Sulfur dioxide (SO2) is a molecule of considerable interest for both atmospheric chemistry and astrophysics. In the Earth's atmosphere, it enters in the sulfur cycle and it is ubiquitous present in polluted atmospheres, where it is responsible for acid rains. It is also of astrophysical and planetological importance, being present on Venus and in interstellar clouds. In this work the collisional broadening of a number of ν1 ro-vibrational lines of SO2 perturbed by N2, O2 and He are investigated at room temperature in the 9 μm atmospheric region by means of high resolution tunable diode laser (TDL) infrared spectroscopy. From N2- and O2-broadening coefficients, the broadening parameters of sulfur dioxide in air, useful for atmospheric applications, are derived as well. From the present measurements some conclusions on the quantum number dependence of the N2-, O2- and He-broadening coefficients are drawn. While the J dependence is weak for all the perturbers investigated, different trends with Ka are reported. N2-broadening coefficients show a slight decrease with increasing values of Ka, whereas O2 and He broadening cross sections first increase up to Ka(″)≈6 and then they keep a nearly constant value. A comparison and a brief discussion on the efficiency of self-, N2-, O2- and He-collisional dynamics are given. The data obtained represent a significant analysis on foreign broadening of SO2 useful for atmospheric remote sensing and astrophysical applications.

Sulfur dioxide (SO2) is a molecule of proved atmospheric relevance, the main sources being anthropogenic, which is one of the main causes of acid rains. Besides, it is also of interest in astrophysics, as it is present in the atmosphere of Venus and in star forming regions. For these reasons SO2 is one of the target molecules in all of the most important spectroscopic databases which collect the spectroscopic line-by-line parameters for atmospheric remote sensing, astrophysics soundings, and climate changing investigations. Although over the years the spectroscopic properties of this molecule have been widely studied, and line-by-line listings of line positions and intensities have been compiled, at present an analogous systematic and complete database of broadening coefficients is still lacking. The aim of this work is to fill in this vacancy, starting from self-broadening coefficients, by coupling experimental measurements to theoretical calculations. The laboratory experiments are carried out for 12 pure rotational transitions of the vibrational ground state (and 2 of vibrational excited states) and for 25 ro-vibrational lines of the ν1 band, lying in the 9µm atmospheric window. Theoretical calculations of broadening coefficients are performed employing a semiclassical formalism based on the ATC (Anderson–Tsao–Curnutte) approximation. From the interplay between theory and experiment the vibrational and quantum number dependence of the collisional cross-sections is first assessed and studied and then a complete database of self-broadening coefficients for 1635 transitions in a wide quantum number range (0≤K″a≤16, 2≤J″≤68) is compiled, presented and made available.

The importance of rotational transition rates in the analysis of cold interstellar clouds is well known. We present results, for temperatures ranging from 5 to 80 K, for the hyperfine- resolved rotational transitions of DCO+ induced by collision with helium. Since the isotopic substitution is not expected to introduce significant changes, close-coupling calculations are based on a potential energy surface obtained for He-HCO+ and checked by accurate pressure broadening and shift measurements. The well-grounded assumption that deuterium nuclear spin is not affected by the collisions allowed us to obtain the hyperfine- resolved transition matrix elements as a sum of spin-free transition matrix elements, which account for collision dynamics, multiplied by purely geometrical factors, which account for the hyperfine dependence. The temperature dependence of the rates is weak for downward transitions j -> j ', j ' < j, while for upward transitions (j ' > j) it can be large due to the need of energy to be transferred from translation to rotation. The dependence of the rates on j and j ' and hyperfine propensities is discussed. The rates for quasi-elastic purely hyperfine transitions j, F -> jF ' are also obtained.

Ground-state rotational spectra of HCO+, N2H+, and CF+ (both 12C- and 13C-containing isotopologues) have been recorded in the 1.0–1.6 THz frequency range. Present measurements allowed us to improve the spectroscopic parameters, which in turn enabled the prediction of rotational transitions up to 2.0–2.5 THz with good accuracy. We therefore consider the present results to be of great value in view of the extended spectral coverage made available by the Herschel Space Observatory, Stratospheric Observatory for Infrared Astronomy, and the Atacama Large Millimeter Array. Furthermore, we re-investigated the ground-state rotational spectrum of N2H+ in the 93–750 GHz frequency range, thus addressing the open issue of the frequency of the J = 1 ← 0 transition as well as resolving the hyperfine structure of the J + 1 ← J transitions with J = 1, 2, and 3 for the first time.

A deep and comprehensive investigation of the vinyl fluoride (CH(2)CHF) spectrum in the atmospheric window around 8.7 μm is presented. At first, the ro-vibrational patterns are modelled to an effective Hamiltonian, which also takes into account the coupling of the C-F stretching vibration, ν(7), with the neighbouring vibrational combination ν(9)+ν(12). The obtained Hamiltonian gives very accurate simulations and predictions of the ro-vibrational quantum energies. Then, in the main part of the work, an experimental and theoretical study of vinyl fluoride self-broadening collisions is carried out for the first time. The broadening coefficients obtained experimentally are compared with those calculated by a semiclassical theory, demonstrating a significant contribution of collisional coupling effects between lines connecting pairs of degenerate (or nearly degenerate) rotational levels. Finally, the experimentally retrieved integrated absorption coefficients are used to calculate the absorption cross-section of the ν(7) normal mode, from which dipole transition moments are derived. The obtained results provide a deep insight into the spectral behaviour of vinyl fluoride, in a spectral region of primary relevance for atmospheric and environmental determinations. Indeed, the data presented constitute an accurate model for the remote sensing of vinyl fluoride--a molecule of proved industrial importance which can lead to hazardous effects in the atmosphere and affects human's health.

We discuss the hyperfine effect on the shape of rotational spectral lines of DCO(+) broadened by collisions with helium. Hyperfine scattering matrix is calculated by the recoupling technique from the spin-free scattering matrix which is obtained by close-coupling calculations and by a previously tested potential. Line shape is calculated for different rotational transitions, perturber density values, and collisional energies. As forecast by a semiclassical treatment and contrary to what may happen for a symmetric top absorber, hyperfine effects are small for a linear absorber. In our case they are of about 2%. We could also verify that the two hyperfine effects on the line shape, modification of resolved components and collisional coupling between them, cancel each other at high values of helium density when hyperfine structure collapses into a single line.

We show that the treatment available in the literature for calculating hyperfine effects on collisional line shape is affected by an error making the theory not self-consistent. By correcting such an error we show that the two hyperfine effects on the line shape, modification of the resolved components and collisional coupling between them, cancel each other when hyperfine structure collapses into a single line.

Sulfur dioxide is still the subject of numerous spectroscopic studies since it plays an active role in the chemistry of Earth's atmosphere and it is a molecule of proven astrophysical importance. In the present work we have determined the self-broadening and integrated absorption coefficients for several lines in the nu(1) band spectral region around 9.2 mum. Besides the parameters of the lines belonging to the nu(1) fundamental of (32)SO(2), also those for some rovibrational lines of the nu(1)+nu(2)-nu(2) hot band of the (32)SO(2) isotopologue and the nu(1) band of the (34)SO(2) isotopic species have been determined. The measurements have been carried out at 297 K using a tunable diode laser spectrometer. The self-broadening parameters have also been theoretically determined employing a semiclassical formalism based on the Anderson-Tsao-Curnutte approximation. The study has been completed with the determination of the vibrational cross sections of the three fundamental bands measured from the spectra recorded at a resolution of 0.2 cm(-1) using a Fourier transform infrared spectrometer.

The rates of rotational transitions for HCO+, the most abundant ion in interstellar space, induced by collision with helium are obtained for temperatures ranging from 10 to 80 K. The calculations are based on a new potential energy surface for the He-HCO+ interaction and on a scattering matrix whose accuracy was checked by pressure broadening and shift measurements. The rates q(jj') decrease for increasing values of j and Delta j, with a temperature trend depending on the energy involved in the transitions: if it is small, the rates are almost constant, while an increase with T is found for other cases. Comparison with previous and less accurate results shows an agreement within 50 per cent. Comparison between state-to-state and pressure broadening cross-sections allows us to discuss importance and influence of elastic and inelastic collisions.

Pressure broadening and pressure shift of N(2)H(+) rotational lines perturbed by collisions with He are studied for the first time using experiment and theory. Results are reported from measurements at 88 K for the rotational transitions j = 3<--2, 4<--3, 5<--4 and 6<--5 with frequencies ranging from 0.28 to 0.56 THz. The agreement between experiment and theoretical data derived from close coupling calculations confirms the reliability of a theoretical framework used for state-to-state transition rates of interest in the interpretation of spectroscopic data from interstellar molecular clouds. The influence of hyperfine effects on shifts and widths of the rotational lines is discussed in detail. Although in principle possible, experiment and theoretical considerations lead to the conclusion that hyperfine effects only play a minor role.

The self-, nitrogen- and oxygen-broadening parameters of pure rotational water lines in the 1.0–1.2THz frequency range have been measured by means of a frequency modulated spectrometer. The J=31,2←30.3 (1.097THz), 31,2←22,1 (1.153THz), 63,4←54,1 (1.158THz), and 32,1←31,2 (1.163THz) transitions have been considered. The experimental determination has been supplemented by theoretical calculations within the semiclassical approximation.

For eight HCO+ rotational transitions we compare semiclassical and quantum calculations of the line broadening and shift induced by collisions with argon and helium atoms. A detailed analysis of the results allows better insights into the problem of the accuracy of the semiclassical model commonly used in most line shape studies.

It is well established that water plays a fundamental role in various atmospheric phenomena and that the accuracy of its collisional broadening parameters has a crucial influence on reduction of remote sensing data. Nevertheless, in this field the experimental data are still scarce and consequently the estimates reported in spectroscopic databases are not always reliable and/or accurate. In the view of filling this gap, the self-, N2- and O2-broadening parameters of the J=11,1←00,0 rotational transition of water (1.113THz) have been determined at room temperature. The experimental investigation has also been supported by theoretical calculations.

For atmospheric purposes, the self- and N-2-broadening parameters of the J = 6(1,6) <- 5(2,3) (22.2 GHz) rotational transition of water has been investigated in the temperature range 296-338 K. This investigation should be considered of particular interest in monitoring the Eaxth's atmosphere because water is a fundamental component and it is well established that the accuracy of collisional broadening parameters has a crucial influence on reduction of remote sensing data. Therefore, a particular effort has been made in order to reduce instrumental as well as systematic errors. Experimental determinations have also been supported by theoretical calculations.

Since water is a fundamental component of the atmosphere and it is well established that the accuracy of collisional broadening parameters has a crucial influence on reduction of remote sensing data, we decided to investigate the self-, N2- and O2-broadening parameters of the J=61,6←52,3 (22.2GHz) rotational transition of water in the temperature range 296–338K. Due to the relevance of this water line, this investigation should be considered of particular interest in monitoring the Earth's atmosphere, and therefore a particular effort has been made in order to reduce instrumental as well as systematic errors. Experimental determinations have also been supported by theoretical calculations.

An experimental and theoretical study of the pressure broadening and the pressure shift of three HCO(+) rotational lines (j=4<--3, 5<--4 and 6<--5) perturbed by collisions with Ar is presented. The measurements are carried out at 77 K and are compared to close-coupling calculations performed on an accurate potential energy surface for the Ar-HCO(+) interaction extending from small to very large separations between the ion and the perturber. For the pressure broadening, agreement between experiment and theory is satisfactory for both close-coupling and semiclassical calculations. For the pressure shift, however, close-coupling calculations are superior. The results agree with experiment in sign and order of magnitude, while semiclassical calculations are inaccurate for the shift of the presently studied lines because they neglect the contribution of strong collisions.

Collisional transitions between molecular lines give rise to relevant modifications of the spectral distribution of absorption. We discuss here the consistency of such a coupling effect with the conservation of resonance frequency for the scattered lines, a requirement that, in the frame of the usually adopted impact approximation models, should follow from the conservation of energy in the collisions. This problem, leads either to the need for a more appropriate theoretical treatment of collisional coupling or to the existence of a decoupling between well resolved lines which could heavily affect the line shape in the far wings. A check allowing discrimination between these two events could be obtained by measurement of the microwave far wing absorption of deuterated ammonia.

N2, Air, O2, H2, Ar, and He broadening and shift of the aQ(9,9) transition line of ammonia have been measured in the temperature range 180–380 K with errors of ∼2 and 10%, respectively. All the measurements have been fitted with the well-known power law, which holds very well in the temperature range considered here. N2 pressure effects have also been compared with calculations based on the Anderson–Tsao–Curnutte (ATC) theory with mixed results, anyway much more satisfactory for broadenings than shifts.

Different methods of calculations are compared to experimental results for the collisional lineshape effects on the rotational transitions of molecular ions surrounded by a neutral buffer gas. Capture method and Anderson Tsao Curnutte (ATC) method with straight line trajectories are both not able to reproduce the measurements. A discussion of the origin of such a discrepancy allows to obtain better insights into the different mechanism of collisional relaxation. Quite good agreement is obtained both by the hybrid method proposed by Liao and Herbst or by ATC calculations using a more realistic translational dynamics.