Anomalous Sputtering Behavior Observed by Quantitative Measurements of the Population Partition of Metastable Ni Atoms

Abstract

The population partition of metastable atoms after ion beam sputtering of polycrystalline Ni was studied. Resonant multiphoton ionization using double resonant two-color two-step photoionization was used. This allows the quantitative measurement of the relative population of all metastable states. The high lying metastable states with excitation energies around 2 eV show populations of the order of 10% of the ground state population which is several orders of magnitude higher than expected. The anomalous population partition is interpreted in the resonant electron tunneling model. [S0031-9007(96)02032-7] PACS numbers: 79.20. Rf, 32.80.Fb, 32.80.Rm The fundamental physical processes governing ion beam sputtering can be studied experimentally by examining the ejected particles. The inelastic collisions of energetic ions with solid surfaces result in the ejection of atoms or clusters in different charge states and in different states of electronic excitation. Both the experimental and theoretical situations have been reviewed by Yu [1]. It is generally assumed that the fraction of sputtered atoms on highly excited metastable states is very low [2][3][4]. This assumption is based on quasithermal excitation and nonradiative deexcitation which depend strongly on the excitation energy.However, theoretical models and, albeit scarce, experimental evidence for a substantial population of such states are available. Indeed, the electron transfer model of Veje [5] predicts strong population of all states within an atomic excitation energy window corresponding to the valence band of the solid under study. Experimentally, in a study of Fe atom sputtering using laser induced fluorescence, a considerable population of metastable states at about 1 eV excitation energy was observed [6,7]. Measurements based on fluorescent photon detection, however, suffer from limited sensitivity and can therefore not be generally applied.Recently, studies of population and kinetic energy distributions of sputtered metastable Ni atoms, using resonant ionization spectroscopy (RIS) [8], were reported [3,4]. However, the experimental procedure used in these investigations limited the study of metastable states to excitation energies below 0.5 eV. Also recently, RIS was applied to probe the population of one high lying metastable state of sputtered Ag atoms; a population of about 6% of the ground-state population was deduced from the data [9,10]. The observations on sputtered Ni and Ag were interpreted as evidence for the fact that the sputter yield of metastable atoms depends on the electronic structure of both the solid and the sputtered atom.In this Letter we report on the development and application of a very sensitive experimental procedure for the quantitative measurement of relative populations of all metastable states. The method is based on resonant multiphoton ionization spectroscopy using two-color twostep ionization schemes in combination with time-of-flight mass spectrometry. Application of this technique to the study of metastable N...

Nickel atom and ion densities in an inductively coupled plasma with an internal coil

“…However, the measured populations of a 3 D 3 and a 3 D 2 were 60% more than that of the ground state 3d 8 4s 2 ͑a 3 F 4 ͒ during sputtering of polycrystalline Ni target by Ar + ions with energies 3 -12 keV. 44 More surprisingly, for a single crystal Ni ͑001͒ target under bombardment by 5 keV Ar + ions, the populations of a 3 D 3 and a 3 D 2 were found to be 5.7 and 7.4 times that of a 3 F 4 , respectively. 43 This remarkable disparity in the Ni partition factor could originate either from target crystal structure differences or from differences in experimental techniques.…”

“…[41][42][43][44] A striking result was the pronounced population inversion of a 3 D 3 and a 3 D 2 sublevels in Ni metastable state, with respect to the electronic ground state. The density ratio a 3 F 4 : a 3 D 3 : a 3 D 2 should be close to 9:7:5 with equal Boltzmann factors.…”

Deexcitation of sputtered metastable aluminum atoms

“…For example, the Ni 3 D J state populations, which exhibit strong coupling in the RET model, decrease faster with excitation energy than the 3 F state populations [26] and the Ni 3 D J states have a TOF mass spectrum shifted towards shorter flight times than the 3 F J states [26]. In a similar way, the Al yields from Ni 3 Al decrease more quickly with excitation energy (from 1 to 0.9) than from NiAl (from 1 to 0.95) and the TOF mass spectrum from Ni 3 Al is shifted to shorter flight times, suggesting a greater coupling to Ni 3 Al.…”

“…RMPI can perform state-selective detection of desorbed species, and can detect almost any ground or metastable excited state atoms [18]. RMPI has been used to study the population and kinetic energy distribution of In [18,19], Rh [20,21], Ti [22], Ni [23][24][25][26], Co [23] and Ag [27,28]. A review of these studies has been made by Garrison et al [29].…”

“…A resonant electron transfer (RET) model has also been used by the Leuven group [23,26,31] to explain population partitions and velocity distributions of sputtered species. In this model, the sputtered particle crosses the metal surface as a positive ion (i.e.…”