Infrared spectra of isotopically mixed CO2 crystals

Infrared spectra are reported for 12C16O2/13C16O2, 12C16O2/12C18O2, and 12C18O2/13C16O2 mixed crystals throughout their complete concentration ranges. The changes with concentration in both the frequencies and the line shapes are in reasonable agreement with current theories of the effects of impurities on exciton states. An apparent exception is an anomalously rapid broadening with concentration in impurities whose transition energies are larger than that of the host. Effects due to interference between guest and host spectral bands, reflection, and possibly particle shape are observed. An analysis of the frequencies of dilute impurities indicates that the ν2 vibrational exciton band has a highly asymmetric density of states with a second moment of about 30 cm−2 and a total spread of about 22 cm−1.