A. Padilla

Vibrorotational Raman and infrared spectra of polar diatomic molecules in inert solutions. I. Spectral theory

A unified non-Markovian theory for the vibrorotational Raman and infrared spectra of polar diatomic molecules diluted in nonpolar fluids is presented. From this theory, the physical basis of the spectra can be interpreted in terms of a few molecular properties of the isolated diatomic and of the time autocorrelation functions determining the collective effects of the solvent molecules on the vibrorotational dynamic of the diatomic. The spectrum is obtained as a diagonal part, constituted by an additive superposition of lines accounting for the integrated intensity, and an (exact) nondiagonal part accounting for the redistribution of intensity due to interbranch and intrabranch mixing effects. This theory generalizes previous theoretical frames based on a secular contribution modified by an (approximate) interference term. Also it allows the comparative analysis of the Raman and infrared spectra, and gives a clear and consistent interpretation of the theoretical lines building up the spectra.

Experimental analysis and modified rotor description of the infrared fundamental band of HCl in Ar, Kr, and Xe solutions.

We report an experimental study of the rotovibrational fundamental PQR-band shapes in the IR absorption spectra of HCl dissolved in condensed rare gases in a wide range of temperatures. The effective vibrational frequencies are determined from analysis of the fine rotational structure partially resolved in the band wings. The central Q-branch components appear redshifted with respect to the effective vibrational frequencies, their shifts in different solvents found to match the HCl stretching mode shifts in binary Rg...HCl van der Waals heterodimers. Theoretical quasi-free rotor and modified rotor models are applied to describe evolution of the band profiles at changing thermodynamic conditions. Both models are shown to reproduce equally well the observed spectral density distributions in the band wings. However, the modified rotor formalism that accounts for depopulation of the lower-energy rotational solute states provides better agreement with the experiment in the range of the P- and R-branch maxima. We surmise that the Q branches separated from the measured spectral profiles are formed by transitions between rotationally hindered states of diatomic molecules coupled to the solvent by the local anisotropy of the interaction potential.

Vibrorotational Raman and infrared spectra of polar diatomic molecules in inert solutions. II. Rotational spectroscopy of HCl in liquid SF6

The non-Markovian spectral theory presented in the preceding paper is applied to the calculation of the pure rotational spectroscopy of HCl in liquid SF6 and its comparison with available experimental far-infrared and anisotropic S-branch results. A quantitative and qualitative study of memory and interference effects on the overall infrared and anisotropic Raman spectra has been carried out. The relation of these effects to the time scales of the dephasing processes and the strong cancellation effects between interbranch and intrabranch mixing are especially stressed. The statistical parameters required for the theoretical calculation of the profiles have been obtained from a microscopic extended cell model for the liquid.

First vibrational overtone bandshape of HCl in fluid SF6 : An experimental and theoretical study

The first overtone band profiles are recorded in the IR spectra of the HCl molecular probes diluted in SF6, both below and above the critical point of the solvent. The spectral density distribution is considered as an additive superposition of the contributions due to the quasifree rotating and the rotationally-hindered probes. A recently proposed spectral theory is applied for description of the quasifree contribution. Good agreement is obtained between the experimental and calculated spectral profiles. The vibration–rotation coupling constant for the HCl molecules is found to decrease in high-density fluids. The central Q-branch components associated with the hindered solute states are separated from the measured spectra and their transformation is studied in a broad range of well-controlled thermodynamic conditions. We discuss the possible influence of the interaction-induced dipole transitions on the absorption in the vicinity of the band center.

Vibrorotational Raman and infrared spectra of polar diatomic molecules in inert solutions. III. Isotropic and anisotropic Raman spectra of HCl in liquid SF6

The isotropic and anisotropic Raman spectra of HCl diluted in liquid SF6 have been analyzed and compared with available experimental data. The theoretical profiles have been calculated with a non-Markovian spectral theory in which the statistical parameters of the isotropic and the anisotropic interaction were obtained from a microscopic extended cell model for the liquid. Memory and interference effects are qualitatively and quantitatively analyzed in terms of the time scales involved in the Raman spectra. A detailed study of intrabranch, interbranch, and cancellation mixing effects is carried out together with the analysis of the contribution of the different relaxation channels.

Non-Markovian near-infrared Q branch of HCl diluted in liquid Ar

By using a non-Markovian spectral theory based in the Kubo cumulant expansion technique, we have qualitatively studied the infrared Q branch observed in the fundamental absorption band of HCl diluted in liquid Ar. The statistical parameters of the anisotropic interaction present in this spectral theory were calculated by means of molecular dynamics techniques, and found that the values of the anisotropic correlation times are significantly greater (by a factor of two) than those previously obtained by fitting procedures or microscopic cell models. This fact is decisive for the observation in the theoretical spectral band of a central Q resonance which is absent in the abundant previous researches carried out with the usual theories based in Kubo cumulant expansion techniques. Although the theory used in this work only allows a qualitative study of the Q branch, we can employ it to study the unknown characteristics of the Q resonance which are difficult to obtain with the quantum simulation techniques recently developed. For example, in this study we have found that the Q branch is basically a non-Markovian (or memory) effect produced by the spectral line interferences, where the PR interferential profile basically determines the Q branch spectral shape. Furthermore, we have found that the Q resonance is principally generated by the first rotational states of the first two vibrational levels, those more affected by the action of the dissolvent.

Infrared fundamental bandshape of HCl in fluid SF6: A modified-rotor description

The fundamental bandshape of HCl diluted in fluid SF6 has been studied in the temperature range 283–340 K at two constant densities of the solvent. It is found that the previously developed quasi-free rotor theory realistically reproduces the experimental band area-normalized intensity distribution in the band wings, but overestimates intensity near the maxima of the P- and R-branches. A concept of a potential barrier hindering rotation of the solute molecules is introduced and a new modified-rotor version of the spectral theory is formulated, which takes into account depopulation of the lower-energy rotational states. We show that the modified-rotor theory significantly improves agreement with the experiment. The rotationally-hindered contribution to the bandshape is separated from the measured spectral profiles and its temperature and density dependence is discussed.

The fundamental vibrational shift of HCl diluted in dense Ar and Kr

The fundamental infrared band vibrational shifts of HCl diluted in dense Ar and Kr have been calculated by means of molecular dynamics simulation techniques and compared with the experimental data reported by Pérez et al. (J. Chem. Phys. 122, 194507 (2005)). The results have been analysed along the liquid–vapour coexistence line as in several supercritical states, with good agreement between the theoretical and experimental values.

A study of molecular rotation in dense fluids from the first vibrational overtone band shape of HCl in Xe fluid

Spectral profiles of the (2 ← 0) vibration-rotation band of HCl dissolved in fluid Xe are recorded in the 267–333 K temperature range at a constant density of 345 Amagat and compared with predictions of an earlier developed modified rotor theoretical model [Bulanin et al. PCCP, 2003, 5, 285]. An improved agreement with the measured distributions is demonstrated in the range of the P- and R-branches. The results obtained are compared with previous analyses of the (1,2 ← 0) bands in HCl–SF6 and (1 ← 0) band in HCl–Xe systems. The observed behaviour of the separated central Q-branch is consistent with the presence in the fluid of long-lived solute–solvent spatial correlations caused by the anisotropy of the binary interaction potential, which in a low-density limit leads to formation of van der Waals complexes.

Isotropic Raman scattering of HF in dense gas and liquid SF6: Study of the motional narrowing effect

The isotropic Raman spectra of HF diluted in dense gas and liquid SF6 have been calculated and compared with existing experimental data. The motional narrowing that arises in the transition from dense gas to liquid phase has been studied globally and line-by-line. The influence of mixing and memory effects and the role played by the different terms of the solute–solvent interaction are especially considered.