Measurement of the electron affinity of the lanthanum atom

Atomic lanthanum anion ${\mathrm{La}}^{-}$ is the best candidate for laser cooling of any atomic anions found so far. The bound-bound electric dipole transitions of ${\mathrm{La}}^{-}$ have been identified and measured. Theoretical studies have shown that ${\mathrm{La}}^{-}$ has a very complicated electronic structure, which cannot be well resolved by the traditional photoelectron spectroscopy. In the present work, we report high-resolution photoelectron spectroscopy of ${\mathrm{La}}^{-}$ via the slow-electron velocity-map imaging method in combination with an ion trap. The electron affinity of La was determined to be $4496.97\left(20\right){\mathrm{cm}}^{-1}$ or 0.557 553(25) eV. In addition, the energy levels of ${\mathrm{La}}^{-}$, ${}^{3}F_3^e$, ${}^{3}F_4^e$, ${}^{1}D_2^e$, ${}^{1}D_2^o$, ${}^{3}P_0^e$, ${}^{3}P_1^e$, and ${}^{3}P_2^e$, were also determined.

Figure 1

Photoelectron spectra of ${\mathrm{La}}^{-}$ at photon energy $h\nu =11595.57{\mathrm{cm}}^{-1}$ obtained with the trap-on mode (a) and the trap-off mode (b). In the trap-on mode, the ions are trapped for 45 ms, and are then thrown out for further analyzing. In the trap-off mode, the ions directly fly through the ion trap as the trap is turned off. The red curve shows the weak peaks multiplied by a factor of 6 for a clearer view. The sticks below the spectra indicate the binding energy of photodetachment channels from the states labeled on the left or right sides. The related transitions are illustrated in Fig. 2.

Figure 2

Partial energy levels of La and ${\mathrm{La}}^{-}$. The transitions’ labels are corresponding to the peaks observed in Fig. 1. Transition $a$ is used to measure the EA value of La.

Figure 3

The photon energy hν versus the squared radius $r^2$ of the measured electron spherical shell for transition $a$. The solid line is the linear least squares fitting. The intercept $11508.88{\mathrm{cm}}^{-1}$ is the binding energy of photodetachment channel $a$ (a). The binding energy of transition $\mathrm{La}\left({}^{2}F_{5/2}^e\right)\leftarrow {\mathrm{La}}^{-}\left({}^{3}F_2^e\right)$ as a function of the kinetic energy of photoelectrons. The dashed lines indicate the uncertainty of $\pm 0.20{\mathrm{cm}}^{-1}$ (b). The uncertainty includes the statistical and fitting error. The systematic error is estimated to be $0.1{\mathrm{cm}}^{-1}$