Knowledge Bank

Dipole moment measurements for a number of van der Waals complexes made up of non- polar constituents have been made, giving results ranging from 0.03 D to 0.5 D. However, the signs of these induced dipole moments have not been determined. The sign of the induced dipole moment can be determined for a complex which contains a monomer having a small dipole moment of known sign. Complexes containing a non-polar molecule and carbon monoxide provide the means to study induced moments with relatively few effects from internal motions. The dipole moment of $CO2-CO$ has been obtained from radio frequency transitions measured using molecular beam electric resonance spectroscopy. A 20 v/cm electric field is sufficient to produce a 1 MHz Stark shift in the $220\to 221$ transition. The dipole moment from these measurements is $\mu =0.2493$ D. Combining this result with the CO moment of $\mu =0.1098$ gives an induced moment in the $CO2-CO$ complex of 0.1395 D. The orientation of the induced moment is in the same direction as that of the CO moment, which makes the $CO2$ monomer positive relative to the CO monomer. Calculations using a distributed polarizability model are currently in progress.