Journal of Chemical Physics, Vol.116, No.23, 10447-10457, 2002
Charge transfer in hyperthermal surface collisions of C-60(0) and C-60(-): Experiment and model calculations
In this paper we address the issue of electron exchange between a large molecular projectile and a surface during a hyperthermal collision. Hyperthermal neutral C-60 molecules with well-defined average vibrational energy E-v=8+/-0.5 eV were scattered from a graphitized (monolayer-covered) nickel surface for both near normal and near grazing incidence angles. The yield of C-60(-) negative ions was measured for the impact energy range 8-33 eV and was found to scale exponentially with the inverse of the normal component of the scattered molecule velocity. Normal and tangential velocities of the scattered C-60 are very low, (1.3-5.6)x10(-4) and (0.8-3.1)x10(-4) a.u. correspondingly. The fact that nearly the same slope (characteristic velocity) was extracted from the semilog plots for both near normal and near grazing incidence angles shows that tangential velocity effects are negligible. The rate of electron tunneling from C-60(-) to the surface was calculated quasi-classically assuming that the excess (active) electron in C-60(-) is moving in the highly screened short range attractive potential of the neutral C-60(0) core, which is approximated by a radial delta function (Dirac bubble). The transmission coefficient was calculated for C-60(-) located at the center of a spherical metallic cavity with image charge potential barrier for the active electron. The preexponential frequency factor was obtained by solving the quasi-stationary Shrodinger equation for the same system but without the image charge barrier. Finally, the decay rate was corrected for the case of C-60(-) in front of a conducting plane. The experimental results were analyzed using the calculated tunneling rates and a critical ion formation distance was obtained. Independence of the scattering dynamics and negative ion yield on the incoming charge state is demonstrated by using incident C-60(-) at the impact energy range of 40-100 eV. The same relative energy losses were measured for incident C-60(-) ions as for neutral C-60(0) thus extending the range of linear dependence of the scattered kinetic energy on impact energy up to 100 eV.