Photoionization of the $3s^{2}3p^4{}^{3}P$ and the $3s^{2}3p^4{}^{1}D,{}^{1}S$ states of sulfur: Experiment and theory

Photoionization of neutral atomic sulfur in the ground and metastable states was studied experimentally at a photon energy resolution of 44 meV (full width at half maximum). Relative cross section measurements were recorded by using tunable vacuum ultraviolet radiation in the energy range 9–30 eV obtained from a laser-produced plasma and the atomic species were generated by photolysis of molecular precursors. Photoionization of this atom is characterized by multiple Rydberg series of autoionizing resonances superimposed on a direct photoionization continuum. A wealth of resonance features observed in the experimental spectra are spectroscopically assigned and their energies and quantum defects tabulated. The cross section measurements are compared with state-of-the-art theoretical cross section calculations obtained from the Dirac Coulomb $R$-matrix method. Resonance series in the spectra are identified and compared, indicating similar features in both the theoretical and experimental spectra.

Figure 1

Time-of-flight mass spectra of carbon disulfide $\left(\mathrm{C}{\mathrm{S}}_2\right)$ recorded at different excitation conditions. (a) photoionization at $\lambda =115.87\text{nm}$ (10.70 eV); (b) primary photoexcitation 193 nm (6.424 eV) and subsequent photoionization (10.70 eV); (c) photoexcitation 193 nm (6.424 eV) without subsequent photoionization, indicating multiphoton ionization caused by 193 nm (6.424 eV) radiation; (d) difference (b)–(c); (e) similar to (d), but photoionization at 10.25 eV.

Figure 2

Photoionization of $\mathrm{S}\left(3s^{2}3p^4{}^{3}P\right)$ in the energy range from threshold up to 30 eV. The various Rydberg series limits are illustrated, namely ${\mathrm{S}}^{+}\left[3s^{2}3p^3\left({}^2{D}_{3/2,5/2}^o\right)\right]$ and ${\mathrm{S}}^{+}\left[3s^{2}3p^3\left({}^2{P}_{1/2,3/2}^o\right)\right]$ by vertical dashed lines. The prominent window resonances are clearly visible. (a) The experimental cross section data are taken at a nominal energy resolution of 44 meV. (b) Theoretical cross sections for 100% of the $\mathrm{S}\left(3s^{2}3p^4{}^{3}P\right)$ initial state from the 512-level DARC calculations (level averaged), convoluted with a Gaussian profile having a FWHM of 44 meV. The corresponding series limits $E_{\infty }$ of Eq. (2) for each series are indicated by vertical dashed lines.

Figure 3

An overview of the present single photoionization cross section measurements as a function of the photon energy below 13.5 eV. The experimental measurements were made at a nominal energy resolution of 44 meV. The assigned Rydberg series limits are indicated as vertical lines grouped by horizontal lines. The corresponding series limits $E_{\infty }$ of Eq. (2) for each series are indicated by vertical dashed lines at the ends of the line groups. See text for a discussion of these resonance features.

Figure 4

An overview of the experimental measurements on $\mathrm{S}\left({}^{1}D\right)$ as a function of the photon energy. (a) Experiment for the $\mathrm{S}\left(3s^{2}3p^4{}^{1}D\right)$ metastable state over the energy range from threshold to 30 eV. Primary photolysis of $\mathrm{C}{\mathrm{S}}_2$ was carried out at $\lambda =193$ nm (6.424 eV). Contributions from ionic fragmentation photolysis of $\mathrm{C}{\mathrm{S}}_2$ are not subtracted. The curve is normalized to the VUV-photon flux. (b) Theoretical cross section results for 100% of the $\mathrm{S}\left(3s^{2}3p^4{}^{1}D\right)$ metastable state from the DARC calculations convoluted at 44 meV. The resonance series limits $E_{\infty }$ of Eq. (2) for each series are indicated by vertical dashed lines at the ends of the line groups. Resonance features from this figure are tabulated in Table 2.

Figure 5

Single-photon ionization of sulfur as a function of the photon energy. (a) The experimental cross section is recorded with a nominal energy resolution of 44 meV. The assigned Rydberg series are indicated by vertical lines grouped by horizontal or inclined lines. The corresponding series limits $E_{\infty }$ of Eq. (2) for each series are indicated by vertical dashed lines at the end of the line groups. The first few values of $n$ for each series is displayed close to its corresponding vertical line in each group. (b) Single photoionization of atomic sulfur as a function of the photon energy in the $3s^{2}3p^4{}^{1}D$ metastable state from threshold to 12.5 eV. Theoretical cross section calculations were carried out with the DARC codes and convoluted with a Gaussian having a profile of 44 meV. The assigned Rydberg series are indicated as vertical lines grouped by horizontal or inclined lines. The corresponding series limits $E_{\infty }$ of Eq. (2) for each series are indicated by a vertical dashed lines at the ends of the line groups. Resonance energies and quantum defects for the series lying below the ${\mathrm{S}}^{+}\left({}^2{D}_{3/2,5/2}^o\right)$ thresholds are tabulated in Table 2.

Figure 6

Theoretical cross sections from the 512-level DARC calculations for sulfur $3s^{2}3p^4{}^3{P}_{2,1,0}$ and $3s^{2}3p^4{}^1{D}_2$ and $3s^{2}3p^4{}^1{S}_0$ initial states convoluted with a Gaussian profile of 44 meV. Single photoionization cross sections of the sulfur atom as a function of energy over the photon energy from thresholds to 30 eV, illustrating strong resonance features in the spectra below 14 eV. (a) $3s^{2}3p^4{}^3{P}_2$, (b) $3s^{2}3p^4{}^3{P}_1$, (c) $3s^{2}3p^4{}^3{P}_0$, (d) $3s^{2}3p^4{}^{3}P$ level averaged, (e) metastable $3s^{2}3p^4{}^1{D}_2$, and (f) metastable $3s^{2}3p^4{}^1{S}_0$ cross sections. The corresponding series limits $E_{\infty }$ of Eq. (2) for the window resonance converging to $3s^{2}3p^4{}^{4}P$ and $3s^{2}3p^4{}^{2}P$ sulfur ion thresholds are indicated by vertical dashed lines.

Figure 7

Theoretical cross sections results from the 512-level DARC for the sulfur $3s^{2}3p^4{}^3{P}_{2,1,0}$ and $3s^{2}3p^4{}^1{D}_2$ and $3s^{2}3p^4{}^1{S}_0$ initial states, in the photon energy region 17–24 eV, convoluted with a Gaussian having a profile of 44 meV. (a) $3s^{2}3p^4{}^3{P}_2$, (b) $3s^{2}3p^4{}^3{P}_1$, (c) $3s^{2}3p^4{}^3{P}_0$, (d) $3s^{2}3p^4{}^{3}P$ level averaged, (e) metastable $3s^{2}3p^4{}^1{D}_2$, and (f) metastable $3s^{2}3p^4{}^1{S}_0$ cross sections. In this energy region the prominent window resonances converging to the singly ionized sulfur ion threshold $3s^{2}3p^4{}^{4}P$ (lowest vertical dashed line) are clearly visible in the cross sections. See text for further details.

Figure 8

Single photoionization of atomic sulfur as a function of the photon energy in the $3s^{2}3p^4{}^{1}D$ metastable state from threshold to 12.5 eV. Theoretical cross sections were carried out with the DARC codes and convoluted with a Gaussian having a profile of 44 meV. The assigned Rydberg series are indicated as vertical lines grouped by horizontal or inclined lines. The corresponding series limits $E_{\infty }$ of Eq. (2) for each series are indicated by vertical dashed lines at the ends of the line groups. The first few values of $n$ for each series is displayed close to its corresponding vertical line in each group. Resonance energies and quantum defects for the various series are tabulated in Table 2.