An investigation of electric field ionisation of neutral indium atoms in Rydberg states

Abstract

The authors have studied phenomena related to the ionisation of indium Rydberg atoms by an external electric field. The indium atoms are excited in an atomic beam apparatus by pulsed dye laser light. The high-lying states are populated in one step from the ground state by an ultraviolet, frequency-doubled dye laser or by stepwise excitation using two dye lasers in the visible region. Shortly after the excitation, an electric field is applied in the interaction region which causes ionisation of the highly excited atoms. The authors have investigated (i) the lowest adiabatic ionisation threshold over a wide range of n values and compared the results with model predictions, (ii) the structure in the adiabatic part of the ionisation spectrum, and (iii) the transition from adiabatic to diabatic ionisation as a function of the slew rate of the ionising field pulse and the quantum defect and quantum numbers of the atomic state concerned. The ionisation signal is demonstrated to shift considerably towards higher field strengths with increasing slew rate. A qualitative explanation is presented and consequences for the detection efficiency are mentioned. Differences in the field ionisation behaviour between nearly degenerate excited states are demonstrated and exploited to achieve selective detection of fine-structure and even hyperfine-structure states.