Observation of an Electric Quadrupole Transition in a Negative Ion: Experiment and Theory

The first direct experimental observation of an electric quadrupole ($E2$) absorption transition between bound states of an atomic negative ion has been made. The transition was observed in the negative ion of bismuth by resonant ($1+1$) photon detachment from ${\mathrm{Bi}}^{-}$ via ${}^3{P}_2\to {}^3{P}_0$ excitation. The $E2$ transition properties were completely independently calculated using a hybrid theoretical approach to account for the strong multilevel electron interactions and relativistic effects. The experimental and ab initio theoretical results are in excellent agreement, providing valuable new insight into this complex system and forbidden transitions in negative ions more generally.

Figure 1

Energy levels for the two bound states of ${\mathrm{Bi}}^{-}$ and the ground state of Bi with arrows showing the two measurements performed: (a, red) single-photon threshold detachment from ${\mathrm{Bi}}^{-}$ ${}^3{P}_0$ and (b, blue) resonant ($1+1$) photon detachment via the ${\mathrm{Bi}}^{-}$ ${}^3{P}_2\to {}^3{P}_0$ $E2$ transition.

Figure 2

Measured photodetachment threshold from the ${\mathrm{Bi}}^{-}$ ${}^3{P}_0$ excited state to the Bi ${}^4{S}_{3/2}$ ground state: circles, data; line, $s$-wave Wigner law fit.

Figure 3

Measured peak for resonant ($1+1$) photodetachment via the ${\mathrm{Bi}}^{-}$ ${}^3{P}_2\to {}^3{P}_0$ $E2$ transition: circles, data; line, Lorentzian fit.