Weakly-bound clusters of atmospheric molecules: infrared spectra and structural calculations of (CO 2 ) n -(CO) m -(N 2 ) p , ( n , m , p ) = (2,1,0), (2,0,1), (1,2,0), (1,0,2), (1,1,1), (1,3,0), (1,0,3), (1,2,1), (1,1,2).

Structural calculations and high-resolution infrared spectra are reported for trimers and tetramers containing CO 2 together with CO and/or N 2 . Among the 9 clusters studied here, only (CO 2 ) 2 -CO was previously observed by high-resolution spectroscopy. The spectra, which occur in the region of the ν 3 fundamental of CO 2 (≈2350 cm -1 ), were recorded using a tunable optical parametric oscillator source to probe a pulsed supersonic slit jet expansion. The trimers (CO 2 ) 2 -CO and (CO 2 ) 2 -N 2 have structures in which the CO or N 2 is aligned along the symmetry axis of a staggered side-by-side CO 2 dimer unit. The observation of two fundamental bands for (CO 2 ) 2 -CO and (CO 2 ) 2 -N 2 shows that this CO 2 dimer unit is non-planar, unlike (CO 2 ) 2 itself. For the trimers CO 2 -(CO) 2 and CO 2 -(N 2 ) 2 , the CO or N 2 monomers occupy equivalent positions in the 'equatorial plane' of the CO 2 , pointing toward its C atom. To form the tetramers CO 2 -(CO) 3 and CO 2 -(N 2 ) 3 , a third CO or N 2 monomer is then added off to the 'side' of the first two. In the mixed tetramers CO 2 -(CO) 2 -N 2 and CO 2 -CO-(N 2 ) 2 , this 'side' position is taken by N 2 and not CO. In addition to the fundamental bands, combination bands are also observed for (CO 2 ) 2 -CO, CO 2 -(CO) 2 , and CO 2 -(N 2 ) 2 , yielding some information about their low-frequency intermolecular vibrations.