New millimeter wave and infrared spectra of the CO dimer: Assignment and precise location of quantum states

Millimeter wave and infrared spectra of the weakly bound dimer (CO)(2) have been studied in low-temperature pulsed supersonic jet expansions. Twenty-f ive new millimeter wave transitions have been observed and assigned, mostly in the 78-107 GHz region. Combined with previous data, they enable the rel ative energies of most of the known rotational levels (28 out of 31) in the ground vibrational state (upsilon (CO)=0) of the dimer to be determined wi th "microwave" accuracy (less than or similar to0.1 MHz). Four new subbands in the infrared spectrum have been assigned in terms of two new stacks of rotational levels in the excited vibrational state (upsilon (CO)=1), one st ack with K=0 and the other with K=1. Energies for these levels have been de termined with "infrared" accuracy (less than or similar to 10 MHz). These r esults contribute significantly to the considerable body of precise experim ental information available for a system that is ripe for further theoretic al investigation. (C) 2000 American Institute of Physics. [S0021-9606(00)01 840-7].