  We report observation of new infrared bands of (CS2)2 and (CS2)3 in the
region of the CS2 {\nu}1+ {\nu}3 combination band (at 4.5 cm-1) using a quantum
cascade laser. The complexes are formed in a pulsed supersonic slit-jet
expansion of a gas mixture of carbon disulfide in helium. We have previously
shown that the most stable isomer of (CS2)2 is a cross-shaped structure with
D2d symmetry and that for (CS2)3 is a barrel-shaped structure with D3 symmetry.
The dimer has one doubly degenerate infrared-active band in the {\nu}1+ {\nu}3
region of the CS2 monomer. This band is observed to have a rather small
vibrational shift of -0.846 cm-1. We expect one parallel and one perpendicular
infrared-active band for the trimer but observe two parallel and one
perpendicular bands. Much larger vibrational shifts of -8.953 cm-1 for the
perpendicular band and -8.845 cm-1 and +16.681 cm-1 for the parallel bands are
observed. Vibrational shifts and possible vibrational assignments, in the case
of the parallel bands of the trimer, are discussed using group theoretical
arguments.
