Two-color excitation of NO in a supersonic free jet. Autoionization of high rydberg states

High Rydberg states of NO above the ionization limit have been measured for the isolated molecule in a supersonic free jet by two-color multiphoton ionization. Three Rydberg series (ns, np and nf) were identified, which appeared by rotational and the vibrational autoionization. The rotational structures of the 13s(υ = 1), 13p(υ = 1) and 12f(υ = 1) states have been analyzed in detail. The fluorescence dip spectra for the intermediate A²Σ⁺(3sσ) state have been measured simultaneously and the cross sections of the one-photon absorption to the high Rydberg states from the A²Σ⁺(υ = 1) state have been determined.     

L-shell ionization in Au by O5+-and Ni5+-ion impact

TheL X-ray production cross sections in gold by 60 to 72 MeV O⁵⁺ ions and 58 to 87 MeV Ni⁵⁺-ions have been measured. TheL-subshell ionization cross sections derived from these experimental results have been compared with the direct ionization theories viz. plane wave Born approximation (PWBA) theory and modified perturbed-stationary-state theory with energy loss, Coulomb deflection and relativistic effects (MECPSSR). A new procedure has been described to account for the change in the yield ratioLβ₁/Lβ_{2, 15} with energy, for Ni⁵⁺-ion impact on gold. TheL sub-shell ionization cross sections have been derived fromLβ₁,Lβ₄ andLβ_{2, 15} lines of theLβ group in addition to those calculated by the conventional Datz TRY3 technique usingLα,Lγ₁ andLγ_{2, 3} X-ray lines. From the shift in the energies of variousL X-ray lines and changes in their intensities, 3 and 5 spectator vacancies in theM- andN-shells in gold have been estimated with O⁵⁺-ion impact while 7, 20 and 4 to 6 spectator vacancies have been inferred in theM-,N-andO-shells respectively with Ni⁵⁺-ion impact in the energy range of the projectiles undertaken in the present studies.     

Resonant and Off-resonance ionization of laser-excited sodium vapor

Electrons produced in a sodium effusive beam irradiated by pulsed laser light have been energy-analyzed. For resonant excitation of the atoms to the 3p state, we have studied associative ionization of excited Na(3p) (AI), Penning ionization (PI) and photoionization of highly-excited Na(n l),n l=3d, 4p, 5s, 4d/4f, by measurements of electron energy spectra. Our interpretation of the spectra is supported by a numerical analysis of the data which leads to estimates of the cross sections compatible with values previously measured in other experiments. Off-resonance ionization is considered at a particular wavelength. It is presumed to involve laser-excited dimers Na ₂ ^** . Spectra show that highly-excited atoms are produced by photodissociation of Na ₂ ^** or from Na ₂ ^** -Na collisional energy transfer, and that these atoms support the dominant off-resonance ionization channel in our laser excitation conditions.     

Oscillator strength measurements of the 5s5p 3P1 → 5snd 3D2 Rydberg transitions of cadmium

We report oscillator strengths distribution in the bound region of cadmium corresponding to the 5s5p ³P₁ → 5snd ³D₂ (21 ≤ n ≤ 52) Rydberg transitions. The experiment was performed using two frequency doubled dye lasers simultaneously pumped by a common Nd:YAG laser in conjunction with an atomic beam set-up. The absolute photoionization cross section of the 5s5p ³P₁ intermediate state at the 5s ²S_1/2 ionization threshold has been measured as 20 (4) Mb using the saturation technique. The measured value of the photoionization cross section at threshold is used to extract the f-values of the above mentioned Rydberg transitions.     

Determination of D0o(BaI) from the chemiluminescent reaction Ba + I2

A study is made of the visible chemiluminescence resulting from the reaction of an atomic beam of barium with I_Z under single-collision conditions (～ 10^?4 torr). The resulting spectrum consists of the BaI C²Π → X²Σ emission on top of an underlying “continuum”. The variation of the BaI emission intensity with Ba and I₂ flux is investigated, and it is concluded that the reaction is bimolecular. The total phenomenological cross section for barium atom removal is determined to be 86 A², which agrees well with the total reactive cross section calculated assuming an electron jump mechanism. The short wavelength cutoff is identified as the transition from the υ′ = 41 level of the BaI C²Π_{$\text{3}\text{2}$} state to the υ′ = 41 level of the BaI X²Σ state. A strict lower bound D^o₀(BaI) ? 102 ± 0.7 kcal/mole for the ground state dissociation energy of BaI is obtained from this short wavelength cutoff. The value D^o₀(BaI) = 102 ± 1 kcal/mole is recommended, where the error estimate includes the possible contribution from the final relative translational energy of the products.     

Angular distributions in the double ionization of noble gases by electron impact

Electron-impact double ionization of noble gases is investigated theoretically for the case of high incident energies (5 keV). An ab initio calculation is made including partial correlation in the initial state as well as in the final state. The results of the calculations are compared with those of other theories and with the first available (e, 3e) experimental data on krypton 4p ⁶.     

Electron impact ionization and dissociation of neutral and charged fullerenes

Three different types of electron impact ionization experiments have been performed, involving neutral and charged C₆₀ and C₇₀. 1) We have determined absolute partial ionization cross sections for formation of parent ions C ₆₀ ^z+ and C ₇₀ ^z+ in charge states up to z = 4, and of singly and multiply charged fragments of size n ≥ 44 and n ≥ 50 from C₆₀ and C₇₀ neutral precursors, respectively. 2) Previous appearance energy measurements of C₇₀ have been improved and extended to z = 5; ionization energies are found to depend linearly on the charge state of the precursor, in agreement with theoretical predictions. 3) A beam of mass selected C ₆₀ ²⁺ has been crossed with an intense electron beam; the induced reactions (fragmentation, post- ionization, and dissociative post-ionization) have been analyzed.     

Microwave ionization and excitation of Ba Rydberg atoms

We have investigated ionization and excitation of the Ba 6s n s ¹ S ₀ and 6s n d ^1,3 D ₂ series in strong microwave fields. The observed microwave ionization threshold fields, scaling as 0.28n ^?5, and the state mixing fields cannot be completely explained in terms of a single cycle Landau-Zener model. However, by taking into account multiphoton resonant transitions driven by many cycles of the microwave field we have been able to interpet the data. In particular, multi-photon transitions have been found to be responsible for apparent resonance structures and for the unexpectedly low mixing fields. Not surprisingly, doubly excited valence states introduce irregularities into both the microwave ionization and the state mixing field values.     

Indirect processes in the electron-impact ionization of Li-like ions

Detailed measurements of electron-impact ionization cross sections have been made in the vicinity of the excitation-autoionization thresholds of Li-like B²⁺, C³⁺, N⁴⁺, O⁵⁺ and F⁶⁺ ions. With an energy spread of 0.4 to 1% of the electron energy and statistical uncertainties as low as 0.1% we could clearly resolve thresholds for excitation of individual terms in 1s2s2l configurations. Numerous resonance features were found which are due to dielectronic capture of the incident electron with subsequent two-electron emission. In particular, dielectronic capture processes involving Δn=2 excitations of a 1s electron provide the dominant resonance contributions to the measured cross sections. Rydberg series 1s2snln′l′ of resonances withn=3 (Δn=2) andn=4 (Δn=3) are resolved up to principle quantum numbersn′ equal to 6 or 7.     

Electron impact and single photon ionization cross sections of neutral silver clusters

Neutral silver atoms and small clusters Ag _n (n=1...4) were generated by sputtering, i.e. by bombarding a polycrystalline silver surface with Ar⁺ ions of 5 keV. The sputtered particles were ionized by a crossed electron beam and subsequently detected by a quadrupole mass spectrometer. In alternative to the electron impact ionization, the same neutral species were also ionized by single photon absorption from a pulsed VUV laser (photon energy 7.9 eV), and the photoionization cross sections were evaluated from the laser intensity dependence of the measured signals. By in situ combining both ionization mechanisms, absolute values of the ratio σ _e (Ag _n )/σ _e (Ag) between the electron impact ionization cross sections of silver clusters and atoms could be determined for a fixed electron energy of 46 eV. These values can then be used to calibrate previously measured relative ionization functions. By calibrating the results using literature data measured for silver atoms, we present absolute cross sections for electron impact ionization of neutral Ag₂, Ag₃ and Ag₄ as a function of the electron energy between threshold and 125 eV.     

Measurements of the oscillator strengths for the 6p7s (1/2,1/2)1 → 6pnp (1/2,3/2)2 Rydberg transitions of lead

We report experimentally determined oscillator strength distribution in the bound region of lead corresponding to the 6p7s (1/2, 1/2)₁ → 6pnp (1/2, 3/2)₂ (20 ≤ n ≤ 52) Rydberg transitions. The absolute value of the photoionization cross section from the 6p7s (1/2, 1/2)₁ excited state at the first ionization threshold 6p (²P_1/2) has been determined as 27 ± 4 Mb using the saturation absorption technique. The threshold value of the photoionization cross section is used to calibrate the oscillator strengths of the above mentioned Rydberg transitions. Moreover, oscillator strengths in the bound region smoothly connect with the differential oscillator strength density at the first ionization threshold, and verify the fundamental condition of quantum defect theory.     

Excitation energy transfer in the Li-Cs collision: Li*(2P)+Cs(6S)→Li(2S)+Cs*(5D)

The measurement of the collisional cross section for the process Li*(2P)+Cs(6S)→Li(2S)+Cs*(5D) are reported. The technique of resonant Doppler-limited two-photon laser excitation with thermionic detection is applied. The population density of the Cs*5D state is probed by photoionization, and the signals of the Cs(6S)→Cs*(5D) and the Li(2S)→Li*(2P) transitions are compared. The value for cross section of 30 Ų is measured, with an accuracy of 45%.     

Doubly autoionizing capture resonances ine + Mg+ collisions

Fine details in the cross section for electron impact ionization of Mg⁺ ions (configuration 1s ² 2s ² 2p ⁶ 3s) have been measured with an energy resolution of 0.3 eV. Structure on top of a smooth direct-ionization “background” reflects the presence of indirect ionization mechanisms. Such contributions in a Na-like ion involvenon-resonant excitation of a 2p electron to singly autoionizing intermediate states subsequently decaying into the channel of net single ionization of the parent ion. We observe even stronger indirect contributions fromresonant excitation of a 2p electron with simultaneous dielectronic capture of the projectile electron into doubly autoionizing states which decay by successive emission of two electrons.     

Change in ionization of barium in calcium and direct determination of barium in calcium by flame atomic absorption spectrometry using the Ba ion resonance line

Partial pressures of barium at various concentrations of calcium are calculated, and degree of ionization of barium in the nitrous oxide-acetylene flame are determined experimentally and theoretically using the Saha equation. The ionization of barium (50 μg ml^?1 ) in the presence of 10000 μg ml^?1 of calcium was about one third of the theoretically expected value. Using the ion resonance line of barium permits the direct determination of barium down to 3 ppm in calcium matrices by atomic absorption spectrophotometry.     

Rotational energy transfer in NO (A2Σ+, v = 0 and 1) studied by two-color double-resonance spectroscopy

Collision-induced rotational relaxation in the A²Σ⁺, v = 0 and 1 states of NO has been measured by using step-wise double-resonant ionization spectroscopy. Multiple quantum rotational energy transfer is occurring to at least |ΔJ| = 6 and the observed cross sections ranging from tens to hundreds of A² are larger than the gas-kinetic collisional cross section. The energy-transfer efficiency is slightly enhanced by the vibrational excitation. Energy-based scaling laws are successfully applied to reproduce the observed rotational distribution.     

Accurate comparison of Hel,NeI photoionization and He(23, 1 S), Ne(3s 3 P s,3 P 0) penning ionization of argon atoms and dimers

Using crossed beams and mass spectrometric ion detection, we have investigated the ionization of argon atoms and dimers in a skimmed supersonic beam by HeI (58.4 nm) and NeI (73.6, 74.4 nm) photons and by He(2^3,1 S) and state selected Ne(3s ³ P ₂,³ P ₀) metastable atoms. The cross section ratioq ₂₂/q ₁ (i.e. the cross sectionq ₂₂ for formation of Ar ₂ ⁺ ions from Ar₂ divided by the total ionization cross sectionq ₁ for Ar atoms), arbitrarily normalized to 1 for HeI impact, is found to vary weakly as follows: HeI/NeI/He(2^{3, 1} S)/Ne(³ P ₀)Ne(³ P ₂)=1/1.136(9)/0.893(4)/1.034(12)/0.985(9). The results are qualitatively interpreted using available information on the intermolecular potentials and the two different ionization processes. The observation thatq ₂₂/q ₁ is 5% larger for Ne(³ P ₀) than for Ne(³ P ₂) is attributed to anomalies in the respective branching ratios for formation of the Ar+(² P _3/2)/Ar⁺(² P _1/2) ion states in conjunction with differences in the stability of the formed Ar-Ar⁺(² P _3/2) and Ar-Ar⁺(² P _1/2) molecular ions.     

Population transfer between the fine-structure levels 5s5p 3 P 1 and 5s5p 3 P 0 of114Cd atoms by collisions with molecular gases

Fine-structure mixing of excited¹¹⁴Cd 5³ P _J atoms induced by collisions with various molecular gases (N₂, H₂, D₂, CO, CH₄, C₂H₆) has been investigated by a combined method of absorption and fluorescence spectroscopy. After pulsed optical excitation of the Cd(5³ P ₁) level, the time dependence of the population densities has been measured simultaneously both for the 5³ P ₁ state and for the collisionally populated 5³ P ₀ state. By analyzing the signal curves at different molecular gas pressures cross sections for collisional transfer between the¹¹⁴Cd 5³ P ₁ and 5³ P ₀ levels as well as the quenching cross sections for the 5³ P levels have been obtained.     

Experimental and theoretical cross sections for photoionization of metastable Xe*. (6s 3 P 2, 3 P 0) atoms near threshold

Using high resolution laser photoelectron spectrometry we have determined absolute cross sections σJ ₀ J ₁ and the electron angular distribution parameter for one photon ionization of metastable Xe^*(6s ³ P _J0, J ₀ = 2, 0) atoms to the resolved Xe₊ (² P _J1, J ₁ = 3/2, 1/2) ion states at several wavelengths near threshold. For comparison with the present and future experimental data we have calculated partial cross sections and ß-parameters for photoionization of Xe^*(6s ³ P _J0, J ₀ = 2, 0) and of the analogous alkali atom Cs(6s) over the photoelectron energy range (0–5) eV. We have used both a term-dependent Pauli-Fock (PF) approach and a configuration interaction method involving Pauli-Fock atomic orbitals (CIPF). Through the PF method we include relativistic effects on the atomic orbitals; the CIPF method was designed to take into account the important electron correlation effects which are found to be essential for obtaining good agreement between the theoretical and experimental results.     

Absolute cross sections for electron impact ionization of Ag2

The previously measured relative cross section function for electron impact ionization (EII) of neutral Ag₂ has now been calibrated quantitatively by combining the electron impact ionization with in situ non resonant two photon ionization (NR2PI). By comparing the NR2PI saturation intensities measured for Ag ₂ ⁺ and Ag⁺ with the corresponding EII intensities, the ratio between the electron impact ionization cross sections (EIICS) of neutral Ag₂ and Ag was determined to be σ_Ag2/σ_Ag=1.53 for an electron energy of 46 eV. This result agrees well with the geometricn ^2/3-rule \((\sigma X_n \sim n^{2/3} )\) commonly proposed for the dependence of the EIICS of clustersX _n on the cluster sizen.     

Improved study of the ionization of hydrogen atoms in thermal energy collisions with metastable He(23 S) atoms

The first electron spectrometric study of the ionizing reaction of metastable He(2³ S ₁) atoms with ground state hydrogen atoms has been carried out with sufficiently high resolution to partially resolve the rotational structure due to formation of rovibrationally excited HeH⁺ (v, J) ions at two different beam source temperatures (300 K and 90 K). The electron energy spectrum has been reproduced in model quantum calculations, using a new large scale ab initio calculation of the He(2³ S)+H(1² S)²Σ-potential. The imaginary part has been adjusted to yield a satisfactory fit to the measured spectrum. The collision energy dependence of the associative ionization electron spectra and of the total and partial ionization cross sections is discussed in some detail. No significant signs for limitations of the used local complex potential method, indicated by results of an earlier study of the He(2³ S)+H(1² S) system, have been found in the present work, in which the calculations were carried out with an improved and corrected program.