Search

We investigated the magnitude and temperature dependence (T dependence) of the dimer absorption in the region of 0–600 cm-1 and the collision-induced absorption (CIA) in the region of 0–1150 cm-1. Together with our previous study of the self water-vapor continuum contributions resulting from far-wing line shapes of the allowed H2O lines in the infrared window between 800 and 1150 cm-1, we find that the three mechanisms have completely different T dependence behaviors. The dimer absorption has the strongest negative T dependence and the continuum absorption from far wings of the allowed lines has a moderately strong negative one. Meanwhile, the CIA exhibits a mild T dependence. In addition, their T dependence patterns are quite different. The T dependence of the far-wing theory varies significantly as the frequency of interest ω varies. For CIA, in general, its T dependence is mildly negative, but becomes slightly positive in the window region between the H2O bands. In contrast, the T dependence of the dimer absorption varies slightly as ω varies. In the microwave and submillimeter region, its T dependence becomes uniform. Concerning the relative importance for each of these three mechanisms, we find that in the infrared widow, the far-wing contributions are the dominant source of the self-continuum. Within the band, its contributions are definitely responsible for the measured continuum data. But, it is impossible to draw quantitatively conclusions on its relative importance unless one is able to improve the accuracy of the local line calculations significantly. On the other hand, within the pure rotational band, the dimer absorptions are a minor contributor to the self-continuum measurements, and its role becomes more important in the microwave and submillimeter regions. Finally, based on our study we conclude that contributions to the self-continuum from CIA in the frequency region of 0–1150 cm-1 are negligible. [ABSTRACT FROM AUTHOR]

It is well known that the water-vapor continuum plays an important role in the radiative balance in the Earth’s atmosphere. This was first discovered by Elsasser almost 70 years ago, and since that time there has been a large body of work, both experimental and theoretical, on this topic. It has been experimentally shown that for ambient atmospheric conditions, the continuum absorption scales quadratically with the H2O number density and has a strong, negative temperature dependence (T dependence). Over the years, there have been three different theoretical mechanisms postulated: Far wings of allowed transitions, water dimers, and collision-induced absorption. Despite the improvements in experimental data, at present there is no consensus on which mechanism is primarily responsible for the absorption. The first mechanism proposed was the accumulation of the far-wing absorption of the strong allowed transitions. Later, absorption by water dimers was proposed and this mechanism provides a qualitative explanation for the strong, negative T dependence. Recently, some atmospheric modelers have proposed that collision-induced absorption is one of the major contributors. However, based on improvements in the theoretical calculation of accurate far-wing line shapes, ab initio dimer calculations, and theoretical collision-induced absorptions, it is now generally accepted that the dominant mechanism for the absorption in the infrared (IR) windows is that due to the far wings. Whether this is true for other spectral regions is not presently established. Although all these three mechanisms have a negative T dependence, their T dependences will be characterized by individual features. To analyze the characteristics of the latter will enable one to assess their roles with more certainty. In this paper, we present a detailed study of the T dependence of the far-wing absorption mechanism. We will then compare our theoretical calculations with the most recent and accurate experimental data in the IR windows. The results of our calculations are found to agree very well with measurements in the 800–1200 cm-1 region. We conclude from this work that the T dependence in the IR window region predicted by the far-wing theory is negative and moderately strong. Its pattern is not simple and it could vary significantly as the frequency of interest varies. [ABSTRACT FROM AUTHOR]

Two polarizable six-dimensional water dimer intermolecular potential surfaces have been determined by fitting the distributed multipole ASP (anisotropic site potential) potential form to microwave, terahertz, and midinfrared cavity ringdown (D[sub 2]O)[sub 2] spectra via a rigorous calculation of the water dimer eigenstates with the PSSH (pseudo-spectral split Hamiltonian) method. The fitted potentials accurately reproduce most ground-state vibration-rotation-tunneling spectra and yield excellent second virial coefficients for both H[sub 2]O and D[sub 2]O. The calculated dimer structure and dipole moment are close to those determined from microwave spectroscopy and high level ab initio calculations, except that the O-O distance (2.952 Å) is significantly shorter than the currently accepted experimental value. The dimer binding energy (4.85 kcal/mol) is considerably smaller than the accepted experimental result, but in excellent agreement with recent theoretical results, as are the acceptor switching and donor-acceptor interchange tunneling barriers and the cyclic water trimer and tetramer structures and binding energies. [ABSTRACT FROM AUTHOR]

Part II. Reports on the fully coupled six-dimensional calculations of the water dimmer vibration-rotation-tunneling (VRT) states with split Wigner pseudospectral (SWPS) approach. Feasible tunneling motions of the water dimmer and how they are manifested in the microwave and VRT spectra; Overview on the SWPS method and improvements applied to water dimmer.

In a preceding paper [J. Chem. Phys. 92, 247 (1990)], we reported a preliminary three-dimensional quantum calculation of the dissociation of the ozone molecule in the Hartley band. We present here a fully converged calculation of the autocorrelation function on the excited 1B2 potential energy surface of Sheppard and Walker. The study has been reformulated in terms of hyperspherical coordinates, in order to make use of the permutation-inversion symmetry of the ozone molecule. The wave-function has been discretized on a three-dimensional hyperspherical grid. The autocorrelation function 〈[lowercase_phi_synonym]0|[lowercase_phi_synonym]t> has been computed within the time formulation, by means of the Lanczos algorithm. A calculation performed for the total angular momentum value J=0 shows a good agreement with the experimental results of Johnson and Kinsey, except for the overall intensity of the recurrence peaks. The calculated photodissociation spectrum reproduces the same oscillation pattern as observed experimentally. [ABSTRACT FROM AUTHOR]

We use our adiabatic pseudospectral method (APS) to calculate energy levels of the H2O2 molecule up to 5000 cm-1. Reasonably high accuracy (a few wave numbers) is achieved for a fully six dimensional calculation in a few hours of CPU time on an IBM 580 workstation. This contrasts with previous calculations on the same system which required 50–100 times more computational effort for a similar level of accuracy. The method presented here is both general and robust, and will allow routine studies of six dimensional potential surfaces and the associated spectroscopy, while making calculations on still larger systems feasible. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A time-dependent wave packet propagation method based on the split operator technique is used to describe the complete history of photodissociation. Three successive steps of the dynamics are discussed by setting the analogy with the energy dependent version of the quantum theory: namely the absorption, the Raman emission, and the fragments internal state analysis. Informations concerning early (absorption), intermediate (Raman emission), and long time dynamics (fragments internal distribution) are related to the parameters of the initial ground and final dissociative potential energy surfaces and to the excitation laser wavelength. The photodissociation of ICN(C 1A’) is taken as a numerical illustration and comparisons are presented with previous calculations carried in the energy frame. [ABSTRACT FROM AUTHOR]