Electronic stopping of slow molecular ions in solids

Energy loss measurements were made for 12.5-130 keV per nucleon H+ and H: on carbon and aluminium foils. For incident H i , both H + and H i are transmitted; the energy per nucleon of the latter being lower than that of transmitted H + , at low energies. The theory shows this is due to interference effects in the binary excitation of target electrons by the spatially correlated protons and suggests that transmitted H: results from di-protons travelling inside the solid with the internuclear axis aligned close to the direction of motion. It has been known for some time that fast H; ions moving in solids do not lose energy at the same rate as two non-interacting protons (for a review, see Brandt and Ritchie 1976). The ratios of stopping power per nucleon R = $dE/dx),?/(dE/dx),+ have been found to be larger than unity for projectile velocities larger than 1.73 au. This has been attributed to be due mainly to interference effects in the correlated excitation of long-wavelength plasmons in the solid which are prominent when the internuclear distance in the molecular ion, r, is much smaller than the adiabatic distance for collective excitations, zl/wp. Here z1 is the velocity of the projectiles and Amp the plasmon energy. Determinations of energy loss of H l ions in thin foils have so far been done through measurements of the energies of the incident projectiles and of emerging protons resulting from the dissociation of the H i molecules in the foils. H l molecules have also been observed to be transmitted through thin foils (Lambert 1976) but no measurements of stopping powers on these molecules have been reported in the literature. This Letter reports first measurements of energy losses per nucleon for H i ions incident on thin foils at low velocities (0-76-2.3 au) and emerging as protons and H; molecules. We show that interference effects in the excitation of target electrons by the correlated protons occur even at velocities where plasmon creation does not contribute significantly to the stopping power. Furthermore, the results suggest that transmitted molecular ions have travelled inside the foil with their internuclear axes aligned close to the direction of motion. The experimental method has been described in detail before (Valenzuela et ul 1972, Eckardt 1978). H+ and H l ion beams were produced in the Bariloche Kevatron accelerator and sorted in mass by a 90" magnetic analyser. The energy of the ions was measured with a precision, 90" electrostatic energy analyser with its slits set to give an energy resolution of 0.4 % FWHM. The energy losses were determined by measuring the energy of the beam without and with a thin foil placed in its path. The energy analyser 0022-3719/78/0021-9851 $01.00 @ 1978 The Institute of Physics L851 L852 Letter to the Editor accepts particles transmitted through the foils within vertical and horizontal angles of 0.043" and 0.36" respectively. Under these conditions, the influence of energy losses by elastic atom-atom collisions can be neglected. In the experiments we used three carbon foils ( 150A thick) and five aluminium foils (-200A thick). The foil thicknesses were determined using the measured energy losses for protons and dE/dx data (Andersen and Ziegler 1977). About one-third of the thickness of the A1 foils is estimated to correspond to surface oxide layers formed during their exposure to atmosphere. Two liquid-nitrogen traps, coaxial with the beam, were located immediately before and after the foils to prevent significant foil thickening due to the build-up of contaminant layers. Figure 1 shows normalised energy spectra obtained for 267 keV H,f incident on an A1 foil. The proton peak is seen to be broader than that for transmitted Hat ; this is due to the repulsion between the dissociation fragments (Gemmell et (11 1975). Two values of I I 8LOO 880017000 17L00 Energy analyser voltage Figure 1. Normalised energy spectra for 267 keV Hf incident on an AI foil the mean energy loss per nucleon can be derived from spectra for incident H i , such as shown in figure 1. If E , is the mean energy of the incident H l , and E , and E , those of transmitted protons and Hat respectively, we may define AE(Hl), = $E, E , and AE(H:), = $(E, E J , and the ratios R l , 2 = AE(Hi),,JAE(H+), where AE(H+) is the energy loss measured with protons incident at the same velocity as the H,f ions used to derive AE(Hl). Figure 2 shows R , and R , for carbon and aluminium foils as a function of projectile velocity, together with their experimental errors. The points result from averaging measurements over foils of nearly equal thickness (within 10 %). Interference effects ( R # 1) can be seen to persist down to the lowest velocities. It can also be observed that R, is smaller than R , at low velocities. The transmitted H l fraction was found to be more than 100 times larger for A1 than for C foils, but the electronics used did not allow us to get more quantitative measurements. The stopping power results can be analysed in the framework of the theory of the energy loss of correlated charges in an electron gas (Arista 1978). Of the targets used in these experiments, A1 is the most suited for comparison since it is free-electron like (we neglect here the influence of the surface oxide layers). The energy loss of protons in A1 in our energy range will be due mainly to excitation of valence electrons. The contribution of the A1 L-shell can be estimated using measured values of the cross section for L-shell excitation (Renazeth et ul 1977) and (EL), the average excitation energy of these Letter to the Editor