The authors prepare n{sup 3}S He Rydberg atoms with selected values n {ge} 25 in a fast beam using CO{sub 2} lasers and double-resonance excitation. They then fly through a TE{sub 121} mode cavity, exposing them to a half-sine pulse (about 350 field osc.) of 9.904 GHz electric field whose polarization can be varied; linear (LP), elliptical (EP), and circular (CP). Making EP close to LP can lead to substantial changes in microwave-power-dependent transitions to nearby bound states. In at least one case, a sharp dip in the LP signal is transformed by EP into a pattern reminiscent of Stueckelberg oscillations, previously observed with LP at higher frequencies. Calculations suggest that pulse-shape-induced dynamics at Floquet avoided-crossing(s) explain the LP behavior. Changing the field to EP clearly must modify this behavior. The authors will discuss this kind of data as well as the polarization dependence of microwave ionization of n{sup 3}S He Rydberg atoms.
At scaled frequencies {Omega}{sub o} = n{sub o}{sup 3}{omega}{much_lt} 1, extreme sensitivity to elliptical polarization (EStoEP) occurs (9.904 GHz, half-sine pulse, 154 field osc.) for certain ranges of n{sub o}, the principal quantum number; n{sub o}=43,42,41 ({Omega}{sub o}=0.1197 to 1.1038), show EStoEP close to circular polarization (CP), while n{sub o} = 33,32,31 ({Omega}{sub o} = 0.0541 to 0.0449) show it farther from CP. Coherent control schemes usually exploit interference of quantal amplitudes, but here 3d classical Monte Carlo calculations (3dCL) are quantitatively accurate. Approximate classical theory reveals underlying resonance physics. In the corotating frame (CRF), a low-{Omega}{sub o} CP/g field of amplitude F induces (Stark) motion at {omega}{sub s} = 1.5n{sub o}F. For {Omega}{sub o} {much_lt} 1, classical ionization requires F{sub o} = n{sub o}{sup 4}F {ge} F{sub cl} = 0.13. In CRF, EP near CP gives driving terms at 2{omega}; EStoEP at n{sub o}=43-41 occurs when this resonates with the {omega}{sub s} motion and F{sub o} {ge}F{sub cl} EStoEP at n{sub o}=33-31 occurs when 4{omega} is close to {omega}{sub s}. At even larger F{sub o}, n{sub o}=31 data show yet more EStoEP reproduced by 3dCL. This is likely the 6{omega} resonance.
We observe and reproduce theoretically, Stueckelberg oscillations in the microwave excitation of helium (n=28)
3S Rydberg atoms. They are produced by the coherent evolution along two isolated Floquet potential curves as the microwave electric field amplitude varies along a half-sine-wave pulse shape. The measured visibility and separation of the oscillations depends sensitively on the Floquet structure, on the atomic beam velocity, and particularly on additive noise, which smears and broadens the pattern.
Circular Rydberg states with
n=70 have been prepared in helium using a modified version of the crossed-fields method. This approach to the preparation of high-
n circular Rydberg states overcomes limitations of the standard crossed-fields method which arise at this, and higher, values of
n. The experiments were performed with atoms traveling in pulsed supersonic beams that were initially laser photoexcited from the metastable
1s2s3S1 level to the
1s73s3S1 level by resonance-enhanced two-color two-photon excitation in a magnetic field of 16.154 G. These excited atoms were then polarized using a perpendicular electric field of 0.844 V/cm, and transferred by a pulse of microwave radiation to the state that, when adiabatically depolarized, evolves into the
n=70 circular state in zero electric field. The excited atoms were detected by state-selective electric field ionization. Each step of the circular state preparation process was validated by comparison with the calculated atomic energy-level structure in the perpendicular electric and magnetic fields used. Of the atoms initially excited to the
1s73s3S1 level, $\ensuremath{\sim}80%$ were transferred to the
n=70 circular state. At these high values of $n, \mathrm{\ensuremath{\Delta}}n=1$ circular-to-circular Rydberg state transitions occur at frequencies below 20 GHz. Consequently, atoms in these states, and the circular state preparation process presented here, are well suited to hybrid cavity QED experiments with Rydberg atoms and superconducting microwave circuits.
We study experimentally and theoretically the detailed field-amplitude dependence of the multiphoton ionization and excitation probability of highly excited
n0 3S helium atoms in a 9.924-GHz linearly polarized microwave electric field. For ionization, with principal quantum numbers in the range
n0=25--32, we use a quasistatic analysis that employs integration of the time-dependent Schr\"odinger 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
n0 3S\ensuremath{\rightarrow}
n0 3L, 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.
We investigate excitation of He 28{sup 3}S atoms and find that a 2nd microwave field lifts the degeneracy of certain Floquet states describing the [atom + single microwave field] system. As a result a microwave pulse produces two patterns of Stueckelberg oscillations. They depend sensitively on the frequencies and amplitudes of both microwave fields and are influenced differently by added by added noise. We present two-frequency (2-f) Floquet maps showing the origin of the intererferences. We are able theoretically to reproduce reasonably well our observed 2-f Stueckelberg oscillations for (stronger) 30 GHz and (much weaker) 36 GHz 2-field. The case of both fields strong is much more complicated. It still remains theoretically to explain why the noise influences the two patterns of Stueckelberg oscillations differently.
We report on an experimental study of the frequency dependence of the microwave ``ionization'' probability for laser-excited Rb(np
)2P3/2(\ensuremath{\Vert}
MJ\ensuremath{\Vert}=1/2) Rydberg atoms. Most data are for n=84. The microwave field was produced by a low-noise frequency synthesizer, and the laser-excited atoms subsequently driven by microwaves were kept in an environment cooled to 77 K in order to minimize the perturbing effects of the thermal, blackbody-radiation field. The driving frequency covered 4--14-MHz intervals around three frequencies between 8.87 and 16.14 GHz. Data are shown for 1- and 5-\ensuremath{\mu}s interaction times, which correspond to the driving frequency being resolved by the atoms to about 1 and 0.2 MHz, respectively. The turnon and turnoff parts of the microwave pulse shape were slow, lasting about 6 ns (circa 70 field oscillations). For the narrow frequency intervals used, the microwave field amplitude was calibrated in situ with use of two-photon Rabi-nutation measurements, which also directly demonstrated the spatial uniformity of the driving field amplitude and its coherence over at least a 1-\ensuremath{\mu}s time scale. Within the frequency intervals studied experimentally, we have observed no sharp, frequency-dependent structure outside of that expected from experimental counting statistics. We give physical arguments based on experimental data and previous experimental and theoretical work on microwave ``ionization'' that show that the present experiments have been in the range of the scaled frequency above 1. Thus, these experimental data examine in a continuous fashion the frequency dependence of microwave ``ionization'' in the high-scaled-frequency regime. Finally, we discuss our results in the light of published theoretical discussions of what might be expected to be the frequency dependence of the microwave ``ionization'' probability for various quantal models for the one-dimensional hydrogen atom in this high-frequency regime.