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Nuclear Instruments and Methods 194 (1982) 285-289 North-Holland Publishing Company

H ~ TRAVERSING

ULTRA-THIN

CARBON

285

FOILS: EXITING MOLECULAR

STATES AT

12.5 A N D 25 keY/ainu * T.R. FOX, K. L A M a n d R. L E V I - S E T T I The Enrico Fermi Institute and Department of Physics, The Universi(v of Chicago, Chicago, Illinois 60637, U.S.A.

We determine the yields of the bound and unbound components emerging from 0.3-1.1 ~,g/cm 2 C foils bombarded by 12.5 and 25 keV/amu ground state H f ions from a field ionization source. Bound H ~- and unbound H +H 0 (coincidence) final states are selected and momentum analyzed with a magnetic sector spectrometer incorporated in a scanning transmission ion microscope (STIM). Recombined H~- yields as high as 22% are observed for the thinnest foils. These are reconciled with the existing data at much higher energies, in a unified parameterization in terms of proton neutral fraction and inferred exit r.m.s, internuclear separation. The H +H ° spectra are decomposed into a central peak and two side wings. The central peak is complementary to the bound state yield in thickness dependence. The side-wing yield is independent of foil thickness. The features of the H +H ° and inferred H +H + spectra are discussed in terms of explosion energy and cluster orientation.

1. Introduction

2. Observation of recombined molecular ions

Our group at Chicago has been studying the interactions between H~- molecular beams (12.5 to 25 k e V / a m u ) and ultra-thin carbon foils (0.3 to i.1 / z g / c m 2) [1-4]. In order to work with such extremely thin films, we have exploited the finely focused beam in the scanning transmission ion microscope (STIM) in our laboratory. The thinnnest foils used can support themselves only over areas a few micrometer across and are mounted on a fenestrated plastic film ( - 1 0 0 nm thick). Examples of such targets are shown in fig. 1. The submicron beam used to image these specimens can be switched rapidly between a carbon-covered hole and an open hole in the fenestrated film, to simultaneously measure the incident and transmitted particle beams. A magnetic sector spectrometer analyzes the transmitted beam; a straight path through the magnetic field allows the detection of neutralized particles. The charged particle spectra can be taken in coincidence with the neutral particle detector, as well. The hydrogen ion beam is produced by a field-ion (FI) source. H ~ ions from a F I source are predominantly in the ground vibrational state, since excited states dissociate in the distorted molecular potential near the FI tip [5]. The results of our earlier work have already been published [I,3]. In this paper we summarize our results for the transmission and dissociation of H~- ions in thin carbon foils.

We detect and measure the m o m e n t u m spectra of H ~ ions in the beam emerging from the foil. Compared with yields measured at higher energies (e.g. < 10-3 at 800 k e V / a m u [6]) we observe much higher H~- yields, up to 22% at 25 k e V / a m u on our thinnest foil (0.3 /xg/cm2). Our results are tabulated in table 1. Our strongest evidence that these ions result from recombination of diss0~iated molecules (rather than from transmission of the original molecules) is the energy lost by the ion while traversing the foil. As discussed previously [2,7], the energy lost by the H~- ion is approximately twice that lost by a single proton of the same velocity. These measurements are described further in these Proceedings [8]. A comprehensive description of our approach to the analysis of these yields may be found in ref. 3. We make

* Work supported by the Air Force Office of Scientific Research under Contract F49620-80-C-0074 and by the National Science Foundation Ceramics Program under Grant DMR-8007978. 0029-554X/82/0000-0000/$02.75

© 1982 North-Holland

Table 1 Recombination yields Y(Hf ) measured in thin C foils Dwell time (fs)

rms sep. (nm)

Y(H~- )

Ref.

12.5 keV/amu, F0 =0.73 3.62 0.46 2.63 0.29 1.65 0.18 3.62 0.46

0.03 -+0.0 I 0.06 -+0.02 0.12 --+-0.04 0.03 ± 0.0 I

I21 [21 [21 [31

25 keV/amu, F0 =0.53 2.56 0.26 1.57 0.16 1.03 0.13

0.07±0.01 0.19±0.03 0.22-'-0.04

[31 I3l (this exp.)

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Fig. I. Focused ion beam images of C foil targets on fenestrated plastic films. (a) 50 keV transmitted H " ions, 0.51 # g , / c m ~m full scale. (b) Secondary. electrons from 60 keV Ga + ions, 0.32 gg/cm 2 (" foil, 67/~m full scale.

the H 2 yields at different energies comparable with each other by dividing them by the proton neutral fraction F0 [9] (equilibrium neutral fraction for protons of the same velocity, i.e. the proton's electron capture probability). We then plot these normalized H2+ yields as a function of inferred r.m.s, internuclear separation at the foil exit [1]. The r.m.s, separation is dominated by multiple scattering rather than Coulomb explosion [1,2]. Graphing the normalized high-energy [6] and low-energy H + yields versus r.m.s, separation successfully unifies the data over an hundredfold range of energy and four I@@

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orders-of-magnitude in yield. Fig. 2 is a slight revision of the graph in ref. 3. The high-energy yields that rise steeply above the low-energy points are explained by the Lyon group [10] as resulting from transmission of the original molecule. Since this occurs with an appreciable probability (compared with the low recombination probabilities) only for very short dwell times, the internuclear separation is near the initial r.m.s, value and the plot rises precipitously at low separation. This transmission is not apparent at our energies, where the recombination probabilities with which the transmission must be compared are much higher. Cue et al. [6] have calculated the yields at high energies, considering the distribution of internuclear velocity and separation resulting from both Coulomb explosion and multiple scattering. The treatment also normalizes the yields by the proton neutral fraction, and fits their 800 k e V / a m u data well. It is not clear, however, that their approach can be extended down to our energy range with yields as high as 22%.

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tral H atom. The solid curves are manual fits to the data points, and are decomposed into the sum of a central peal and two side wings. The central peak is not arbitrary: the curve shown is the energy straggling distribution measured for single protons of the same velocity in the same film. We have found that the straggling curves given by Symon [11] fit these proton measurements well, using an empirical choice of Symon's asymmetry parameter X and the width. We keep the asymmetry and width from the single proton measurement, adjusting only the position and height. The side wings are quite symmetric. In order to compare the dissociation and recombination yields, we have calculated the fractions shown in table 2. We assumed that the total yield for diproton clusters with exactly one electron (i.e. H~- or H ÷ H °) is Table 2 Percentage yields of transmitted ions from H f traversing thin C foils, Y(Hf )=recombination. Yc =central peak of H + H °, and Yw=side wings of H +H ° Energy (keV/amu)

Dwell time (fs)

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given by the probability for two independent protons to capture one electron: Y = 2Fo(l - F0), where Fo is again the proton neutral fraction. This Y is then the sum of the recombination yield Y ( H ~ - ) and the coincidence yield Y(H ÷ H ° ) ; the latter is split proportionately into the central peak yield Y~ and the side-wing yield Yw. The non-coincidence spectra arise from both H ÷ H ° and H + H + final states. We cannot measure the latter directly, but we can subtract the curve in fig. 3d (with a small scaling adjustment to account for the neutral

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288

T.R. Fox et aL / H, 4 traversing ultra-thin carbon foils: molecular stute~'

coincidence efficiency) from that in fig. 3c to approximate an H + H + spectrum. The result is shown in fig. 4. Since the parent curves in figs. 3c and 3d are quite similar, it is not surprising that the curve in fig. 4 has the same basic shape. The side wings are somewhat more evident, however. The dissociation of the H j molecule is accompanied by an energy release Q in the center-of-mass (CM) frame. In the laboratory (lab) frame, this contributes a kinetic energy excess A E = -+ (TQ) I/2 cos 0, where T is the total kinetic energy of the H ~ molecule and 0 is the CM angle between the beam line and the internuclear axis. To this A E is added the energy loss suffered during foil traversal, which is given by the energy straggling distribution. Two extreme cases should be noted. For 0 - - 9 0 ° (transverse orientation), cos0 = 0 and all dissociation products will have A E = 0, regardless of Q, and will therefore contribute to the central peak. For 0 = 0 (longitudinal orientation), A E is maximal and will contribute to the side wings for Q > 0 . 2 eV or so (at 25 keV/amu). All low-Q events, regardless of 0, go into the central peak. Elsewhere in these proceedings [8], we discuss evidence from our energy loss measurements that indicates preferential transverse alignment for those that recombine. Restricting the discussion to the consequences of these two extreme orientations will help the qualitative understanding of our laboratory spectra (figs. 3 and 4).

4. Discussion

A major difference between our energy range and much higher energies ( - I M e V ) is the presumed absence of Coulomb explosion of the cluster while traversing the foil. For our velocity range, the screening of the two protons by the electrons of the medium should be essentially complete for separations greater than about 0.05 nm, assuming static screening. This is smaller than the initial separation of the cluster when entering the foil. Thus, the CM energy release Q results from explosion after exiting the foil. The relative velocity of the two protons due to scattering in the foil is essentially transverse and therefore does not contribute to the laboratory A E. Another major difference is the large electron capture probability ( F 0 ~ 0.5 at 25 keV/amu). Consequently, the H + H ° and H S final states are much more probable than at higher energies. The coincidence events represent a sizeable fraction of the total dissociation events; e.g. fig. 3d (where the coincidence detection efficiency is ~ 80%) corresponds to roughly half of the events in fig. 3c. Central peaks in dissociation spectra have been the subject of much discussion [12]. It should be noted that our spectra are not restricted to small angles about zero,

so their interpretation is more complicated than from small-angle experiments. We can see from the 25 k e V / a m u yields in table 2 that the H ~ H ° central peak and the H ~ recombination yields are related. Since the side wing yield results are nearly constant, the sum of the recombination and central peak yields is also constant. This suggests that there exists a component of our coincidence central peaks that represents potential recombinanants, presumably in the lsog molecular state. As the target thickness increases, scattering increases the internuclear separation and relative velocity, reducing the probability that these lsog clusters recombine. Correspondingly, the lsag clusters that do not recombine will dissociate with a relatively low Q value, and will increasingly contribute to the central peak as observed. The other significant contributions to the central H -~H ° peak are from repulsive states (i.e. 2po u and 2 p % ) formed by the two protons when they capture one electron at the foil exit. As noted above, the energy loss data imply that the H + H ° central peaks preferentially result from clusters aligned transversely. Thus, the "gentle" explosion of the repulsive states can still contribute to the central peak, as will the repulsive states formed at large separations (low Q). Without angular resolution, we cannot separate these contributions. The longitudinally-aligned repulsive states with Q > 0.2 eV will contribute to the side wings. Since the longitudinal separation is not affected by multiple scattering, it is not surprising to observe that the side wing yield is insensitive to the target thickness. In the inferred H ~ H ~ spectra of fig. 4, the molecular explosion is replaced by the stronger Coulomb explosion, but the transverse clusters still feed the central peak. The H * H ~ Q values are larger than the H ' H ° values, so the H + H * side wings (from the longitudinal clusters) are somewhat larger and broader than the H * H ° wings.

References

[I] W.H. Escovitz, T.R. Fox and R. Levi-Setti, IEEE Trans. Nuc. Sci. NS-26 (1979) 1395. [2] W.H. Escovitz, PhD Thesis, University of Chicago (1979) unpublished. [31 T.R. Fox, Nucl. Instr. and Meth. 179 (1981) 407; T.R. Fox, PhD Thesis, University of Chicago (1980) unpublished. [4] R. Levi-Setti and T.R. Fox, Nucl. Instr. and Meth. 168 (1980) 139. [5] G.R. Hanson, J. Chem. Phys. 62 (1975) 1761. J.R. Hiskes, Phys. Rev. 122 ( 1961 ) 1207. [6] N. Cue, N.V. de Castro Faria, M.J. Gaillard, J.C. Poizat, J. Remillieux, D.S. Gcmmell and I. Plesser, Phys. Rev. Lctt. 45 (1980) 613. [71 WH. Escovitz, T.R. Fox and R. Levi-Sctti, IEEE Trans. Nuc. Sci. NS-26 (1979) 1147.

T.R. Fox et a L / H + traversing ultra-thin carbon foils." molecular states

[8] R. Levi-Setfi, K. Lain and T.R. Fox, these Proceedings, p. 281. [9] M.J. Gaillard, J.C. Poizat, A. Ratkowski and J. Remillieux, Nucl. Instr. and Meth. 132 (1976) 69. [10] J. Remillieux, Nucl. Instr. and Meth. 170 (1980) 31. [11] K.R. Symon, Ph.D. Thesis, Harvard University (1948)

289

unpublished; B. Rossi, High energy particles (Prentice Hall, New York, 1952) p. 29. [12] See, for example, E.P. Kanter, P.J. Cooney, D.S. Gemmell, K.O. Groeneveld, W.J. Pietsch, A.J. Ratkowski, Z. Vager and B.J. Zabransky, Phys. Rev. A20 (1979) 834.

VI. MOLECULAR PROJECTILES