p hydrogen clusters

Nuclear Instruments and Methods in Physics Research B28 (1987) 497-501 Nosh-Holl~d, Amsterdam

497

FOIL DISSOCIATION OF 40-120 keV/p HYDROGEN CLUSTERS B. MAZUY,

A. BELKACEM,

M. CHEVALLIE~

Institut de Physique NucGaire (and IN2P3), 69622 ViIkurbanne Cedex, France

UniversiJ

M.J. GAILLARD,

Claude Bernard Lyon-l,

J.C. POIZAT and J. ~MILLIEUX

43, Bd du II Novembre 1918,

Received 1 June 1987

We report on measurements of angular and charge state distributions of hydrogen fragments resulting from the dissociation of fast Hz clusters (n ~13) in a carbon foil. The proximity effects on the fragment neutralization have been investigated for beam velocities above and around the Bohr velocity. At a given velocity the angular width and the yield of neutral atoms are observed to saturate at n 2 5 and n 2 7, respectively. The interpretation of this behaviour provides some insight into the collective aspects of the collisions and into the structure of hydrogen clusters.

It is well known that the charge state distribution of emerging fragments resulting from the passage of fast molecular ions through a thin solid target is different from what one would get with atomic projectiles of the same velocity [l-3]. When a molecule penetrates into a condensed medium most of its electrons are stripped off and the binding between the atoms constituting the molecule is disrupted. The ionic fragments are repelled from each other by a screened Coulomb interaction while they fly through the solid. When they emerge from the exit surface of the foil some of these fragments bind electrons that may originate from the incident projectiles or that have been picked up from the target. The first process is dominant in the case of very thin foils and is directly related to the transmission probability of the projectile electrons. In particular, previous studies [2] in our laboratory have shown that the transmission probability of Ho atoms decreases exponentially with the target thickness for short dwell times (less than lo-‘* s for MeV projectiles) and that for longer dwell times the yield of neutral atoms from incident Ho is the same as obtained from H+ projectiles, which means that neutralization results from the pick-up of a target e1ectron.b Many authors [4-S] have also reported that, in this “pick-up regime”, the close proximity of the molecular fragments at the exit surface of the solid target tends to decrease the average charge state. In the particular case of hydrogen molecules, these effects increase the probability of neutralization of fragments. It has been also shown in our laboratory that the excess of Ho atoms is about twice as high with H: projectiles as with H: projectiles of the same velocity in the 0.1-1.2 MeV/p energy range, which means that the collective effect depends strongly on the number of neighbouring frag0168-583X/87/$03~50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

ments during the neutralization process. However, de Castro Faria et al. [8] have with D: projectiles shown that the neutral atom production in the pick-up regime does not vary monotonically with target thickness but presents a maximum because the formation of neutral atoms at emergence from a very thin target is inhibited by the high probability of molecular recombination. The purpose of this paper is to report on experimental results obtained with hydrogen cluster projectiles Hi (n s 13). We have measured the angular distribution and the yield of Ho atoms emerging from a selfsupporting 3.4 pg cmm2 carbon foil bombarded with Hz projectiles of velocities ranging from 1.2 to 2.2~~ (u. = e2/fi = 2.18 X 1O’cm s-l being the Bohr velocity). In this velocity range the emergent fragments are mainly Ho and H+. We note that the thickness of the target allows to consider that all the emergent Ho result from a capture of a target electron. Hydrogen cluster beams of energies ranging from 40 to 600 keV are currently delivered by the Co&oft cluster accelerator of the Institut de Physique NuclCaire de Lyon [9,10]. Hz bursts of duration 60 ms are produced at a repetition rate of 0.2 Hz. The hydrogen clusters produced and observed up to now are singly charged species of odd mass numbers (except Hz). A sketch of the experimental setup is shown in fig. 1. The accelerated ion cluster beam is selected in energy and mass by electrostatic (E) and magnetic (Bl) analyzers. The selected HT beam passes through two collimators Dl and D2 which define an angular divergence of O.l” and a cross-section of 3 mm2 at the target. The pressure in the beam line and in the target chamber is about low6 Torr and does not induce significant dissociation of incident clusters. Due to Coulomb explosion and also to multiple scattering the

498

B. Mmuy et al. / Foil dissociation of 40- 120 keV/p S

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hydrogen clusters I

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I

HT *C

I

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TARGET

Fig. 1. Experimental setup (see text for details)

fragments emerge from the foil within a wide cone. A surface barrier detector (SB) placed downstream from the target, is used to measure their angular distribution. A collimator D3 (0 = 0.5 mm) placed in front of SB defines an angular acceptance of 0.034 O. SB and D3 are mounted together on a two-axis system that can rotate both in the horizontal and the vertical planes around a center located close to the target position. The angular scans have been performed along the vertical plane. A permanent magnet B2 can be placed between the target chamber and the detector in order to deflect the charged fragments in the horizontal plane and to allow the measurement of the angular distribution of the Ho atoms. The vertical slit S (6 x 20 mm2) bound to the detection system and placed just downstream from B2 lets the only neutral fragments reach the detector. During the scans the beam is monitored by measuring the target current due to secondary electron emission. This current is proportional to the number of incident projectiles. The proportionality factor depends on the mass of the cluster, its velocity and the tilt angle of the target relative to the beam direction, parameters that remain unchanged during an angular scan. Our range of data is limited in mass and energy of the incident clusters, on one side, by the energy threshold of the detector SB (- 40 keV), and on the other side by the maximum voltage of the accelerator (600 kv). As an illustration, we show in fig. 2 the angular distributions of H+ and Ho fragments resulting from the dissociation of 80 keV/p HT clusters in the carbon foil. The solid and the dashed curves give the angular distributions of the totality of the fragments (Ho + H+) and of the neutral atoms (HO), respectively. The dot-dashed curve, obtained by subtraction, gives the angular distribution of the protons. The fact that protons have an angular distribution larger than Ho atoms in explained as follows: the kinetic energy gained by each proton in their mutual repulsion depends strongly on its initial location with respect to the others, and also on the dynamical screening in the foil, that can be characterized by a screening length ads. In our experimental range of projectile velocities ads is given [ll], with a reasonable accuracy, by the ratio of the projectile velocity on the plasma

,_

EMERGENCE

ANGLE

8

( DEGREES)

Fig. 2. Angular distribution of emerging fragments resulting from the collision of 80 keV/p Hq clusters with a 3.4 pg cm-’ carbon foil. The carves have been drawn only to guide the eye. 9: and 6’: are the fwhm of the angular of emergent Ho and H+, respectively.

distributions

frequency in carbon and varies from 0.7 to 1.2 A. Then, the projectile travels inside the solid as a swarm of individual protons, the volume of which increases with the penetration depth. Because of multiple scattering and energy straggling the fragments become progressively randomly distributed inside this volume and in particular at emergence where their mutual separation settles the proximity effects on neutralization. We can consider that, at emergence, the protons continue to repel each other in vacuum whereas, at a first approximation, the Ho atoms do not interact anymore with the other fragments. Consequently, the angular distribution of Ho atoms reflects the combined effects of multiple scattering and screened Coulomb explosion in the foil only. In fig. 3 we show the variation of the fwhm 8,” of the Ho fragment distributions with the mass number n of the incident clusters, for various projectile velocities. The 0: values are deduced from the theoretical predict-

B. Mazuy et al. / Foil dissociation of 40- I20 keV/p I

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7

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I

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11 13 9 MASS NUMBER (n)

Fig. 3. Variation with the cluster mass number of the fwhm of the angular distribution of Ho atoms emerging from a 3.4 (lg cm-* carbon foil, for various projectile energies. The (n =l) values are calculated (see text)

ions of Sigmund for proton multiple scattering [12]. We observe that at a given projectile velocity 02 first increases with increasing n and then tends to saturate for n > 5, for all velocities. Since in our experimental conditions the screening distance is of the order of the mean proton separation in the initial cluster (- 1 A), we may consider that each proton interacts only with its closest neighbours. Therefore, the saturation of 0,” at n - 5 means that the average number of close neighbours for protons in a cluster of high mass number of only 4, a rather low value for condensed matter [13,14]. This is also indicates that the density of a cluster does not depend on its mass number, in good agreement with previously published experimental results obtained from the investigation of cluster dissociation in gas targets [15,16]. We observe also that for a given mass number n, 13,” decreases when the cluster velocity increases, which could be expected. However, the variation of 0,” with velocity seems not to depend on n, which means that it is dominated by multiple scattering. The contribution of the screened Coulomb explosion to the variation of 0,” with velocity seems to be lessened by two opposite

499

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effects on the final kinetic energy of the fragments: this energy is proportional to the repulsion force between the fragments and to the time during which this force is applied; increasing the velocity reduces the repulsion time (for a given target thickness) but increases the dynamical screening length and then the repulsion force. From the angular distributions one deduces the fraction +t of neutral atoms and its variation with n by integrating the angular distributions over the total volume of the explosion cone. Before studying proximity effects on the charge state of the fragments, we found it useful to study the velocity dependence of the neutral fraction in the simple case of atomic incident projectiles. We measured the “atomic” neutral fraction $J: by using a cluster beam incident on two successive foil targets. The cluster projectiles explode in the first foil, and the distance between the resulting fragments when they reach the second foil is so large that they behave like atomic projectiles (in matter of charge exchange). However, it must be noted that the angular distribution of the fragments is mainly determined by the repulsion effects in the first foil. In fig. 4 we show the measured velocity dependence of & deduced from the study of the angular distributions of H+ and Ho fragments. The values are in very good agreement with other measurements in carbon by Kreussler and Sizmann [17], shown for comparison, and by the Chateau-Thierry and Gladieux [18] (which provides a good test of the procedure used here to measure neutral fractions). Note that the velocity appearing in fig. 4 and also in the data to be shown later is

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3

VELOCITY (IN UNITS OF V,) Fig. 4. Variation of the “atomic” neutral fraction with the projectile velocity at emergence from a 3.4 pg cm-* carbon foil. The experimental results by Kreussler and Sirmann (KS.), shown for comparison, are taken from ref. [17].

B. Mazuy et al. / Foil dissociation of 40- 120 keV/p hydrogen &stem

500

the velocity at emergence. The small correction due to proton slowing down in carbon has been done using tabulated stopping powers. In fig. 5 we show the variation of & the neutral fraction per incident proton, as a function of n, for various projectile velocities. We observe that, at a given velocity, & increases first linearly with n, and then tends to saturate for n 1’7 (unfortunately the ~turation can be seen only for the two lower velocities). These features can be explained in the following way: one knows that molecular effects on the charge state distribution at emergence are directly related to the distance between the fragments at the exit surface of the foil. Due to the shortness of ads and the magnitude of multiple scattering effects it would be rather complicated to calculate the separations between the protons at emergence for a given geometrical structure of the incident cluster. However, the average separation can be estimated from the calculation of the screened Coulomb explosion if one assumes that multiple scattering induces mainly the randomization of the respective positions of the protons in the swarm. We have calculated that for 40 keV/p Hi ions the average distance between the two protons is about 3 A at emergence from a 3.4 pg cm-* carbon foil. Previous

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We acknowledge the kind assistance of F. Rochigneux and J. Martin for efficiently preparing the cluster beam.

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studies [2] have shown that the relative excess of neutral atoms A&/#$, i.e. (& - &/+,, corresponding to this separation has a constant value of about 0.5, in the 0.5-2 MeV energy range. In our case the value of A&/+~ is only 0.06, which means that at low velocity this ratio decreases rapidly with decreasing projectile velocity. Therefore one cannot make use here of the simple g~met~cal model discussed in ref. [2]. More systematic measurements of A& in a wider range of velocity and target thicknesses are needed to clear up the matter. The linear increase with n of +z for clusters of low mass numbers is in qualitative agreement with that previously observed [2] for H+, Hl and H: projectiles. This feature may mean that the average distance between the protons at the exit surface of the foil is the same whatever the cluster mass is. Thus, the density of the emerging swarm, for a given velocity and target thickness, does not depend on n. In addition, the neutralization of a given proton has been shown to be affected by molecular effects for internuclear distances shorter than about 10 _& Then, the value of n at which +z is observed to saturate may reflect the mean number of protons that are contained at emergence in a sphere of radius 10 A. In conclusion, we have shown that the information one can extract from cluster-solid collision experiments are of two kinds. The angular distribution of the emergent fragments depends on what happens in the first layers of the foil and sheds some light on the structure of the incident clusters. In particular, we have shown that the density of hydrogen clusters does not depend on the mass number and also that the mean number of closest neighbours to each proton of the cluster is about 4, which is a rather low value. The charge state distribution at emergence depends strongly on the swarm size in the last layers of the foil and may allow to help understanding the fundamental processes in the interaction of ionic projectiles with solid targets. New experiments are in progress that will extend this type of meas~~ents to hydrogen clusters of lower velocities and higher mass numbers.

tn1

Fig. 5. Variation with n, the cluster mass number, of the neutral fraction of fragments emerging from a 3.4 pg cm-* carbon foil for various cluster velocities. The incident energy per proton and u,,,, the mean velocity of the fragments at emergence, expressed in units of the Bohr velocity, are given for each set of data.

References PI B.T. Meg&

K.G. Harrison, and M.W. Lucas, J. Phys. B6 (1973) L-362. PI M.J. Gaillard, J.C. Poizat, A. Ratkowski, J. Remillieux, and M. Auzas, Phys. Rev. A16 (1977) 2323. [31W.H. Escovitz, T.R. Fox, and R. Levi-Setti, IEEE Trans. Nucl. Sci. 26 (1979) 1149.

B. Mazuy et al. / Foil dissociation of 40- 120 keV/p [4] M.J. GaiUard, J.C. Poizat, A. Ratkowski, J. Remillieux, Nucl. Instr. and Meth. 132 (1976) 69. [5] D. Maor, P.J. Cooney, A. Faibis, E.P. Kanter, W. Koenig, and B.J. Zabransky, Phys. Rev. A32 (1985) 105. [6] N.V. de Castro Faria, F.L. Freire Jr., J.M.F. Jeronymo, E.C. Montenegro, A.C. de Pinho, and D.P. Almeida, Nucl. Instr. and Meth. B17 (1986) 321. [7] A. Clouvas, These Doct. &SC. Phys., Univ. Lyon-I (1985). [8] N.V. de Castro Faria, F.L. Freire Jr., E.C. Montenegro, and A.G. de Pinho, J. Phys. B19 (1986) 1781. [9] H.O. Moser, J. Martin, and R. SaIin, J. Phys. (Paris) 38 (1977) 215. [lo] Y. Chanut, J. Martin, R. Salin, and H.O. Moser, Surf. Sci. 106 (1981) 563. [ll] W. Brand& in: Atomic Collisions in Solids, eds., S. Datz,

[12] [13] [14] [15] [16] [17] [18]

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B.R. Appleton and C.D. Moak (Plenum Press, New York, 1975), vol. 1, p. 261. P. Sigmund, and K.B. Winterbon, Nucl. Instr. and Meth. 119 (1974) 541. H. Huber, J. Chem. Phys. 70 (1980) 353. Y. Yamaguchi, J.F. Gaw, and H.F. Shaefer III, J. Chem. Phys. 78 (1983) 4074. M. ChevaIIier, A. Clouvas, H.J. Frishkom, M.J. Gaillard, J.C. Poizat, and J. RemiBieux, 2. Phys. D2 (1986) 87. A. Van Lumig, and J. Reuss, Int. J. Mass Spectrom. Ion Phys. 25 (1977) 137. S. Kreussler, and R. Sirmann, Phys. Rev. B26 (1982) 520. A. de Chateau-Thieny, and A. Gladieux, Nucl. Instr. and Meth. 132 (1976) 553.