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# Semiclassical Methods in Atomic and Molecular Physics

#### Abstract

Semiclassical approximation which is the marriage of classical and quantum mechanics can be applied to a large variety of problems in quantum physics. In addition to being a highly intuitive approach, often the semiclassical results can be as accurate as the exact quantum calculations. This dissertation explores in detail and comparatively the application of classical, semiclassical, and quantum theories to some selected electron-ion and electron-molecule interactions. We first study classically the laser-assisted spontaneous electron bremsstrahlung in a combined Coulomb and laser (ac) fields. Due to chaotic scattering, the radiation probability as a function of the impact parameter and the constant phase of the laser field exhibits fractal structures. At low incident electron kinetic energy, the Coulomb focusing leads to a substantial extension of the range of impact parameters enhancing the bremsstrahlung cross section as compared to the pure Coulomb case. Next, we implement the Classical trajectory Monte Carlo (CTMC) method to compute charge-transfer cross section in low-energy positronium-proton collisions. The CTMC approach gives accurate and reliable cross sections having the same scaling law as obtained by exact quantum calculations. However, we observe a suppression in the quantum cross section which is mainly due to the low-impact parameter behavior of the probabilities governed by the quantum uncertainty principle. By treating photodetachment in combined high-frequency laser and low-frequency fields, we investigate semiclassically the temporal and spatial interference structure in the electron current density. The time-dependent Green’s function is used for calculation of electron flux on a detector. The caustic divergences in the electron flux are removed by using the uniform Airy approximation. Finally, we treat the dissociative electron attachment (DEA) to polyatomic molecules by developing the multi-mode nonlocal theory of DEA. The nonlocal theory is complemented by a semiclassical approach. With the inclusion of two vibrational modes, we demonstrate the application of the new theory to a generic molecule of the type CY3X, where Y denotes the H or F atom and X denotes the halogen atom.

#### Subject Area

Theoretical physics|Quantum physics|Atomic physics|Molecular physics

#### Recommended Citation

Ambalampitiya, Harindranath B, "Semiclassical Methods in Atomic and Molecular Physics" (2021). *ETD collection for University of Nebraska-Lincoln*. AAI28418425.

https://digitalcommons.unl.edu/dissertations/AAI28418425