Electronic structure and angular momentum coupling as reflected in electron excitation out of rare-gas metastable levels: Excitation cross sections of krypton

We present measurements of electron-impact excitation cross sections into levels of the $4p^{5}5p$ configuration from the $J=0$ and $J=2$ metastable levels of krypton. Metastable-atom targets were generated using two different sources, a hollow-cathode discharge and via charge-exchange collisions between a fast ${\mathrm{Kr}}^{+}$ beam and Cs atoms. The metastable atoms are excited to $4p^{5}5p$ levels by a monoenergetic electron beam and the fluorescence from the levels are used to determine the excitation cross sections. Laser quenching of the hollow-cathode target is used to separate the signal contributions from excitation of the two metastable levels. Like excitation from the metastable levels of Ar, cross sections for dipole-allowed excitations are generally larger than ones for dipole-forbidden excitations. Krypton differs from Ar and Ne, however, in having a larger spin-orbit coupling for the $4p^5$ core so that the energy levels of each excited configuration segregate into two tiers based on the value of the core angular momentum. Cross sections for dipole-allowed excitation with a change in the core angular momentum are not only much smaller than their core-preserving counterparts, but also have different energy dependence. The measured cross sections are compared with recent theoretical calculations and with previous experimental work.

Figure 1

Energy levels of the $4p^{5}5s$ and $4p^{5}5p$ configurations of krypton.

Figure 2

Spatial profiles of the electron beam (at an energy of $100\mathrm{eV}$) and the metastable krypton beam.

Figure 3

Spatial variation of the $2p_9$ excitation signal at $50\mathrm{eV}$ as a function of the distance between the electron gun and center of the optical viewing region. For the static gas target, this is the convolution of the electron beam width and width of the viewing region. For the 2.5-keV fast beam target, the profile is shifted due to the motion of the atoms in the fast beam target between the point where they are excited by the electron beam and where they decay.

Figure 4

Core-preserving cross sections for excitation into the lower tier levels $(j_c=3∕2)$ from the $1s_5$ level $(j_c=3∕2)$ as a function of the incident electron energy. Error bars are statistical only. Lines represent the different theoretical calculations from Ref. 8 (BP51, BP15, DW-1, DW-2) and the $R$-matrix calculation (RMEB) from Ref. 19.

Figure 5

Core-preserving cross sections for excitation into the upper tier levels $(j_c=1∕2)$ from the $1s_3$ level $(j_c=1∕2)$. Error bars are statistical only. Lines represent the theoretical calculations from Refs. 8, 19.

Figure 6

Core-changing cross sections for excitation into the upper tier levels $(j_c=1∕2)$ from the $1s_5$ level $(j_c=3∕2)$. Error bars are statistical only. Lines represent the theoretical calculations from Refs. 8, 19.

Figure 7

High-energy cross sections results for excitation into the $2p_6$ and $2p_9$ levels from the $1s_5$ level. Error bars are statistical only. Lines represent the theoretical calculations from Refs. 8, 19.

Figure 8

Comparison of normalized excitation cross sections: $●$ $\mathrm{Kr}(1s_5\to 2p_9)$ (this experiment), $◻$ $\mathrm{Rb}(5S\to 5P)$ from Ref. 28. Error bars are statistical only.