OBSERVATION OF COMBINATION BANDS INVOLVING INTERMOLECULAR VIBRATIONS OF N$_2$O-N$_2$, N$_2$O-OCS AND N$_2$O-CO$_2$ COMPLEXES USING AN EXTERNAL CAVITY QUANTUM CASCADE LASER

Abstract

Spectra of the weakly-bound N$_2$O-CO$_2$, N$_2$O-OCS, and N$_2$O-N$_2$ complexes in the region of the N$_2$O $\nu_1$ fundamental band ($\sim$2224~cm$^{-1}$) are observed in a pulsed supersonic slit jet expansion probed with a quantum cascade laser. One new band is observed for each complex: two combination bands involving the intermolecular in-plane bending for N$_2$O-CO$_2$ and N$_2$O-N$_2$ complexes, and the out-of-plane torsional vibration for N$_2$O-OCS. The resulting intermolecular frequencies are 34.17, \textbf{138}, 044302 (2013).} 17.11 and 22.33 cm$^{-1}$ for N$_2$O-CO$_2$, N$_2$O-OCS, and N$_2$O-N$_2$ complexes, respectively. The intermolecular vibrations provide clear spectroscopic data against which theory can be benchmarked. These results will be discussed, along with a brief introduction to our pulsed-jet supersonic apparatus which has been retrofitted by an infrared cw external-cavity quantum cascade laser (QCL) manufactured by Daylight Solutions. The QCL is used in the rapid-scan signal averaging mode. Although the repetition rate of the QCL is limited by its PZT scan rate, which is 100 Hz, we describe a simple technique to increase the effective repetition rate to 625 Hz. In addition, we have significantly reduced the long term frequency drift of the QCL by locking the laser frequency to the sides of a reference line.