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AIP conference proceedings

Laser spectroscopy of doubly-excited states in barium

G.J. Kuik, Robert van Leeuwen, R. de Graaf, W. Vassen, Wim Ubachs, W. Hogervorst

Published Jan 1, 1993• 0 citations

Abstract

With pulsed and cw laser radiation we investigated doubly‐excited, autoionizing states of Ba, 4fnf series were excited from metastable 5d2 states using two pulsed dye lasers and 5dnf intermediate states. To explain the data on high‐lying states knowledge of the wave functions of the intermediate states and their perturbers is prerequisite. In preparation of experiments on double circular states such as 4f2, 4f5g, 5g2 etc., we studied the 5d5g multiplet, excited with two pulsed lasers from 6s5d metastable states. Furthermore we present results of a study of 6pnl states with large l, excited with two narrowband cw lasers. 6snl states were populated in a beam from metastable 5d2 states in the presence of an electric field. The atomic motion was used to reduce the field adiabatically to zero before applying the second excitation step (Stark switching). The results show that further excitation to 7dnl and even Nfnl states is feasible, giving possibilities to study the three‐body Coulomb problem with cw laser resolution.

DOI: 10.1063/1.43784

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With pulsed and cw laser radiation we investigated doubly‐excited, autoionizing states of Ba, 4fnf series were excited from metastable 5d2 states using two pulsed dye lasers and 5dnf intermediate states. To explain the data on high‐lying states knowledge of the wave functions of the intermediate states and their perturbers is prerequisite. In preparation of experiments on double circular states such as 4f2, 4f5g, 5g2 etc., we studied the 5d5g multiplet, excited with two pulsed lasers from 6s5d metastable states. Furthermore we present results of a study of 6pnl states with large l, excited with two narrowband cw lasers. 6snl states were populated in a beam from metastable 5d2 states in the presence of an electric field. The atomic motion was used to reduce the field adiabatically to zero before applying the second excitation step (Stark switching). The results show that further excitation to 7dnl and even Nfnl states is feasible, giving possibilities to study the three‐body Coulomb problem with cw laser resolution.

We report the observation of unusual doubly excited states of barium: Ba (5dj, 21c). One of the valence electrons of Ba is prepared in an oriented circular Rydberg state (n = 21, l = m = 20, denoted 21c) by the adiabatic crossed-fields method. The other electron is excited to a 6pj state which decays radiatively to a 5dj metastable level. The measured autoionization lifetime of Ba (5d5/2, 21c) is T ~ 1 μ s , whereas for Ba (5d3/2, 21c), τ is estimated to be longer than 400 μs. This very large difference of lifetimes between the fine-structure states of the inner electron has been analysed.

Using the adiabatic crossed-field method we have prepared for the first time doubly excited barium atoms with one valence electron in an oriented circular Rydberg state (n=21, l=m=20, denoted 21c) and the other electron in a

{6\mathrm{p}}_{\mathrm{j}}

state. In a fraction of these atoms the inner electron decays radiatively to a ${5\mathrm{d}}_{\mathrm{j}\ensuremath{'}}$ metastable level. The obtained circular-metastable doubly excited atoms are very stable. For the (${5\mathrm{d}}_{3/2>}$21c) state the autoionization lifetime \ensuremath{\tau} is longer than 400 \ensuremath{\mu}s; for the (${5\mathrm{d}}_{5/2}$21c) state, \ensuremath{\tau}=1\ifmmode\pm\else\textpm\fi{}0.2 \ensuremath{\mu}s. the specific role of the 5d electron is demonstrated. \textcopyright{} 1990 The American Physical Society

Using a three-step laser saturation excitation technique, the saturation effects on the Ba 6p n s ( J = 1) and 6p n d ( J = 1, 3) autoionization spectra are observed systemically in zero field. These saturation spectra are introduced to determine the high n members of 6p nl ( l = 0, 2) autoionizing series and are used to analyse the channel interactions among the autoionizing series in zero field. Furthermore, the saturation excitation technique is applied to the electric field case, in which the saturation spectra of Ba 6p nk (| M | = 0, 1) autoionizing Stark states are measured. Most of these saturation spectra are observed for the first time so far as we know, which indicate the mixing of the autoionizing states in the electric fields.

Energy-pooling collisions between low-lying excited states of Ba have been studied. We have used a CW dye laser to excite the level in Ba. Radiative decay and collisions with Ba or buffer-gas atoms (Ar, He) populate also the , and metastable levels. The spatial density distributions of these excited levels were measured by the absorption of lines from a Ba hollow-cathode lamp as a function of laser frequency. When the laser was on the exact atomic resonance we observed fluorescence from the , , , , , , and states populated by energy-pooling collisions of Ba( , ). By changing the buffer gas pressure the fluorescence intensity ratio between - and - transitions changes dramatically. This is found to be due to redistribution of population among metastable levels, both in magnitude and space. In addition, we have found that the and metastable levels are also populated when the laser is tuned to the blue from the 553.5 nm line under which conditions a pronounced red-shifted light cone (cone emission) is observed.