Effect of the Impact Angle on the Kinetic Energy and Angular Distributions of β-Carotene Sputtered by 15 keV Ar Projectiles.

Molecular dynamics (MD) computer simulations are used to model ejection of particles from β-carotene samples bombarded by 15 keV Ar. The effect of the incidence angle on the angular and kinetic energy distributions is investigated. It has been found that both of these distributions are sensitive to the variation of the incidence angle, particularly near the normal incidence. For impacts along the surface normal, material ejection is azimuthally symmetric, and a significant emission occurs along the surface normal. The kinetic energy distribution of intact molecules has a maximum near 1 eV and terminates below approximately 2 eV. An increase of the incidence angle breaks the azimuthal symmetry. Most of the intact molecules become ejected in the forward direction. The maximum in the polar angle distribution shifts toward large off-normal angles. In addition, the most probable kinetic energy of ejected molecules is significantly increased. The mechanisms of molecular emission responsible for the observed changes are delineated. The implications of the observed ejection characteristics for the utilization of large gas cluster projectiles in secondary neutral mass spectrometry are discussed.