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Journal of Physics B Atomic and Molecular Physics

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

Jaap H. M. Neijzen, A. Dönszelmann

Published Jul 14, 1982• 5 citations
Publisher

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.

DOI
10.1088/0022-3700/15/13/015

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Half-cycle microwave ionization of Rydberg atoms: beyond diabatic ionization

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Ionization of Rydberg atoms by short, half-cycle electrical pulses is investigated experimentally in the intermediate-pulse duration regime where the ionization threshold fields are determined both by the slew rate of the ionizing pulse and its duration. Electrical pulses with durations ranging from 140 ps to 9 ns and amplitudes up to are applied to rubidium and lithium atoms prepared in high Rydberg states by laser excitation (n ranges from 42 to 82). The amplitude of the 140 ps pulse required for ionization of the Rb and Li Rydberg population is found to be a factor of two higher than the expected amplitude based on diabatic field ionization and recent theoretical calculations by Reinhold et al (1993 26 L659). Using the longer pulses we studied the transition from the diabatic/adabatic ionization towards ionization in the impulse-kick regime.

Journal of Physics B Atomic Molecular and Optical Physics•Mar 28, 1997•16 citations
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.
The phenomenon of diabatic ionisation of indium Rydberg atoms by an external electric field has been investigated. The indium atoms are excited in an atomic beam apparatus to high lying p states by stepwise laser excitation. The authors have studied the dependence of the ionisation processes on the slew rate of the applied electric field. The observation of a new diabatic ionisation feature for high slew rates is reported and an explanation is presented. Consequences for the ionisation efficiency are mentioned.
The authors describe the details of an experiment using a sodium atomic beam and two dye lasers. By stepwise excitation they have selectively populated highly excited ns, nd and, using Stark mixing, np states fron n=20-50. The excited atoms were detected by field ionisation. The results obtained for the ionisation threshold fields for these states are presented and compared with the theoretical classical law F c =(2n*) -4 . A suitable field shape and detection technique allows the observation of multiple thresholds with high sensitivity. Multiple thresholds for nd states have been observed, as expected. Some ns singlet states also show multiple thresholds. Using suitably polarised laser beams the mod m L mod dependence of the ionisation threshold fields for the 20d state has been observed. The approach proposed recently by Gallagher for predicting the threshold fields is generally confirmed.
The outer electron of a Rydberg atom is so weakly bound to the atomic core that a relatively small electric field may cause the electron to be stripped from the atom. The field strength at which this ionization happens is strongly dependent on the state of the Rydberg atom. Pulsed electric field ionization serves, therefore, not only as a means by which a Rydberg atom may be efficiently detected, but also a means by which the state of the Rydberg atom may be analyzed.\nThe first observation of predominantly diabatic passage to field ionization of a non-hydrogenic Rydberg atom has resulted in an improved understanding of the adiabatic and diabatic ionization processes. The magnitudes of the various field ionization features have been measured and found to agree with theory, and by detailed analysis, it is now possible to label the various field ionization signal features with the appropriate (VBAR)m(,l)(VBAR) quantum numbers. A much better understanding of the laser excitation process, the Rydberg state production, and the response of the excited state to an increasing electric field has enabled measurement of some Rydberg fine-structure intervals, through observation of field ionization quantum beats.
A beam of gold atoms was produced by heating the gold-containing material by an electro-thermal method. Two UV laser beams then irradiated the atomic beam perpendicularly, which excited the Au atoms to very high Rydberg states from their ground states (6S1/2). A high voltage pulsed electric field was then applied to the interaction region approximately 200 ns after the laser pulse, ionising the Au atoms in Rydberg states. By scanning the wavelength of the laser over a certain range, the positions of 38 energy levels of Au atoms in the n 2D3/2 (n=18-38) and n 2S1/2 (n=21-34, 36-38) series have been measured. The ionisation limits of the n 2D3/2 and n 2S1/2 series were determined by parametric fitting to be Ed=74 409.8 (3) cm-1 and Es=74 410.0 (2) cm-1, respectively. The quantum defects of all these levels were also calculated. This measurement is the first for Au in the energy range and channels mentioned above.
A frequency stabilized ring dye laser, with a linewidth of (TURN)250 kHz, has been utilized to selectively excite Rb(N('2)S, N('2)D) states in the range N = 20 - 100 by a two photon process. The highly excited atoms were detected by the technique of electric field ionization. Excited states with (VBAR)M(,L)(VBAR) < 3 have been observed to ionize along predominantly adiabatic paths and states with (VBAR)M(,L)(VBAR) (GREATERTHEQ) 3 have been observed to ionize along predominantly diabatic paths for the range of N investigated. The electric field dependence of the ionization threshold has been determined to be in good agreement with that calculated from a simple model based on the energies and ionization rates of hydrogen.\nCollisions between highly excited rubidium atoms and neutral xenon atoms, which result in a change in the N and L of the excited state of the rubidium atom, have been investigated. The N of the final state has been shown to be limited to those states which lie closest in energy to the initial state, in good agreement with theory. The distribution of L of the final states has been shown to be completely statistical or biased towards production of high L. The distribution of final L is definitely not biased toward production of states with low values of L.
Ionization of Rydberg atoms by short, half-cycle electrical pulses is investigated experimentally in the intermediate-pulse duration regime where the ionization threshold fields are determined both by the slew rate of the ionizing pulse and its duration. Electrical pulses with durations ranging from 140 ps to 9 ns and amplitudes up to are applied to rubidium and lithium atoms prepared in high Rydberg states by laser excitation (n ranges from 42 to 82). The amplitude of the 140 ps pulse required for ionization of the Rb and Li Rydberg population is found to be a factor of two higher than the expected amplitude based on diabatic field ionization and recent theoretical calculations by Reinhold et al (1993 26 L659). Using the longer pulses we studied the transition from the diabatic/adabatic ionization towards ionization in the impulse-kick regime.