Core Concepts

Even with realistic frequency spread in the ionizing radiation, the nondipole angular asymmetry parameters in neon photoionization exhibit significant enhancement near specific dipole and quadrupole autoionizing resonances, making them detectable in experiments.

Abstract

**Bibliographic Information:**Dolmatov, V. K., & Manson, S. T. (2024). On Resonance Enhancement of E1 −E2 Nondipole Photoelectron Asymmetries in Low-Energy Ne 2p-Photoionization. arXiv preprint arXiv:2410.02869v1.**Research Objective:**This study investigates the impact of frequency spread in ionizing radiation on the nondipole angular asymmetry parameters (γ2p, δ2p, and ζ2p) in neon 2p photoionization near the 2s → 3p, 2s → 4p (dipole), and 2s → 3d (quadrupole) autoionizing resonances. The authors aim to determine if these parameters remain experimentally detectable after considering frequency spread.**Methodology:**The researchers employ the Random Phase Approximation with Exchange (RPAE) method, a well-established theoretical framework in atomic physics, to calculate the photoionization cross sections and angular asymmetry parameters. They incorporate the effect of frequency spread using a Gaussian function with a full-width at half-maximum (FWHM) of 5 meV, representing a typical experimental condition.**Key Findings:**The calculations reveal that while the frequency spread does influence the resonance enhancement of γ2p, δ2p, and ζ2p, these parameters retain significant values near the 2s → 3d quadrupole and 2s → 3p and 2s → 4p dipole resonances. The reduction in magnitude due to frequency spread is not substantial enough to render these parameters undetectable.**Main Conclusions:**The study concludes that the resonance enhancement of nondipole angular asymmetry parameters in neon 2p photoionization persists even after accounting for realistic frequency spread in the ionizing radiation. This finding suggests that experimental observation of these enhanced nondipole effects should be feasible.**Significance:**This research holds significance for both theoretical and experimental atomic physics. It provides further validation for the theoretical predictions of enhanced nondipole effects in photoionization near autoionizing resonances. Moreover, it encourages experimentalists to undertake measurements to confirm these predictions, potentially leading to a deeper understanding of electron correlation and nondipole dynamics in atomic systems.**Limitations and Future Research:**The study focuses specifically on neon and a particular set of resonances. Investigating other atomic systems and exploring the influence of varying frequency spreads would be valuable avenues for future research. Additionally, experimental verification of these theoretical findings is crucial for further advancing the field.

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Stats

The frequency spread in the ionizing radiation is assumed to be 5 meV at the half-maximum of the radiation's intensity.
The maximum value of γ2p, without accounting for frequency spread, is approximately 0.12.
Accounting for frequency spread reduces the maximum value of γ2p to approximately 0.06, which is about 6% of β2p.
The maximum value of ζ2p, without accounting for frequency spread, is approximately 0.22.
Accounting for frequency spread reduces the maximum value of ζ2p to approximately 0.12, which is about 12% of β2p.

Quotes

"We demonstrate that the frequency spread in the ionizing radiation does quantitatively affect the resonance spikes in γ2p, δ2p, and ζ2p. Nevertheless, the spikes remain sufficiently strong to be experimentally detected."
"In contrast, Ne is a noble gas, for which conducting experiments is easier. This is why we focus on the photoionization of Ne in the present work."

Key Insights Distilled From

by Valeriy K. D... at **arxiv.org** 10-07-2024

Deeper Inquiries

Employing theoretical methods beyond the Random Phase Approximation with Exchange (RPAE) could indeed lead to variations in the predicted magnitudes of nondipole effects in photoionization. Here's a breakdown of how different methods could introduce these variations:
Methods accounting for higher-order correlations: While RPAE effectively captures some degree of electron correlation, more sophisticated methods like
Coupled-Cluster (CC) methods,
Configuration Interaction (CI), or
Many-Body Perturbation Theory (MBPT) at higher orders
can provide a more accurate representation of electron-electron interactions. These methods could refine the calculated dipole and quadrupole transition amplitudes, potentially leading to either enhancement or suppression of the predicted nondipole effects depending on the specific atomic system and the energy range considered.
Relativistic effects: For heavier atoms, relativistic effects become increasingly important.
The Dirac-Fock (DF) method or its relativistic many-body counterparts, such as the Relativistic Random Phase Approximation (RRPA), incorporate these relativistic effects. These methods could significantly alter the calculated nondipole parameters, especially for inner-shell photoionization where relativistic effects are more pronounced.
Time-dependent methods: The interaction of an atom with a time-varying electromagnetic field, as in the case of a photon, can be more accurately described using time-dependent methods like the
Time-Dependent Density Functional Theory (TDDFT) or the
Time-Dependent R-Matrix (TD-RM) method. These methods can capture dynamic electron correlations and could provide a more accurate picture of the photoionization process, potentially leading to different magnitudes of nondipole effects compared to time-independent methods like RPAE.
In essence, the choice of the theoretical method depends on the specific atomic system under investigation, the energy range of interest, and the desired level of accuracy. While RPAE provides a reasonable starting point, incorporating higher-order correlations, relativistic effects, or employing time-dependent methods could lead to more accurate predictions of nondipole effects in photoionization.

Yes, the presence of external electric or magnetic fields during photoionization can indeed significantly alter the observed nondipole angular asymmetries. Here's how these fields influence the process:
Breaking of symmetry: The application of an external field breaks the inherent spherical symmetry of the atom. This symmetry breaking mixes different angular momentum states, leading to modifications in the selection rules for photoionization. Consequently, transitions that are forbidden in the field-free case become allowed, potentially enhancing or suppressing certain nondipole channels.
Stark and Zeeman effects:
Electric fields induce Stark shifts in the atomic energy levels, leading to energy level splitting and mixing of states with different parities. This mixing can significantly alter the interference between dipole (E1) and quadrupole (E2) transitions, directly impacting the nondipole angular asymmetry parameters.
Magnetic fields, on the other hand, introduce Zeeman splitting and modify the angular momentum quantization axis. This can lead to a rotation of the photoelectron angular distribution and affect the observed nondipole asymmetries.
Field-induced modifications of the continuum states: External fields can distort the wavefunctions of the outgoing photoelectrons in the continuum. This distortion can further influence the interference between different photoionization channels, leading to variations in the nondipole angular distributions.
The magnitude of these field-induced modifications depends on the strength and orientation of the applied field relative to the polarization direction of the ionizing radiation. By carefully controlling these parameters, one can manipulate the nondipole angular asymmetries, providing a valuable tool for probing the dynamics of the photoionization process and gaining deeper insights into the electronic structure of atoms.

The successful measurement and control of enhanced nondipole effects in photoionization could pave the way for exciting practical applications, particularly in the development of novel light sources and advanced imaging techniques:
Novel Light Sources:
Generation of customized polarization states: By manipulating the nondipole angular asymmetries through external fields or tailored laser pulses, it might be possible to generate light with specific, customized polarization states. This could be particularly valuable for applications requiring precise control over light polarization, such as in optical communication, quantum information processing, and high-resolution spectroscopy.
Production of short-wavelength radiation: Nondipole effects become increasingly important at higher photon energies. Harnessing these effects could potentially lead to new methods for generating short-wavelength radiation, such as extreme ultraviolet (EUV) or X-ray light, which are crucial for lithography, microscopy, and materials science.
Advanced Imaging Techniques:
Enhanced spatial resolution in microscopy: Nondipole effects introduce additional angular dependencies in the photoelectron emission patterns. Exploiting these dependencies could lead to enhanced spatial resolution in photoemission microscopy techniques, allowing for more detailed imaging of nanoscale structures and materials.
Element-specific and chemical state imaging: The sensitivity of nondipole angular asymmetries to the electronic structure of the target atom or molecule could be utilized for element-specific or chemical state imaging. By analyzing the angular distribution of photoelectrons, one could potentially differentiate between different elements or chemical states within a sample, providing valuable information for materials characterization and biological imaging.
Tomographic reconstruction of electron density: The angular information encoded in the nondipole photoelectron distributions could be used for tomographic reconstruction of the electron density within atoms or molecules. This could provide a powerful tool for visualizing the three-dimensional electronic structure of matter with unprecedented detail.
While these applications are still in the realm of exploration, the successful measurement and control of enhanced nondipole effects in photoionization hold significant promise for advancing various fields, ranging from fundamental atomic physics to applied photonics and imaging technologies.

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