Knowledge Bank

The far-infrared spectrum of the NO radical in its electronic ground state exhibits both pure rotational transitions and magnetic dipole transitions in {P}-, {Q}-and R-branches between the two spin states $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Pi 1/2$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Pi 3/2$. We have measured 150 transitions using tunable far-infrared spectroscopy to supply accurate far-infrared frequencies for the stronger pure rotational and magnetic dipole transitions as well as to check on intensity predictions for perturbation allowed electric dipole transitions between the spin states. Two stabilized $CO2$ laser frequencies were mixed with a tunable microwave frequency in a metal-insulator-metal (MIM) diode to generate far-infrared radiation. To obtain accurate center frequencies Voigt profiles were adjusted to the observed absorption spectra by a least-squares procedure. This technique was shown to supply center frequencies accurate to better than 10kHz for the CO $moleculea$. Fits of the obtained transition frequencies to an effective Hamiltonian of the type introduced by Brown $etalb$. have been performed. Standard deviations somewhat higher than 10kHz have been found for these fits up to now. The reason for that is under investigation and will be discussed at the meeting. The preliminary value for the spin-orbit coupling constant is 3691813.743(21) MHz, which is a factor of 50 more accurate than the one determined from the most recent Fourier transform $measurementc$.