Highly Excited Hydrogen Atoms in Strong Microwaves

Abstract

Consider an atom that is highly excited to a state with principal quantum number n near 60. Such an atom’s electron binding energy is a few meV. External electric fields of modest field strength, in the range 10–100 V/cm, can have such a strong influence on such atoms that they can be rapidly ionized, even within a few classical electron orbit periods. For microwave fields at near-ionizing strengths, many quantum states are coupled and the problem of energy absorption by the atom from the external field can be addressed as a nonlinear dynamics problem. As both quantum numbers and numbers of photon absorption/emission events can be large, a semiclassical picture based upon an underlying classical electron dynamics is expected to be useful. The problem when viewed classically is in the class of externally-driven nonlinear oscillators. This class of problems has much in common with the class of two coupled nonlinear oscillators, as one of the two is just replaced by an external oscillator of fixed amplitude an frequency. An excited hydrogen atom in a sufficiently strong static magnetic field exhibits electron motion involving a strong competition between Larmor precession in the plane perpendicular to the field and Coulombic motion along the field. This is two coupled nonlinear motions, as in two coupled nonlinear oscillators.