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Physical Review A

Coherence, ionization, and recombination in a microwave field

Vincent Carrat, Eric Magnuson, Thomas F. Gallagher

Published Dec 16, 2015• 3 citations
Publisher

Abstract

An amplitude-modulated near-infrared laser pulse synchronized to a 14-GHz microwave field is used to excite atoms to the vicinity of the ionization limit at specific phases of the microwave field. When the laser is tuned above and below the ionization limit, phase-dependent modulation is observed in the recombination and ionization, respectively. The phase-dependent modulation is 10% of the total excitation, far greater than the 0.1% modulation observed with single-picosecond laser pulse excitation, and in agreement with calculations based on coherent excitation over several microwave cycles [Phys. Rev. Lett. 95, 013001 (2005)].

DOI
10.1103/physreva.92.063414

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An amplitude-modulated near-infrared laser pulse synchronized to a 14-GHz microwave field is used to excite atoms to the vicinity of the ionization limit at specific phases of the microwave field. When the laser is tuned above and below the ionization limit, phase-dependent modulation is observed in the recombination and ionization, respectively. The phase-dependent modulation is 10% of the total excitation, far greater than the 0.1% modulation observed with single-picosecond laser pulse excitation, and in agreement with calculations based on coherent excitation over several microwave cycles [Phys. Rev. Lett. 95, 013001 (2005)].
Using a laser intensity modulated synchronously with a 15.9 GHz microwave field we have examined laser excitation of Li atoms to the vicinity of the ionization limit in the presence of the microwave field and a parallel static field. The static field breaks the symmetry of ionization in the two microwave half cycles, and as the intensity modulated laser beam is delayed relative to the microwave field, maxima in the ionization are observed every microwave field cycle, not every half cycle, as in the absence of the static field. When the laser is tuned below the ionization limit there is an unexpected phase reversal of the modulation in the ionization, the origin of which is clarified by a classical model.
begin{wrapfigure}{l}{0pt} includegraphics[scale=0.38]{Energy_exchange.eps} caption{Electrons gaining or losing energy during the first microwave cycle depending of the initial launching direction. Here the phase of the microwave field is adjusted to provide a maximum energy transfer. The oscillations in energy are synchronized with the 14GHz microwave field.} end{wrapfigure} Exciting an atom with high-frequency radiation in the presence of a low frequency field can result in energy transfer between the photoelectron and the low frequency field, depending on the phase of the low frequency field when the excitation occurs. We excite Li atoms with IR lasers in the presence of a microwave field. In a previous experiment, detection of highly excited states with excitation by a ps laser tuned above the limit clearly showed a phase dependence. The variation of the signal due to a phase change reach 0.1% of the total excitation in that case. We are using a new excitation scheme with a CW amplitude modulated laser, the modulation being phase locked to the microwaves. We now observe a signal variation of 10% of the total excitation. The ps pulses spreads the population over a broad energy spectrum while the modulated excitation keeps it in narrow bands. The modulated laser frequency can be tuned to couple one band to the highly excited states, enhancing the collection efficiency, additionally it is closer to the limit. Furthermore, the modulated laser allows the observation of phase dependent transfer to both higher and lower energies. The observations can be described with relatively simple models.
Using picosecond laser photoionization of Li in a microwave field we have observed phase-dependent reccombination of the photoelectrons with their parent Li+ ions. Recombination occurs at phases of the microwave field such that energy is removed from the photoelectron in the first microwave cycle after excitation, and there are two maxima in the recombination in each microwave cycle. These observations are consistent with observations made using an attosecond pulse train phase locked to an infrared pulse and with the "simpleman's" model, modified to account for the fact that the photoelectrons are produced in a Coulomb potential.
We have excited Li atoms to Rydberg states with a picosecond laser in the presence of a phase-locked microwave field. We have observed energy transfer to higher- and lower-lying Rydberg states depending on the phase of the microwave field at which the picosecond laser excitation occurs. These observations confirm the prediction of a classical picture of the energy transfer and demonstrate that the phase-dependent energy transfer is not limited to processes in which energy transfer occurs across the ionization limit.
We report on an experimental study of the frequency dependence of the microwave ``ionization'' probability for laser-excited Rb(np)2{)}^{2})2P3/2{\mathit{P}}_{3/2}P3/2​(\ensuremath{\Vert}MJ{\mathit{M}}_{\mathit{J}}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.