R. Neugebauer

Dynamics of secondary ion emission: Novel energy and angular spectrometry

A new spectrometer has been developed based on the combination of standard time-of-flight technique and position sensitive delay line detectors. The basic features of the spectrometer, particularly of the multi-hit capable detector, are described. To demonstrate the performance of this new system, the dynamic emission characteristics, i.e. the threedimensional velocity distribution, of desorbed H þ from Al target by Ar 0 impact (570 keV) is presented. It is found that the desorption yield is maximum for radial and axial emission velocities at 1.2 and 12 km/s respectively, corresponding to 1.5 eV ions emitted at 57 to normal (following the projectile radial direction). The initial energy distribution spreads out over 16 eV. 2002 Elsevier Science B.V. All rights reserved.

The influence of energy density inside the nuclear track on the secondary-ion emission

Abstract Secondary ion emission yields from polycrystalline LiF and deuterated amorphous carbon a-C:D, bombarded by isotachic projectiles at 1.4 MeV/u, were studied as a function of the projectile atomic number (C, N, Ar, Kr, Sn) and consequently, as a function of the electronic stopping power. The emitted, positively charged secondary ions were analysed by a time of flight mass spectrometer. The H + ions originate on the top surface near the track core, while heavier secondary ions (e.g. hydrocarbons) originate from the region of the track halo. Emission of Li + from LiF was observed for all projectiles. On the other hand, only Sn projectiles were able to produce D + emission from a-C:D. The results are discussed considering the deposited energy density.

Atomic collisions in secondary ion production by hydrogen near the electronic stopping power maximum

Abstract H + projectiles from a 2.5 MV van de Graaff accelerator were used to bombard a carbon target (500 A) under high vacuum conditions. By varying the H + projectile energy (50–160 keV/u) it was possible to scan the low energetic L indhard– S charff– S chiott region (LSS), the maximum, and the high energetic B ethe– B loch region (BB) of the electronic stopping power (d E /d x ) e . The secondary ion yields Y (H + ,C 2 H x + ,C 3 H x + ,C 4 H x + ,C 5 H x + ) from the beam entrance surface were measured with a time of flight spectrometer (TOF). The measurements reveal a non-linear behaviour between the electronic stopping power (d E /d x ) e calculated with TRIM, in the maximum and the Bethe–Bloch region, and the secondary ion yields. However, the experimental results show good agreement with a new Pereira et al. [Int. J. Mass Spectrom. Ion Proc. 174 (1998) 179] effective energy loss model.

Electronic sputtering by swift highly charged ions of nitrogen on amorphous carbon

Abstract Electronic sputtering with heavy ions as a function of both electronic energy loss d E /d x and projectile charge state q was studied at the French heavy ion accelerator GANIL. Amorphous carbon (untreated, and sputter-cleaned and subsequently exposed to nitrogen) was irradiated with swift highly charged ions ( Z =6–73, q =6–54, energy 6–13 MeV/u) in an ultrahigh vacuum scattering chamber. The fluence dependence of ion-induced electron yields allows to deduce a desorption cross-section σ which varies approximately as σ ∼(d E /d x ) 1.65 or σ ∼ q 3.3 for sputter-cleaned amorphous carbon exposed to nitrogen. This q dependence is close to the cubic charge dependence observed for the emission of H + secondary ions which are believed to be emitted from the very surface. However, the power law σ ∼(d E /d x ) 1.65 , related to the electronic energy loss gives the best empirical description. The dependence on d E /d x is close to a quadratic one thus rather pointing towards a thermal evaporation-like effect.

VELOCITY EFFECT IN SECONDARY ELECTRON EMISSION BELOW AND ABOVE THE ELECTRONIC STOPPING POWER MAXIMUM

We studied secondary electron emission yields γ from a carbon surface with helium projectiles near the electronic stopping power maximum (0.2–2 MeV). We ask the question: “Does γ depend on the electronic stopping power (dE/dx)e only or is there an additional dependence on the projectile velocity VP?” A velocity effect is indeed found for backward emission. The results are discussed in the framework of recent electron transport and track models and compared with the yield of secondary ions from the same collision system.

Secondary-ions from atomic collision processes in solid surfaces

Abstract Using time-of-flight technique (TOF) we investigated the secondary ion emission from an uncleaned carbon surface induced by fast helium bombardment near the maximum of electronic stopping power ( d E d x ) e . We examined the production of secondary ions dependend on projectile properties only, as specific projectile energy ( 50 keV/u ≤ E P m P ≤ 500 keV/u ) and incident charge state (0 ≤ qi ≤ 2), but not on target properties, as surface conditions. The “pre-equilibrium stopping power” plays a major role in secondary ion production in the examinated projectile velocity regime.

Secondary Molecular Ion Emission In Binary Projectile-Surface Collisions

Secondary molecular ions, emitted from a LiF target bombarded by a MeV argon beam, are analyzed by a XY‐TOF detection system. This new method allows, for each emitted ion, simultaneous measurement of its time‐of flight (TOF) and its impact coordinates (XY) on the detector surface, after acceleration by a homogenous electric field. Angular distributions and initial velocities for atomic (H+ , Li+, C+) and for molecular (Hn+, CmHn+) ions are determined. The analysis reveals different emission processes, among them a unexpected emission of fast molecular ions from binary collisions.

Ionizing beam tracking in gaseous chambers for field mapping

The combination of the time-of-flight (TOF) technique with two-dimensional position-sensitive (XY) ion detection is employed for tracking projectile-gas collisions. The main goal is to determine the detailed distribution of local electric fields, which accelerate the gas ions produced in the collisions. A low-pressure target gas spectrometer for this goal is described and all necessary XY- TOF expressions concerning the ion dynamics are presented. Experimental results of a 150 keV neutral H beam colliding with a He–Ne–Ar mixture are compared with the calculations. The new method can also be used for mapping magnetic fields or in large collision gas cells, e.g., to increase resolution in mass spectrometry and to perform initial momentum measurements of the produced ions.

QUASIFREE ELECTRON SCATTERING ON ATOMS IN THE INVERSE KINEMATIC SYSTEM

Abstract The measurement of elastic electron scattering is a sensitive tool to test ionic and atomic potentials. Because a dense ionic target is difficult to produce, we use inverse kinematics, i.e. we exchange the roles of projectile and target and transform the cross sections. The absolute singly and doubly differential cross sections for the projectile ionization of He 0 (0.1 MeV/u) scattered by Ne 0 , measured for the direct and the inverse system in the whole angular range from 0° to 180°, show the validity of the transformation. We find a pronounced generalized Ramsauer-Townsend minimum in the angular-dependent singly differential cross section. Changing to the Ne 3+ target the loss peak is absent in our measurements for larger emission angles. This may be explained by a large capture cross section into bound states of the neon ion, which depends on the impact parameter.