Infrared bands of CS₂ dimer and trimer at 4.5 μm

We report observation of new infrared bands of (CS₂)₂ and (CS₂)₃ in the region of the CS₂ ν₁ + ν₃ combination band (at 4.5 µm) 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 D_2d 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 ν₁ + ν₃ region of the CS₂ monomer. This band is observed to have a rather small vibrational shift of -0.844 cm^-1. We expect one parallel and one perpendicular infrared-active band for the trimer but observe two parallel bands and one perpendicular band. 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.