OBSERVATION OF TWO COMBINATION BANDS INVOLVING TORSION AND ASYMMETRIC BENDING MODES OF THE NON-POLAR N$_2$O DIMER

Abstract

The non-polar N${2}$O dimer has four low frequency intermolecular vibrational modes: van der Waals stretch (A${g}$ symmetry), symmetric and asymmetric in-plane bends (A${g}$ and B${u}$ symmetry, respectively), and out-of-plane torsion (A${u}$ symmetry). There are two possible intramolecular dimer vibrations which correlate with the $\nu {1}$ monomer stretch. One of these is an in-phase vibration of the two monomers, which has A${g}$ symmetry and is not infrared active. The other is an out-of-phase vibration, which has B${u}$ symmetry and gives rise to the infrared active dimer band at 2229.48 cm$^{-1}$. In the present work, we report the observation of an N${2}$O dimer band with c-type rotational structure, assigned as a combination of the A${g}$ intramolecular N${2}$O $% \nu {1}$ stretching vibration and the A${u}$ intermolecular out-of-plane dimer torsional vibration. The vibrational origin for this band is measured to be 2249.360 cm$^{-1}$, giving an estimated torsional frequency of 21.5 cm$% ^{-1}$. A second combination band with a- and b-type rotational transitions has also been measured and assigned as the combination of the same A${g}$ intramolecular vibration and the B$_{u}$ asymmetric van der Waals bend. The measured band center is 2264.37 cm$^{-1}$, giving an asymmetric bending frequency of about 36.5 cm$^{-1}$. A search for the other two combination bands is currently underway.