NANOSECOND SWITCHING, FIELD REVERSAL EVIDENCE OF RYDBERG ATOM DESORPTION FROM SURFACES

The emission of Rydberg states of alkali atoms by thermal desorption a t high-temperature surfaces can be studied by the field reversal (FR) rapid kinetics method. This method is based on molecular beam steady-s tate conditions with periodic reversals of the electric field outside the hot surface to modulate the desorbing flux. Such a modulation is p ossible if the ion desorption channel is larger than or of the same si ze as the neutral desorption channel. The desorption of K was studied on carbon-covered Ir and partly oxidized Re surfaces with rise and fal l times of the field of less than 10 ns, and with field strengths as l ow as 5 V cm(-1). The existence of Rydberg states K was verified from the very large, 300-ns-broad first ion peak after field reversal to a ccelerating field. The size of this peak was sensitive to the conditio ns during the retarding phase, which shows that it was caused by ioniz ation of K emitted during the retarding phase. The formation of Rydbe rg species was also verified by extraction of ions even during the ret arding phase, with the temporal dependence following the accumulation of K-atoms on the surface. The ion extraction in this case was due to field ionization of desorbing K at very low field strengths. The ion formation was not due to field penetration, since the typical FR time dependence during the retarding phase was retained. The positive curre nt extracted at low emitter voltages under constant field conditions a lso supports this view. Electron current emission studies in fields wi th reversed polarity neither showed any field penetration. The thermal excitation barrier was found to be 4.31 +/- 0.24 eV, which agrees wit h the excitation energy for Rydberg states K. (C) 1998 Elsevier Scien ce B.V. All rights reserved.