High Principal Quantum Number Research Articles

Rovibrational structure of NO+2 and state-to-state dynamics in the high-resolution threshold photoionization of NO2

Triple-resonant zero-kinetic-energy (ZEKE) photoelectron spectroscopy is employed to characterize the rovibrational structure of NO+2. Relative threshold positions determine vibrational frequencies and rotational constants; while anomalous, and in some cases forbidden, intensities of transitions provide information on rotational and vibrational coupling between the NO+2 core and electronic degrees of freedom associated with Rydberg orbitals of very high principal quantum number. Trends in the strengths of anomalous threshold photoionization features establish that mode selectivity, which has a pronounced effect on the vibrational autoionization dynamics of NO2, also operates to determine the envelope of rotational-line intensities in threshold photoionization. Competition between rotationally and vibrationally mediated coupling leads to a dependence of the rotational profile on the mode of vibrational motion of the core.

Preparation of high-principal-quantum-number "circular" states of rubidium.

We describe a rubidium circular-state-preparation technique suited to high principal quantum numbers. It combines an excellent selectivity and a good efficiency. Preliminary spectroscopic tests on the n=50\ensuremath{\rightarrow}n=51 circular to circular transition at 51 GHz are reported.

Zero kinetic energy (ZEKE) photoelectron spectroscopy of ammonia by nonresonant two-photon ionization from the neutral ground state

Rotationally resolved zero kinetic energy-pulsed field ionization (ZEKE-PFI) photoelectron spectra of NH3 were obtained by nonresonant two-photon ionization from the X̃(1A1′) electronic ground state. The ZEKE spectra were recorded up to an internal ion energy of 9500 cm−1 by pulsed field ionization of long-lived Rydberg states of very high principal quantum numbers. Rovibronic bands of the NH3+ cation with v2+=1–9 (umbrella mode), v4+=1, and for the first time the mode ν1 in the combination band v+=1121 of the X̃+(2A2″) electronic ground state are observed in the ZEKE spectra. Rotational constants and band origins are determined in the analysis. The adiabatic ionization energy obtained by a recent ZEKE measurement is confirmed at 82 159±1 cm−1.

Magneto-optical spectra of lithium and sodium

Measurements of Faraday rotation spectra for the elements lithium and sodium are reported. The observed patterns are all obtained and analysed under full Paschen-Back coupling conditions. These are believed to be the first accurate measurements for these elements extending to high principal quantum numbers. The course of f-values in the principal series is compared with available calculations, which take account of core polarization effects by the quantum defect orbital method. The authors' measurements reveal that the influence of the magnetic field on the f-values of the high series members grows as B2n4. When this is corrected for, the data extrapolate to absorption cross sections in the base of the continuum which agree well with earlier measurements. The authors also apply this correction to earlier data for potassium (Nawaz et al., 1992).

Anomalous n dependence of low-energy electron capture from atomic hydrogen by multicharged ions.

A theory of electron capture from states of high principal quantum number {ital n} by multicharged ions is presented. We predict a capture cross section that depends upon the {ital n} of the initial state as {ital n}{sup {beta}} where {beta} is as large as 7 in some cases. This surprisingly high {ital n} dependence explains why a small (less than 0.1%) fraction of hydrogen in high {ital n} states contributes 30% of the capture yield in O{sup 5+}+H collisions. The theory also predicts a strong dependence on the dipole moment of the initial state.

Rotationally resolved zero-kinetic-energy photoelectron spectrum of nitrogen.

Rotationally resolved zero-kinetic-energy (ZEKE) photoelectron spectra of the X $^{2}\mathrm{\ensuremath{\Sigma}}_{\mathrm{g}}^{+}$(${\mathrm{v}}^{+}$=0,1) \ensuremath{\leftarrow}X $^{1}\mathrm{\ensuremath{\Sigma}}_{\mathit{g}}^{+}$(v=0) bands of nitrogen have been recorded. The spectra show unexpected rotational-line intensities. The intensity distortions are explained by channel interactions. The strong S and U branches [(${\mathit{N}}^{+}$-J'')=+2 and +4, respectively] of the (1-0) band borrow intensity from a nearly degenerate Rydberg state, converging to an electronic excited state of the ion, by a complex resonance mechanism. Field-induced rotational autoionization is shown to enhance the O branch and M branch [(${\mathit{N}}^{+}$-J'')=-2 and -4] intensity of the (0-0) band. Recording ZEKE spectra with extraction fields of 10--100 V/cm appears to be an attractive and powerful way of obtaining spectroscopic information on Rydberg states with simultaneously high principal quantum number n and high total-angular-momentum quantum number J.

A note on Δn ≠ 0 Stark transitions in hydrogenlike atoms

AbstractIn a gaseous helium or hydrogen target slow muons or antiprotons are captured into orbits with a high principal quantum number (n = 15 to 50) to form (μ− α)+ ions, (pα)+ ions, or (pp) atoms respectively. In the subsequent deexcitation process Stark mixing of the intermediary states plays an important role. The successful Mainz Cascade Model assumed Δn = 0 for the Stark transitions, although formally no such selection rule exists. This note examines the reasons why Δn ≠ 0 Stark transitions play only a negligible role in the deexcitation cascade.

Threshold-field-ionization photoelectron spectroscopy and delayed forced autoionization of HCl.

A time-of-flight photoelectron spectrometer is oriented coaxially with a pulsed molecular beam to detect high Rydberg states of HCl with lifetimes as long as 7 \ensuremath{\mu}s. Two classes of such states are found. Delayed extraction following double-resonant excitation field-ionizes Rydberg states of very high principal quantum number (n\ensuremath{\ge}100) that converge to successive rotational limits for producing ${\mathrm{HCl}}^{+}$, effectively yielding a rotationally resolved threshold photoelectron spectrum. Surprisingly, excitation under more rigorously field-free conditions (100 mV/cm) produces additional resonances which can be assigned to series of long-lived Rydberg states extending to principal quantum numbers as low as 10. The lower-principal-quantum-number resonances isolated by their long field-free lifetimes appear to constitute a high-angular-momentum nonpenetrating subset of the autoionizing Rydberg series that converge both to accessible rotational levels of the upper $^{2}\mathrm{\ensuremath{\Pi}}_{1/2}$ threshold and excited rotational limits of the $^{2}\mathrm{\ensuremath{\Pi}}_{3/2}$ ground state of ${\mathrm{HCl}}^{+}$.

Predissociation of H3 near its ionization threshold induced by very weak electric fields.

We report the discovery of predissociation channels in H{sub 3} at energies within a few meV of the lowest ionization threshold. The predissociation affects selected {ital p}- and {ital d}-Rydberg states with high principal quantum numbers. For the {ital d} states, the predissociation process is induced by very weak electric fields, of order less than 1 V/cm. The predissociations are due to interaction with vibrationally excited states of low principal quantum number that exhibit significant coupling to the repulsive ground-state surface of H{sub 3}, 2{ital p}{sup 2}{ital E}{prime}. We discuss the possible significance of these predissociation channels for decay paths in dissociative recombination of H{sub 3}{sup +} with low-energy electrons.

Electrostatic level shift in a hydrogen atom inserted between perfectly conducting plates

The electrostatic energy displacement for a hydrogen atom inserted between two parallel, perfectly conducting plates is calculated by means of perturbation theory, starting from the free-space solution of the Dirac equation for the hydrogen atom. The correction to the Coulomb potential of an electric point charge due to the influence of the boundary conditions imposed by the plates is determined using different methods. The resulting level shift is calculated in first order of perturbation theory with results given in terms of multipole expansions. The leading terms reproduce results already known, giving rise to a level shift of roughly in approximately=-n4 10-6 Hz (1 mm/a)3 for high principal quantum number n.

Collisions between Sr Rydberg atoms of intermediate and high principal quantum numbers and noble gases at thermal energies

Pressure shift and broadening measurements of 5snd 1D2 and 5snd 3D2 Rydberg states of Sr I due to Xe have been performed in the range 8<or=n<or=35 by means of Doppler-free two-photon excitation, combined with thermionic detection and the use of a molecular frequency standard. The data show striking differences between pressure shifts and broadenings of singlet and triplet states, which up to now have not been predicted by theory.

Recombination of Neon-Like and Adjacent Ions in Plasmas

The neon-like and sodium-like ions of medium Z elements, chromium and nickel are identified as abundant or strongly radiating species in the transport dominated region of the JET plasma. Also in plasma produced by laser ablation of multilayer targets stimulated emission has been identified from neon-like selenium. For both observations accurate theoretical electronic recombination rate coefficients contribute to spectral interpretation, to unambiguous separation of diffusive and atomic processes in impurity transport and to derivation of metal concentrations from VUV lines.A new calculation is presented of recombination of fluorine-like, neon-like and sodium-like ions of chromium, iron, nickel and selenium. Radiative recombination and dielectronic recombination rates to angularly resolved energy levels of both low and high principal quantum number are determined using combination of multiconfiguration perturbation theory and Correspondence Principle techniques. The modifying effects of collisional redistribution in finite density plasma are evaluated and effective net recombination coefficients and effective recombination coefficients to excited levels leading to radiative emission obtained.The theoretical radial distribution of nickel ions across the JET torus is obtained by combining the recombination results with a transport model.

Excitation of Swift Heavy Ions in Foil Targets. III. Initial Populations of Rydberg States

Results are reviewed of experiments in which Rydberg states with high principal quantum numbers (n :&lt;:: 20) are excited by foil targets in transmitted heavy ions. A uniform procedure is described for analysis of the decays downstream of such states using a Monte Carlo method. Previously observed X-ray decays of 130 MeV Br Rydberg ions into L vacancies are analysed and numerical populations of initially generated states are deduced. It is found that the initial states may be described as mainly statistical, i.e. all substates having equal weights, with 63% of initial ions in such states. Some X-ray lines are found to come from high-n (:&lt;:: 50), low-/ (;S; 2) states. There is evidence for an unknown post-foil mechanism feeding the 3s electron state.

Experimental and theoretical study of QED corrections in the ground state of heliumlike iron

The energy of the Lyβ and Lyγ lines of heliumlike iron has been measured with a precision of 25 ppm. The study of these transitions involving levels of high principal quantum number (n=3 andn=4) provides us with informations on the ground state 11S0 of the ion. The experimental values are compared to MCDF calculations including QED two-body corrections.

Determination of accurate quantum defects and wavefunctions for alkali Rydberg states with high principal quantum numbers

Quantum defects and the corresponding wavefunctions are determined for lithium, sodium and potassium using a new method that searches for the poles of the Schwinger T matrix along the negative real energy axis. A Coulomb Green's function is used to factor out the effect of the long-range potential. Model potentials are employed to include the effects of core polarisation and correlation. Quantum defects accurate to 1% are obtained with small basis sets and small grids. The method has been tested and found to be numerically stable for states with principal quantum number n as high as 80.

Coherent states and the Kepler-Coulomb problem.

The Kustaanheino-Stiefel transformation is used to transform the Coulomb problem to a four-dimensional isotropic harmonic oscillator with a constraint. Ordinary coherent states are introduced over this oscillator and are shown to evolve in a fictitious time. When projected back into the physical space, the states follow the classical Kepler orbits in physical time. The resulting quasiclassical picture is obtained without the Bohr correspondence principle of taking only contributions from high principal quantum numbers.

The electron-electron interaction in doubly-excited states of atoms

Doubly-excited states of the isoelectronic sequence of H−, in which both electrons are in high principal quantum numbers, are examined on the basis of alternative pictures of the correlations between the two electrons. Restricting ourselves to the lowest1 S states with both electrons in principal quantum numbern, we parametrise the electron-electron interaction on the basis of these pictures and compare the resulting simple expressions with more elaborate numerical calculations. This provides further understanding of the nature of correlations in such states.

Observation of lines above 2000 Å in O VIII and C VI in the Princeton Large Torus due to charge-exchange processes: Diagnostic applications

Hydrogenlike oxygen and carbon lines from transitions (Δn=1) between levels of high principal quantum number n, with wavelengths above 2000 Å have been observed. Observations of such transitions were possible due to charge-exchange processes during neutral beam injection of hydrogen atoms into the Princeton Large Torus tokamak. The lines are O VIII 2976 Å (8–7 transitions), C VI 3434 Å (7–6), and C VI 5291 Å (8–7). Application of these lines for ion temperature measurements and initial observations of neutral beam vertical distributions in the plasma are presented.

Centrifugal barrier effects in the high Rydberg states and autoionising resonances of neon

The photoabsorption spectrum of neon in the range 500-700 AA has been investigated at high resolution using synchrotron radiation and a 3 m normal incidence vacuum spectrograph equipped with holographic gratings. All of the five Rydberg series 2p5(2P32/)ns(3/2)10, nd(1/2)10 and 2p5(1P12/)ns(1/2)10, nd(3/2)10 have been extended to high principal quantum numbers yielding accurate values for the ionisation potentials (2P32/=173 929.6+or-0.2 cm-1 and 2P12/=174 709.8+or-0.2 cm-1). In contrast with the corresponding features of heavier rare gases, the ns-nd interchannel interactions and autoionisation broadening are shown to be weak. The behaviour is discussed with reference to multichannel quantum defect and Hartree-Fock theory, and is shown to result from centrifugal barrier effects in the nd channels.

Three-photon excitation of hydrogen Rydberg states

A three-photon process using radiation at 2430 and 3660 A and with a 1s-2s two-photon resonance is used to excite atomic hydrogen from the ground state to Rydberg states of high principal quantum number. Collision-induced ionization is used to monitor the excitation.