We present a detailed experimental study of the frequency-modulated excitation of a two-level atom, using a microwave field to drive transitions between two Rydberg Stark states of potassium. In the absence of a modulation the interaction is the standard model of the Rabi problem, producing sinusoidal oscillations of the population between the two states. In the presence of a frequency modulation of the interacting field, however, the time evolution of the system is significantly modified, producing square wave oscillations of the population, sinusoidal oscillations at a different frequency, or even sinusoidal oscillations built up in a series of stair steps. The three responses described above are each found in a different regime for the frequency of the modulation with respect to the unmodulated Rabi frequency: the low-, high-, and intermediate-frequency regimes, respectively. @S1050-2947~98!03209-0# PACS number~s!: 42.50. Hz, 32.80.2t, 32.30.Bv, 42.50.Md

We present in situ measurements of temperatures inside multi-atmosphere spin-exchange optical pumping cells using Raman scattering from the N2 quenching gas. Under conditions usually prevailing in spin-exchange optical pumping experiments, we find that gas temperatures can be elevated hundreds of degrees above ambient, and that convection plays a very important role in the heat transport of the system.

, "Classical subharmonic resonances in microwave ionization of lithium Rydberg atoms," Phys. Rev. A. 62, 063401 (2000). We have studied the ionization of lithium Rydberg atoms by pulsed microwave fields in the regime in which the microwave frequency is equal to or a subharmonic of the classical Kepler frequency of the two-body Coulomb problem. We have observed a series of resonances where the atom is relatively stable against ionization. The resonances are similar to those seen previously in hydrogen, but with significant quantitative differences. We also present measurements of the distribution of states that remain bound after the microwave interaction for initial states near one of the classical subharmonic resonances. Classical subharmonic resonances in microwave ionization of lithium Rydberg atoms

We report the first studies of magnetic decoupling of the spin relaxation of alkali-metal atoms due to binary collisions with buffer gases. When binary collisions are the dominant relaxation mechanism, the relaxation and its magnetic decoupling are well described by the S-damping rate Gamma(SD) due to the spin-rotation interaction gammaN.S, the spin exchange rate Gamma(EX) for collisions between alkali atoms, and a new "Carver rate" Gamma(C), due to the pressure-shift interaction deltaAI.S, which can substantially broaden the magnetic decoupling curve while having no influence on the zero-field rates.

With this experiment we demonstrate that excitation of a two-state system with radio-frequency fields differing in phase by 90°produces nonintuitively different results, even for very long pulses. In addition, we show how the phase dependence of the transition probability of long pulses can be easily understood by using the single cycle time propagator. Finally, we have found surprising results for real pulses in the strong-field regime, i.e., pulses having appreciable rise and fall times.