IR-active matrix-isolated molecules (CO and CO2) to probe host crystal (N2) quality

The matrix isolation technique is a powerful technique for specific questions. But due to relatively high impurity concentrations (10(-2)-10(-4)) the impurity is not in a real matrix-isolated case, as in theoretical descriptions. In addition, due to sample preparation (cold deposition) the matrix resembles more an amorphous thin film rather than a good crystal, in terms of solid state physics. As a consequence, many spectroscopic data or phenomena were misinterpreted in the past. Therefore, we studied the real matrix-isolated case (similar to 10(-7)) in pretty good bulk matrix material. Looking at impurity spectra (CO, CO2, and their isotopes) and analyzing bandwidths, frequencies, and intensities as a function of temperature, we could characterize the quality of structural phase transitions of matrix and its hysteresis; we could unambiguously assign crystal field splitting to orientations of matrix-isolated particles in the host crystal; we could separate homogeneous from inhomogeneous bandwidth and discuss crystal quality of matrix; due to carefully determined integrated absorption intensities and known absorption coefficients, we were able to determine the real concentration of impurities in the matrix etc. Because of the improved FTIR technic (high sensitivity, resolution, accuracy) we studied in addition the overtone region of molecular excitations and investigated the kinetics of phase transitions; finally, we found spectroscopic evidence of impurity clusters and bands. Consequently, we are able to determine solubility limits of impurities in matrix (in our specific case this limit is about ppm).