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ELECTRIC FIELD IONIZATION OF SODIUM RYDBERG ATOMS

THOMAS H. JEYS

Published Jan 1, 1982• 0 citations

Abstract

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.

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ELECTRIC FIELD IONIZATION OF HIGHLY EXCITED RUBIDIUM ATOMS (COLLISIONS, RYDBERG)

G. B. McMillian

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.

•Jan 1, 1985•0 citations

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.

Selective field ionization (SFI) is frequently used to detect atoms in high Rydberg states and to analyze Rydberg population distributions. In order to realize the full potential of this technique, however, a detailed understanding of the behavior of Rydberg atoms during passage from low to ionizing fields is required. The factors influencing this passage are discussed quantitatively and are illustrated by means of detailed SFI data obtained for

\mathrm{Na}34d

atoms.

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.

Pulsed dye lasers are used to excite sodium atoms into a coherent superposition of high Rydberg

\mathrm{nd} (^{2}\mathrm{D}_{\frac{3}{2}},^{2}D_{\frac{5}{2}})

states. Analysis of the field ionization behavior of these atoms reveals the time evolution of the superposition state. Rapid application of a weak electric field projects different components of the superposition state onto states having identifiably different field ionization behavior. Analysis of the signal from subsequent field ionization as a function of the time of weak-field application reveals fine-structure quantum beats. These beats result from the time development of the original superposition state. Measurements of the fine-structure quantum beat frequency are presented for the principal quantum numbers

n=32, 34, 35, 36, 38, \mathrm{and} 40

.

Utilizing the technique of high resolution selective field ionization it is possible to observe ionization structure as narrow as 2 V/cm for Na(nS,nD) atoms. Field ionization structure this narrow is characteristic of atoms evolving predominantly adiabatically in a rapidly increasing electric field. By increasing the slew rate of the applied field it is possible to induce departures from the adiabatic path at specific avoided level crossings. These departures result in multiple ionization thresholds which may be explained by arguments based on a simple quantum mechanical model of adiabatic field ionization.

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.