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Bulletin of the American Physical Society

Polarization control of microwave ionization of H atoms

Patrick M. Koch, M. R. W. Bellermann, D Richards

Published May 1, 1996• 0 citations

Abstract

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.

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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.

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.

We report the first observation of the ionization of Rydberg atoms by a circularly polarized 8.5-GHz microwave field. The observed threshold field for ionization is E=1/16

{\mathrm{n}}^{4}

, a value higher than that required for ionization by a linearly polarized microwave field. The origin of this threshold can be understood by transforming to a frame rotating at the microwave frequency. In the rotating frame no transitions occur, and level mixings alone are responsible for the ionization.

A classical study of the dynamics of atomic Rydberg states interacting with both elliptically and circularly polarized microwave fields is presented. In recent experiments, enhanced ionization rates were observed in elliptically as opposed to circularly polarized fields. A classical phase-space simulation provides good qualitative agreement with experimental findings. These results are explained in terms of the breakdown of almost conserved quantities as the polarization of the field is changed from circular to elliptical.

In circularly polarized microwave fields of frequencies in the vicinity of 10 GHz ionization of Na Rydberg atoms occurs at a field E=1/16n4, up to principal quantum number n=50. This field is equal to the classical field for ionization and the static field required to ionize a Na atom. On the other hand, this field is far below the linearly polarized 10 GHz field required to ionize Na atoms, E=1/3n5. If the problem is transformed to a frame rotating with the microwave field, the field becomes a static field. In this case it is straightforward to calculate the ionization field classically. However, it is far lower than the experimentally observed field, a discrepancy which may be due to an angular momentum barrier.

Electric field ionization of excited Na atoms by microwave fields between 12 and 15 GHz exhibits several interesting phenomena. The

|m|=0 \mathrm{and} 1

states may be ionized with a very low microwave field, $\ensuremath{\sim}\frac{1}{{3n}^{5}}$, corresponding to the point at which the

n

and

n+1

Stark manifolds of principal quantum numbers

n

and

n+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

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.

Quantitative results are presented for stabilization of atoms against ionization by strong laser fields over a wide range of frequencies for circular and linear polarization. With circular polarization, stabilization occurs for all frequencies. At high frequency (\ensuremath{\omega}>1 a.u.), the stabilization intensity is predicted by the occurrence of photon thresholds. This mechanism gives excellent numerical agreement both with present calculations and the results of Pont and Gavrila [Phys. Rev. Lett. 65, 2362 (1990)]. A different mechanism operates for \ensuremath{\omega}1 a.u. Linear polarization differs strongly from circular polarization in many respects.