Understanding the infrared continuum spectrum of the N2O dimer and other van der Waals complexes at low temperatures

Recent diode laser measurements of the N2O dimer 4.5 μm absorption band revealed a continuous band shape even though the experimental spectral resolution was 7×10−4 cm−1. The theory of rotational relaxation and dimer formation in supersonic molecular beams is used to establish a lower bound to the (N2O)2 rotational temperature of approximately TR?10 K for a source pressure of 17 atm. A band model approach is used to predict the temperature dependence of the band shape and line density for (N2O)2. The calculated band shape gives a good fit to the measured (N2O)2 spectrum for rotational and vibrational temperatures of 30 K. The results obtained show that the line density is of the order 104–105 lines/cm−1 for rotational and vibrational temperatures from 10 to 30 K. This range of line densities will yield a continuous band shape even if a spectral resolution, limited by beam divergence, of 2×10−5 cm−1 could be attained. It is shown that the smallest predissociative lifetime which could yield a discrete N2O dimer spectrum is approximately 10−7 s. A simple expression for quickly estimating average line densities is presented and applied to a sampling of van der Waals complexes.