Abstract:Semiclassical calculations of rate constants for energy transfer in an HF gas from 300 to 3500 K are reported. The potential hypersurface used was an analytic fit to recent SCF calculations plus the experimental dispersion potential. Good agreement with experiments was obtained without further adjustment of the potential parameters.
“…The existence of several, narrow resonances has been confirmed as the main origin for the fast self-relaxation of gaseous HF, while the coupling with rotational degrees of freedom appears the main cause of slope inversion in the p7 behavior at high temperature, in keeping with several earlier suggestions [25,26,291. A further test of the theory in predicting the correct isotopic behavior of the present system has been carried out successfully and will be reported elsewhere [ 5 5 ] .…”
Section: Computational Results and Discussionsupporting
confidence: 68%
“…More recently, Billing and collaborators [26] have examined the temperature dependence of the H F vibrational relaxation by evaluating the rate constants of the process of PTvib.…”
AbstractsA numerical fit to computed ab initio points of the HF-HF potential energy surface is here employed in its simplest spherical form to solve the corresponding close-coupling equations that yield vibrationally inelastic collision cross sections for the V / T process in the gas phase. The existence of a deep potential well in the interaction is examined here for its effect on near-threshold cross section behavior and evidence is found, through the present calculations, for the relative importance of several (orbiting) shape resonances that contribute to the Ql,o cross sections at low collision energies (50.4 eV). The bearing of such lengthened interaction times on the corresponding low temperature relaxation times is shown in the ensuing calculation of p~ in the T range between 300 and 4000 K. In spite of the simplicity of the present model, quite good agreement is in fact found with experiments in the low-T region, while the lack of V , R / T coupling explains the departure of the computed p~, above 1000 K, from the experimental findings.Une adaptation numerique aux points d'une surface de potentiel HF-HF calcults ab initio est utilisie dans sa forme spherique la plus simple pour resoudre les equations de couplage fort correspondantes, qui fournissent des sections efficaces intlastiques du point de vue des vibrations pour le processus V / T dans la phase gazeuse. L'existance d'un puits de potentiel profond dans I'interaction est examinee du point de vue de son effet pour le comportement de la section efficace prks du seuil. Nos calculs dtmonstrent I'importance relative de plusieurs resonances de forme qui contribuent aux sections efficaces Ql,a ii des energies de collision basses (50.4eV). L'influence de ces temps d'interaction prolongis sur les temps de relaxation correspondants pour les basses temperatures est demontrte dans un calcul de pr pour T entre 300 et 400 K. Malgrt la simplicit6 du modde on trouve un accord assez bon avec les donntes experimentales pour les basses temperatures. D'autre part I'absence de couplage V , R / T explique la difference entre les valeurs de PT calcultes au-dessus de 1000 K et les valeurs exptkimentales.Eine numerische Anpassung zu ab-initio-berechneten Punkten auf einer HF-HF-Potentialenergieobedache wird in ihrer einfachsten spharischen Form benutzt, urn die entsprechenden Gleichungen fur starke Kopplung zu losen, die inelastische Schwingungsquerschnitte fur das V/ T-Prozess in der Gasphase liefern. Die Existenz eines tiefen Potentialtopfes in der Wechselwirkung wird mit Riicksicht auf seinen Effekt auf das Verhalten des Querschnitts in der Nahe der Schwelle untersucht. Unsere Berechnungen zeigen die relative Bedeutung von mehreren "Shape"-Resonanzen, die zu den Ql
“…The existence of several, narrow resonances has been confirmed as the main origin for the fast self-relaxation of gaseous HF, while the coupling with rotational degrees of freedom appears the main cause of slope inversion in the p7 behavior at high temperature, in keeping with several earlier suggestions [25,26,291. A further test of the theory in predicting the correct isotopic behavior of the present system has been carried out successfully and will be reported elsewhere [ 5 5 ] .…”
Section: Computational Results and Discussionsupporting
confidence: 68%
“…More recently, Billing and collaborators [26] have examined the temperature dependence of the H F vibrational relaxation by evaluating the rate constants of the process of PTvib.…”
AbstractsA numerical fit to computed ab initio points of the HF-HF potential energy surface is here employed in its simplest spherical form to solve the corresponding close-coupling equations that yield vibrationally inelastic collision cross sections for the V / T process in the gas phase. The existence of a deep potential well in the interaction is examined here for its effect on near-threshold cross section behavior and evidence is found, through the present calculations, for the relative importance of several (orbiting) shape resonances that contribute to the Ql,o cross sections at low collision energies (50.4 eV). The bearing of such lengthened interaction times on the corresponding low temperature relaxation times is shown in the ensuing calculation of p~ in the T range between 300 and 4000 K. In spite of the simplicity of the present model, quite good agreement is in fact found with experiments in the low-T region, while the lack of V , R / T coupling explains the departure of the computed p~, above 1000 K, from the experimental findings.Une adaptation numerique aux points d'une surface de potentiel HF-HF calcults ab initio est utilisie dans sa forme spherique la plus simple pour resoudre les equations de couplage fort correspondantes, qui fournissent des sections efficaces intlastiques du point de vue des vibrations pour le processus V / T dans la phase gazeuse. L'existance d'un puits de potentiel profond dans I'interaction est examinee du point de vue de son effet pour le comportement de la section efficace prks du seuil. Nos calculs dtmonstrent I'importance relative de plusieurs resonances de forme qui contribuent aux sections efficaces Ql,a ii des energies de collision basses (50.4eV). L'influence de ces temps d'interaction prolongis sur les temps de relaxation correspondants pour les basses temperatures est demontrte dans un calcul de pr pour T entre 300 et 400 K. Malgrt la simplicit6 du modde on trouve un accord assez bon avec les donntes experimentales pour les basses temperatures. D'autre part I'absence de couplage V , R / T explique la difference entre les valeurs de PT calcultes au-dessus de 1000 K et les valeurs exptkimentales.Eine numerische Anpassung zu ab-initio-berechneten Punkten auf einer HF-HF-Potentialenergieobedache wird in ihrer einfachsten spharischen Form benutzt, urn die entsprechenden Gleichungen fur starke Kopplung zu losen, die inelastische Schwingungsquerschnitte fur das V/ T-Prozess in der Gasphase liefern. Die Existenz eines tiefen Potentialtopfes in der Wechselwirkung wird mit Riicksicht auf seinen Effekt auf das Verhalten des Querschnitts in der Nahe der Schwelle untersucht. Unsere Berechnungen zeigen die relative Bedeutung von mehreren "Shape"-Resonanzen, die zu den Ql
“…The overall agreement with experiment in the high temperature range is quite satisfactory, especially when the large variation of experimental data is taken into consideration. Our quantum rate coefficient indicates that all previous theoretical results, which were obtained using a semiclassical trajectory method on a much less accurate PES 31,32 , underestimate at all temperatures. It is not clear what are the sources of the errors, but it is known that these theoretical results were obtained using less accurate PESs and approximate dynamical methods.Fig.…”
Section: Resultssupporting
confidence: 53%
“…The result is shown in Fig. 2, together with the available experimental 23–29 and previous theoretical 31,32 data. Our quantum mechanical rate coefficient initially decreases with temperature from 100 to 500 K, but increases slowly with temperature from 500 to 1200 K, with a minimum value of 2.1 × 10 −12 cm 3 s −1 mole −1 at 500 K. The calculated rate coefficient (3.03 × 10 −12 cm 3 s −1 mole −1 ) is in quite good agreement with the experimental result of 3.0 × 10 −12 cm 3 s −1 mole −1 at 200 K, reported by Hancock and Green 23 .…”
Inelastic collisions involving molecular species are key to energy transfer in gaseous environments. They are commonly governed by an energy gap law, which dictates that transitions are dominated by those between initial and final states with roughly the same ro-vibrational energy. Transitions involving rotational inelasticity are often further constrained by the rotational angular momentum. Here, we demonstrate using full-dimensional quantum scattering on an ab initio based global potential energy surface (PES) that HF–HF inelastic collisions do not obey the energy and angular momentum gap laws. Detailed analyses attribute the failure of gap laws to the exceedingly strong intermolecular interaction. On the other hand, vibrational state-resolved rate coefficients are in good agreement with existing experimental results, validating the accuracy of the PES. These new and surprising results are expected to extend our understanding of energy transfer and provide a quantitative basis for numerical simulations of hydrogen fluoride chemical lasers.
“…The V -T, R transfer probability for HF-HF, a system with strong hydrogen bonding, is about 1 ~o at room temperature and increases with decreasing temperature (cf. Poulson et al, 1978). For CO2-H, the large rates even at high energies have been ascribed to the presence of a chemically reactive channel, CO + OH -~ CO 2 + H, and to the formation of a stable intermediate radical, HCO 2 (Flynn and Weston, 1986).…”
We review here observations and models related to the chemical and thermal structures, airglow and auroral emissions and dynamics of the Venus thermosphere, and compare empirical models of the neutral densities based in large part on in situ measurements obtained by the Pioneer Venus spacecraft.Observations of the intensities of emissions are important as a diagnostic tool for understanding the chemical and physical processes taking place in the Venus thermosphere. Measurements, ground-based and from rockets, satellites, and spacecraft, and model predictions of atomic, molecular and ionic emissions, are presented and the most important sources are elucidated. Coronas of hot hydrogen and hot oxygen have been observed to surround the terrestrial planets. We discuss the observations of and production mechanisms for the extended exospheres and models for the escape of lighter species from the atmosphere. Over the last decade and a half, models have attempted to explain the unexpectedly cold temperatures in the Venus thermosphere; recently considerable progress has been made, although some controversies remain. We review the history of these models and discuss the heating and cooling mechanisms that are presently considered to be the most important in determining the thermal structure. Finally, we discuss major aspects of the circulation and dynamics of the thermosphere: the sub-solar to anti-solar circulation, superrotation, and turbulent processes.
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