Q-branch line mixing in N2O: Effects of l-type doubling

Abstract

Rotational collisional narrowing, or line mixing, has been observed in the Q branch of the ν2+ν3Π–Σ band of N2O near 2798 cm−1 using a difference-frequency spectrometer. Self-broadened spectra were recorded at pressures ranging from 20 to 747 Torr. The broadening coefficients, derived from the lower pressures before the lines significantly overlap, are in close agreement with prior measurements of P- and R-branch widths in Σ–Σ bands. At higher pressures where the Q-branch lines are blended, the band contours deviate from purely additive component line shapes, exhibiting stronger peak absorptions and weaker wings characteristic of line mixing. A simple rotational energy gap scaling law is used to model the off-diagonal relaxation matrix elements needed to calculate the observed collisional narrowing. Spectra calculated using several trial sets of collisional selection rules are presented. We find evidence for a parity conserving collisional selection rule and/or elastic-reorientation collisions.