Infrared spectrum and energy levels of the CO dimer: Evidence for two almost isoenergetic isomers

The CO dimer has been studied in the region of the CO stretching vibration (2139-2152 cm(-1)) with a pulsed supersonic jet and a tuneable diode laser probe. By using both pinhole and slit jet configurations, and varying the distance downstream of the laser probe, the dimer spectrum was obtained over a range of effective rotational temperatures from about 1 to 12 K. Using this temperature dependence and the technique of combination differences, over 120 transitions belonging to 13 subbands were assigned in terms of 24 ground state (v(CO) = 0) and 36 excited state (v(CO) = 1) rotational energy levels of (CO)(2). The levels fall into two groups, corresponding to isomers with effective intermolecular separations of either about 4.4 or 4.0 Angstrom. The 4.4 Angstrom isomer is the ground state, while the 4.0 Angstrom isomer is a low-lying (0.88 cm(-1)) excited state. This energy ordering is inverted when upsilon(CO) = 1. Previous calculations suggest that both forms are planar and roughly T-shaped, with the 4.4 Angstrom isomer in a C-bonded configuration and the 4.0 Angstrom isomer in an O-bonded configuration. Measurements on an isotopically mixed dimer, (CO)-C-13-O-16-(OO)-O-12-O-16, indicate that C-O vibrational coupling between the monomer units in a dimer is very weak. (C) 1999 American Institute of Physics. [S0021-9606(99)01540-8].