Laser–radio-frequency double-resonance spectroscopy of ${}^{84-87}\mathrm{Rb}$ isotopes trapped in superfluid helium

In this paper, we report on a laser spectroscopy measurement of ${}^{84-87}\mathrm{Rb}$ isotopes in superfluid helium (He ii) at 1.8 K using laser–radio-frequency double-resonance spectroscopy. Rb ion beams $(>60$ MeV/u) provided by the RIKEN projectile fragment separator (RIPS) were injected and trapped into He ii. The stopping position of atoms in He ii was precisely confirmed by laser spectroscopy. By optically pumping the trapped Rb isotopes, large atomic spin polarization (up to 40%) of each observed isotope in the ground state was achieved. The laser–radio-frequency double-resonance spectra were observed for stable ${}^{85,87}\mathrm{Rb}$ isotopes as well as for unstable isotopes ${}^{84,84m,86}\mathrm{Rb}$ by scanning a weak magnetic field with a fixed-frequency RF field. From the measured Zeeman splitting, nuclear spin values for ${}^{84m,84-87}\mathrm{Rb}$ isotopes were determined with reasonable accuracy. The number of ions injected into He ii for the resonance spectra measurement was on the order of $10{}^4$ particles per second. This work may open new opportunities for the study of various particles trapped in condensed helium in several fields.

Figure 1

Schematic view of the experimental setup. Energetic ion beams produced by a projectile fragment separator are injected into the experimental setup. An energy degrader and the first plastic scintillator (PL1) are used to adjust the beam energy and count the injected ions, respectively. The second plastic scintillator (PL2) in the cryostat is used to check the stopping position of the beams in He ii. The trapped atoms in He ii are irradiated by circularly polarized laser light. Two sets of coils are used, one to apply a magnetic field $B_0$ and one to apply a RF field. The photon-detection system, as shown on the right, which detects the laser-induced fluorescence photons emitted from the laser-excited atoms, is located under the cryostat.

Figure 2

(a) (left) Schematic of optical pumping to achieve polarized atoms. Taking the ground-state energy level structure of ${}^{87}\mathrm{Rb}$ as an example, the polarization of atoms is reflected by the population of the atomic states being concentrated at the maximum quantum number $m_F$ = +2. (a) (right) Vector relation between propagation of the laser, applied magnetic field $B_0$, and RF field, and laser-induced fluorescence photons. (b) Block diagram showing measurement of LRDR spectra. The spectrum was recorded by scanning the magnetic field $B_0$ with a fixed RF field (1.8 MHz in the current spectrum) applied to the atoms.

Figure 3

Trapping results for ${}^{84-87}\mathrm{Rb}$ isotopes in He ii shown by the relation between the ratio of the number of detected laser-induced fluorescence photons $N_L$ to the number of injected beams $N_B$ and the stopping position of atoms in He ii. Here, the stopping position was calculated from the thickness of the energy degrader using the lise++ program, as described in detail in Ref. [22]. The peak position $(\mathit{x}$ = 0) suggests the perfect overlap of the laser beam and the trapped atoms. All the Y axes are normalized by the maximum number of counts. The scales of the horizontal axes are closely related to the degrader thickness, as explained in the text.

Figure 4

(a) Directly observed LRDR spectra, as explained in Fig. 2. The red dashed line is the applied magnetic field. (b) LRDR spectra of ${}^{84-87}\mathrm{Rb}$, presented as the relation between the intensity of the laser-induced fluorescence photon and the scanned magnetic field $B_0$. The red solid line shows the fitting using Eq. (6) and a Lorentz function.

Figure 5

LRDR for ${}^{84,85}\mathrm{Rb}$ isotopes and isomer ${}^{84m}\mathrm{Rb}$. The inset shows the directly recorded spectrum using the measurement pattern presented in Fig. 2.