A.A. Vigasin

Water vapor continuous absorption in various mixtures: possible role of weakly bound complexes

Abstract The model is suggested to describe the temperature dependence of mm-wave and infrared continua absorption in terms of absorption by weakly bound binary complexes. Present approach is applicable to a mixture of water vapor with various gases. It is shown that accurate consideration of mass action law for (H2O)2 dimers and H2O–N2, H2O–CO2, and H2O–Ar complexes allows to reproduce temperature dependencies of continua absorption measured at different wavelengths in mixtures containing various amount of water vapor. The model can be generalized to rigorously account for details of potential energy and dipole surfaces relevant to water–water and water–foreign gas interactions.

Direct absorption spectroscopy of water clusters formed in a continuous slit nozzle expansion

In this article, we report on a Fourier transform infrared study of absorption bands belonging to small-sized water clusters formed in a continuous slit nozzle expansion of water vapor seeded in argon carrier gas. Clear signatures of free and H-bonded OH vibrations in water aggregates from dimer to pentamer are seen in our spectra. Following an increase in argon backing pressure, the position of the cluster absorption bands varies from those characteristics of isolated water aggregates in the gas phase to those known for clusters trapped in a static argon matrix. These variations can be interpreted in terms of sequential solvation of the water clusters by an increasing number of argon atoms attached to water clusters. Our measured spectra are in good agreement with those obtained previously either for free or Ar coated small-sized water clusters using pulsed slit-jet expansions. Our results are equally in accord with those originating from a variety of tunable laser based techniques using molecular beams or free jets or from the study of water aggregates embedded in rare gas matrices. Distinctions are reported, however, and discussed. Ab initio calculations have made it possible to speculate on the average size of an argon solvation shell around individual clusters as well as on the development of the OH stretch vibrational shifts in mixed (H(2)O)(m)Ar(n) clusters having different compositions and architectures.

Vibrational frequency shifts caused by weak intermolecular interactions

Abstract Vibrational frequency shifts of HF monomers and hydrogen-bonded aggregates of HF and H 2 O are analyzed in terms of their correlation with the square root of the critical temperature of the matrix material. This empirical correlation, justified by simple model considerations, may prove useful in establishing proper assignments of vibrational bands in the spectra of van der Waals and hydrogen-bonded species trapped in cryogenic matrices, in host clusters, and even in the gas phase.

VIBRATIONAL FREQUENCY SHIFTS AND THERMODYNAMIC STABILITIES OF (HF)N ISOMERS (N=4-8)

Abstract Based on the spectroscopic data available for the HF dimer and trimer and taking into account the cooperativity effect, the force fields of larger (HF)n clusters (n=4–8) have beeen determined for various isomeric structures and used to calculate the frequency shifts for the stretching vibrations. The comparison with recent experimental results, obtained with size selection, suggests that tailed ring structures (i.e. symmetric rings with additional HF units attached to them) might play an important role in the cluster distribution in expansion-cooled molecular beams. Thermodynamic calculations show that tailed ring structures have comparably high stabilities.

On the possibility to quantify contributions from true bound and metastable pairs to infrared absorption in pressurised water vapour

The present paper aims to evaluate the proportions of true bound and metastable water dimers from an analysis of laboratory observations of the integrated absorption in the OH stretching band of pressurised water vapour. Knowledge of the statistical weights of true bound and metastable dimers is of crucial importance for understanding the nature of bimolecular absorption by water vapour. It is particularly relevant for understanding the nature of the so-called water vapour continuum absorption in the atmosphere, the cause of which has been debated over many decades. Our consideration is essentially based on the recent laboratory measurements of water dimer spectroscopy data as well as independent measurements of the density variations in absorptivity of pressurised steam.

ON THE NATURE OF COLLISION-INDUCED ABSORPTION IN GASEOUS HOMONUCLEAR DIATOMICS

Abstract The phenomenon of collision-induced absorption in equilibrium gases is considered by accounting for the statistical-physical partitioning of bound, metastable and free bimolecular states. The importance of molecular anisotropy and dimers formation is emphasized. A line-mixing model is suggested to describe the overall collision-induced bandshapes of nitrogen and oxygen fundamentals, which effectively accounts for the perturbation of monomers rotational movement.

Contribution of bound, metastable and free states of bimolecular complexes to collision-induced intensity of absorption

Abstract The collision-induced absorption intensity is subdivided into bound, metastable and free contributions. The averaging over the phase space domains is performed making use of the whole potential surface for two interacting linear molecules. A new efficient algorithm for multidimensional integration is formulated by generalizing the Harris—Engerholm—Gwinn quadrature method.

H2O−N2 collision-induced absorption band intensity in the region of the N2 fundamental: ab initio investigation of its temperature dependence and comparison with laboratory data

The present paper aims at ab initio and laboratory evaluation of the N2 collision-induced absorption band intensity arising from interactions between N2 and H2O molecules at wavelengths of around 4 μm. Quantum chemical calculations were performed in the space of five intermolecular coordinates and varying N−N bond length using Møller–Plesset perturbation and CCSD(T) methods with extrapolation of the electronic energy to the complete basis set. This made it possible to construct the intermolecular potential energy surface and to define the surface of the N−N dipole derivative with respect to internal coordinate. The intensity of the nitrogen fundamental was then calculated as a function of temperature using classical integration. Experimental spectra were recorded with a BOMEM DA3-002 FTIR spectrometer and 2 m base-length multipass White cell. Measurements were conducted at temperatures of 326, 339, 352 and 363 K. The retrieved water–nitrogen continuum significantly deviates from the MT_CKD model because the relatively strong nitrogen absorption induced by H2O was not included in this model. Substantial uncertainties in the measurements of the H2O−N2 continuum meant that quantification of any temperature dependence was not possible. The comparison of the integrated N2 fundamental band intensity with our theoretical estimates shows reasonably good agreement. Theory indicates that the intensity as a function of temperature has a minimum at approximately 500 K.

Thermally averaged effective dissociation energy of dimers

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