New infrared spectra of the nitrous oxide trimer

Infrared spectra of $N2O$ trimers are studied using a tunable diode laser to probe a pulsed supersonic slit-jet expansion. A previous observation by Miller and Pedersen [J. Chem. Phys. 108, 436 (1998)] in the $N2O$ $ν1+ν3$ combination band region $(∼3480cm−1)$ showed the trimer structure to be noncyclic, with three inequivalent $N2O$ monomer units which could be thought of as an $N2O$ dimer (slipped antiparallel configuration) plus a third monomer unit lying above the dimer plane. The present observations cover the $N2O$ fundamental band regions $ν3$ $(∼1280cm−1)$ and $ν1$ $(∼2230cm−1)$⁠. In the $ν3$ region, two trimer bands are assigned with vibrational shifts and other characteristics similar to those in the $ν1+ν3$ region, but in the $ν1$ region all three possible trimer bands are observed. Relationships among the various bands are considered with reference to their rotational intensity patterns, their vibrational shifts, and the properties of the related $N2O$ dimer, with results that generally support the conclusions of Miller and Pedersen. Three trimer bands are also observed for the fully $N15$-substituted species in the $ν1$ region, and these results should aid in the detection of the as-yet-unobserved pure rotational microwave spectrum of the trimer.