS rates) coefficients. In the thermal range investigated, 60–260 K, the STS rates decrease with increasing temperature and with increasing change in the rotational quantum number. Other quantities of interest for fluid dynamics, such as the rotational collision number, the relaxation cross section, and the bulk viscosity, have been derived from the STS rates.

A laboratory study of state-to-state rate coefficients (STS rates) for H₂:H₂ inelastic collisions in the v = 0 state is reported. The study, which spans the 295–20 K thermal range, is based on the use of a kinetic master equation. It describes the time-space evolution of populations of H₂ rotational levels as induced by inelastic collisions. It is applied here to a supersonic jet of natural H₂. This medium bears a large amount of relevant data that allows for the establishment of best values and confidence margins for the dominant STS rates of H₂:H₂ inelastic collisions on an experimental basis. The primary experimental data derived from the supersonic jet are the local number density, the populations of the H₂ rotational levels, and their gradients along the jet by means of high-sensitivity Raman spectroscopy with superb space resolution. First, two sets of theoretical STS rates from the literature have been tested against the experiment. The set that shows a better agreement with the experiment has then been scaled to derive an improved set of experiment-scaled STS rates (ES rates). They allow the reproduction of more than 50 experimental population gradient data within a standard deviation 2:H₂ ground-state inelastic collisions between 300 and 20 K are presented in machine-readable format. Other (incomplete) sets of theoretical rates from the literature are discussed.

Rotationally inelastic collisions of O₂ with He in the 10-34~K thermal range are investigated by means of an experimental procedure based on supersonic gas jets probed by Raman spectroscopy. The procedure employs a kinetic master equation (MEQ) which describes the time evolution of the rotational populations of O₂ along supersonic jets of O₂ and He mixtures. The MEQ is expressed in terms of experimental quantities (number density and rotational populations) measured here, and state-to-state rate coefficients for the O₂:He inelastic collisions calculated here, plus those for O₂:O₂ collisions from the literature. An agreement with the experiments is accomplished for temperatures between 10 and 34 K. Within this thermal range, the role of the fine structure due to electron spin in the collision dynamics of O₂ is discussed.

The torsional Raman spectra of two astrophysically detected isotopologues of dimethyl ether (DME, ¹²CH₃O¹²CH₃ and ¹³CH₃O¹²CH₃) have been recorded at room temperature and cooled in a supersonic jet and interpreted with the help of highly correlated ab initio calculations. DME displays excited torsional and vibrational levels at low energy that can be populated at the temperatures of the star-forming regions, obliging to extend the analysis of the rotational spectrum over the ground state. Its spectrum in the THz region is rather complex due to the coupling of the torsional overtones 2ν ₁₁ and 2ν ₁₅ with the COC-bending mode and the presence of many hot bands. The torsional overtones are set here at 2ν ₁₁ = 385.2 cm⁻¹ and 2ν ₁₅ = 482.0 cm⁻¹ for ¹²CH₃O¹²CH₃ and 2ν ₁₁ = 385.0 cm⁻¹ and 2ν ₁₅ = 481.1 cm⁻¹ for ¹³CH₃O¹²CH₃. The new assignment of 2ν ₁₁ is downshifted around ∼10 cm⁻¹ with respect to the literature. All the other (hot) bands have been reassigned consistently. In addition, the infrared-forbidden torsional fundamental band ν ₁₁ is observed here at 197.8 cm⁻¹. The new spectral characterization in the THz region reported here provides improved values of the Hamiltonian parameters to be used in the analysis of the rotational spectra of DME isotopologues for further astrophysical detections.

Raman vibrational shifts of small parahydrogen (pH₂), orthodeuterium (oD₂), and paratritium (pT₂) clusters with respect to the free molecules are calculated by combining a first order perturbation theory approach with Langevin equation Path Integral Ground State (LePIGS) simulations [ J. Phys. Chem. A 2013 , 117 , 7461]. Our theoretical predictions are compared to existing cryogenic free jet expansion results for pure (pH₂)_N clusters [ Phys. Rev. Lett. 2004 , 92 , 223401] and to new measurements for (oD₂)_N clusters reported here. This method has been successfully used before to predict the Raman vibrational shifts of (pH₂)_N clusters [ J. Chem. Phys. 2014 , 141 , 014310]. The 6-D interaction potential of Hinde [ J. Chem. Phys. 2008 , 128 , 154308] is reduced to 1-D using the Adiabatic Hindered Rotor approximation to yield effective pair potentials for both molecules being in the ground vibrational state, and for one of them carrying one quantum of vibrational excitation. These reduced 1-D potentials are fitted to a Morse Long Range analytic form for later convenience. Good agreement between experiment and theory is found for the smaller clusters, but significant deviations remain for the larger ones.