Landau Quantization and Time Dependence in the Ionization of Cold, Strongly Magnetized Rydberg Atoms

The electric-field-ionization and autoionization behavior of cold Rydberg atoms of ${}^{85}\mathrm{Rb}$ in magnetic fields up to 6 T is investigated. Multiple ionization potentials and field-ionization bands reflecting the Landau energy quantization of the quasifree Rydberg electron are observed. The time-resolved and state-selective field-ionization study provides evidence of $m$ mixing and spin flips of the Rydberg electron. Spin-orbit coupling combined with $m$ mixing gives rise to a Feshbach-type autoionization of metastable positive-energy atoms.

Figure 1

Density plot of the electron signal vs time and excitation energy in wave numbers for linear laser polarization ($\perp \mathbf{B}$) and $B$ values of (a) 6 and (b) 2.9 T. In the time range $t\lesssim 11\mu \mathrm{s}$, no electric field is applied to the atoms. In the range $t\gtrsim 11\mu \mathrm{s}$, an approximately linear field-ionization ramp is applied. The FI ramps are sketched on top. Ionization potentials (IPs) are indicated by horizontal white lines. The IP indicated by the solid white line coincides with the field-free ($B$-independent) ionization energy.

Figure 2

Evidence of time-dependent state mixing. In panels (a) and (b), atoms in the $[m=0,m_s=1/2]$ subspace are excited using ${\sigma }^{-}$ polarization at time $t=0$ and probed by FI pulses with delays of $\approx 1$ and $\approx 31\mu \mathrm{s}$, respectively. The dotted line in (c) shows the electron signal vs time for a FI pulse starting at $\approx 10\mu \mathrm{s}$ for excitation into $m=0$ states at an excitation energy of $-0.5\hslash {\omega }_c$ [as indicated by the dashed line in (a) and (b)]. The solid line shows an analogous result for excitation into $m=2$ states with an energy of $1.3\hslash {\omega }_c$.

Figure 3

Approximate effective potentials of the $[m,m_s]$ subspaces and relevant couplings between states on these potentials for an initial laser-excited state with $m=0$. Each Coulomb binding potential contains densely spaced energy levels.