Vibrational Overtones and Rotational Structure of HCl in Rare Gas Matrices

Rotational/librational energy levels of diatomic molecules in an external (crystal) field of octahedral symmetry

Volker Berghof, Michael Martins, Burkhard Schmidt, and Nikolaus Schwentner

The rotational structure in the vibrational transitions from ν=0 to ν=1,2,3,4 of H35Cl and H37Cl is studied in Xe, Kr and Ar matrices with high spectral resolution. A consistent set of rotational constants Bv for the vibrational levels ν=0 to 4 is derived. B0 decreases with the tightness of the cage from 9.78 cm-1 in Xe to 8.83 cm-1 in Ar for H35Cl (gas phase 10.44 cm-1). The values for B0 to B4 decrease linearly with v due to the vibration-rotation-coupling constant α which increases from 0.37 cm-1 in Xe to 0.479 cm-1 in Ar (gas 0.303) according to the cage tightness. The splitting of the R(1) transition which originates from the hindering of rotation is analyzed in Xe using the T2g-T1u and T2g-Eg transition energies. A comparison with force field calculations yields a dominant contribution of the 6th spherical harmonic YA1g6 of the octahedral matrix potential. The modulation of the potential takes a value of K6/B=17 which corresponds to a barrier for the rotation of 160 cm-1. The splitting increases with the vibrational level v which can be interpreted as a weak admixture of the YA1g4 spherical harmonic. A large isotope effect and a reduction of the T1u-A1g transition energy (R(0)-transition) beyond the crystal field value are attributed to an excentric rotation with a displacement of the center of mass of the order of 0.05 Å. The vibrational energies ωe show an opposite trend with matrix atom size and decrease with polarizability from 2970 cm-1 in Ar to 2945.4 cm-1 in Xe (gas 2989.9 cm-1) while the anharmonicity ωeχe of the free molecule lies close to the Kr value and thus between that of Ar and Xe.

J. Chem. Phys. 116 (21), 9364-9373 (2002)