Knowledge Bank

Laser-Stark spectra showing well resolved hyperfine structure on 13 transitions in the $\nu 4$ and 2 $\nu 2$ bands of $\mathrm{<mrow\; data-mjx-texclass="ORD">14</mrow>}NH3$ are obtained and analyzed. Direct least-squares fits of the spectra yield determinations of the nuclear quadrupole coupling constant eqQ in the various states, accurate to about $\pm 1$ (40 kHz) in favorable cases. To this accuracy, eqQ is the same in the ground and $\nu 4$ states, but it shows sizable changes in the $2\nu 2$ state, in agreement with the predictions of Spirko. The data are obtained using a CO laser and two precision Stark cells. One cell, placed inside the laser cavity, is used to Lamb-dip stabilize the laser on an $NH3$ transition Stark-tuned into resonance with it. The other cell, placed in an external cavity, is used to record the data. Spectra of isolated lines, fit to a Lorentzian lineshape, show widths of 600 kHz (FWHM), a factor of 250 below the Doppler width. The key parameters needed to obtain such linewidths are the laser frequency stability, which is found to be $\pm 10$ kHz for periods of an hour or more, and the homogeneity of the Stark field, which is demonstrated through an analysis of broadening mechanisms to be $\pm 1.7\times 10-4$.