Cluster Desorption Angular Distribution from Frozen H2O and CO2 Bombarded by Fast Ions

Abstract:

The secondary ion desorption of H2O and CO2 ices bombarded by 1, 7 MeV N 2+ ions (45 incidence) are analyzed using a time-of-flight mass spectrometer. The ice is condensed on a thin metal substrate, kept at 80 K. The" stop" detector is equipped with a position sensitive anode enabling the measurement of the energy and angular distributions for the secondary ions, event by event. The results clearly show changes in the angular distribution of the desorbed clusters as a function of the number of constituents. In figs. 1a and 1b, orthographic projections illustrate the angular distribution for water clusters (H2O) nH+, with n= 1 and n= 13. The dependence of the ionic emission yield on the azimutal (Θ) and polar (Φ) angles is displayed in figs. 1c and 1d. Clusters with few constituents are emitted almost symmetrically (regarding reflections on the detector plane). As the number of constituents increase, the distributions become progressively asymmetric over the projectile axis direction. Similar behaviour is found for the desorption of CO2 ice targets. This result shows a" memory" of the ionic emission regarding the projectile direction that becomes more important as the mass of the molecule increase.

Año de publicación:

2005

Keywords:

Fuente:

google