Infrared photodetachment of ${\mathrm{Ce}}^{-}$: Threshold spectroscopy and resonance structure

The negative ion of cerium is investigated using tunable laser photodetachment threshold spectroscopy. The relative cross section for photodetachment from ${\mathrm{Ce}}^{-}$ is measured over the photon energy range $0.61–0.75\mathrm{eV}$ using a crossed laser-beam–ion-beam technique. The spectrum of neutral atom production reveals a photodetachment threshold at $0.65\mathrm{eV}$, which is interpreted as the threshold for the ${\mathrm{Ce}}^{-}$ $\left(4f5d^{2}6s^2{}^{4}H_{7∕2}\right)$ to Ce $\left(4f5d6s^2{}^{1}G_4\right)$ ground-state to ground-state transition yielding the electron affinity of Ce to be 0.65(3) eV. At least five narrow peaks are observed in the cross section over the range $0.62–0.70\mathrm{eV}$ due to negative ion resonances, and their energies and widths are measured. The results are compared to other recent experimental and theoretical studies of ${\mathrm{Ce}}^{-}$.

Figure 1

(a) Negative ion mass spectrum obtained with a double-layer ${\mathrm{CeO}}_2+\mathrm{W}$ cathode in the sputtering source. The mass scale was calibrated using the peaks of ${\mathrm{Cs}}^{-}$ $\left(133\mathrm{amu}\right)$, ${}^{140}\mathrm{CeO}^{-}$ $\left(156\mathrm{amu}\right)$, and ${\mathrm{W}}^{-}$ isotopes (182, 183, 184, and $186\mathrm{amu}$). (b) Expanded view of the ${\mathrm{Ce}}^{-}$ region of the mass spectrum, showing the two isotopes of Ce (140, $142\mathrm{amu}$). Note the slight tails on the high-mass sides of the peaks due to small amounts of the hydride ions ${\mathrm{CeH}}^{-}$.

Figure 2

(Color online) Measured normalized neutral production signal (arbitrary units) for photodetachment from ${\mathrm{Ce}}^{-}$ obtained as the average of multiple sweeps over the entire threshold region. Five resonance peaks are labeled $A$—$E$. Finely stepped spectra separately measured over peaks $A$, $B$, $D$, and $E$ are included with the coarsely stepped spectrum. Error bars are one standard deviation of the mean of the individual signals for the multiple sweeps at each photon energy.

Figure 3

(Color online) Measured normalized neutral production signal for photodetachment from ${\mathrm{Ce}}^{-}$ in the vicinity of peaks $A$ and $B$ (see Fig. 2). Open circles, measured data; solid line, fit of the Fano function [Eq. (2)] to the data. Note the change in photon energy scale between the graphs.

Figure 4

(Color online) Measured normalized neutral production signal for photodetachment from ${\mathrm{Ce}}^{-}$ in the vicinity of peaks $C$, $D$, and $E$ (see Fig. 2). Open circles, measured data; solid line, fit of the Fano function [Eq. (2)] to the data. Note the change in photon energy scale between the graphs.

Figure 5

(Color online) (a) Relative cross section (arbitrary units) for photodetachment from ${\mathrm{Ce}}^{-}$ in the near-threshold region. Open circles, measured data; heavy solid line (red), simulated $p$-wave Wigner threshold curve [Eq. (1)] with fixed threshold energy of $0.65\mathrm{eV}$ indicated by the vertical arrow. For the $p$-wave curve, only the scaling factor $a$ was adjusted. (b) Residual differences between the data and the simulated curve. The large peaks at $0.634\mathrm{eV}$ and $0.648\mathrm{eV}$ are resonance peaks $B$ and $C$, respectively. Additional smaller resonance peaks are at $0.68\mathrm{eV}$ and $0.73\mathrm{eV}$.