  The infrared spectrum of the weakly-bound C2H4-CO2 complex is investigated in
the region of the nu3 fundamental band of CO2 (~2350 cm-1), using a tunable OPO
laser source to probe a pulsed supersonic slit jet expansion. The spacing of
the various K-subbands in this perpendicular spectrum is very irregular, and
the pattern of irregularity is quite different from that observed previously in
another C2H4-CO2 band by Bemish et al. [J. Chem. Phys. 103, 7788 (1995)]. But
by allowing for the different symmetry of the nu3 (CO2) upper vibrational
state, both results can be strikingly well explained using the 'double internal
rotor' model as described by Bemish et al.
