5. 6.4 Temperature Dependence

The sample could be heated to temperatures up to 300 C. Measurements were done at two different temperatures. At temperatures above 80 C the surface became sufficiently conductive to remove surface charges fast enough. For measurements using an ion beam the surface was heated up to 150 C to avoid surface charging as was done by [ 33 ]. During beam adjustments using the target current as a parameter, a particle flux of the order of ions was used. During time of flight measurements the flux was reduced to ions by the beam pulsing system. At this beam intensity surface charging was small but noticeable and the surface was kept at 150 C to avoid problems. When using a neutral primary beam heating was not necessary due to the weaker beam ( particles ). For neutral hydrogen or carbon the ionization efficiency into a negative charge state was approximately equal to the ionization efficiency into a positive charge state resulting in a very small net current imposed on the sample. Even for neutral oxygen as primary particle where no positive ions were observed after reflection from the surface no surface charging could be observed. This allowed the temperature for neutral primary particles to be kept low (room temperature 20-30 C), close to the operating temperature in a future instrument where no permanent heating or no heating at all will be available for the surface.