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Le Journal de Physique Colloques

ION DETECTION TECHNIQUES APPLIED TO RYDBERG ATOMS IN EXTERNAL FIELDS

Francis Jean Penent, Christian Chardonnet, Dominique Delande, F. Biraben, J. C. Gay

Published Nov 1, 1983• 2 citations
Publisher

Abstract

The simultaneous development of single mode c.w. dye laser excitation and thermoionic detection over the last five years has supplied with one of the most powerful experimental technique for atomic physics studies under vapour phase conditions. One of the most interesting application concerns problems of fundamental physics which can be tackled on Rydberg atoms especially the ones dealing with the atomic properties in external fields. Several selected examples are discussed to the light of generalized vectorial models of diamagnetism, linear Stark effect and crossed electric and magnetic fields effects. Although the experimental results are highly illustrative of the pure hydrogenic behaviour in external fields, none of them have been obtained on the hydrogen atom itself.

DOI
10.1051/jphyscol:1983716

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The simultaneous development of single mode c.w. dye laser excitation and thermoionic detection over the last five years has supplied with one of the most powerful experimental technique for atomic physics studies under vapour phase conditions. One of the most interesting application concerns problems of fundamental physics which can be tackled on Rydberg atoms especially the ones dealing with the atomic properties in external fields. Several selected examples are discussed to the light of generalized vectorial models of diamagnetism, linear Stark effect and crossed electric and magnetic fields effects. Although the experimental results are highly illustrative of the pure hydrogenic behaviour in external fields, none of them have been obtained on the hydrogen atom itself.
In this work, numerical calculations were implemented based on the diagonalisation of a matrix representation of the Stark Hamiltonian on a subset of the basis given by the zero-field Hamiltonian. Dipole matrix elements between the involved states were calculated, which allow for the prediction of transition strengths to Stark shifted Rydberg states. The applicability of the numerical methods were confirmed experimentally by precision spectroscopy in a rubidium vapour cell with electrodes using electromagnetically induced transparency (EIT). The numerical methods were then extended to predict ionisation rates of Rydberg states in the highly Stark shifted regime beyond the classical ionisation threshold. For this purpose, a complex absorbing potential (CAP) was specifically adjusted to the Stark Hamiltonian. An experiment was conducted, in which cold atoms from a magneto-optical trap (MOT) were excited to Stark shifted Rydberg states and the arising ions were detected. The obtained ionisation spectra agree well with the results from the numerical calculations. Another extension of the numerical methods was implemented for crossed electric and magnetic fields. This was used for approximate calculations of the motional Stark effect (MSE), which acts on atoms moving through static magnetic fields and thereby experiencing a Lorentz electric field. In a precision spectroscopy, once more using EIT in a vapour cell, the MSE was observed in low magnetic fields for the first time in rubidium. The spectral shift, which is caused by the MSE, matches the prediction from the numerical calculations. In a final experiment, the practical use of the knowledge of several properties of Stark shifted Rydberg states was illustrated. The numerical methods including the CAP were used to specifically select states for their optical accessibility, permanent electric dipole moments and ionisation rates. In the experiment, atoms from a MOT were excited to these states to observe different characteristics of the dipole blockade effect. The detection scheme for this experiment was realised by controlled ionisation using only small changes in the external electric field.
Because of their unusual properties, Rydberg atoms, i.e., atoms with one electron occupying a highly excited orbit, have attracted the interest of many physicists. For example, due to their large size, such atoms are particularly sensitive to external fields. Fifty years ago, Segre and co-workers1 investigated the Stark effect and atomic diamagnetism of Rydberg states of alkalis with principal quantum numbers up to n = 30. During the last decade, Rydberg states have been studied extensively2 due to the availability of tunable laser radiation.
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Although the hydrogen atom has naturally served as the basis for extensive theoretical studies of the physics of highly excited atoms in strong external fields, experiments with this fundamentally important species in magnetic fields have not been reported, except for the most recent ones carried out in our laboratory.1,2 We report new results of studies on the spectroscopy of the atom in static homogeneous magnetic fields of strength B ≲ 6 T at excitation energies from high Rydberg states (n ≳ 20) into the ionization region (E ≲ +30 cm−1).
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.