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Clusters of helium, hydrogen, and nitrogen are reflected at a polished stainless steel plate at temperatures ranging from 80 to 550 K. The incident clusters contain on the average about 1.5×105 atoms of helium or molecules of hydrogen or 104 molecules of nitrogen, as measured by time-of-flight mass spectrometry. The angular distributions of the average sizes, the velocities, and the molecular intensities of the reflected cluster beams show that in the investigated range of reflector temperatures the reflection of the helium clusters corresponds to the hydrogen cluster reflection at higher reflector temperatures while the nitrogen cluster reflection corresponds to the hydrogen cluster reflection at lower reflector temperatures. The transition between the two regimes of reflection as observed with hydrogen clusters is marked by an optimum reflector temperature leading to a maximum intensity of the reflected beam, a minimum loss of clustered material, and a distinct angular separation of incident cluster sizes. At a grazing incidence angle of 84.3° the measured optimum reflector temperature for the hydrogen cluster reflection is 215 K and increases with decreasing angle of incidence. The two regimes of high-temperature and low-temperature reflection of clusters exhibit close phenomenological relationship to the regimes of thermal and structure dominated scattering of atoms from single crystal planes. In both cases the transition region is characterized by a maximum angle of reflection and a minimum divergence of the reflected beam. The features of high-temperature cluster reflection are explained by a semiempirical model based on the evaporation recoil of the cluster molecules ablating after contact with the comparatively hot reflector surface.

Single Ar n or (CO 2 ) n (nӍ960) cluster impacts on a diamond ͑111͒ surface are studied by large-scale molecular dynamics simulations in order to investigate highly energetic cluster-surface interactions. For a cluster impact energy E a of 100 keV, a hemispherical crater and multilayered shockwaves are observed. Rebounding hot fluidized carbon material is seen to replenish the transient crater very quickly, with a central peak appearing as a long time phenomenon in the case of a CO 2 cluster impact. Transient craters develop also for lower impact energies of 30рE a р75 keV while only an elastic deformation is observed for E a ϭ10 keV. The volume of the transient crater is approximately proportional to E a while the volume of the plastically deformed region and the kinetic energy transfer via the shockwave are linear functions of E a minus a threshold energy of about 10 keV. At an impact energy of 100 keV, the number of carbon atoms emitted from the target is much larger for a CO 2 cluster impact than for an Ar cluster impact with a factor of about 3.35. The reactive enhancement of the surface erosion in the CO 2 case is also proven by a strong CO signal in the spectrum of the emitted fragments. On the other hand, the surface of the relaxed crater is more densely packed and smoother in the case of the Ar cluster impact.

Spinpolarized 3He nuclear targets and metastable 4He atoms by optical pumping with a tunable, Nd:YAP laser Metastable electronic states of helium clusters of 100 to 10' atoms of either isotope, or of neon clusters of some 10' atoms, are observed to be excited by the impact of electrons of 30 to 100 eV energy. Being detected through the release of electrons which results from the impact of the excited neutral or ionized clusters onto the first dynode of an electron multiplier, these metastable excitations allow the time-of-flight spectrometry of speeds and sizes of large clusters. The energy dependence of the excitation probability of the helium clusters indicates the initial excitation of the metastable atomic triplet state while the observed flight times of about 10-3 s point to the metastable triplet molecular state as the final helium cluster excitation.

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