Abstract :
The emission of Rydberg states of alkali atoms by thermal desorption at high-temperature surfaces can be studied by the field reversal (FR) rapid kinetics method. This method is based on molecular beam steady-state conditions with periodic reversals of the electric field outside the hot surface to modulate the desorbing flux. Such a modulation is possible if the ion desorption channel is larger than or of the same size as the neutral desorption channel. The desorption of K was studied on carbon-covered Ir and partly oxidized Re surfaces with rise and fall times of the field of less than 10 ns, and with field strengths as low 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 accelerating field. The size of this peak was sensitive to the conditions during the retarding phase, which shows that it was caused by ionization of K* emitted during the retarding phase. The formation of Rydberg species was also verified by extraction of ions even during the retarding 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 current extracted at low emitter voltages under constant field conditions also supports this view. Electron current emission studies in fields with reversed polarity neither showed any field penetration. The thermal excitation barrier was found to be 4.31±0.24 eV, which agrees with the excitation energy for Rydberg states K*.
