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Physical Review A

Atomic photoabsorption process controlled by static and oscillating magnetic fields

Takeshi Shirahama, Xiao‐Min Tong, Ken-ichi Hino, Nobuyuki Toshima

Published Oct 23, 2009• 6 citations
Publisher

Abstract

We present a theoretical method to study atomic photoabsorption processes in a temporally periodic external field. The time development of the atomic dipole moment is propagated by numerical time propagation, and the photoabsorption cross sections is obtained directly from the Fourier transform of the autocorrelation function without summing up over the Floquet-Fourier components. As an example, we study the photoabsorption spectra of hydrogen atoms exposed in a mixture of static and periodically oscillating magnetic fields for the transition from $1s\ensuremath{\rightarrow}2p$. The atomic energy structure and photoabsorption spectra show interesting dependence on the strength of the oscillating magnetic field. Tuning the combination of the magnetic field strengths, we can control the photoabsorption process. The proposed method is general and can be applied to processes in which the number of relevant high harmonic components of the Fourier expansion is so large that the conventional Floquet method is intractable.

DOI
10.1103/physreva.80.043414

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Communications in Theoretical Physics•Aug 1, 2009•4 citations
We present a theoretical method to study atomic photoabsorption processes in a temporally periodic external field. The time development of the atomic dipole moment is propagated by numerical time propagation, and the photoabsorption cross sections is obtained directly from the Fourier transform of the autocorrelation function without summing up over the Floquet-Fourier components. As an example, we study the photoabsorption spectra of hydrogen atoms exposed in a mixture of static and periodically oscillating magnetic fields for the transition from 1s\ensuremath→2p1s\ensuremath{\rightarrow}2p1s\ensuremath→2p. The atomic energy structure and photoabsorption spectra show interesting dependence on the strength of the oscillating magnetic field. Tuning the combination of the magnetic field strengths, we can control the photoabsorption process. The proposed method is general and can be applied to processes in which the number of relevant high harmonic components of the Fourier expansion is so large that the conventional Floquet method is intractable.
A robust time-dependent approach to the high-resolution photoabsorption spectrum of Rydberg atoms in magnetic fields is presented. Traditionally we have to numerically diagonalize a huge matrix to solve the eigen-problem and then to obtain the spectral information. This matrix operation requires high-speed computers with large memories. Alternatively we present a unitary but very easily parallelized time-evolution method in an inexpensive way, which is very accurate and stable even in long-time scale evolution. With this method, we perform the spectral calculation of hydrogen atom in magnetic field, which agrees well with the experimental observation. It can be extended to study the dynamics of Rydberg atoms in more complicated cases such as in combined electric and magnetic fields.
We propose a theoretical method to calculate the infrared (IR) laser-assisted photoabsorption cross sections over a broad energy range by a single calculation. In this method we define an initial wave function as the product of the dipole operator and the atomic ground state, propagate the initial wave function in the IR laser field with different initial phases, then calculate the generalized autocorrelation function, which is defined as the averaged value of the autocorrelation function over the initial phase from 0 to $2\ensuremath{\pi}$ (or one IR optical cycle). The IR laser-assisted photoabsorption cross sections are obtained by the Fourier transform of the generalized autocorrelation function. We apply this method to study the IR laser-assisted photoabsorption of He atoms. From the simulation results, we see that the IR laser field affects not only the resonant position but also the lifetime or the width of the bound states. This photoabsorption cross section is an important quantity to analyze the IR-laser-assisted dynamical processes by an attosecond pulse, a pulse train, or a free-electron laser.
We study theoretically resonance absorption of \ensuremath{\gamma} rays by nuclei exposed to an external radio-frequency (rf) magnetic field. The external field is taken to couple directly to the nuclear magnetic moments. Perturbation theory is developed for M\"ossbauer spectroscopy in terms of Floquet eigenstates that incorporate the rf field exactly. The present treatment allows for calculation of the conventional time-averaged absorption spectrum, as well as analysis of the time dependence of absorption, resolved on the scale of a period of the rf field. An efficient numerical implementation of the theory is described and comparisons with the equally general approach of Salkola and Stenholm [M. Salkola and S. Stenholm, Phys. Rev. A 41, 3838 (1990)] are made. We demonstrate that even at off-resonance frequencies a strong modulation leads to shifts of the original static hyperfine lines. Examples of line splittings of NMR's associated with Rabi flopping are also shown. \textcopyright{} 1996 The American Physical Society.
In systems of ultracold atoms, pairwise interactions are resonantly enhanced by the application of an oscillating magnetic field that is parallel to the spin-quantization axis of the atoms. The resonance occurs when the frequency of the applied field is precisely tuned near the transition frequency between the scattering atoms and a diatomic molecule. The resulting cross section can be made more than two orders of magnitude larger than the cross section in the absence of the oscillating field. The low momentum resonance properties have a universal description that is independent of the atomic species. To arrive at these conclusions, we first develop a formal extension of Floquet theory to describe scattering of atoms with time-periodic, short-range interaction potentials. We then calculate the atomic scattering properties by modeling the atomic interactions with a square well potential with oscillating depth and then explicitly solving the time-dependent Schrodinger equation. We then apply the Floquet formalism to the case of atoms scattering with a contact interaction described by a time-periodic scattering length, obtaining analytic results that agree with those obtained by solving the time-dependent Schrodinger equation.
During recent years, interest has been rising for applications of vector light beams toward magnetic field sensing. A series of experiments were performed to extract information about properties of static magnetic fields from absorption profiles of light passing through an atomic gas target. In this paper, we propose an extension to this method for oscillating magnetic fields. To investigate this scenario, we carried out theoretical analysis based on the time-dependent density matrix theory. We found that absorption profiles, even when averaged over typical observation times, are indeed sensitive to both strength and frequency of the time-dependent field, thus opening the prospect for a powerful diagnostic technique. To illustrate this sensitivity, we performed detailed calculations for the $5s{\phantom{\rule{0.16em}{0ex}}}^{2}{S}_{1/2}\phantom{\rule{0.16em}{0ex}}(F=1)\ensuremath{-}5p{\phantom{\rule{0.16em}{0ex}}}^{2}{P}_{3/2}\phantom{\rule{0.16em}{0ex}}(F=0)$ transition in rubidium atoms, subject to a superposition of an oscillating (test) and a static (reference) magnetic field.
By combining the B-spline basis set with model potential (B-spline + MP), we present oscillator strength spectra of Rydberg Li atoms in external fields. The photoabsorption spectra are analyzed. Over the narrow energy ranges considered in this paper, the structure of the spectra can be independent of the initial state chosen for a given atom. Our results are in good agreement with previous high-precision experimental data and theoretical calculations, where the R-matrix approach together with multichannel quantum defect theory (R-matrix+MQDT) was used. It is suggested that the present methods can be applied to deal with the oscillator strength spectra of Rydberg atoms in crossed electric and magnetic fields.