Electronic level structure of ${\mathrm{Th}}^{+}$ in the range of the ${}^{229m}\mathrm{Th}$ isomer energy

Using resonant two-step laser excitation of trapped ${}^{232}{\mathrm{Th}}^{+}$ ions, we observe 166 previously unknown energy levels of even parity within the energy range from 7.8 to 9.8 eV and angular momenta from $J=1/2$ to $7/2$. We also classify the high-lying levels observed in our earlier experiments by the total angular momentum and perform multiconfiguration Dirac-Fock calculations to compare their results with the observed level density. The observed levels can be relevant for the excitation or decay of the ${}^{229m}\mathrm{Th}$ isomeric nuclear state which lies in this energy range. The high density of electronic levels promises a strongly enhanced electronic bridge excitation of the isomer in ${}^{229}{\mathrm{Th}}^{+}$.

Figure 1

Two-step laser excitation scheme in ${}^{}{\mathrm{Th}}^{+}$, starting from the ground state through different odd-parity intermediate levels (middle panel) to high-lying levels within the indicated energy ranges (upper panel). Levels are labeled by their energy in ${\mathrm{cm}}^{-1}$ and their total angular momentum $J$.

Figure 2

Experimental setup for laser excitation of trapped ${}^{}{\mathrm{Th}}^{+}$ ions. The first step excitation is provided by the second harmonic (SHG) or third harmonic (THG) radiation of a nanosecond Ti:Sa laser, locked to a Fizeau wavemeter. The second excitation step is provided by the THG of a second ns-Ti:Sa laser. ${}^{}{\mathrm{Th}}^{+}$ ions are loaded via laser ablation (ns-Nd:YAG laser at 1064 nm). Molecular compounds of ${}^{}{\mathrm{Th}}^{+}$ are photodissociated by pulses from a $Q$-switched diode-pumped solid-state laser ($Q$-DPSS) at 266 nm.

Figure 3

Two-step excitation of the level $74195.{13}_{}{\mathrm{cm}}^{-1}$ via the intermediate level $35156.9_{5/2}{\mathrm{cm}}^{-1}$. Graph (a) shows the decrease of the fluorescence signal of the intermediate level (wavelength range from 325 to 650 nm) due to three-photon ionization during the scan from negative to positive detunings. The slight increase of the signal after the excitation occurs due to redistribution of ${}^{}{\mathrm{Th}}^{+}$ ions in the trap. Graph (b) shows the simultaneously recorded VUV fluorescence signal of the decay of the high-lying level. The line shape is determined by the multimode spectrum of the Ti:Sa laser radiation. The start of the scan at 0GHz laser detuning corresponds to the wave number $39039.{15}_{}{\mathrm{cm}}^{-1}$ of the second excitation step laser radiation.

Figure 4

Histogram of the number of electronic levels in ${}^{}{\mathrm{Th}}^{+}$ with $J=1/2$ to $7/2$ and even parity in the energy range up to 80 000 ${\mathrm{cm}}^{-1}$. The calculated level numbers are shown in red and the measured level numbers in blue (Ref. [3]) and green (this work).