Atomic-structure investigations of neutral einsteinium by laser resonance ionization

Excited atomic states of neutral einsteinium were investigated by resonant laser ionization using 10 pg $(2\times {10}^{10}$ atoms) of ${}^{254}\mathrm{Es}$. Ten transitions from the ${5f}^{11}7s{}^2{}^{4}I_{15/2}^o$ ground state to even-parity states were investigated in detail, via studies of lifetimes and further excitation steps into higher-lying odd-parity states below and above the first ionization potential. This led to the identification of 37 previously unknown odd-parity energy levels in the region between $37000$ and $42500\mathrm{cm}{}^{-1}$ and a number of autoionizing states, which ensure a high ion yield in resonant ionization. Rydberg states were identified and the corresponding Rydberg series converging either to the ionic ground or an excited state were analyzed to determine the first ionization potential of einsteinium to a value of $E_{\text{IP}}=51364.58{\left(14\right)}_{\mathrm{stat}}{\left(50\right)}_{\mathrm{sys}}\mathrm{cm}{}^{-1}$, improving the previous result by a factor of four.

Figure 1

Sketch of the RISIKO mass separator and the solid-state laser system. More details are given in the text and in Ref. [24].

Figure 2

Left: Es I level scheme: Blue lines indicate all previously known energy levels from [15, 21], green-shaded regions were studied in this paper with newly identified levels indicated by orange lines. Around the IP (red line), only the strongest AIS as given in Table 1 are indicated. Right: Sketch of the different ionization schemes studied in this paper. Detailed information on the individual excitation schemes is given in Table 1.

Figure 3

Wavelength scan on ground-state transitions in Es I showing ten FES resonances. The ionization laser was operated at fixed frequency, ${\lambda }_2=400$ nm and ${\lambda }_2=350$ nm for the high- and low-energy region in this scan, respectively, as indicated in the spectrum. The solid-orange lines represent known FES with assigned configurations from literature [15, 21]. The dashed-orange lines are also listed there, but were not seen in this scan. The dotted green lines represent known transitions from literature, which could previously not be assigned to energy levels.

Figure 4

Spectra showing odd-parity states of einsteinium obtained in a two-step ionization scheme by scanning the second step around the IP for the ten different FES No. 1 to No. 10 as indicated and specified in Table 1. On the left $\mathit{y}$ axis the count rate during the laser scans is shown. On the right $\mathit{y}$ axis an enhancement factor is given by normalizing the count rates to their individual baseline of nonresonant ionization. All spectra show a specifically high density of resonances in the region between the IP and the first excited state of the ${\mathrm{Es}}^{+}$ ion, ascribed to a large extend to converging Rydberg series. The highlighted regions are shown in more detail in Fig. 8.

Figure 5

Scan for higher-lying odd-parity second excited states in a three-step excitation scheme in Es I starting from different FES with their energies added to the scanning laser wavenumber. The dotted lines indicate where energy levels were assigned. The intensities in all scans are normalized to one and an arbitrary offset for each trace is added for distinction. About half of the excited levels were measured starting from at least two different FES. In many spectra, the second excited state at 37 915 ${\mathrm{cm}}^{-1}$ shows a particularly high count rate, as the ionization laser accidentally hits an AIS.

Figure 6

Determination of the lifetime for two different excited states in a two-step scheme, with the subscript referring to the FES from Table 1. The data are given by the blue (FES No. 1) and green (FES No. 4) dots. The subset of the data considered in the fit for FES No. 1 is shown by the red circles. Solid lines represent fits to the data. For details see text.

Figure 7

Fractional part of the quantum defect ${\delta }_{\text{frac}}$ as function of the effective principal quantum number $n^{*}$ for the excitation scheme starting from FES No. 2 at 24 338.2 ${\mathrm{cm}}^{-1}$. Panel (a) refers to the spectrum below the IP, while panel (b) shows all observed energy levels up to the first excited state in the ${\mathrm{Es}}^{+}$ ion $(\mathit{cf}$. Fig. 4 panel No. 2). Assigned Rydberg levels are highlighted in the plot in red. More details on the assignment are provided in the text.

Figure 8

Magnification of the obtained spectra [cf. Fig. (4) of scheme No. 2 (blue) and 3 (red) with an added offset of 200 counts per second for better visualization. The $d$-series shows a shift between both spectra, which can be explained by different fine structure configurations of the corresponding states.

Figure 9

Rydberg-Ritz fit to the $\mathit{s}$- and $d$-series extracted from Fig. 7 converging to the ionic ground state and the lowest-lying excited state in the ${\mathrm{Es}}^{+}$ ion.

Figure 10

Comparison of the frequency scans with higher resolution (blue) and the data obtained in the long-range scan (orange). All FES are drawn relative to their centroid (dashed-blue line) as given in Table 1. The dashed-orange line indicates the centroid determined in the broadband laser scan by just applying a Gaussian fit, the deviation in all 10 cases is below 0.25 ${\mathrm{cm}}^{-1}$.

Figure 11

Hyperfine structure splitting of the ground state (GS) and lowest-lying excited state (ES) in the ${}^{254}\mathrm{Es}^{+}$ ion. The total splitting is given on the right.