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Title: Harmonic generation in time-dependent R-matrix theory
Author: Brown, Andrew Christopher
Awarding Body: Queen's University Belfast
Current Institution: Queen's University Belfast
Date of Award: 2013
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We present the extension of time-dependent R-matrix (TDRM) theory to describe harmonic generation in general multielectron systems. The process of harmonic generation is of current interest because of its use as a probe of ultrafast electron dynamics and as a source of ultrashort (sub-femtosecond) laser pulses. Modelling harmonic generation- the production of high frequency light stimulated by laser-matter interaction- within the highly accurate and extensive TDRM theory allows us to investigate multielectron and multichannel effects as they influence the process. We detail recent advances in the field of attosecond physics, and give an overview of the theoretical methods used. A comprehensive explanation of the TDRM method, and the extensions made by this author, are provided. Calculations of harmonic generation in helium provide a test of the reliability of the method. We compare our results with those from the benchmark HELIUM method, finding striking agreement. We also assess the different methods of calculating the harmonic spectrum, using either the dipole length, velocity or acceleration operators. e We report the first evidence of multielectron interference in harmonic generation in argon. There are two mechanisms by which the fifth harmonic can be produced- excitation of electrons into the continuum, or resonant excitation of electrons into bound states. Interference between the continuum and resonant pathways gives rise to an asymmetric resonance in the fifth harmonic intensity, suggesting that the two pathways are of comparable importance. The final two results chapters explore harmonic generation in singly ionised argon, and uncover multichannel interference (interference between intermediate ion states) as well as multielectron effects in harmonic generation. We find that the dominant contribution to harmonic generation arises from excited states of Ar+. We also investigate the effect of the magnetic quantum number, finding that M=1 Ar+ exhibits a four-fold increase in harmonic generation over Ar+ with M=O.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available