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Journal of Physics B Atomic and Molecular Physics

Threshold effects in strong-field photoionisation of atoms

Andrzej Raczyński, Jarosław Zaremba

Published Dec 14, 1986• 18 citations
Publisher

Abstract

Near-threshold ionisation of an atom is investigated using a non-perturbative realistic model. The influence of the Rydberg states on the dynamics of the ground-state population is discussed. In particular, details of its non-exponential decay and of the population trapping as well as the shift of the threshold are calculated.
DOI
10.1088/0022-3700/19/23/010

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Photoionisation by a strong laser tuned to the neighbourhood of an autoionising level is discussed. An explicit model for the continuum dipole strength is used which reproduces the important features of a real atom. Within the context of this model the problem is solved exactly. The exact solution is related to the usual pole approximation via a Taylor expansion which also yields an improved pole approximation. Population trapping is observed to occur and the general condition for it is derived. An interpretation of population trapping as the strong field relic of the Fano minimum is given. Some remarks are made on the consequences of population trapping for electron-ion scattering in the presence of a laser.
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.
A model of multiphoton ionisation is presented which can be used when the turn-on of the atom-field interaction is adiabatic or near adiabatic. It is shown that in the adiabatic limit, the probability of the atom not being ionised is given by a single exponential decaying in time. A time-independent perturbation series is developed which can be used to calculate the adiabatic limit ionisation rate to any desired order. A time-dependent perturbation series is used to calculate the ionisation probability in the near-adiabatic limit. Comparison is made between results obtained with this model and those obtained using the more common sudden approximation to the turn-on.
A simple theory of the Rydberg atoms ionisation by electromagnetic pulses and microwave field is presented. The analysis is based on the scale transformation which reduces the number of parameters and reveals the functional dependencies of the processes. It is shown that the observed ionisation of Rydberg atoms by subpicosecond electromagnetic pulses scale classically. The threshold electric field required to ionise a Rydberg state may be simply evaluated in the photonic basis approach for the quantum dynamics or from the multiphoton ionisation theory.