The rapid progress in research on chemical lasers has led to increased interest in the development of theoretical models of molecular relaxation processes. In this review, recent applications of various theories of nonreactive collisions between diatomic molecules, from semiclassical to quantal, are surveyed in light of experimental data. The intention is to provide the theoretician with a back-ground of those features of the collision problems that require more accurate treatments, and to provide the experimentalist with a spectrum of models available for handling his data.

Vibrational relaxation theories and measurements

https://iopscience.iop.org/article/10.1088/0963-0252/25/4/043002/pdf

Erratum: Kinetics of plasma-assisted combustion: effect of non-equilibrium excitation on the ignition and oxidation of combustible mixtures

Using ab initio Hartree–Fock SCF, configuration interaction, and approximate coupled pair techniques, an extensive quantum chemical study of the infrared absorption intensities of the water molecule is reported. In an attempt to establish the necessary requirements for the quantitative prediction of infrared intensities four major effects were examined: (a) the magnitude of electron correlation effects, (b) the applicability of the Hellmann–Feynman theorem to the calculation of dipole moments functions, (c) basis set requirements, and (d) the accuracy of the customary harmonic approximations. The calculated infrared intensities for the water molecule and several of its isotopic derivatives are in good agreement with the available experimental data. In addition to the fundamentals the intensities of several overtone and combination bands were also calculated resulting in generally very small values that strongly depend on the potential energy surface used in the generation of vibrational wave functions.

The infrared absorption intensities of the water molecule: A quantum chemical study

H2O–HCl is studied using a number of basis sets including 6‐31G** and variants which are augmented by a diffuse sp shell and a second set of d functions on O and Cl. Optimization of the geometry of the complex is carried out including explicitly electron correlation and counterpoise correction of the basis set superposition error (BSSE) at both the SCF and correlated levels. Correlation strengthens and shortens the H bond while BSSE correction leads to an opposite trend; these two effects are of different magnitude and hence cancel one another only partially. ΔH°(298 K) is calculated to be −4.0 kcal/mol, 1/4 of which is due to correlation. Formation of the complex causes the strong intensification and red shift of the H–Cl stretching band normally associated with H bonding, whereas the internal vibrations of H2O are very little affected, except for a doubling of the intensity of the symmetric stretch. With respect to the intermolecular modes, the bends of the proton donor are of higher frequency than thos...

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Structure, energetics, and vibrational spectrum of H2O–HCl

We have studied the effects of oxygen atoms on the vibrational relaxation of shock‐heated N2 over the temperature range from 1200 to 3000 °K, using the CO tracer technique The measured relaxation times pτN2–O vary from 08 to 2 atm · μsec in this range with an uncertainty of ± 50%, in excellent agreement with existing high‐temperature shock tube and low‐temperature flow tube results The weak temperature dependence of all these results is in marked contrast to theoretical predictions Some limited measurements of the relaxation of CO by oxygen atoms yield values of pτCO–O of approximately 004 atm · μsec, in good agreement with other recent measurements This rate is approximately 25 times as fast as the N2–O rate and, coincidentally, is approximately the same as that for the relaxation of O2 by O atoms The greater rapidity of the CO–O and O2–O relaxation, compared with that of N2–O, may be the result of atom‐exchange processes in the former cases

Vibrational relaxation of shock‐heated N2 by atomic oxygen using the ir tracer method

Shock tube measurements of the rates of vibrational relaxation of N2 by H2O and the vibrational exchange between N2 and CO are described. Simultaneous measurements of H2O v2 ν2 band and CO fundamental band ir emission were performed in mixtures of 1%–10% CO+∼1% H2O+N2. Measurement of H2O emission by a calibrated optical system allowed an accurate determination of the concentration of H2O for each run. The CO emission had two, almost decoupled, regions of behavior which allowed determination of two relaxation times: Pτ NCe≈ 1.6 atm· μsec (V–V exchange relaxation time between N2 and CO) and Pτ NH≈ 1.5 atm· μsec (T–V relaxation of N2 by H2O), both for 960 〈T〈2200°K.

Shock Tube Vibrational Relaxation Measurements: N2 Relaxation by H2O and the CO–N2 V–V Rate

Water vapor emission between 4.7 and 10 μ from 1300 to 3600°K has been measured using a shock tube to heat the gas and an optical system calibrated by a black body to measure the radiation. The optically thin data are interpreted in terms of v2-band emission with some contribution, at the higher temperatures, from the pure rotational band. Calculable concentrations of H2O were made behind incident shocks from the reactions of H2 and O2 highly diluted in argon. There is excellent agreement between data taken at the equilibrium and “partial equilibrium” states, thereby providing additional justification for the existence of the partial equilibrium state and Schott's method for calculating it. An integrated band intensity of 338 (+17 per cent - 7 per cent) ama-1 cm-2 at 2000°K is deduced, in good agreement with earlier workers using different techniques. Above 2500°K the measured radiation is less than we calculate for our bandpass, assuming contributions from both the v2 and the rotational bands of H2O; possible reasons for this discrepancy are discussed.

K. N. C. Bray

Papers

Shock Tube Vibrational Relaxation Measurements: N2 Relaxation by H2O and the CO–N2 V–V Rate

Absolute H2Ov2-band intensity obtained from reacting H2 + O2 mixtures behind shock waves