Absolute Photoionization Cross-section Measurements Research Articles

Valence Photoionization and Autoionization of the Formyl Radical.

We have used 308 nm photolysis of acetaldehyde to measure a photoionization spectrum of the formyl (HCO) radical between 8 and 11.5 eV using an 11 meV FWHM photoionization energy resolution. We have confirmed that the formyl radical is the carrier of the spectrum by generating an identical spectrum of the HCO product in the Cl + H2CO reaction. The spectrum of HCO and its deuterated isotopologue (DCO) have several resolved autoionizing resonances above the Franck-Condon envelope, which we assign to autoionization after initial excitation into neutral 3sσ and 3p Rydberg states converging to the first triplet excited state of HCO+(ã 3A'). The quantum defects for these states are δ3sσ = 1.06 ± 0.02 and δ3p = 0.821 ± 0.019. We report absolute photoionization cross-section measurements of σHCOPI(9.907 eV) = 4.5 ± 0.9 Mb, σHCOPI(10.007 eV) = 4.8 ± 1.0 Mb, σHCOPI(10.107 eV) = 6.0 ± 1.2 Mb, σHCOPI(10.107 eV) = 5.7 ± 1.2 Mb, and σHCOPI(10.304 eV) = 10.6 ± 2.2 Mb relative to the photoionization cross section of the methyl radical. The absolute cross-section measurements are a factor of ∼1.5 larger than those determined in past studies, although the presence of strong autoionizing features supports a dependence on photoionization energy resolution. We propose that the semiempirical model of Xu and Pratt for estimation of free radical photoionization cross sections is more accurate when applied with a reference species containing the same atoms as the free radical rather than isoelectronic species with different atoms.

Photoionization measurements for the iron isonuclear sequence Fe3+, Fe5+, and Fe7+

Cross sections for single photoionization of Fe${}^{3+}$, Fe${}^{5+}$, and Fe${}^{7+}$ ions were measured at spectral resolutions of 0.04, 0.15, and 0.13 eV, respectively, by merging mass- and charge-selected ion beams with a beam of monochromatized synchrotron undulator radiation. The measurements span photon energy ranges beginning at the ionization thresholds and extending several tens of electron volts to include the most important resonant contributions due to $3p$--$\mathit{nd}$ transitions to autoionizing states. The photoion yield spectra are characterized by narrow resonances and also broad features in Fe${}^{3+}$ and Fe${}^{5+}$ that are believed to result from unresolved fast super-Coster-Kronig transitions following $3p$--$3d$ excitation. Absolute photoionization cross-section measurements were also performed using ion beams containing undetermined fractions of ions in their ground and metastable states. A Rydberg series attributed to $3p$--$\mathit{nd}$ transitions from the ${}^{2}{G}_{1}$ metastable state in Fe${}^{3+}$ was identified. The data are compared with recently published measurements on Fe${}^{3+}$ and Fe${}^{5+}$ using a similar technique at lower spectral resolution.

Experimental photoionization cross-section measurements in the ground and metastable state threshold region of Se+

Absolute photoionization cross-section measurements are reported for Se+ in the photon energy range 18.0–31.0 eV, which spans the ionization thresholds of the 4S3/2 ground state and the low-lying 2P3/2, 1/2 and 2D5/2, 3/2 metastable states. The measurements were performed using the Advanced Light Source synchrotron radiation facility. Strong photoexcitation–autoionization resonances due to 4p → nd transitions are seen in the cross-section spectrum and identified with a quantum-defect analysis.

Valence-shell photoionization of the Cℓ-like ion Ar+: Experiment and theory

Absolute photoionization cross-section measurements for Ar+ ions and theoretical calculations performed within the semi-relativistic Breit-Pauli R-matrix approximation are reported for the photon energy range 27.4 eV to 60.0 eV. The resonance features are assigned spectroscopically, and their energies and quantum defects tabulated with excellent agreement being obtained between experiment and theory.

Absolute photoionization cross-section measurements of the Kr I isoelectronic sequence

Photoionization spectra have been recorded in the $4s$, $4p$, and $3d$ resonance regions for the Kr I isoelectronic sequence using both the dual laser produced plasma (DLP) technique (at DCU) to produce photoabsorption spectra, and the merged ion beam and synchrotron radiation technique (at ASTRID) to measure absolute photoionization cross sections. Profile parameters are compared for the $4s\text{\ensuremath{-}}np$ resonances of ${\mathrm{Rb}}^{+}$ and ${\mathrm{Sr}}^{2+}$. Many $4p\ensuremath{\rightarrow}ns$, $md$ transitions are identified with the aid of Hartree-Fock calculations, and consistent quantum defects are observed for the various $ns$ and $md$ Rydberg series. Absolute single and double photoionization cross sections recorded in the $3d$ region for ${\mathrm{Rb}}^{+}$ and ${\mathrm{Sr}}^{2+}$ ions show preferential decay via double photoionization. This is only the second report to our knowledge where both the DLP technique and the merged-beam technique have been used simultaneously to record photoionization spectra, and the advantages of both techniques (i.e., better resolution in the case of DLP and values for absolute photoionization cross sections in the case of the merged-beam technique) are highlighted.

Absolute photoionization cross sections forXe4+,Xe5+, andXe6+near13.5nm: Experiment and theory

Absolute photoionization cross-section measurements for a mixture of ground and metastable states of ${\mathrm{Xe}}^{4+}$, ${\mathrm{Xe}}^{5+}$, and ${\mathrm{Xe}}^{6+}$ are reported in the photon energy range of $4d\ensuremath{\rightarrow}nf$ transitions, which occur within or adjacent to the $13.5\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ window for extreme ultraviolet lithography light source development. The reported values allow the quantification of opacity effects in xenon plasmas due to these $4d\ensuremath{\rightarrow}nf$ autoionizing states. The oscillator strengths for the $4d\ensuremath{\rightarrow}4f$ and $4d\ensuremath{\rightarrow}5f$ transitions in ${\mathrm{Xe}}^{q+}$ $(q=1--6)$ ions are calculated using nonrelativistic Hartree-Fock and random phase approximations. These are compared with published experimental values for ${\mathrm{Xe}}^{+}$ to ${\mathrm{Xe}}^{3+}$ and with the values obtained from the present experimental cross-section measurements for ${\mathrm{Xe}}^{4+}$ to ${\mathrm{Xe}}^{6+}$. The calculations assisted in the determination of the metastable content in the ion beams for ${\mathrm{Xe}}^{5+}$ and ${\mathrm{Xe}}^{6+}$. The experiments were performed by merging a synchrotron photon beam generated by an undulator beamline of the Advanced Light Source with an ion beam produced by an electron cyclotron resonance ion source.

Absolute measurements of photoionization cross-sections for ions

A merged-beam set-up for absolute measurements of photoionization cross-sections of ions is described. The facility is capable of recording cross-sections as low as 10 −19 cm 2 and has been used to study a large number of singly- and multiply-charged, atomic and molecular, positive and negative ions. It is based on a synchrotron radiation beam line fitted with an undulator at the storage ring ASTRID and a low-energy (∼2 keV) ion beam line. Photons in the energy range 15–200 eV are merged co-linearly with the target ions over a distance of 50 cm, and the absolute photoionization cross-section is determined from the resulting photoion yield with a typical accuracy of 10%. Different types of ion sources are available, thus permitting a large number of positive and negative, atomic and molecular, singly- and multiply-charged ions to be investigated. Emphasis is put on accurate determination of the absolute cross-sections, requiring calibration of photodiode and particle detectors together with measurements of the photon–ion overlap.

Photoexcitation of a Volume Plasmon inC60Ions

Neutral C60 is well known to exhibit a giant resonance in its photon absorption spectrum near 20 eV. This is associated with a surface plasmon, where delocalized electrons oscillate as a whole relative to the ionic cage. Absolute photoionization cross-section measurements for C+60, C2+60, and C3+60 ions in the 17-75 eV energy range show an additional resonance near 40 eV. Time-dependent density functional calculations confirm the collective nature of this feature, which is characterized as a dipole-excited volume plasmon made possible by the special fullerene geometry.

A critical test of many-body theory: The photoionization cross section of Cl as an example of an open-shell atom.

The present work reports the first absolute photoionization cross-section measurements for the open-shell atom chlorine. Measurements were made from the $^{1}\mathrm{S}$ ionization threshold at 16.4 eV to 75 eV. Agreement with several calculations is good at the higher photon energies. We have observed a Cooper minimum at about 44 eV as predicted by theory. However, near threshold the various calculations differ considerably with each other and with the present results.

Photoionization of atomic nitrogen

Relative photoionization cross sections were measured for atomic nitrogen from 870 to 600 Å at a wavelength resolution (FWHM) of 0.83 Å. Five members of the autoionizing Rydberg series 2s 2p3(5S0)np 4P were observed, and values of the line shape parameters q, Γ, and ρ2 were determined for the two lowest energy transitions (n = 3 and 4). Oscillator strengths were estimated for both transitions after normalizing the present data to absolute photoionization cross section measurements. The results are q = −1.7±0.1, Γ=0.0275±0.003 eV ρ2=0.51±0.04, and f =0.00580±0.00109 for the n = 3 transition and q = − 1.7 ± 0.1, Γ = 0.010 ± 0.003 eV, ρ2 = 0.58 ± 0.15, and f =0.00240±0.00100 for the n =4 transition. The photoionization cross section showed no other structure in the wavelength region from the NI ionization threshold to 600 Å.