The far-infrared absorption coefficient of HCl diluted in liquid Ar has been calculated by using a mixed classical-quantum stochastic simulation approach. The simulated spectra have been compared with the available experimental data at different thermodynamic conditions without using ad hoc fitting parameters. Despite the fact that some discrepancies can be observed in the high frequency side of the far-infrared bands, a reasonable agreement has been found between the theoretical and the experimental spectral profiles. Both, classical and quantum simulated line shapes were comparatively analyzed, determining the time scales involved in the rotational spectra. 1. Introduction Infrared (IR) spectroscopy has been a useful tool in chemistry and physics to study and characterize a large variety of molecular systems. Because it is a nondestructive method, it is useful to study the structure of complex systems such as biological molecules, proteins, DNA, and membranes [1, 2]. In the last decade, IR spectroscopy has been used in the study of healthy and nonhealthy human tissues [2], and as well it has been employed in the industry field for quality control. On the other hand, IR spectroscopy has been highly successful in measuring the degree of polymerization in polymer manufacture [1, 2] and also has a forensic purpose, being used in the analyze of substances, such as alcohol, drugs, fibers, blood, and paints [2, 3]. Far-infrared spectroscopy is closely related to the rotational dynamics of a wide range of molecular systems. In particular, the study of far-infrared absorption spectra of diatomic polar molecules diluted in inert solvents gives relevant information about intermolecular interactions and the dynamical properties of the solvent variables involved in the diatomic relaxation [4, 5]. The rotational diatomic relaxation in dense fluids determines the basic properties of the far-infrared absorption bands, which, depending on the diatomic molecule and the thermodynamic conditions of the solution, can present a fine rotational structure associated with the R-branch of the gas phase spectra. In that case, as it happens with the HCl immerse in dense Ar [6], a quantum model for the diatomic rotation is necessary in order to explain the spectral properties of the far-infrared bands. In a recent work [6], we have developed a mixed classical-quantum stochastic simulation (MCQSS) for the rotational and translational degrees of freedoms of both the solute HCl molecule and the Ar solvent atoms. In this study, the diatomic was treated as a quantum rotor which is
References
[1]
T. Theophanides, Ed., Infrared Spectroscopy—Material Science, Engineering and Technology, InTech, Vienna, Austria, 2012.
[2]
T. Theophanides, Ed., Infrared Spectroscopy—Life and Biomedical Sciences, InTech, Vienna, Austria, 2012.
[3]
J. Uddin, Ed., Macro to Nano Spectroscopy, InTech, Vienna, Austria, 2012.
[4]
J. R. Birch and J. Yarwood, Spectroscopy and Relaxation of Molecular Liquids, D. Steele and J. Yarwood, Eds., Elsevier, Amsterdam, The Netherlands, 1991.
[5]
M. O. Bulanin, N. D. Orlova, and G. Ya Zelikina, Molecular Cryospectroscopy. Advances in Spectroscopy, R. J. Clark and R. E. Hester, Eds., Wiley, Chichester, UK, 1995.
[6]
A. Padilla, J. Pérez, W. A. Herrebout, B. J. Van der Veken, and M. O. Bulanin, “A simulation study of the vibration-rotational spectra of HCl diluted in Ar: rotational dynamics and the origin of the Q-branch,” Journal of Molecular Structure, vol. 976, no. 1–3, pp. 42–48, 2010.
[7]
R. Zare, Angular Momentum, Wiley, New York, NY, USA, 1988.
[8]
S. L. Holmgren, M. Waldman, and W. Klemperer, “Internal dynamics of van der Waals complexes. I. Born-Oppenheimer separation of radial and angular motion,” Journal of Chemical Physics, vol. 67, no. 10, Article ID 4414, 9 pages, 1977.
[9]
S. L. Holmgren, M. Waldman, and W. Klemperer, “Internal dynamics of van der Waals complexes. II. Determination of a potential energy surface for ArHCl,” Journal of Chemical Physics, vol. 69, no. 4, Article ID 1661, 9 pages, 1978.
[10]
G. Guelachvili, P. Niay, and P. Bernage, “Infrared bands of HCl and DCl by Fourier transform spectroscopy. Dunham coefficients for HCl, DCl, and TCl,” Journal of Molecular Spectroscopy, vol. 85, no. 2, pp. 271–281, 1981.
[11]
P. H. Berens and K. R. Wilson, “Molecular dynamics and spectra. I. Diatomic rotation and vibration,” Journal of Chemical Physics, vol. 74, no. 9, Article ID 4872, 11 pages, 1981.
[12]
W. A. Herrebout, B. J. Van Der Veken, A. Medina, A. Calvo Hernández, and M. O. Bulanin, “Experimental and theoretical study of the far-infrared spectra of HCl dissolved in liquid Ar, Kr, and Xe,” Molecular Physics, vol. 96, no. 7, pp. 1115–1124, 1999.
[13]
J. M. Hutson, “Vibrational dependence of the anisotropic intermolecular potential of Ar-HCl,” The Journal of Physical Chemistry, vol. 96, no. 11, pp. 4237–4247, 1992.
[14]
M. O. Bulanin, K. Kerr, A. Padilla, J. Pérez, and A. C. Hernández, “First vibrational overtone bandshape of HCl in fluid SF6: an experimental and theoretical study,” Physical Chemistry Chemical Physics, vol. 2, no. 23, pp. 5375–5382, 2000.
[15]
L. Bonamy and P. N. M. Hoang, “Far infrared absorption of diatomic polar molecules in simple liquids and statistical properties of the interactions. I. Spectral theory,” The Journal of Chemical Physics, vol. 67, pp. 4423–4430, 1977.
[16]
K. G. Tokhadze and Z. Mielke, “About origin of Q component on the vibration-rotation band of HF in simple solvents,” The Journal of Chemical Physics, vol. 99, no. 7, pp. 5071–5077, 1993.
[17]
A. Medina, J. M. M. Roco, A. C. Hernández et al., “Vibration-rotation spectra of HCI in rare-gas liquid mixtures: molecular dynamics simulations of Q-branch absorption,” Journal of Chemical Physics, vol. 116, no. 12, pp. 5058–5065, 2002.
[18]
A. Medina, J. M. M. Roco, A. C. Hernández, and S. Velasco, “Infrared Q-branch absorption and rotationally-hindered species in liquids,” Journal of Chemical Physics, vol. 119, no. 10, pp. 5176–5184, 2003.
[19]
A. Medina, J. M. M. Roco, A. C. Hernández, and S. Velasco, “Infrared spectral profiles in liquids and atom-diatom interactions,” Journal of Chemical Physics, vol. 121, no. 13, pp. 6353–6360, 2004.
[20]
A. Medina, J. M. M. Roco, A. C. Hernández, and S. Velasco, “Dynamical characterization of rotationally hindered species in liquids,” Journal of Chemical Physics, vol. 123, no. 23, Article ID 234509, 7 pages, 2005.
