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Journal of Physics B Atomic and Molecular Physics

Microwave multiphoton n-decreasing transitions in electrically polarised, highly excited hydrogen atoms

J. E. Bayfield, Lal A. Pinnaduwage

Published Feb 14, 1985• 36 citations
Publisher

Abstract

Microwave n-decreasing multiphoton transitions have been observed, for the first time, in n=60 and n=63 hydrogen atoms initially formed static-field polarised in their lowest energy Stark states with electric quantum number n e =1-n. Resonant n changing down one or two units can dominate the overall microwave transition probability. Regions of microwave frequency exist where n changing down two to four units significantly reduces the microwave ionisation probability.

DOI
10.1088/0022-3700/18/3/004

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Microwave n-decreasing multiphoton transitions have been observed, for the first time, in n=60 and n=63 hydrogen atoms initially formed static-field polarised in their lowest energy Stark states with electric quantum number n e =1-n. Resonant n changing down one or two units can dominate the overall microwave transition probability. Regions of microwave frequency exist where n changing down two to four units significantly reduces the microwave ionisation probability.
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A previous theory of atomic transitions in ultrastrong electromagnetic fields is applied to the problem of the ionization of highly excited hydrogen atoms. The magnitude of the fields necessary for the validity of the theory is greatly reduced by using excited hydrogen in the initial state, and the requirement of only multiphoton ionization limits the application to lowfrequency (microwave) fields. The requirement can probably be satisfied with currently available equipment. A result of the calculation is that the average number of photons absorbed is twice the minimum number.
We study experimentally and theoretically the detailed field-amplitude dependence of the multiphoton ionization and excitation probability of highly excited n0{\mathit{n}}_{0}n0​ 3^{3}3S helium atoms in a 9.924-GHz linearly polarized microwave electric field. For ionization, with principal quantum numbers in the range n0{\mathit{n}}_{0}n0​=25--32, we use a quasistatic analysis that employs integration of the time-dependent Schr\"odinger equation using basis states of the static field Hamiltonian. The calculated results are used to interpret the observed ionization threshold structure. For excitation, the results of n0{\mathit{n}}_{0}n0​ 3^{3}3S\ensuremath{\rightarrow}n0{\mathit{n}}_{0}n0​ 3^{3}3L, L>2 excitation experiments are explained quantitatively and precisely using a theory of multiphoton resonances. We present maps of quasienergy levels that allow the study of the dynamics of the field-switching transients. These transient effects are analyzed along the lines of standard atomic collision theory and are shown to determine the shape of the observed resonances.
We report the observation of sequences of from 1- to as many as 28-photon transitions between potassium (n+2)s(n+2)s(n+2)s states and the lowest-energy Stark states of the nnn manifolds for $n=15\ensuremath{-}18$. The sequences are observed by the scanning of a static electric field from the field at which these pairs of levels nearly cross, to zero field where they are well separated. The maximum number of photons absorbed is observed to be proportional to the microwave field.
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