The CO dimer millimeter wave spectrum: Detection of tunneling transitions

A portion of the CO dimer millimeter wave absorption spectrum has been stud ied by using our highly sensitive intracavity-jet OROTRON spectrometer in t he frequency range from 131 to 174 GHz. By varying the CO concentration in the Ne/CO gas mixture feeding the supersonic jet expansion, the effective t emperature of the beam could be changed, revealing a correlation between th e observed line intensity and the relative energy of the respective lower s tate energy levels. Using this temperature dependence and the technique of combination differences together with the data from the infrared study of B rookes and McKellar [J. Chem. Phys. 111, 7321 (1999)], out of over 200 obse rved transitions, a total of 19 lines could be assigned. All assigned milli meter-wave transitions are tunneling transitions. They belong to four subba nds, which connect seven lower energy levels with A(+) symmetry to ten prev iously unknown upper energy levels with A(-) symmetry. The A(+) and A(-) se paration signifies the tunneling splitting of the CO stretching ground stat e v(CO) = 0 energy levels. The energies of all levels were determined to mi crowave accuracy. The discovered energy levels fall into two substates, cor responding to the projection K=0 and to K = 1 of the total angular momentum J onto the intermolecular axis. The effective intermolecular CO-CO separat ion for these new K = 0 and K = 1 states is 4.26 and 4.17 Angstrom, respect ively. (C) 2000 American Institute of Physics. [S0021-9606(00)02432-6].