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

Abstract

Millimeter wave and infrared spectra of the weakly bound dimer (CO)2 have been studied in low-temperature pulsed supersonic jet expansions. Twenty-five new millimeter wave transitions have been observed and assigned, mostly in the 78–107 GHz region. Combined with previous data, they enable the relative energies of most of the known rotational levels (28 out of 31) in the ground vibrational state (vCO=0) of the dimer to be determined with “microwave” accuracy (≲0.1 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 (vCO=1), one stack with K=0 and the other with K=1. Energies for these levels have been determined with “infrared” accuracy (≲10 MHz). These results contribute significantly to the considerable body of precise experimental information available for a system that is ripe for further theoretical investigation.

Millimeter-Wave Spectra of the CO Dimer: Observation and Assignment of 20 New Transitions

“…The millimeter-wave spectra studied here and in (1,4,5) can be thought of as transitions between tunneling states analogous to those observed, for example, in the HF and HCl (6) dimers. But the nature of the tunneling motions is still not clear for the CO dimer.…”

“…Of these, 16 are definitively assigned on the basis of millimeter-wave combination differences, and 3 are assigned with reasonable confidence on the basis of infrared measurements. A few new or changed assignments of previous transitions (1,4,5) are also proposed. As a result of the new assignments, we newly locate the energies of seven levels of the CO dimer with microwave accuracy.…”

“…(1). Briefly, from studies in the infrared (1-3) (C-O stretch, ∼2145 cm −1 ) and millimeter-wave (1, 4, 5) (75-180 GHz) regions, it has been possible to characterize six distinct "stacks" of rotational levels in the ground C-O stretching state (v CO = 0), as well as eight stacks in the excited state (v CO = 1).…”

New CO–CO interaction potential tested by rovibrational calculations

“…In the labeling of the stacks, the experimental assignments are followed. [7][8][9][10][11][12]14 Figure 4 shows the calculated stacks as a function of their effective intermolecular distance R eff ϭ(2 B) Ϫ1/2 . In Fig.…”

Higher Energy States in the CO Dimer: Millimeter-Wave Spectra and Rovibrational Calculations

“…During the last years significant progress has finally been achieved toward securing reliable experimental data in the IR and millimeter-wave regions, thereby establishing for the first time an understanding of the rovibrational and rotational spectra of the CO dimer [12][13][14][15][16][17][18] and its isotopologues. 13,[20][21][22][23][24] There has also been parallel progress in accurate ab initio calculations of a reliable potential energy surface combined with calculations of the rovibrational states of the dimer.…”