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We have studied high resolution beryllium Auger electron spectra in the 120–160 eV projectile electron energy range following intermediate energies (1.2 and 1.5 a.u. impact velocities) in Be + + CH 4 ion–molecule single collisions [Comments At. Mol. Phys. D 14 (1984) 117]. The major focus of this study is the double K-shell excitation of the Be + projectile forming triply excited BeII*** states (hollow atoms) and BeIII** K shell ionization plus excitation of BeII doubly excited states with two inner shell vacancies. In such intermediate velocity ion-molecular collisions, the CH 4 target plays an important role in the enhancement of the population of BeII*** triply excited states when compared to a more tightly bound He (1s 2 ) 1 S ground state target. Due to the pioneering theoretical work of Gou and Chung [Phys. Rev. A 29 (1996) 6103] most line structures can now be unambiguously identified. The main triply excited states are associated with the 2s 2 2p, 2s2p 2 and 2p 3 and 2 l 2 l ′ n ′ l ″ ( n =3 and 4) configurations decaying to different final BeIII (1snl) ionic states. The line identification method used here is based on Auger energy levels, decay channels and branching ratios.

The excitation energies and the probabilities for radiative and radiationless decays of hollow atom states belonging to the 2s22p, 2s2p2 and 2p3 configurations have been calculated on the basis of perturbation theory method (1/Z-expansion) in a broad range of values of atomic number Z. Our results agree well with the first observation of these states and the theoretical and experimental results obtained recently for neutral Li atoms. It should be noted that the 1/Z expansion method gives more accurate results for ions than for neutral systems, because of the limited number of coefficients of 1/Z expansion calculated in this work. In this case the good agreement provided by our results for neutral system allows us to believe that our data for the same triply excited states with Z>3 should be used for prediction of new experiments with these states.

Cross sections have been measured for the production of He{sup +} ({ital np}){sup 2}{ital P}{sup {ital o}} states, {ital n}=2,3,4, by proton impact on helium over a projectile velocity range of 1.42{endash}2.45 a.u. (50 {le}{ital E}{le}150 keV). Cross sections were determined by measuring the extreme ultraviolet photons emitted from excited He{sup +} ions. The data indicate a lower energy than expected for the maximum cross section. A comparison of the present results in terms of projectile energy dependance with the cross sections for excitation to He (1{ital snp}){sup 1}{ital P}{sup {ital o}}, ionization, and total electron capture suggests the primary mechanism for the production of excited He{sup +} at low energies is transfer excitation, with ionization excitation being the dominant mechanism at higher energies. {copyright} {ital 1996 The American Physical Society.}

We present the first EUV emission spectra following single and multiple-electron capture for the 80 keV Ar8++N2 multi charged ion-molecular collision system. Our analysis provides evidence of single and double capture processes leading to radiative deexcitation of Ar7+*(nl) n=3–6 and Ar6+*(3lnl′) n=3–5 projectile states. Furthermore we have identified numerous Nq+ (q=2,3) ionic target lines in the EUV spectral range (10–18 nm), owing to multiple ionization of the N2 molecule and consecutive dissociation plus excitation of the fragments. The subsequent photon emission arises from high lying states of the target such as N2+*(1s22s2lnl′) 2,4L,n=2–10 and N3+*(1s22lnl′) 1,3L,n=2–5 with excitation energies up to 52 eV and 78 eV, respectively. The N2+ and N3+ excited dissociated fragments are due to multiply electron capture processes involving up to six electrons.

In this study, we present high resolution electron spectra and new identifications of numerous triply excited states of the Li-isoelectronic sequence produced by intermediate velocity Li+ and Be+ projectiles interacting with CH4 molecules under single collision conditions. In particular, we show here as typical examples high resolution lithium and beryllium projectile Auger electron spectra following double K-shell excitation in 200 keV Li++CH4 and 300 keV Be++CH4 collisions. For the beryllium case, the impact velocity is 1.15 a.u. Many new lines appearing in these spectra have been assigned to triply excited Li and Be+ and doubly excited Li+ and Be2+ states. Only recently due to the pioneering theoretical predictions by Chung and coworkers (1), such collisional line structures can now be unambiguously identified. We have not only identified the lowest lying, most prominent 2s22p, 2s2p2 and 2p3 triply excited states (2), but also new excitation and deexcitation channels of higher lying 2l2l′nl′ hollow ato...

The electron spectra of autoionizing states of Li I and Li II were studied using H+ and He+ bombardment. Excitation energies of core-excited levels in Li I and Li II were measured, some of them to within +or-10 meV accuracy. For the metastable state (1s2s2p)4P0 an excitation energy of 57.385 eV was obtained.

We report the first measurement of Na and Na/sup +/ autoionization spectra following collisions of 1-MeV He/sup +/ ions on selectively excited Na 3p /sup 2/P/sub 3/2/ target atoms. The atomic states were populated with a c.w. dye laser which was tuned to the Na 3s /sup 2/S/sub 1/2/..-->..Na 3p /sup 2/P/sub 3/2/ transition. The simultaneous laser-ion beam excitation technique produces significant intensity variations particularly for the decay of the Na/sup +/ 2s2p /sup 6/3s /sup 1,3/S and the Na/sup +/ 2s2p /sup 6/3p /sup 1,3/P states. The technique may provide additional information for line identification, excitation mechanisms, and angular distribution measurements.

Metastable, autoionizing high-spin states are studied using the ion-beam time-of-flight method in connection with single-collision excitation. High-resolution, time-delayed spectra are obtained from Li, Be, and B excited in 200-keV impact on CH/sub 4/. Evidence is found for the (1s2s2p/sup 2/)/sup 5/P state in B II. Excitation energies for the observed quartet and quintet states are given, and the branching ratio associated with the BII (1s2s2p/sup 2/)/sup 5/P decay is estimated.

In the last decade it has been established that charged particle interaction with gaseous or solid targets may exhibit substantial anisotropic population of magnetic substates, which manifests itself in the polarisation of the emitted radiation as well as in the angular distribution of the ejected autoionisation electrons. Due to the short collision time (Δt ≤ 10-15s) in typical ion-atom collision experiments fine- and hyperfine-structure components are generally expected to be coherently excited (Macek 1969, Andra 1979). Consequently fine- and hyperfine-structure modulations in the time resolved spectroscopy should be observable using the projectile electron spectroscopy method (Rodbro et al. 1979). The observability of such quantum beats depends both on the lifetime of the atomic states under investigation and on the time resolution of the experimental facility. Because of the short lifetime of autoionising states decaying via Coulomb interaction these quantum beats remain in general unresolved and are expected to depolarise the time-averaged angular distribution. Note that a depolarisation of angular correlations, owing to the lack of information with respect to unobserved internal degrees of freedom is a concept being central to the analysis of angular correlation measurements in many fields of physics.

Be Auger spectra are studied by 25-500 keV Be+ ion impact on H2, He, CH4 and Ne. Several single and doubly core-excited states not previously seen are identified by comparison with calculated transition energies. In particular metastable autoionising Be II (1s2s2p)4P0 states are strongly populated.

High resolution projectile Auger spectra of singly and doubly core-excited oxygen ions are studied as a function of incident beam energy (2--18 MeV), exciter gas (helium, neon, argon, and krypton) or foil (carbon), incident charge state (1+--4+), and delay time. Auger electron production from excited oxygen states varies considerably for collisions with different beam energies and exciter gases. A striking similarity in both the linewidths and relative Auger line intensities is observed for single collision gas and multiple collision foil excitation. The incident ion charge state does not appreciably affect the excitation cross sections and the charge state of the outgoing projectile. In order to assign the measured Auger peaks numerous excitation and Auger transition energies for singly and doubly core-excited states are calculated using semiempirical and ab initio methods. Calculated transition energies are compared with experimental line structures. Possible excitation and deexcitation mechanisms are suggested and discussed within a quasimolecular single-particle model.

The ejected-electron spectra of highly excited autoionising levels of Li, Be, B and C have been studied by using the projectile electron spectroscopy method. Singly and doubly core-excited states are strongly populated in 100 to 500 keV Li+, Be+, B+ and C+ single collisions with He and CH4. High resolution is obtained by selecting a small observation angle (6.4 degrees ) with respect to the beam axis. Numerous states, previously not seen, have been resolved and identified by comparison with existing theoretical data. For a large number of core-excited states, absolute Auger transition and excitation energies are given. In particular, isoelectronic sequences for singly core-excited lithium-like (Z=2-16) and doubly core-excited helium-like (Z=1-14) autoionising states are studied.

We have measured high-resolution electron decay-in-flight spectra of highly excited carbon ions, produced by 2-MeV ${\mathrm{C}}^{+}$ beams emerging from carbon foils. An almost complete identification for the resolved spectral lines is suggested for the prompt and time-delayed electron emission spectra. To assign the observed structures excitation energies for various core-excited states have been calculated using the generalized Brillouin theorem-multiconfiguration method of Schwarz and Chang. Experimental term energies for Coulomb and metastable autoionizing states in two-, three-, and four-electron carbon ions are tabulated and compared to calculated transition energies. Because of line blending most of the experimentally deduced term energies are uncertain to \ifmmode\pm\else\textpm\fi{}0.4%. Some spectral features in the high-energy portion of the prompt spectrum can be attributed to decays of doubly excited He-like carbon ions.

We have used the beam-foil interaction mechanism to populate highly excited states in lithium by passing 300-keV ${\mathrm{Li}}^{+}$ ions through 8-\ensuremath{\mu}g/${\mathrm{cm}}^{2}$-thick carbon foils. The energies of the electrons emitted via autoionization were analyzed. By moving the foil along the beam axis, prompt and delayed electrons due to decays of Lii and Liii states were identified. These spectra are interpreted with the help of detailed theoretical calculations. The excitation energies of the $\mathrm{LiI}(1s2{s}^{2})^{2}S^{e}$ and $\mathrm{LiII}(2s2p)^{1,3}P^{o}$ terms were determined to 0.5% accuracy. We have observed decays of doubly excited Liii states above the $n=2$ threshold. On the basis of our theoretical estimates, we assign a peak in the high-energy portion of the spectra as due to deexcitation of triply excited $\mathrm{LiI}2{s}^{2}2p$ and $2s2{p}^{2}$ configurations.

Highly excited single and double K-vacancy states in boron are studied using the projectile electron-spectroscopy method. The prominent features of the ejected-electron spectrum of boron are identified and compared with theoretical estimates.

Little is known experimentally and theoretically about cascade induced repopulation of multiply excited states with high orbital, spin, and angular momentum quantum numbers. Such states are often created with surprisingly large cross sections by the beam-foil or beam-gas interaction. Recent progress in beam-foil and beam-gas1,2 electron spectroscopy offers the possibility to study excitation and deexcitation mechanisms for highly excited ionic projectile states via the Auger decay. This time-of-flight technique allows to analyze the decay of long-lived metastable autoionizing states which are frequently populated by a sequence of cascades that incorporates optical and Auger transitions.

Angular distributions of Auger electrons emitted from Be II 1s(2s2p 3P0) 2P0, 1s(2s2p 1P0) 2P0, and (1s2p2)2D states excited in 100–500-keV Be+ on He and Be+ on CH4 collisions have been measured. For the 2P0 states the source-frame anisotropy coefficients are deduced, and the alignment parameters are calculated. The Be+ on He collisions lead to strong alignment where the ML = 0 substates are preferentially populated. Weaker alignment is obtained in Be+ on CH4 collisions.

The only measurements for super-narrow two-electron (2, 3c)3P0 and (2, 4c)3P0 states in doubly ionized beryllium are those recently reported by Andersen et al. There they measured the optical decays (2, 2a)3P-(2, 3c)3P0 and (2, 2a)3P-(2, 4c)3P0 in the extreme UV following beam-foil excitation. In this work we present an analysis of additional decay channels of the (2, 3c)3P0 and (2, 4c)3P0 levels in Be III as well as a discussion of their fundamental properties.

Measurements of the Auger decay of beam-foil excited Be II and Be I levels are reported along with a proposed assignment of the experimental spectra. The Li I, Be II and Be III (1s2s 2 ) 2 S → (1s 2 2s) 2 S Auger transitions as presented in this letter represents the first observation of such states in positive ions with Z ⩽5.

High resolution projectile Auger spectra of singly and doubly core-excited oxygen ions are studied as a function of incident beam energy (2 to 18 MeV), exciter gas (helium, neon, argon, and krypton) or foil (carbon), incident charge state (1+ to 4+) and delay time. Auger electron production from excited oxygen states varies considerably for collisions with different beam energies and exciter gases. Calculated transition energies are compared with experimental line structures. The ratio of single to double K-vacancy production is estimated within the framework of the molecular orbital model.