First Observation of Atomic Levels for the Element Fermium ($Z=100$)

The atomic level structure of the element fermium was investigated for the first time using a sample of $2.7\times {10}^{10}$ atoms of the isotope ${}^{255}\mathrm{F}\mathrm{m}$ with a half-life of 20.1 h. The atoms were evaporated from a filament and stored in the argon buffer gas of an optical cell. Atomic levels were sought by the method of resonance ionization spectroscopy using an excimer-dye-laser combination. Two atomic levels were found at wave numbers $(25099.8\pm 0.2)$ and $(25111.8\pm 0.2){\mathrm{c}\mathrm{m}}^{-1}$. Partial transition rates to the $5f^{12}7s^2$ ${}^{3}H_6^e$ ground state have been determined from their saturation characteristics. Multiconfiguration Dirac-Fock calculations suggest that the leading orders of these levels could be the $5f^{12}7s7p$ ${}^{5}I_6^o$ and $5f^{12}7s7p$ ${}^{5}G_5^o$ terms.

Figure 1

Experimental setup. The four sections are evacuated by turbo molecular pumps (TMP) with relatively low pumping speeds as indicated. QMS is the quadrupole mass spectrometer and LPM is the laser power meter. The $\alpha$ detectors are mounted perpendicular to the plane of drawing.

Figure 2

Wave number scan of the dye laser at ${\varphi }_1=7.9\times {10}^{14}\mathrm{\text{photons}}/({\mathrm{c}\mathrm{m}}^2\mathrm{\text{pulse}})$ for mass number $A=255$. The excimer photon number flux was $6.5\times {10}^{15}\mathrm{\text{photons}}/({\mathrm{c}\mathrm{m}}^2\mathrm{\text{pulse}})$. The full line shows the best fit of the power broadened line profile to the data.

Figure 3

Mass spectrum of the atoms ionized at fixed dye laser wave number of $25111.8{\mathrm{c}\mathrm{m}}^{-1}$.

Figure 4

Saturation characteristic of the optical transition at ${\overline{\nu }}_1=25111.8{\mathrm{c}\mathrm{m}}^{-1}$. Shown is the measured ion number $N({\varphi }_1)$ at mass $A=255$ as a function of the photon flux ${\varphi }_1$ of the first excitation step normalized at ${\varphi }_{1\max }$. The full lines show best fits of model calculations to the data from which the partial transition rate $A_{ki}$ can be inferred. Insets: line shapes at fluxes of $0.5\times {10}^{14}$ and $7.9\times {10}^{14}\mathrm{\text{photons}}/({\mathrm{c}\mathrm{m}}^2\mathrm{\text{pulse}})$, as indicated.