Studies of ionization and wavepacket dynamics of very-high-n Rydberg atoms using half-cycle pulses

[摘要] The response of potassium Rydberg atoms with principal quantum number, $n ge 400$, to fast, unidirectional electric field pulses, termed half-cycle pulses (HCPs), is investigated experimentally and the results compared to classical trajectory Monte Carlo calculations. The durations, $Tsb{p}$, of the HCPs span the transition from the short pulse regime, where the classical orbital period of the initial state $Tsb{n} >> Tsb{p}$, to the long pulse regime, where $Tsb{p}z >> Tsb{n}$. In experiments with single HCPs, ionization probabilities measured as a function of pulse width and amplitude agree with theory on an absolute scale without any adjustable parameters, providing a benchmark test for the validity of the classical limit of ionization. HCPs are also used to create and probe very high-n Rydberg wavepackets. Application of a HCP with $Tsb{p} > Tsb{n}$ leads to population of a coherent superposition of Stark states. In each case, the time-evolution of the wavepacket is examined by applying a second short HCP after a variable time delay. This probe pulse ionizes a fraction of the atoms present and the survival probability exhibits pronounced oscillations that are well reproduced by CTMC simulations. The CTMC calculations show that the observed quantum beats reflect the time-evolution of the distribution of the z component, $psb{z}$, of the momentum of the excited electron. The first experimental studies of excitation and ionization by a sequence of short HCPs are also reported. Application of a sequence of identical pulses results in a multi-step excitation followed by ionization. The ionization probability of atoms with $nsb{i}sim 388$ subject to a sequence of identical, equally-spaced 2 ns HCPs were measured as a function of HCP height and pulse repetition frequency, $omegasb{p}$. The experimental data show that the Rydberg atoms are considerably more stable against ionization when the frequency of the perturbation scaled to the orbital frequency of the initial state, $omegasb0$ = $nsbsp{i}{3}omegasb{p}$, is $omegasb0 sbsp{sim}{>} 1$ than when $omegasb0 << 1$. This stability is not as pronounced in a CTMC calculation that uses $delta$-function impulses.

[发布机构] Rice University