UNDERSTANDING THE INFRARED CONTINUUM SPECTRUM OF THE N2O DIMER AND OTHER VANDERWAALS 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 104-10 5 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. © 1979 American Institute of Physics.