 
Summary: ,
22 I. Barth and J. Manz
2.1 Introduction
Recent quantum dynamics simulations predict that circularly polarized laser pulses
may induce stationary ring currents or timedependent charge circulations in model
systems, with applications from the hydrogen atom or oneelectron atomic ions
[1, 2] to molecules [313], see also [14, 15], circular carbon clusters [16], nano
rings [1720], and nanotubes [21]; for pioneering applications to Rydberg states
see [2227]. These ring currents and charge circulations in turn induce stationary
[2, 4, 79, 28] and timedependent [28] magnetic fields in the model systems,
respectively. From a theoretical perspective, a stationary ring current is described
in terms of a single complex stationary degenerate eigenfunction of the system,
carrying angular momentum; different eigenstates correspond to different ring cur
rents and, therefore, to different induced stationary magnetic fields [28]. As a rule,
the corresponding stationary densities and charge distributions often have typical
toroidal or neartoroidal shapes [2, 79, 11, 28]; exceptions from this rule (e.g.,
for ringshaped molecules) are documented in [4, 28], see also Fig. 2.3b below.
Recently, this characteristic signature has been observed experimentally in ionized
states of atoms induced by welldesigned circularly polarized laser pulses [29]. In
contrast, charge circulations, i.e., nonstationary ring currents, are represented by
