Isotope shifts and hyperfine structure of the $\text{Fe}\text{I}$ 372-nm resonance line

We report measurements of the isotope shifts of the $3d^{6}4s^2$ $a{{}^{5}D}_4-3d^{6}4s4p$ $z{{}^{5}F}_5^o$ $\text{Fe}\text{I}$ resonance line at 372 nm between all four stable isotopes ${}^{54}\text{F}\text{e}$, ${}^{56}\text{F}\text{e}$, ${}^{57}\text{F}\text{e}$, and ${}^{58}\text{F}\text{e}$, as well as the complete hyperfine structure of that line for ${}^{57}\text{F}\text{e}$, the only stable isotope having a nonzero nuclear spin. The field and specific mass shift coefficients of the transition have been derived from the data, as well as the experimental value for the hyperfine structure magnetic dipole coupling constant $A$ of the excited state of the transition in ${}^{57}\text{F}\text{e}$: $A\left(3d^{6}4s4pz{{}^{5}F}_5^o\right)=81.69\left(86\right)\text{MHz}$. The measurements were carried out by means of high-resolution Doppler-free laser saturated absorption spectroscopy in a Fe-Ar hollow cathode discharge cell using both natural and enriched iron samples. The measured isotope shifts and hyperfine constants are reported with uncertainties at the percent level.

Figure 1

(Color online) Experimental arrangement used for the observation of Doppler-free saturated absorption spectra of the 372-nm resonance line in neutral iron (see text for an explanation of all elements).

Figure 2

(Color online) Typical Doppler-free laser saturated absorption spectrum recorded with a hollow cathode made of natural iron, illustrated on two different scales. All lines refer to the $3d^{6}4s^2$ $a{{}^{5}D}_4-3d^{6}4s4p$ $z{{}^{5}F}_5^o$ $\text{Fe}\text{I}$ transition at 372 nm [9] for the different stable isotopes ${}^{54}\text{F}\text{e}$, ${}^{56}\text{F}\text{e}$, ${}^{57}\text{F}\text{e}$, and ${}^{58}\text{F}\text{e}$. The isotope ${}^{57}\text{F}\text{e}$ is the only one to possess a hyperfine structure. Two hyperfine components of the investigated transition are visible in the present spectrum [denoted (1) and (2) in decreasing order of intensity]. As expected, a third hyperfine component predicted more than 50 times weaker than the first component is not visible. The frequency axis is referenced with respect to the central ${}^{56}\text{F}\text{e}$ peak.

Figure 3

(Color online) Hyperfine structure components (1), (2), and (3) of the $3d^{6}4s^2$ $a{{}^{5}D}_4-3d^{6}4s4p$ $z{{}^{5}F}_5^o$ $\text{Fe}\text{I}$ transition at 372 nm for the isotope ${}^{57}\text{F}\text{e}$ (nuclear spin $I=1/2$). The quoted excited state energy and air wavelength of the transition are the accurate values of Ref. [5].

Figure 4

(Color online) Same Doppler-free laser saturated absorption spectrum as in Fig. 2 recorded with an enriched sample of iron ($\approx 10\%{}^{54}\text{F}\text{e}$, $\approx 10\%{}^{56}\text{F}\text{e}$, $\approx 70\%{}^{57}\text{Fe}$, and $\approx 10\%{}^{58}\text{Fe}$). In addition to the lines observed in Fig. 2, all cross-over (co) resonances between the ${}^{57}\text{F}\text{e}$ hyperfine components are visible in the spectrum.

Figure 5

(Color online) King plot of the modified residual isotope shift ${\mu }_{A,A^{\prime }}\delta {\nu }_{A,A^{\prime }}^{\text{RIS}}$ with respect to the modified difference in mean square nuclear charge radii ${\mu }_{A,A^{\prime }}\delta {⟨r^2⟩}_{A,A^{\prime }}$ for the different isotope pairs 58–56, 57–56, and 56–54. The origin value and slope of the linear regression line yield the specific mass shift $k_{\text{SMS}}$ and field shift $F$ coefficients, respectively.