Near-threshold Ionization Research Articles

Photoionization and excitation energies of an Al atom in Ar N clusters

Photoionization measurements on Al · ArN clusters (N < 150) in a cold cluster beam have been used to determine ionization threshold energies El, th(N) of the metal atom in a variable-size dielectric medium. The El, th(N) values descend from the atomic value [5.99 eV (1 eV ≈ 1.602 × 10–19 J)] to below 5.5 eV near N≈ 55. The El, th(N) functional form is compared to dielectric continuum predictions and to N-specific Monte Carlo calculations incorporating polarization and model-potential interactions. Satisfactory agreement is obtained when a strong neutral Al–Ar interaction is assumed. Measurements of the slow fragmentation probability indicate that near-threshold ionization of smaller clusters (N < 25) produces negligible fragmentation, resulting in N-specific threshold measurements. Comparison of neutral cluster distributions, measured by threshold photoionization to the corresponding cold Al+·ArN distribution (produced in the source) supports a predominant charge-invariance of the icosahedral-based structures, also found in Monte Carlo simulations. By contrast, photo-absorption spectra in bound states, as exemplified by R2PI spectra of the Al 3p → 4s transition, exhibit only a small fraction (ca. 0.12 eV) of the bulk shift (ca. 0.6 eV). This result may be partly explicable in terms of the distinction between sudden and adiabatic polarization energies in the two experiments.

MQDT analysis of radiationless decay rates of autoionizing rydberg states of polyatomic molecules

Fast intramolecular radiationless decay processes which compete with autoionization in the decay of high Rydberg levels of polyatomic molecules are taken into account by an absorptive imaginary part of the reactance matrix in the multi-channel-quantum-defect-type (MQDT) formalism. Calculations for a simple model involving two electronic ionization continua and a single vibrational mode reveal a dramatic impact of the decay processes on the near-threshold ionization cross section, in particular, a strong suppression of autoionizing series. The model calculations provide a semiquantitative explanation of the characteristic features of autoionization spectra recently obtained by two-color near-threshold photoionization in several polyatomic molecules.

Autoionisation and threshold ionisation of Ba+ by energy-resolved electrons

Detailed measurements have been made of the ionisation of Ba+ by energy-resolved electrons, drawn from a double 127' electron energy selector, for energies between threshold and 30 eV. Several autoionisation thresholds have been resolved and their energies are compared with values obtained by others from optical and ejected-electron spectra of neutral barium. Comparisons are also made with results of Hartree-Fock calculations by Hansen (1974-1975). These comparisons enabled the probable configurations of some principal autoionising transitions to be suggested. Detailed measurements have also been made of the near-threshold ionisation of Ba+.

Near-threshold associative ionisation reactions of atoms

The quantum approach to the theory of endoergic associative ionisation reactions of atoms within the framework of the multichannel quantum defect method taking account of configurational interaction between the Rydberg and valence dissociative states is developed. It has been shown that the total cross section of the AI reaction near the ionisation threshold is proportional to the energy E measured from the ionisation threshold ( sigma AI approximately E). The effect of the resonant quasidiscrete valence states on the ionisation spectrum is studied. As an example, the AI cross section for the reaction N+O to NO++e- is calculated. The temperature dependence of the AI thermal rate coefficients over a broad temperature range has been considered. The effect of a strong monochromatic radiation field on the AI reactions has been discussed.

Analysis of e-e angular correlations in near-threshold electron impact ionisation of helium

Using a coincidence technique in a coplanar geometry, triple differential cross sections (TDCS) for electron impact ionisation of helium are measured in the 0.5-2 eV energy range above threshold. As a few (LS Pi ) states (0<or=L<or=2) of the two outgoing electrons are obviously involved in the process, their respective intensities appear as unknown parameters in the theoretical TDCS as deduced in the frame of the Wannier theory. We show that almost all these parameters can be determined through normalisation to the measured TDCS in two specific geometries: in the first one the two electrons are kept in opposite directions while in the second one they remain symmetrical with respect to the incident beam. A comparison with the complete set of data is then performed. The measured TDCS are in agreement with the Wannier theory for the lowest energies (0.5 and 1 eV). The angular correlation predicted by this theory is confirmed but a precise determination of the angular correlation width cannot be achieved. At 2 eV the overall agreement becomes poorer, although some predictions of the Wannier theory still apply: in particular the measured TDCS are almost independent of the energy sharing between the two electrons. Finally specific measurements at 8 eV clearly show from considerations of symmetry that the Wannier theory no longer applies at this energy.

Near-threshold ionization of atomic hydrogen.

We consider near-threshold ionization of atomic hydrogen by strong laser light. Nonperturbative features of the photoelectron spectrum and double optical resonance spectrum (in the V configuration) are obtained and discussed. We show that in contrast to the usual sign of the ac Stark shift the ionization threshold is dynamically lowered due to ``pulling'' by the atoms near threshold Rydberg levels. Dynamical widths of the spectra are also found. Our results are obtained analytically by means of a variant of the single-pole approximation and are qualitatively consistent with experimental results.

Threshold effects in strong-field photoionisation of atoms

Near-threshold ionisation of an atom is investigated using a non-perturbative realistic model. The influence of the Rydberg states on the dynamics of the ground-state population is discussed. In particular, details of its non-exponential decay and of the population trapping as well as the shift of the threshold are calculated.

Comment on "Photoelectron angular distributions for near-threshold two-photon ionization of cesium and rubidium atoms"

A Comment on the Letter by Adila Dodhy, R. N. Compton, and J. A. D. Stockdale, Phys. Rev. Lett. 54, 422 (1985).Received 30 October 1985DOI:https://doi.org/10.1103/PhysRevLett.56.401©1986 American Physical Society

Fragmentation of hydrogen-bonded molecular clusters on photoionization

Photofragmentation mechanisms for the near-threshold ionization of (H2O)m(Ar)n, (CH3NH2)1,2(H2O)n and (N-methylformamide)1,2(H2O)n clusters have been investigated based on the molecular-beam vacuum ultraviolet photoionization mass spectroscopy with Ar (11.83 and 11.62 eV) and Kr (10.64 and 10.03 eV) resonance lamps. In the Ar-lamp ionization of (H2O)m(Ar)n cluster, (H2O)+k(2 ⩽k⩽ 10) ions were observed in addition to (Ar)1,2(H2O)+n(2 ⩽n⩽ 5) clusters. This suggests that excess energy dissipation among H2O–Ar and/or Ar–Ar vibrational modes finally leads to dissociation of the bond(s), preventing the activation of the proton-transfer reaction in an H2O+–H2O hydrogen-bonding unit. In the Kr lamp ionization of methylamine hydrated clusters and N-methylformamide hydrated clusters, proton-transfer reactions were found to be negligible. Even at 11.83 eV, CH3NH+2(H2O)n and HNCH3CHO+(H2O)n ions were dominant fragments. The origin of the protonated ions was attributed to the ionization of the CH3NH2–CH3NH2 or HNCH3CHO–HNCH3CHO dimer unit in the hydrated clusters, based on the solute concentration dependence of the ion signals. When an H2O molecule was ionized directly, the hydrogen atom of an adjacent molecule was abstracted by H2O+, forming an H3O+ ion. An increase of the excess energy did not change the degree of fragmentation as much, but changed the fragmentation pathways to the production of protonated ions. Selective ionization of a solute molecule enabled us to specify the bond which was primarily dissociated. The fragmentation mechanisms were elucidated by identifying the character of the ionized solute molecule in the hydrogen bond: electron donor (base) or hydrogen donor (acid).

Near-threshold ionization of molecules by intermediate energy electrons

Ionisation of molecules XY by electron impact with ejection of a slow electron with energy E ⪡ I ( I is the molecular ionisation potential) is studied. It has been shown that at the scattered electron energy ϵ′ ⪢ E, partial transition amplitudes may be represented in the form of the product of an adiabatic amplitude and a function that reproduces the behaviour of the slow electron in the molecular ion field. The dependence of cross section on the escaping electron energy is determined including the dissociative channel effect. The relative importance of mechanisms of direct and resonance interaction is revealed.

Energy partitioning in near-threshold excitation and ionisation of helium by electron impact

The processes of near-threshold ionisation and excitation of helium by electron impact have been studied experimentally by two different techniques to establish how the available excess energy is partitioned between the resulting free or bound electrons. The partitioning is found to be non-uniform and to be in agreement with that obtained from classical trajectory calculations.

Energy distribution in the near-threshold electron-impact ionization of atoms and ions

A concise overview of the present situation of the near-threshold energy distribution investigations for electron-atom (-ion) ionizing collisions is given. By making use of an essentially perturbative approach, the distribution function has been calculated for two extreme regimes of the twoelectron final state: (i) ${\ensuremath{\epsilon}}_{1}\ensuremath{\approx}{\ensuremath{\epsilon}}_{2}$ and (ii) ${\ensuremath{\epsilon}}_{1,2}\ensuremath{\ll}{\ensuremath{\epsilon}}_{2,1}$, where ${\ensuremath{\epsilon}}_{1,2}$ are the escaping-electron final energies. It is shown that within the classical Wannier theory (linear configuration) one obtains the flat distribution function in both cases. The calculations for case (ii) are compared with recent Coulomb-dipole results. A brief discussion of some conceptual differences between Wannier and Coulomb-dipole theories is also given.

Extensions of the Wannier theory for near-threshold excitation and ionisation of atoms by electron impact

The classical cross sections for excitation and ionisation processes in which the total angular momentum is zero and the magnitude of the total energy E (measured with respect to the ionisation energy) is small are investigated. The cross sections have the form mod E mod 0.127f(E1/ mod E mod ) when the charge of the residual ion is unity, where E1 is the asymptotic energy of the more energetic of the two final electrons and the cross sections are differential in E1. The forms of f(E1/ mod E mod ) for E 0 have been established by accurate trajectory integrations. The results are found to be consistent with those of the various experimental studies of near-threshold excitation of high-n states and near-threshold ionisation. The distribution functions of the mutual angle theta 12 in the ionisation process and of the individual electron angular momenta l in the excitation process have been investigated, and their energy dependences and forms have been found.

Near-threshold electron impact ionisation of helium

Uses an electron trap spectrometer in a fixed incident energy mode of operation to explore near-threshold ionisation of helium. The authors find that the partitioning of the excess energy between the two outgoing electrons is random for incident energies up to 30 eV. The energy dependence of the associated total ionisation cross sections is consistent with that of other experimental measurements.

The classical theory of near-threshold ionisation

The double-escape probability at a near-zero energy E has been analysed within the classical Wannier theory. The method due to Vinkalns and Gailitis (1967) has been generalised for the case of outgoing particles of arbitrary masses and charges. Wannier's threshold law for electron-atom ionisation and Klar's (1981) recent WKB result for a positron-atom collision are obtained as special cases. The threshold behaviour of the distribution of mutual angles for the case of oppositely charged particles reads: ( theta 12)max approximately E1/4 for all Z>0, where Z is the charge of the remaining ion. The validity of the classical model as well as the approximations made within the classical description of the near-threshold behaviour are discussed. Further possible applications of the method are also quoted.

A New Class of Atomic States: The 'Wannier-ridge' Resonances

The evidence for the existence of systems that are dominated by electron-electron correlations is briefly reviewed, with particular attention being paid to (i) the processes of near-threshold electronimpact ionization and excitation of atoms and (ii) the structure of atomic doubly excited states and Feshbach resonances. New evidence is then presented for the existence of atomic resonances in which two excited electrons both have high val. es of the principal quantum number n (up to n = 8), and in which these two electrons reside on or near the Wannier ridge (i.e. with r1 ~ -rz).

Near-threshold ionization of excited Na(3 2P) by collisions with O 2 and SO 2

Positive ions are produced in crossed beams of eV-range sodium atoms and thermal O 2 or SO 2. Resonant laser light is used to excite a small fraction of the sodium to Na*(3 2P 1 2 or 3 2P( 3 2 ). Product ions are observed at lower reagent kinetic energies with Na* than with Na.

Measurements of cross sections for the ionization of Ba+by energy resolved electrons

The crossed beam method for observing the ionisation of ions by electrons has been refined by the inclusion of a 127 degrees electrostatic electron energy selector. This enhances the resolution of the measurements, by an order of magnitude, to about 0.15 eV. With this apparatus, autoionization thresholds have been resolved in the ionization function for, Ba++e to Ba2++2e, at energies of 15.8, 16.8, 19.2 and 21.9 eV. Observations have also been made of the near-threshold ionization of K+.