Experimental studies of partial photodetachment cross sections in K${}^{-}$ below the K(7 ${}^{2}P$) threshold

A collinear beams apparatus has been used to determine photodetachment cross sections for K${}^{-}$ in the photon energy range 4.250–4.360 eV. State-selective detection, utilizing a resonance ionization scheme, was applied to measure partial cross sections for those channels which leave the residual K atoms in the excited $7{}^{2}S$, $5{}^{2}F$, and $5{}^{2}G$ states. The energy region studied encompassed the openings of the aforementioned channels, as well as the channel that leaves the K atom in the $7{}^{2}P$ state. Two previously unobserved resonances were seen in all three partial cross sections between the K($5{}^{2}G$) and K($7{}^{2}P$) thresholds. It is shown that a more reliable determination of resonance parameters can be made if the same resonances are observed in several channels. In the region below the K($5{}^{2}F$) threshold, three previously observed resonances were investigated [Kiyan et al., Phys. Rev. Lett. 84, 5979 (2000)]. A greatly increased modulation of the signal was obtained by detecting in the K($7{}^{2}S$) channel instead of the K($5{}^{2}S$) channel used in the previous study. Furthermore, the shapes of the cross sections in the threshold regions are discussed. A detailed description of the apparatus and the experimental procedure employed is presented in the paper.

Figure 1

Partial energy level diagram (not to scale) of the K${}^{-}$ and K systems. A K${}^{-}$ ion absorbs a UV photon (${\gamma }_{\mathrm{UV}}$) and is promoted to a doubly excited state (dashed energy levels), which decays via autodetachment (dashed arrows). State selectivity in the detection is achieved by resonant excitation using an IR photon (${\gamma }_{\mathrm{IR}}$) followed by field ionization (FI).

Figure 2

Schematic of the collinear beams apparatus. Two laser beams and an ion beam are superimposed in a collinear copropagating geometry. The field ionizer separates the beam particles from the interaction region.

Figure 3

Two-dimensional projection of the field-ionized Rydberg atoms in state $24{}^{2}D$ from measurement of the K($5{}^{2}F$) channel. The selected signal region is shown in black. The hatched regions are used to estimate the background in the signal region.

Figure 4

Partial cross section for the K$\left(7{}^{2}S\right)$ channel. Vertical dashed lines indicate threshold energies. Panel (a) shows the full energy region between the K$\left(7{}^{2}S\right)$ and the K$\left(5{}^{2}F\right)$ thresholds. Panel (b) shows a close-up of the region just below the K$\left(5{}^{2}F\right)$ threshold. Error bars (blue online) indicating the one sigma statistical uncertainty are shown for a few representative data points. The solid curve (red online) is a fit of Eq. (6) with three resonances. Resonance energies are shown as short vertical lines, while the horizontal lines indicate the widths. The lowest lines (magenta online) represent the previous experimental results by Kiyan et al. [11], the middle lines (red online) represent the present data, and the upper lines (blue online) are the theoretical values by Liu [18].

Figure 5

Partial photodetachment cross sections for the K$\left(7{}^{2}S\right)$, K$\left(5{}^{2}F\right)$, and K$\left(5{}^{2}G\right)$ channels, shown in panels (b), (c), and (d), respectively. Error bars (blue online) indicating the one sigma statistical uncertainty are shown for a few representative data points. Representative fits to the data are shown as solid curves (red online). Energies and widths of the resonances extracted from the three fits are indicated in the corresponding panels. Panel (a) shows the weighted mean of the resonance parameters (lower line, red online) as well as the values by Liu (upper line, blue online). Threshold energies are indicated by vertical dashed lines.