Autoionization Lifetime Research Articles

Intraconfiguration interactions in barium 6p1/2nf autoionizing states.

Measurements are reported of the energies and linewidths of the J=2, 3, and 4 components of the 6${\mathit{p}}^{1/2}$14f and 6${\mathit{p}}_{1/2}$19f states of barium. These states are anomalously broad, having autoionization lifetimes of approximately 1/2 of a classical Rydberg orbit period, and previously measurements have discerned no differences among the various J components. This work shows that the differences can be observed, and are consistent with calculations. The variations between states would be large if the states were LS coupled, but the large 6p ionic fine-structure forces recoupling into jj configurations. The spherical symmetry of the 6${\mathit{p}}_{1/2}$ ionic state minimizes the differences between J states, but does not eliminate them.

Observation of Circular-Metastable Doubly Excited States of Barium.

Using the adiabatic crossed-field method we have prepared for the first time doubly excited barium atoms with one valence electron in an oriented circular Rydberg state (n=21, l=m=20, denoted 21c) and the other electron in a ${6\mathrm{p}}_{\mathrm{j}}$ state. In a fraction of these atoms the inner electron decays radiatively to a ${5\mathrm{d}}_{\mathrm{j}\ensuremath{'}}$ metastable level. The obtained circular-metastable doubly excited atoms are very stable. For the (${5\mathrm{d}}_{3/2>}$21c) state the autoionization lifetime \ensuremath{\tau} is longer than 400 \ensuremath{\mu}s; for the (${5\mathrm{d}}_{5/2}$21c) state, \ensuremath{\tau}=1\ifmmode\pm\else\textpm\fi{}0.2 \ensuremath{\mu}s. the specific role of the 5d electron is demonstrated. \textcopyright{} 1990 The American Physical Society

Photoionization and photoelectron spectroscopy of O2 with coherent vacuum ultraviolet radiation

The photoionization of O2 near its ionization limit has been studied with coherent vacuum ultraviolet radiation produced by third harmonic generation in free jet expansions of the rare gases. High resolution (∼2 cm−1) photoionization spectra were obtained in the ionization threshold region from 103 to 98 nm which includes three vibrational levels of the H 3Πu (3sσ) Rydberg state. The H, v=0 photoionization spectrum was assigned by simulating the H 3Πu←X 3∑+g Rydberg excitation, yielding spectroscopic constants as well as the overall autoionization lifetime. The v=1 and 2 levels have distinctly different rotational band contours which reflect perturbations with bound and dissociative levels of nearby ‘‘dark’’ states. The photoionization dynamics were probed further through measurements of photoelectron angular distributions for the v+=0 and 1 vibrational levels of O+2. In addition to strong variations in the asymmetry parameter (β) across the H state autoionization resonances, spectrally narrow variations in β were found in the surrounding continuum. These latter results suggest the presence of weak resonance features imbedded in the background continuum which nonetheless strongly influence the photoelectron ejection dynamics.

MQDT analysis of rovibrational interactions and autoionization in Na 2 Rydberg states

The nd Rydberg series of the Na 2 molecule have been excited by optical-optical double resonance. The experimental spectra are analysed in the framework of the multichannel quantum defect theory including rotational and vibrational channels. The rovibrational interactions in the Rydberg series are calculated and good agreement with experimental energy levels is obtained. The dynamical aspect of the autoionization process is theoretically investigated and interference effects are discussed for Δν=2 autoionization. The autoionization lifetimes are experimentally measured. Agreement between theory and experiment is only qualitative because the autoionization lifetime of the Δν=2 process depends drastically on the variation of the quantum defects with the intemuclear distance. Moreover, the lifetimes are found to be very sensitive to an external electric field.

Continuum versus discretized wavefunctions. The importance of being well normalized

We illustrate, for some resonance widths of helium-like systems, and a Fano-type expression recently proposed, the importance of discretized continuum wavefunctions being well re-normalized. Our conclusions bear on the use of L 2 techniques to evaluate matrix elements such as autoionization and predissociation lifetimes, photoionization probabilities, and unimolecular and bimolecular reaction probabilities.

Mechanism of thermal electron attachment to NO2

The mechanism of thermal electron attachment to NO2 has been reexamined by observing the dependence of the attachment rates on the nature and the pressure of the environmental gases. Measurements for mixtures of NO2 with rare gases, H2, D2, N2, CO2, and n-C4H10 all showed two-body pressure dependence of the attachment rates at buffer-gas pressures of about 10 to 100 Torr. They gave the same two-body rate constant of (1.13±0.07)×10−10 cm3 molecule−1 s−1. The latter result disagrees with the data reported by Mahan and Walker in 1967. The present results indicate that the collisional electron detachment process introduced previously to interpret the effect of the nature of environmental gases should be negligible. We have also observed the decrease of the two-body rate constants at pressures below about 10 Torr for all the mixtures studied. This strongly suggests that the attachment mechanism is an ordinary two-step three-body process. The three-body rate constants then obtained are mostly of the orders of 10−27 cm6 molecule−2 s−1 and do not differ much with nature of the third bodies. An autoionization lifetime of 1×10−8 s has been estimated for the transient-negative ion of NO2. It has been found that even room light could cause appreciable decrease of the rate constants, probably through decomposition of NO2 molecules. The discrepancy between the present results and the previous ones may be due to such an effect.

Positron annihilation in the doubly excited states of positronium negative ions.

The two-photon-annihilation rates for some $^{1}\mathrm{S}^{\mathrm{e}}$ doubly excited states of positronium negative ions are calculated. Hylleraas-type wave functions are used to represent the system. The doubly excited wave functions are obtained by use of a stabilization method. In comparing the annihilation lifetimes with the autoionization lifetimes (related to the inverse of the resonance widths that were obtained previously by using the method of complex-coordinate rotation), it is concluded that the decay mode of these lower-lying doubly excited $^{1}\mathrm{S}^{\mathrm{e}}$ states is dominated by autoionization processes.

A simple method for calculating resonance widths

We present a simplification of a method we have recently proposed for calculating (non-relativistic) autoionization lifetimes. We point out that, in our approach, the Z −1 scaling of resonance energies and widths follows automatically. As an illustration, results are presented for the first four 1,3D resonances of the helium isoelectronic series.

Constrained variational procedure for the calculation of autoionization lifetimes

The linear homogeneous constrained variational procedure due to Weber and Handy is used in the approximation of autoionization lifetimes with Miller's “golden rule formula”.

Mechanism of thermal electron attachment in N2O and N2O–hydrocarbon mixtures in the gas phase

The attachment of thermal electrons to nitrous oxide at room temperature has been studied, following pulse radiolysis, by a microwave conductivity technique. For pure N2O at pressures from 10 to 300 torr, the results are explained by a combination of two-body attachment followed by reactions leading to partial electron detachment, a two step three-body process, and a process giving overall four-body behavior. The results for mixtures of N2O with alkanes (C2H6, C3H8, n-C4H10, iso-C4H10, n-C5H12, and neo-C5H12) and butenes (1-, 2-cis-, 2-trans-, and iso) are also explained in the same way, but with no electron detachment. Common values of 5×10−15 cm3/molecule sec for the two-body rate constant and 4.6×10−33 cm6/molecule2 sec for the three-body rate constant (with N2O as the third body) explain the data. The three-body rate constants increase with molecular complexity (6×10−34 cm6/molecule2 sec for C2H6 to 1.55×10−31 cm6/molecule2 sec for neo-C5H12). The four-body rate constants range from ∼10−53 to ∼10−51 cm9/molecule3 sec. The branched compounds such as neopentane and isobutene have higher three-body rate constants than the linear isomers. The attachment rates of mixtures of those compounds with the higher three-body rate constants appear to saturate as pressures increase. From the results for N2O-neo-C5H12 mixtures a value of (5.8±0.6) ×10−13 cm3/molecule sec has been determined for the rate constant of the initial two-body electron capture by N2O to form a short-lived N2O−. The autoionization lifetime of N2O− is estimated to be 1.8×10−10 sec or greater. The problem of excess nitrogen in N2O–hydrocarbon radiolysis is discussed in relation to these results.

Feshbach projection operator calculation of the potential energy surfaces and autoionization lifetimes for He(2 3S) –H and He(2 3S) –H2

A new technique has been developed for calculating potential energy surfaces and widths for autoionizing molecular systems. The method is based on the use of the Feshbach projection operators P and Q, which are defined within the space of configurations generated from a basis set of square-integrable orbitals. Autoionizing states are then obtained by diagonalizing a particular subblock of the finite Hamiltonian constructed by the CI procedure. These states decay because they are only approximate eigenfunctions of the full finite Hamiltonian. Their lifetimes are evaluated using continuum functions which are expanded in the basis orbitals. Such an expansion is shown to be sufficiently accurate. The present technique is compared with the stabilization method and is found to illuminate certain features of that procedure. Potential surfaces V* and widths Γ have been calculated for the Penning ionization transitions He(2 3S)+H→He+H++e− and He(2 3S)+H2→He+H2++e−.

Autoionization in diatomics: Measured line shape parameters and predicted photoelectron spectra for some autoionizing states of the hydrogen halides

Using a Hopfield continuum source and a computer-controlled data acquisition system, we have measured relative absorption cross sections for HCl, DCl, HBr, DBr, and HI at 0.2 Å resolution from 1000 to 700 Å, a region dominated by autoionizing Rydberg states converging to the A 2Σ+ state of the HX+ ion. In order to analyze the shapes of autoionizing vibrational bands, we applied Fano’s theory of line shapes to homogeneous autoionization in diatomics, and found that the line shape parameters q, Γ, and ρ2 are independent of rotational quantum number but may depend upon the vibrational quantum number of the autoionizing state. Calculations of this vibrational dependence in HCl shows it to be most pronounced at high v. For progression I in HCl and DCl (n*=2.795) and in HBr and DBr (n*=2.748), we fit computer-generated vibronic band contours to experimental absorption cross sections to obtain q, Γ, and ρ2 which we then used to predict the photoelectron spectra of these autoionizing states. The autoionization lifetime h//Γ in HCl and DCl is 1.1×10−14 sec; in HBr and DBr it is 9.5×10−15 sec. The measured autoionization widths of bands in the next member of the Rydberg series obey the scaling rule Γ∝n*−3. We attribute an additional broadening of levels with v′?3 in progression I of HBr as the onset of predissociation, and estimate the relative rates of predissociation and autoionization. A vibrational reassignment of progression II in HCl and DCl gives n*=2.310 for these states. We saw only a single broad peak (with n*=2.78) in the absorption spectrum of HI.

A study of electron attachment to SF6and auto-detachment and stabilization of SF6-

A study of the autoionization of the SF6- ion has been performed using the technique of ion cyclotron resonance (ICR). This study utilizes the motion of charged particles in the trapping plate plane to effectively separate the SF6- ion and electron currents, thus enabling the direct evaluation of the ion current signal as a function of time. The autoionization process was observed as a function of both the electron-neutral interaction time and the density of the neutral molecule. It was found that the 'apparent' autoionization lifetime, tau a, of SF6- varies as a function of observation time from 50 mu s to 10 ms. Estimates are also made of the effective collisional stabilization frequency of the SF6-*-SF6 collision pair as well as the vibrational fluorescence lifetime of the excited ion.

Nonstatistical emission from metastable autoionizing states of Oi

Emission intensities are reported for multiplet components of three metastable autoionizing OI 3P0 levels with nanosecond lifetimes—the 2s2p5 3P0, 3s″ 3P0, and 3d′ 3P0 states. Emission intensities for the 3s″ 3P0 and 3d′ 3P0 states are nonstatistical, i.e., the observed intensities for components of the multiplets do not correspond to the theoretical statistical absorption intensities. Autoionization, prohibited for these states in a pure Russell−Saunders coupling scheme, proceeds via weaker spin−orbit or spin−spin interactions, primarily through direct interaction with available continua. Previous autoionization studies of all of these states showed that the autoionization intensities are nonstatistical only for the 2s2p5 3P0 and 3s″ 3P0 states. These results are explained in terms of a competition between autoionization and emission based on a comparison of the true lifetimes of the states and the radiative lifetimes calculated from the absorption oscillator strengths. When the autoionization and emission lifetimes are comparable, both decay channels show nonstatistical behavior; when one decay channel dominates, only the minor decay channel shows nonstatistical behavior. The nonstatistical nature of the decay is discussed in terms of selective autoionization of certain total angular momentum levels of the decaying 3P0 states; however, additional theoretical work is required to establish the origin of this effect.

Comparison of doubly-excited helium energy levels, isoelectronic series, autoionization lifetimes, and group-theoretical configuration-mixing predictions with large-configuration-interaction calculations and experimental spectra

For comparison with our recent group-theoretical predictions of configuration-mixing coefficients, we report extensive configuration-interaction calculations for doubly excited states in the helium isoelectronic sequence below the $N=2$ ($S, P, D$ states), $N=3$ ($S, P, D, F, G$ states), $N=4$ ($S, P, D$ states), and $N=5$ ($^{1}P^{\mathrm{o}}$ states) ionization thresholds. Two new quantum numbers label the Rydberg series, and are used to predict three "selection rules" for dipole excitation and autoionization processes which agree with experiment. New autoionization widths are reported for the helium states and confirm our group-theoretically predicted selection rules. Our width of 0.151 eV for the $^{1}P^{\mathrm{o}}$ state at 62.92 eV is in agreement with the recent experimental value 0.132 \ifmmode\pm\else\textpm\fi{} 0.014 eV. Calculations of helium energies and autoionization widths are strongly affected by "avoided crossings" of the doubly excited states as $Z$ is varied continuously. The new quantum numbers project out states in ${\mathrm{H}}^{\ensuremath{-}}$ which correspond to observed $^{1}P^{\mathrm{o}}$ shape resonances above the $N=2 \mathrm{and} 3$ ionization thresholds.

Autoionization lifetime of SF 6− and the calculated electron affinity

The Fano configuration-interaction theory of autoionization is used to calculate the partial photoionization cross-sections for production of diatomic molecular ions in specific vibrational and electronic states. Expressions are presented which are useful in analysing photoelectron spectra when the exciting radiation (sharp lines or continuum) is in the vicinity of autoionizing states. The vibrational distribution in O2+(X2Пg) is calculated for both a uniform excitation continuum and with the solar vacuum-ultraviolet spectrum as an excitation source; the necessary oscillator strengths and autoionization lifetimes are determined from published photoelectron, photoionization, and absorption spectra of O2. Autoionization of the upper states of the Hopfield bands produces highly vibrationally excited ions. About 5 per cent of the O2+ produced by solar photoionization have more than 1 eV of vibrational energy.

Attachment of slow (<1 eV) electrons to O2 in very high pressures of nitrogen, ethylene, and ethane

The results of a recent study on the capture of slow (<1 eV) electrons by O2 embedded in very high pressures of nitrogen (up to 27 500 torr), ethylene (up to 17 000 torr), and ethane (up to 17 500 torr) are presented and discussed. Large changes have been observed in both the magnitude and the energy dependence of the attachment rate (and cross section) with increasing density of these three media. At sufficiently low pressures (≲ 1500 torr for N2; ≲ 2500 torr for C2H4; ≲ 3500 torr for C2H6), low-energy electron attachment to O2 can be treated approximately as a three-body process. However, as the density of each medium increases, each affects the capture process differently, demonstrating the profound effect and importance of the environment on the electron attachment process. Electron capture mechanisms and reaction schemes consistent with the observed dependences of the attachment rates on the density of each medium are presented and discussed. The high-pressure data on O2 in C2H4 and N2 have been successfully related to ``liquid-state'' behavior. From the O2, C2H4 data an autoionization lifetime equal to ∼2 × 10−12 sec has been estimated for O2−*.

A new measurement of the SF −6 autoionization lifetime

A new technique, which uses an ion cyclotron resonance mass spectrometer as an ion trap, is used to measure the autoionization rate of SF − 6. In the present experiments, the SF − 6 ion current is followed from 5 μsec to 800 μsec. The results show that SF − 6 does not decay by a simple, exponential, unimolecular process. Attempts to analyze the data in terms of unimolecular decay of a single state leads to ‘lifetimes’ which increase from < 0.3 msec to ca. 2 msec as the time delay between ion generation and analysis increases.

Atomic oxygen oscillator strengths in the autoionization region. I. The absolute strength of the 5s′ lines

A curve-of-growth method was used to determine the absolute oscillator strength and other line parameters of the autoionized transition OI:2s22p43P2→ 2s22p3(2D5/2,3/20) 5s′ 3D3,2,10. This method was used because the spectral width of the lines is far less than the bandwidth of the spectrometer. Neither lifetime nor emission measurements are applicable because depopulation of the upper state occurs predominantly by autoionization. The atomic oxygen concentrations were detemined by means of the O–NO2 titration. Sets of curves of growth were computed on the basis of the separation of the three levels, 3D3,2,10, as observed with a spectrograph, tabulated values of the relative line strengths based on L–S coupling, and consideration of the different broadening mechanisms operating under the conditions of the experiment. This included consideration of the configuration interaction of the upper states with the adjoining ionization continuum. Comparison of the experimental curve with the computed curves gives f = 0.0021, Σgf = 0.019, and Γ = 0.01 Å, resulting in an autoionization lifetime τ of 2 × 10−11 sec.

Molecular Autoionization Lifetimes and Cross Sections for Penning Ionization: Numerical Results for He* (1s2s 3S) + H(1s 2S)

The width Γ (or lifetime ℏ/Γ) for autoionization of He* (1s2s 3S) + H(1s2S) has been calculated as a function of internuclear distance, and cross sections for Penning and associative ionization (He*+H → He+H++e−, HeH++e−) have been determined for low collision energies. Associative ionization is 22% of the total ionization cross section in the limit of zero collision energy; this fraction decreases with increasing energy, being ·18% at a collision energy corresponding to 300°K. The distribution in energy of the ionized electron is also calculated, and it is seen that measurement of this quantity should lead to a good estimate of the well depth of the He*–H potential. Comparison of these results with those obtained by an orbiting model shows that the model (suitably scaled) is adequate in predicting the total ionization cross section, but is less accurate for the more detailed collision properties.