Doppler-free high-resolution continuous-wave optical UV spectroscopy on the $A{}^2{\mathrm{\Sigma }}^{+}\leftarrow X{}^2{\mathrm{\Pi }}_{3/2}$ transition in nitric oxide

We report on Doppler-free continuous-wave optical UV spectroscopy resolving the hyperfine structure of the $A{}^2{\mathrm{\Sigma }}^{+}\leftarrow X{}^2{\mathrm{\Pi }}_{3/2}$ transition in nitric oxide for total angular momenta $J_X=1.5–19.5$ on the ${\mathrm{oP}}_{12\mathrm{ee}}$ branch. The resulting line splittings are compared to calculated splittings and fitted determining new values for the molecular constants $b$, $c$, $eQ{q}_0$, and $b_F$ for the $A{}^2{\mathrm{\Sigma }}^{+}$ state. The constants are in good agreement with values previously determined by quantum beat spectroscopy.

Figure 1

Schematic of the spectroscopy setup. The output of the UV laser is split into a pump and a probe beam which are both expanded before entering the cell in counterpropagating configuration. The pump beam is modulated with a chopper wheel to employ a lock-in amplifier.

Figure 2

Selection of measured hyperfine spectra for different total angular momenta between (a) $J_{\mathrm{X}}=19.5$ and (h) 5.5. The depicted signal is the output (Out) of the lock-in amplifier (LI) normalized to the highest peak in each spectrum. The peaks correspond to the reduced absorption of the probe laser at the position of the Lamb dips. The data is shown as (violet) dots and was fitted with a Voigt profile of the form given in Eq. (1), shown as solid (orange) lines.

Figure 3

Schematic of the six expected hyperfine transitions for an exemplary rotational transition of the ${\mathrm{oP}}_{12\mathrm{ee}}$ branch. The energy splittings and arrows are not to scale.

Figure 4

(a) Comparison between measured (dots) and calculated hyperfine splittings (squares). The measured splittings were calculated from the fit parameters of the Voigt fit shown in Fig. 2. The error bars include the estimated error of the laser lock and one standard deviation of the fit error. (b) Experimentally measured splittings (dots) as shown in (a) compared to data (squares) calculated from the newly fitted hyperfine constants for the $A{}^2{\mathrm{\Sigma }}^{+}$ state.