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Civil War Book Review

Half -Cycle Pulse Ionization of Diamagnetic Zeeman States in Rydberg Atoms.

Chitra Rangan

Published Jan 1, 2000• 0 citations
Publisher

Abstract

We study the dynamics of highly excited (Rydberg) atoms in laboratory-scale magnetic fields via their ionization by THz half-cycle pulses (HCPs). A half-cycle pulse is a unipolar electromagnetic pulse that can probe an atomic electron at all points along its orbit around the atomic core. The dynamical evolution of diamagnetic Zeeman states subjected to an ultrashort (picosecond) HCP is calculated for both hydrogen and sodium atoms. Diamagnetic Zeeman states are localized parallel and perpendicular to the magnetic field. We use a grid-based pseudo-potential method to determine these eigenstates of the combined Coulomb-diamagnetic potential. We study states with principal quantum number n between 15--30 in the ℓ-mixing regime at magnetic fields of 1--6T. These initial states are subjected to the electric field of an HCP and their subsequent time evolution is calculated by solving the time-dependent Schrodinger equation. The HCP width is varied between the short pulse (impulsive) and the long pulse limit, keeping the total momentum transferred a constant. We calculate the total ionized fraction, and also the spectrum of the ionized photoelectrons. We find that, in the impulsive limit, where the width of the pulse is much smaller than typical time scales in the system, the calculated amount of ionization and the photoelectron spectra are different depending on the localization of the initial state. These differences are shown to be due to the different initial momentum distributions of the parallel and perpendicular states. As the duration of the HCP is made longer, we find that ionization is suppressed as compared with the impulsive limit. The states localized perpendicular to the magnetic field are found to be much more sensitive to the HCP width than the parallel states, reflecting the fact that the two classes of states interact with different parts of the Coulomb-diamagnetic potential during the HCP. The ionization characteristics are shown to scale classically with the principal quantum number. The results are presented in such a manner that they can be verified by experiment.

DOI
10.31390/gradschool_disstheses.7223

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We study the dynamics of highly excited (Rydberg) atoms in laboratory-scale magnetic fields via their ionization by THz half-cycle pulses (HCPs). A half-cycle pulse is a unipolar electromagnetic pulse that can probe an atomic electron at all points along its orbit around the atomic core. The dynamical evolution of diamagnetic Zeeman states subjected to an ultrashort (picosecond) HCP is calculated for both hydrogen and sodium atoms. Diamagnetic Zeeman states are localized parallel and perpendicular to the magnetic field. We use a grid-based pseudo-potential method to determine these eigenstates of the combined Coulomb-diamagnetic potential. We study states with principal quantum number n between 15--30 in the ℓ-mixing regime at magnetic fields of 1--6T. These initial states are subjected to the electric field of an HCP and their subsequent time evolution is calculated by solving the time-dependent Schrodinger equation. The HCP width is varied between the short pulse (impulsive) and the long pulse limit, keeping the total momentum transferred a constant. We calculate the total ionized fraction, and also the spectrum of the ionized photoelectrons. We find that, in the impulsive limit, where the width of the pulse is much smaller than typical time scales in the system, the calculated amount of ionization and the photoelectron spectra are different depending on the localization of the initial state. These differences are shown to be due to the different initial momentum distributions of the parallel and perpendicular states. As the duration of the HCP is made longer, we find that ionization is suppressed as compared with the impulsive limit. The states localized perpendicular to the magnetic field are found to be much more sensitive to the HCP width than the parallel states, reflecting the fact that the two classes of states interact with different parts of the Coulomb-diamagnetic potential during the HCP. The ionization characteristics are shown to scale classically with the principal quantum number. The results are presented in such a manner that they can be verified by experiment.
The ionization of potassium np Rydberg atoms with n∼388n\sim388n∼388 by pulsed unidirectional electric fields, termed half-cycle pulses (HCPs), with various well-characterized shapes (rectangular, triangular, and sawtooth) and durations $T\sb{p}$ comparable to the classical electron orbital period $T\sb{n}$ is investigated. The experimental results are compared to classical trajectory Monte Carlo (CTMC) simulations and the classically-scaled results of quantum calculations undertaken at n=5.n=5.n=5. The data show that in the intermediate regime, $T\sb{p}\sim T\sb{n},$ the threshold field for ionization becomes sensitive to the shape and rise time of the HCP, increasing with increasing rise time. For each pulse shape, the agreement between the experimental measurements and both the CTMC and quantum calculations is very good.
The characteristics of quasi-one-dimensional (1D) high-nnn Rydberg atoms produced by photoexciting, in a weak dc field, extreme members of the n=350n=350n=350 Stark manifold are investigated by studying their ionization when subjected to unidirectional electric field pulses, termed half-cycle pulses (HCP's), of duration Tp⪡Tn{T}_{p}⪡{T}_{n}Tp​⪡Tn​ where Tn{T}_{n}Tn​ is the classical electron orbital period, directed both parallel and perpendicular to the atomic axis. The observed differences in the ionization characteristics provide a measure of the anisotropy of the momentum distribution. It is shown that HCP's applied transverse to the atomic axis provide a particularly sensitive probe of the quasi-1D nature of the state. The experimental data are in good agreement with the results of classical trajectory Monte Carlo simulations.
Nearly unipolar, subpicosecond half-cycle pulses (HCPs) have been used to ionize Ba+{\mathrm{Ba}}^{+}Ba+ ngngng Rydberg ions. The double ionization probability vs HCP field strength data are compared with analogous single ionization measurements for neutral Ba 6snd6snd6snd Rydberg atoms prepared in the same apparatus. We find that neutral and ionic Rydberg states with the same binding energies follow approximately the same ionization curves. Our findings are in agreement with the predictions of an impulsive ionization model and classical trajectory Monte Carlo simulations.
We study the ionization of Rydberg atoms by half-cycle electromagnetic pulses as a function of the duration of the pulses covering a broad range of time intervals connecting the adiabatic and sudden regimes. The threshold fields for ionization lie on a universal classical scaling invariant curve. We present quantum mechanical results which agree well with the universal curve. The n -2 threshold field scaling found experimentally is a linear approximation to the universal curve by the tangent in the region where the pulse duration is of the order of the orbital period of the atom. We predict the existence of an ultrashort pulse limit in which the threshold field scales as n -1 . Effects due to the non-hydrogenic core are negligible.
The characteristics of strongly-polarized quasi-one-dimensional (quasi-1D) Rydberg atoms produced by photoexcitation of selected Stark states in the n = 350 Stark manifold are investigated by studying their ionization when subject to sub-nanosecond unidirectional electric field pulses, termed half-cycle pulses (HCPs), directed both parallel and perpendicular to the atomic axis. The observed differences in the ionization characteristics provide a measure of the anisotropy of the momentum distribution and thus of the quasi-1D nature of the states. The response of such states to a train of HCPs directed parallel and anti-parallel to the axis of polarization of the initial state is also investigated. Pronounced differences in the survival probability are observed confirming theoretical predictions that the dynamical response of the system will be very different for these two conditions. The presence of noise is also observed to play a crucial role in the dynamics. Reasonable agreement between experiment and theory is achieved.
We present the technique and results of our study of the redistribution of Rydberg states in barium by a half cycle pulse (HCP). A survey of previous research concerning the interaction of half cycle pulses with Rydberg atoms reveals a lack of experimental data in the area of state redistribution. This is primarily due to an inability to identify the redistributed states accurately using the currently available tool: selective field ionization (SFI). We present the limitations of SFI as a state identification tool. We then present a survey of isolated core excitation as a state identification tool, along with its limitations in dealing with a mixture of states. Combining SFI and ICE overcomes these limitations and allows for the accurate characterization of a mixture of Rydberg states, as found in HCP redistribution. We present our results for the HCP redistribution of a series of ns and nd initial states, for n from 30 to 40, at various HCP intensities. These results primarily involved single photon transitions, confining us to the linear regime. Analyzing the results of these redistributions with respect to the energy difference between initial and final states yielded information about the frequency structure, after transmission to the interaction region, of the HCP. This led us to postulate on the effects of diffraction on the HCP. We conclude with several suggestions for the improvement of the experimental system, as well as a direction for future research.