Photoionization and electron-impact ionization of ${\mathrm{Kr}}^{3+}$

Absolute photoionization cross sections for ${\mathrm{Kr}}^{3+}$ were measured in the energy range $39.05–143.2\mathrm{eV}$ for single ionization and $120.6–137.7\mathrm{eV}$ for double ionization. For comparison, an electron-impact single-ionization measurement was made in the energy range $43.1–179.1\mathrm{eV}$ and normalized to previously published absolute measurements. The Flexible Atomic Code and Cowan atomic structure codes were used to calculate energy levels, excitation energies and oscillator strengths for $3d\to np$, $3d\to nf$, and $4s\to np$ autoionizing transitions from the ground and metastable states. From the single-photoionization measurements, ionization thresholds of the ${}^{2}P^o{}_{3∕2}$, ${}^{2}D^o{}_{5∕2}$ metastable states and ${}^{4}S^o{}_{3∕2}$ ground state were measured to be $46.62\pm 0.02$, $48.59\pm 0.01$, and $50.70\pm 0.02\mathrm{eV}$, nearly $2\mathrm{eV}$ lower than National Institute of Standards and Technology tabulated values. Within the experimental uncertainty, oscillator strengths determined from the photoionization measurements are in agreement with both calculations. Excitation-autoionization and resonant-excitation double-autoionization features are evident in the electron-impact ionization cross section.

Figure 1

Apparent absolute single-photoionization cross section of ${\mathrm{Kr}}^{3+}$ measured with a photon energy resolution of $100\mathrm{meV}$, showing $4s\to np$ transitions. The large empty circles with error bars show the absolute cross section measurements with their uncertainties. The vertical bars indicate the ionization thresholds for the ${}^{2}P^0{}_{3∕2}$, ${}^{2}P^0{}_{1∕2}$,${}^{2}D^0{}_{5∕2}$ and ${}^{2}D^0{}_{3∕2}$ metastable states and the ${}^{4}S^0{}_{3∕2}$ ground state tabulated in the NIST database.

Figure 2

Apparent single-photoionization cross section of ${\mathrm{Kr}}^{3+}$ from Fig. 1 (lower panel). The middle and upper panels show similar photoionization measurements made at twice and three times the photon energy. Note the large difference in cross-section scales in these three energy ranges and the correspondence of resonance features below $46\mathrm{eV}$ in the lower panel, indicating contributions due to second and third order radiation in the photon beam.

Figure 3

Comparison of single-photoionization cross-section measurements at the same photon energy resolution $\left(100\mathrm{meV}\right)$ made with ${}^{86}\mathrm{Kr}^{3+}$ and ${}^{84}\mathrm{Kr}^{3+}$.

Figure 4

Blowup of the threshold region of Fig. 1. The vertical bars indicate the corrected ionization thresholds for the ${}^{2}P^0{}_{3∕2}$, ${}^{2}P^0{}_{1∕2}$, ${}^{2}D^0{}_{5∕2}$, and ${}^{2}D^0{}_{3∕2}$ metastable states and the ${}^{4}S^0{}_{3∕2}$ ground state. See text for details.

Figure 5

Photoion yield for ${\mathrm{Kr}}^{3+}$ as a function of the photon energy measured at higher resolution $\left(30\mathrm{meV}\right)$, indicating that the ionization threshold for the ${}^{2}D^0{}_{5∕2}$ metastable state is $48.59\pm 0.01\mathrm{eV}$.

Figure 6

Absolute single-photoionization cross section in the energy range of $3d\to 4p$ transitions in ${\mathrm{Kr}}^{3+}$ measured with an energy resolution of $50\mathrm{meV}$. The large open circle with error bar shows an absolute cross-section measurement made at the same energy resolution, with its estimated uncertainty.

Figure 7

Comparison of absolute cross-section measurements for single photoionization of ${\mathrm{Kr}}^{3+}$ with calculated values, shifted by $+0.81\mathrm{eV}$ for Cowan code result and by $+1.56\mathrm{eV}$ for FAC code result.

Figure 8

Single-photoionization cross section in the energy range of the $3d\to np$ $(n>4)$ and $3d\to nf$ $(n>3)$ transitions in ${\mathrm{Kr}}^{3+}$ measured with an energy resolution of $50\mathrm{meV}$ in the energy range $109.1–137.2\mathrm{eV}$, and a resolution of $200\mathrm{meV}$ in the energy range $137.2–143.2\mathrm{eV}$. The large open circles with error bars show individual absolute cross-section measurements and their uncertainties.

Figure 9

Comparison of the absolute double-photoionization cross section for the $3d\to np(n>4)$ and $3d\to nf(n>3)$ transitions in ${\mathrm{Kr}}^{3+}$ measured at an energy resolution of $50\mathrm{meV}$ in the energy range $120.6–137.7\mathrm{eV}$ with the measured single-photoionization cross section for the same transitions at the same energy resolution. The large open circles indicate individual absolute cross section measurements that were used to normalize the spectrum.

Figure 10

Ratio of double to single photoionization cross sections of ${\mathrm{Kr}}^{3+}$ as a function of the photon energy in the energy range $120.6–137.2\mathrm{eV}$.

Figure 11

Energy-scan measurement of the cross section for electron-impact ionization of ${\mathrm{Kr}}^{3+}$ (small dots) normalized to the absolute measurement of Tinschert et al. [6] (open circles). The experimental energy resolution is approximately $1\mathrm{eV}$. Triangles indicate absolute measurements of Gregory et al. [7]. The dotted-line curve is the semiempirical Lotz formula prediction for the direct ionization cross section. The dashed and solid curves are the distorted-wave direct ionization and direct ionization (DI) plus excitation-autoionization (EA) calculations of Loch et al. [8].

Figure 12

Comparison of electron-impact single-ionization cross-section measurements for ${\mathrm{Kr}}^{3+}$ (a) with the single-photoionization cross-section measurements (b) in the energy range $43.1\text{to}75\mathrm{eV}$.

Figure 13

Comparison of electron-impact single-ionization cross-section measurements for ${\mathrm{Kr}}^{3+}$ (a) with the single-photoionization cross-section measurements (b) in the energy range $75\text{to}92\mathrm{eV}$.

Figure 14

Comparison of electron-impact single-ionization cross-section measurements for ${\mathrm{Kr}}^{3+}$ (a) with the single-photoionization cross-section measurements (b) in the energy range $92\text{to}138\mathrm{eV}$.