Knowledge Bank

Rotational collisional narrowing, or line mixing, has been observed in both the $\nu 1+\nu 2$ and $\nu 2+\nu 3$ Q branches of $N2O$ near 1880 and $2800cm-1$ using a difference-frequency spectrometer and a tunable diode laser spectrometer. Spectra were recorded at pressures ranging from 1 to 740 torr, all self-broadened. The observed pressure-broadened widths of Q(1) through Q(33) in $\nu 1+\nu 2$ agree with those reported by Lacome $et.al.1$ for the P and R branches of the $2\nu 1,\nu 1+2\nu 2,2\nu 2$, and $\nu 1$ bands to within several percent. Line-mixing effects are most evident between Q(15) and Q(1) and above the Q-branch head at pressures of 200 torr and greater. Deviations between the observed absorption coefficients and those calculated using the normal isolated Lorentz line model are as large as 50% at 1 atm. A simple energy gap scaling law is used to model the off-diagonal relaxation matrix elements from the observed pressure-broadening coefficients. Spectra calculated from these matrix elements substantially reproduce the experimental data if it is assumed that 1/2 of all collisions place the molecule into doubled rotational states that are not accessed by the Q-branch transitions.