Bound-bound transitions in the emission spectra of ${\mathrm{Ba}}^{+}$-He excimer

We present an experimental and theoretical study of the emission and absorption spectra of the ${\mathrm{Ba}}^{+}$ ions and ${\mathrm{Ba}}^{+*}\mathrm{He}$ excimer quasimolecules in the cryogenic Ba-He plasma. We observe several spectral features in the emission spectrum, which we assign to the electronic transitions between bound states of the excimer correlating to the $6{}^2{P}_{3/2}$ and $5{}^2{D}_{3/2,5/2}$ states of ${\mathrm{Ba}}^{+}$. The resulting ${\mathrm{Ba}}^{+}\left(5{}^2{D}_J\right)\text{He}$ is a metastable electronically excited complex with orbital angular momentum $L=2$, thus expanding the family of known metal-helium quasimolecules. It might be suitable for high-resolution spectroscopic studies and for the search for new polyatomic exciplex structures.

Figure 1

(a) Top view of the experimental setup and (b) vertical cross section of the sample cell.

Figure 2

Spectra of laser-induced fluorescence and rf plasma emission. Blue line, ablation off; red line, ablation on. $T=1.47$ K, $p_{He}=4$ mbar. Positions of the spectral lines we found are indicated in the inset by vertical arrows. The peak marked with an asterisk is an artifact.

Figure 3

Energy levels of a free ${\mathrm{Ba}}^{+}$ ion (right side) and the ${\mathrm{Ba}}^{+}$-He quasimolecule [22] (center). On the left, electron orbitals of ${\mathrm{Ba}}^{+}$ and He are drawn schematically. All orbitals are symmetric with respect to rotations around the horizontal axis.

Figure 4

Calculated Franck-Condon factors for the transitions from $G^2{\mathrm{\Pi }}_{3/2}$ state of ${\mathrm{Ba}}^{+*}\mathrm{He}$ towards $A^2{\mathrm{\Pi }}_{1/2}, B^2{\mathrm{\Sigma }}_{1/2}, C^2{\mathrm{\Pi }}_{3/2}$, and $D^2{\mathrm{\Delta }}_{5/2}$ states. Vertical bars colored in red, blue, black, green, and magenta correspond to the transitions originating from the states with $v=0$, 1, 2, 3, and 4, respectively.

Figure 5

(a) Excitation spectrum of the $\mathrm{Ba}{}^{+}{6}^2{S}_{1/2}\to 6{}^2{P}_{3/2}$ transition. $T=1.37$ K, $p_{He}=2.4$ mbar. Zero detuning corresponds to transition wavelength in vacuum: $\lambda =454.4$ nm. (b)–(d) Emission spectra of the ${\mathrm{Ba}}^{+*}\mathrm{He}$ excimer. $T=1.57$ K, $p_{He}=6.6$ mbar. (b) $G^2{\mathrm{\Pi }}_{3/2}\to X^2{\mathrm{\Sigma }}_{1/2}$ bound-free transition; (c) $G^2{\mathrm{\Pi }}_{3/2}\to A^2{\mathrm{\Pi }}_{1/2}$ and $G^2{\mathrm{\Pi }}_{3/2}\to B^2{\mathrm{\Sigma }}_{1/2}$ bound-bound transitions; (d) $G^2{\mathrm{\Pi }}_{3/2}\to C^2{\mathrm{\Pi }}_{3/2}$ and $G^2{\mathrm{\Pi }}_{3/2}\to D^2{\mathrm{\Delta }}_{5/2}$ bound-bound transitions. Curve 1 (black), experimental data; curve 2 (red), calculated spectrum fitted to the experimental data (see the text); curve 3 (blue), spectrum calculated assuming only $v=0$ state populated.