Interaction between molecular impurities trapped in rare gas crystals. II. Interpretation of the <i>Q</i> branch in the near infrared spectrum of hydracids
Abstract
For non-negligibly low concentrations of hydracid dopant (20≤M/R≤500) in a rare gas matrix, the near infrared spectrum exhibits, besides the well-known rovibrational structure of the monomer, a weak band in the absorption gap around the normally forbidden pure vibrational frequency of the hydracid. The intensity and the profile of this band is concentration and temperature dependent. We give here an interpretation of its structure, based on the calculations performed in the previous paper (paper I). The interactions between dopant monomers located in a wide range of positions (between a√2 and a√10, a nearest-neighbor distance) are shown to be responsible for this absorption, which appears as a distribution of Q branches corresponding to the distribution of dopants inside the crystal. The shape of the sum band and its evolution with concentration and temperature are thus explained in terms of an inhomogeneous broadening process. More particularly, the three maxima observed at 2866.5, 2869.0, and 2870.6 cm−1 for HCl trapped in argon are, respectively, connected to the absorption of second and third nearest-neighbor dopants and to the superposition of all other pair absorption in a frequency range about 1.5 cm−1.