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Journal of Physics Conference Series

Microwave recombination and ionization

Vincent Carrat, Eric Magnuson, Alexandr Arakelyan, Thomas F. Gallagher

Published Sep 7, 2015• 0 citations
Publisher

Abstract

Laser excitation above the limit in the presence of a microwave field can lead to high lying bound states if it occurs at the correct phase of the microwave field, and phase dependent ionization is also observed. The excitation spectra of the high lying states in the presence of the field is a sequence of peaks straddling the limit, spaced by the microwave frequency.

DOI
10.1088/1742-6596/635/9/092065

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International Symposium on Molecular Spectroscopy (ISMS)•Jun 23, 2015•0 citations
Laser excitation above the limit in the presence of a microwave field can lead to high lying bound states if it occurs at the correct phase of the microwave field, and phase dependent ionization is also observed. The excitation spectra of the high lying states in the presence of the field is a sequence of peaks straddling the limit, spaced by the microwave frequency.
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.
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)].
Electric field ionization of excited Na atoms by microwave fields between 12 and 15 GHz exhibits several interesting phenomena. The ∣m∣=0and1|m|=0 \mathrm{and} 1∣m∣=0and1 states may be ionized with a very low microwave field, $\ensuremath{\sim}\frac{1}{{3n}^{5}}$, corresponding to the point at which the nnn and n+1n+1n+1 Stark manifolds of principal quantum numbers nnn and n+1n+1n+1 intersect. The ionization occurs by an atom's making a succession of Landau-Zener transitions to higher-lying states, finally reaching the classical ionization limit. The ∣m∣=2|m|=2∣m∣=2 states, on the other hand, ionize at a very high microwave field, $\ensuremath{\sim}\frac{1}{{9n}^{4}}$. Both these observations and the more familiar static or low-frequency result of ionization by a field of $\ensuremath{\sim}\frac{1}{{16n}^{4}}$ are all consistent with ionization in a completely classical fashion. The apparent differences are due to the dynamics of passing from zero field to a high ionizing field.
In the ionization of Li and Na by a 16.91-GHz microwave field, roughly 20% of the atoms are not actually ionized but trapped in highly excited states within one or, at most, two microwave photons of the ionization limit. The highly excited atoms appear at the same field as the onset of microwave ionization, irrespective of whether ionization occurs at the field E=1/3n5E=1/3{n}^{5}E=1/3n5 or E=1/16n4E=1/16{n}^{4}E=1/16n4. The highly excited atoms are only detected in static fields less than 35 mV/cm, a field which roughly corresponds to field ionization of an atom bound by twice the microwave frequency. We also show that Na ∣m∣=1|m|=1∣m∣=1 atoms require a microwave field of E=1/9n4E=1/9{n}^{4}E=1/9n4 for ionization in zero static field, but the required field for ionization is reduced to E=1/3n5E=1/3{n}^{5}E=1/3n5 if a static field of 1 V/cm is present.
\n \nWe have investigated the electric field ionization of highly excited (ns)\nand (nd) Rydberg states of the barium ion by microwave fields. For states\nwith a principal quantum number nleq40n\\leq 40nleq40 the observed microwave ionization\nthreshold fields show the Z3/3n5Z^3/3n^5Z3/3n5 dependence expected on the basis of\npreviously observed ionization of neutral Rydberg states with Z=1. We show\nthat for states in this n regime microwave ionization is a state selective\ndetection technique. Threshold fields for states with n> 40, however,\ndepart from the expected behaviour, the thresholds become broader and shift\nto higher fields. For a total of 58 states we present the field strengths\nwhere 90% , 50% and 10% ionization occur.\n\n\n
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.