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• W2045548285 abstract "Some specific effects seen in the interactions of swift hydrogenic beams penetrating thin foils a r e presented. We describe f i rs t dynamic effects due to periodi c electron density fluctuations trailing fast charged particles moving in solids which can sustain plasmon oscillations. Studies of non equilibrated charge distribution dows tream very thin foils permit us to separate the role played by projectile a n d target electrons in neutral atom production. The observation of b o u n d molecules emerging from thin target raisesquestions about their formation and t h e processes involved. Finally we show how original measurements on the dissociation of fast molecular ions by thin foils has opened up new possibilities f o r t h e investigation of molecular ion structure INTRODUCTION Recently, considerable interest has centered on the penetration of fast molecular ions through solids since i t has been demonstrated that the propagation of spatially correlated ions through matter is influenced by the electronic polarization wake trailing each ion [ I 1. This paper present some features which have been observed when MeV atomic and molecular hydrogen ions penetrate thin foils. We a r e interested in the interactions occuring inside the solid and in phenomena which involve correlated electrons and ions emerging in a close vicinage from the foils. The f i rs t part i s a survey of some anornalies observed with molecular beams which can be attributed to polarization wake effects. In the second part, we comment on non-equilibrated neutral fractions measured dowstream very thin foils and related processes pertaining to the existence of momolecular states a t emergence. The observation of bound molecular ions dowstream foils is thendiacussed in terms of electron capture a n d w a k e effects. Finally we show that the study of the dissociation of fast molecular ions opens up new possibilities for the investigations of molecular structure. POLARIZATION WAVE EFFECTS Evidences of the electronic-polarization wake were f i rs t seen through the observation of energy and angular distributions of break-up fragments emerging from thin amorphous or crystallit t t ne targets bombarded with H2, H and HeH mole3 cular targets ions of MeV energy range [11 . The ion velocity v being larger than the Bohr velocity v the binding electrons a r e believed to be strip0 0 ped within a few A of the solid. This a consequence of the large cross-section for electron loss due to collisions with target electrons. Then the Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979174 charged components arising from the molecular ion repel each other due to the Coulomb force and the cluster begins to explode. With thin targets, the inter nuclear distances remain smaller than the screening length a = v/w where w is the voP' P lume plasma frequency of the target. The Coulomb repulsion initiated in the foil will be generally completed in vacuum dowstream the foil, and will r e sult in a small isotropic component added to the initial ion velocities. Consequently, the effects of the Coulomb explosion can be seen in the angular and energy distributions of ions detected after emergence. Of course, the distributions depends on the population of vibrational levels of the incident molecular ions [ 2 ). Moreover, multiple scattering, slowing down and straggling due to electronic inte ractions modify this simple description. Earlier results obtained by Golovchenko and ~ a e ~ s ~ a a r d [31 were consistent with the Coulomb explosion model. They measured the distri0 butions for protons transmitted through a 100 A thick carbon foil bombarded with 2 MeV H ' ions. 2 With such a thin foil, most of the repulsion occurs after the foil. Further experiments performed with thicker targets exhibited particular features, and a new force was necessary to explain them and other new effects observed with molecular beams. This force i s related to the existence of a polarization wake trailing ions moving in solids. The concept of electronic polarization wake was predicted in 1955 by R. Ritchie in his thesis and published with Neufeld in 1955 [4I . Soyears ago, Neelavathi, Ritchie and Brandt r5I studied the possibility for an electron to be trapped in the wake created by a moving ion. They expected surf-riding electrons to appear a s a group of electrons with velocity centered about v, the velocity of h e ion. When a fast charged particle i s moving faster than the Ferrni velocity in solid which can sustain well defined plasmon oscillations, the electrons of the solid react in an effort to screen the ion charge and an oscillatory charge density which is stationnary in the projectile f rame trails the ion like a wake behind a ship. This polarization wake sets up a potential in the medium. The resulting force acts a s a brake on the projectile itself and i s responsible for the electronic stopping power of the solid. To probe this potential, i t was tempting to use molecular projectiles with internuclear distan0 ces (* 1 A) short enough to allow vicinage effects through interference between the wakes t r a i 1 i n g behind each partner. The f i rs t observations concerned an alignment effect in the motion of ion clusters moving through solids [ 11 . In one of them, we measured the energy spectra of protons emerging in the beam direction from amorphous carbon foils bomt t barded with 2 MeV Hg and HeH beams (figure 1). As predicted by the Coulomb explosion model, two peaks were observed corresponding to the leading and trailing particles. The low energy peak corresponding to trailing ions i s higher than the high energy peak and the explanation i s that the trailing ions feel an additionnal force the so-'called wake force which attract i t towards the leading ion track. Thus the wake force enhances the emergence of trailing ions a t 0' and gives r ise to the asymmetry observed." @default.
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• W2045548285 date "1979-02-01" @default.
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• W2045548285 title "INTERACTIONS OF SWIFT H, H+2 AND H+3 WITH THIN FOILS" @default.
• W2045548285 doi "https://doi.org/10.1051/jphyscol:1979174" @default.
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