Microwave multiphoton ionization and excitation of helium Rydberg atoms

We study experimentally and theoretically the detailed field-amplitude dependence of the multiphoton ionization and excitation probability of highly excited ${\mathit{n}}_0$ ${}_{}{}^3{}_{}{}^{}$S helium atoms in a 9.924-GHz linearly polarized microwave electric field. For ionization, with principal quantum numbers in the range ${\mathit{n}}_0$=25–32, we use a quasistatic analysis that employs integration of the time-dependent Schrödinger 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 ${\mathit{n}}_0$ ${}_{}{}^3{}_{}{}^{}$S→${\mathit{n}}_0$ ${}_{}{}^3{}_{}{}^{}$L, 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.