Oscillator strength measurements of the 3p ↦ nd Rydberg transitions of sodium

We report new measurements of the oscillator strengths of the 3p ²P_3/2 ↦nd ²D_{5/2, 3/2} and 3p ²P_1/2 ↦ nd ²D_3/2 Rydberg transitions of sodium using a thermionic diode ion detector in conjunction with the Nd:YAG pumped dye lasers. The ns ²S_1/2 and nd ²D_5/2,3/2 Rydberg series have been recorded via two-step excitation, from the 3p ²P_3/2 and 3p ²P_1/2 intermediate states. Employing the saturation technique, the photoionization cross sections from the 3p ²P_3/2 and 3p ²P_1/2 intermediate states at the first ionization threshold are determined as 7.9(1.3) Mb and 6.7(1.1) Mb respectively. The f-values of the Rydberg transitions are calibrated with the photoionization cross section measured at the first ionization threshold and compared with the earlier data.     

Photoionization cross section and oscillator strength distribution in the near-threshold region of strontium

We present experimentally measured absolute values of the photoionization cross sections from the 5s5p ¹P₁ and 5s5p ³P₁ excited states of strontium at the first ionization threshold as 11.4±1.8 Mb and 10.7±1.7 Mb respectively using saturated absorption technique along with a thermionic diode ion detector in conjunction with a Nd:YAG pumped dye laser system. These threshold photoionization cross sections values have been utilized to determine the oscillator strengths of the 5s5p ¹P₁↦5snd ¹D₂ and 5s5p ³P₁↦5snd ³D₂ Rydberg transitions. The oscillator strength densities in the continuum corresponding to the 5s5p ³P₁ excited state have also been determined by measuring the photoionization cross sections at five ionizing wavelengths above the first ionization threshold. Smooth merging of the discrete f-values into the oscillator strength densities has been observed for the 5s5p ³P₁↦5snd ³D₂ series across the ionization threshold.     

Photoionization cross sections and oscillator strengths of neutral cesium

The absolute photoionization cross sections from the 6p ²P_1/2 excited state of cesium at threshold and above the threshold region have been measured using the saturation absorption technique. The photoionization cross section at the ionization threshold is determined as 22.6±3.6 Mb, whereas in the region above threshold its value ranges from 22 to 20 Mb for photoelectron energies up to 0.1 eV. A comparison of the photoionization cross sections with earlier reported theoretical and experimental data have been presented and are in good agreement within the uncertainty. In addition, the oscillator strengths of the 6p ²P_1/2→n d ²D_3/2 (21≤n≤60) Rydberg transitions of cesium have been calibrated using the threshold value of the photoionization cross section. A complete picture of the oscillator strengths from the present work and previously reported data from n=5–60 is presented.     

Two-color three-photon excitation studies of thallium Rydberg states

We present new data on the even-parity Rydberg states of atomic thallium using two-step three-photon laser excitation technique in conjunction with a thermionic diode ion detector. Atoms are excited from the 6p ²P_1/2 ground state to the 7p ²P_1/2 intermediate state via two-photon excitation and subsequently promoted to the high lying ns ² S_1/2 and nd ²D_3/2 Rydberg states. The first ionization potential of thallium is determined as 49,266.66(1) cm^-1 using data for the ns ² S_1/2 (25 ≤ n ≤ 54) and nd ²D_3/2 (24 ≤ n ≤ 65) Rydberg series. This value is believed to be more accurate because the contribution due to the hyperfine structure splitting of the 7p ²P_1/2 state (0.07185 cm^-1) is much smaller as compared to that of the 6p ²P_1/2 ground state (0.711 cm^-1).     

Measurement of the photoionization cross-section of the 3p 2P1/2, 3/2 excited levels of sodium

The photoionization cross-section and number density of the 3p ²P_1/2,3/2 excited levels of sodium have been measured as a function of the laser energy using two-step laser excitation in conjunction with a thermionic diode working in the space charge limited mode. Employing the saturation technique, the cross-sections for the 3p ²P_1/2 and 3p ²P_3/2 levels are determined as 2.16 (43) Mb and 3.74 (74) Mb respectively.     

Photoionization cross-section measurements from the 2p, 3d and 3s excited states of lithium

The photoionization cross-sections from the 2p²P_{1/2, 3/2}, 3d²D_{3/2, 5/2} and 3s²S_1/2 excited states of lithium have been measured at different ionizing laser wavelengths, above the first ionization threshold. The experiments are performed by using a thermionic diode working in the space charge limited mode and the cross-sections are measured by employing the saturation technique. By changing the ionization photon energy, a smooth frequency dependence of the cross-sections has been observed for the 2p and 3d states. The cross-section from the 3s excited state has been measured at a single photon energy. The measured values of the photoionization cross are compared with the available data.     

Loading rate of Yb<Superscript>+</Superscript> loaded through photoionization in radiofrequency ion trap

We investigate the loading rate of Yb⁺ ions loaded through photoionization in a radiofrequency trap. The absolute or relative number of the loaded trapped ions is measured by use of an electric resonance of the secular motion. This method is applicable even in the presence of anharmonicity. In two-color photoionization, where the first-excitation laser drives the ¹S₀–¹P₁ transition in the Yb atom and the second one ionizes the atom from the ¹P₁ state, the loading rate is at its highest by the excitation of the ionization potential. A similar loading rate is observed at the second-laser wavelength around 369.5 nm, which is the wavelength for the cooling transition of Yb⁺. We estimate the loading cross section to be 40(15) Mb for the two-color excitation of the ionization potential. The excitation of the Yb atoms in the Rydberg states is detected by the enhancement of the loading rate. By irradiation with only the first-excitation laser, Yb⁺ is produced at a rate three orders of magnitude smaller than that when the non-resonant two-photon absorption from the ¹P₁ state is the dominant process. We also measure the charge-exchange rate between Yb⁺ and Yb, and discuss its effect on isotope-selective photoionization loading.     

Photoionization cross sections of Fe XXI

Total and partial photoionization cross sections for (Fe XXI+hν→Fe XXII+e) are presented for the ground and excited bound states with n?10 and l?9. Fe XXI is prevalent in high-temperature astrophysical plasmas as well as in photoionized plasmas excited by hard X-rays. Results are reported for the first time for the high-energy photoionization with core excitations to n=2,3 states. Details of photoionization, especially the high-energy features that often dominate considerably over the low energy ones, are illustrated. These prominent features will affect the photoionization and the recombination rates in high-temperature plasmas. Calculations are carried out in the close coupling (CC) approximation using the R-matrix method. A large CC wavefunction expansion for Fe XXII which includes the ground and 28 excited core states from n=2 and 3 complexes and spans over a wide energy range is used. A total of 835 discrete bound states of Fe XXI in the singlet, triplet, and quintet symmetries are obtained. Total photoionization cross sections, σ_PI(nLS), for ionization into all 29 states are presented for all 835 final bound states and partial photoionization cross sections, σ_PI(g,nLS), for ionization into the ground ²P⁰ state of the core are presented for 685 states. While the n=2 core excitations are at relatively lower energy range (within 15 Ry from the ionization threshold), the n=3 excitations lie at considerably higher energy, 73 Ry and above, yet introduce resonant features and enhancements more prominent than those of n=2 states. Larger numbers of resonances are formed due to Rydberg series of autoionizing states converging on to the 29 core states. However, most noticeable structures are formed in the excited state cross sections by the photoexcitation-of-core (PEC) resonances in the photon energy range of 73-82 Ry. All these high-energy features are absent in the currently available results. The present results should enable more accurate modeling of the emission spectrum of highly excited plasma from the optical to far-ultraviolet region.     

Superfluorescence from europium atom at 629.977 nm

Visible superfluorescence at 629.977 nm is observed in europium atom with very high optical conversion efficiency on the transition 5d6p ¹⁰F_7/2 → 5d6s ¹⁰D_7/2. The peak intensity of fluorescence varies as square of the number of atoms in the excited state (N), which shows the superfluorescence character of the transition. The ratio of average superfluorescence power to excitation laser power is observed to be ∼15% in the forward direction.     

The 4d-4f dipole resonance of the Pr atom in an endohedral metallofullerene, Pr@C82

The photoion yield spectra of an endohedral metallofullerene Pr@C₈₂ were measured in the photon energy range of 100-150 eV by using time-of-flight mass spectrometry. Parent ions Pr@C₈₂⁺, Pr@C₈₂²⁺ and Pr@C₈₂³⁺ were observed in the mass spectra. The photoion yield spectra of Pr@C₈₂²⁺ showed a broad peak at 120-140 eV that was assigned to the 4d-4f giant dipole resonance of the encapsulated Pr atoms. Absolute photoabsorption cross sections of Pr@C₈₂ were evaluated from the photoion yield spectra to be 37±12 Mb at 110 eV (off-resonance) and 52±13 Mb at 130 eV (on-resonance). These cross sections of Pr@C₈₂ were compared with the results of Ce@C₈₂, the only metallofullerene whose photoionization properties have ever been studied near the 4d edge of the encapsulated metal atom. The enhancement of photoabsorption due to the giant resonance was found to be similar in Pr@C₈₂ and Ce@C₈₂, but there are marked differences in the peak shapes, which can be explained as due to interference effects between the fullerene cage and the encapsulated metal atoms.     

Electric field induced anomalies in frequency shift and collisional perturbations in the 5snd D2 Rydberg series of strontium in a heatpipe setup with ionization detection

We have examined the effect of weak DC electric field (2-20 V/cm) complimented by foreign gas collisions on the bound J = 2 even-parity 5snd ¹D₂ Rydberg states of neutral strontium. We use resonant two-photon transverse excitation, employing a narrow bandwidth tunable dye laser and an atomic jet in a heatpipe setup with ionization detection. In this paper we report certain anomalies in the observed spectra covering principal quantum number range n = 27-42 indicating a frequency shift reversal with nearly quadratic dependence on the field strength above an anti-resonance region. Furthermore, we have observed the emergence of highly localized doubly-excited 4d² states, including a remarkably broad perturber with large angular momentum, uncovering orbital contraction effect. This non-Rydberg excitation, which intrudes upon the two-photon spectrum with large energy overlap is due to single-photon excitations from the 5s5p ¹P₁ resonance level following molecular dissociation of the Sr₂ dimers suitably governed by binary atomic collisions. Our study which involves laser excitation complimented by electric field and collisions using inexpensive home-made setup opens up the possibilities for a new class of experiments, with considerable simplicity in the choice of excitation schemes for both Rydberg and non-Rydberg transitions, to reach states lying at high energies which cannot otherwise be accessed from the ground state due to parity and selection rules, while allowing one to probe localization properties of atomic wave functions.     

Ultraviolet upconversion luminescence in Er-doped Y2O3 excited by 532 nm CW compact solid-state laser

Ultraviolet upconversion emissions at 262, 276, 308 and 320 nm were observed from Er³⁺-doped Y₂O₃ with a 532 nm continuous wave compact solid-state laser excitation. Power-dependence analysis demonstrates that two-photon upconversion process populates the ⁴D_5/2, ²H_9/2 and ²P_3/2 states. The energy transfer upconversion (ETU) plays an important role in populating ⁴D_5/2 and ²P_3/2 states. It appears that ²P_3/2 state population originates from ETU ²H_11/2+²H_11/2→⁴I_13/2+²P_3/2, moreover, a subsequent excited state absorption (ESA) from the ⁴I_9/2 level.     

Approximation of the electron excitation cross sections for triplet states excited from the 23 S 1 metastable state in helium

The method of approximated four-parameter representation of the electron-impact cross sections for a helium atom excited from the 2³ S ₁ metastable state into higher triplet states is applied and discussed. The approximation consists in interpolation over the whole set of the cross section values for each helium atomic level measured in our experiments and reported by other researchers. The approximation parameters and the cross sections calculated using these parameters for the maxima of the corresponding excitation functions are presented for 19 triplet levels of the S, P, and D HeI series with n=2–10. The interpolated values are compared to the theoretical cross sections. The serial regularities were investigated for the S, P, and D levels studied and a decrease in the cross sections for excitation from the given metastable state within each series, described by the approximate law Q=Cn ^?5, was revealed. Validity of the similarity relationship and the Bethe approximation for cross sections in the 2³ S?n ³ P series was verified. It is shown that the cross sections for a triplet level excitation from the 2³ S ₁ metastable state exceed the corresponding values for excitation from the ground state of helium by a factor of approximately 10³ for n=2 and 3 and 10 2 for the higher levels. It is concluded that the proposed method of representation of the cross sections for the electron-impact excitation of triplet levels from the metastable state increases accuracy and more importantly, reliability of the final results.     

Odd autoionizing 2p<Subscript>1/2</Subscript><Superscript>5</Superscript>n(s<Superscript>′</Superscript>/d<Superscript>′</Superscript>) resonances of Ne excited from the 2p<Superscript>5</Superscript>3p[K]<Subscript>J</Subscript> states

Absolute photoionization cross sections for Ne atoms in the excited levels (Paschen notation ) were calculated at near threshold energies within the configuration interaction Pauli-Fock approach including core polarization. The computed spectra and the lineshape parameters of the odd parity 2p_1/2 ⁵ns^′/d^′ autoionizing resonances are found to be in good agreement with high resolution laser spectroscopic results. Guided by the theoretical results, improved analyses of the measured spectra by superimposed Fano-type profiles were achieved. Theoretical predictions are presented for resonances which have not yet been studied experimentally. In addition, we report the absolute partial photoionization cross sections for the ²P_3/2 and ²P_1/2 channel at photoelectron energies up to 7 eV. Except for the highest lying 2p₁(¹S₀) level, these cross sections monotonically decrease with energy (as reported earlier in single-electron calculations for the Ne(2p⁵3p) configuration) with branching ratios which essentially reflect the core composition of the 2p_x levels. For the 2p₁ level the resonance structure and the partial cross sections are strongly influenced by a Cooper-Seaton minimum in the d_3/2 ^′ channel,located just above the ²P_1/2 ionization limit.     

Many-electron character of Rydberg series converging to the ionization thresholds of the 4<Emphasis Type="Italic">p</Emphasis><Superscript>4</Superscript>5<Emphasis Type="Italic">s</Emphasis> states of KrII

The photoionization cross sections of the 4p shell and the 4s main level and 4p ⁴(³ P) 5s ⁴ P _{5/2, 3/2} satellite subvalence levels of KrII have been calculated in the 4s-near-threshold range of excitation energies from 28.48 to 28.70 eV. The calculation takes into account the core relaxation by the methods of the theory of non-orthogonal orbitals, the interaction between resonant states through autoionization channels by solving the complex secular equation, and the interaction between the channels of the continuous spectrum in all orders of the perturbation theory by the K-matrix method. Good quantitative agreement between the energy-integrated theoretical and experimental photoionization cross sections for the satellite levels has been obtained for the first time. It is shown that only simultaneous consideration of the above-mentioned effects leads to such agreement. The resonant structure of the photoionization cross sections in this excitation energy range is related to the autoionization decay of the 4p ⁴5s(⁴ P _1/2)np and 4p ⁴5s(² P _3/2)np Rydberg series. The specificity of this process is that both series manifest themselves not independently but owing to their strong electrostatic interaction with the prominent 4p ⁴(¹ D)5s ² D _5/2 6p _3/2 resonance, which lies in this excitation energy range.     

Alternate technique for simultaneous measurement of photoionization cross-section of isotopes by TOF mass spectrometer

New measurements of photoionization cross-sections of the lithium isotopes are reported employing a Time of Flight (TOF) mass spectrometer in conjunction with an atomic beam apparatus. Using a two-step selective photoionization and saturation technique, we have simultaneously measured the photoionization cross-section of the 2p excited state of both the isotopes Li⁶ and Li⁷ as 15±2.5 Mb and 18 ±2.5 Mb where as the corresponding number densities have been determined as N₀≈5.3×10¹⁰ atoms/cm³ and N₀≈6.2×10¹¹ atoms/cm³ respectively.     

Three-photon resonance enhanced excitation spectrum of atomic thallium

We present first measurements on the resonance enhanced three-photon excitation in thallium, using a Nd:YAG laser pumped dye laser in conjunction with a thermionic diode ion detector. The even-parity 6s²ns²S_1/2 (15 ? n ? 31) and nd ²D_5/2 (13 ? n ? 42) Rydberg states have been observed. The measured level energies reveal a dynamic shift from the photoabsorption values, which is decreasing with increasing n, while the asymmetry in the line profile is observed to be increasing with increasing n. In addition, an autoionising level (sp²⁴P_3/2) adjacent to the ionization threshold has been observed and quantitatively analyzed using the Fano’s photoionization cross-section relation for an isolated autoionising resonance.     

Cation spectroscopy of 3,4-difluoroaniline by two-color resonant two-photon mass-analyzed threshold ionization

We applied the two-color resonant two-photon mass-analyzed threshold ionization technique to record the vibrationally resolved cation spectra of 3,4-difluoroaniline (34DFA) via the 0⁰, X¹, 6b¹, and I² levels of the S₁ state. The adiabatic ionization energy of this molecule was determined to be 64 195 ± 5 cm⁻¹. Most of the observed active modes of the 34DFA cation in the D₀ state are related to the in-plane ring deformation vibrations. Comparing these data with those of 3-fluoroaniline and 4-fluoroaniline, one can learn the effects of fluorine substitution on the electronic transition and molecular vibration.     

Luminescent properties of trivalent praseodymium-doped lanthanum aluminum germanate LaAlGe2O7

The luminescent characteristics of Pr³⁺-activated LaAlGe₂O₇ were investigated. In response to excitement using 448 nm blue light, the emission spectra involved most of the ³P₀→³H_J transitions. The dominant emission came from the ³P₀→³H₄ transition at 487 nm. ¹D₂ fluorescence quenching was observed in highly doped samples and is related to the cross-relaxation processes among neighboring Pr³⁺ ions. In contrast with conventional Pr³⁺-activated phosphors, the extraordinary excitation spectra showed only intense f-f transition of Pr³⁺ ions, while the 4f-5d transition was eliminated. This is ascribed to photoionization. By analyzing absorption and excitation spectra, it is recognized that no efficient energy transfer occurs between Pr³⁺ and the host lattice in LaAlGe₂O₇.     

Shallow levels in virgin hydrothermally grown n-type ZnO studied by thermal admittance spectroscopy

Three n-type single crystal hydrothermally grown ZnO samples with resistivities of 5.1±0.6, 15±2 and 220±20 Ω cm, respectively, have been electrically characterized using thermal admittance spectroscopy (TAS). The presence of three main donors: two shallow ones D₁ and D₂ and a deeper one D₃ with activation energies of ∼30, ∼50 and ∼290 meV, respectively, are detected. In addition, the TAS spectra reveal the presence of a fourth level, D_X, with a peak amplitude in the conductance spectra that decreases with the temperature occurrence. It is shown that this anomalous behavior is consistent with D_X being a negative-U defect of donor-type. An activation energy of ∼80 meV for the ++/+ transition, a capture cross section equal to ∼3×10⁻¹⁷ cm² and an energy barrier for atomic reconfiguration of ∼0.25 eV, respectively, deduced according to the assignment of D_X to a negative-U defect. A tentative assignment of the D_X defect with oxygen vacancies is discussed.