The Spectral Properties Of A Three-Level Atom Driven By Modulated Fields And The Annealing Process Of C₆₀ Analyzed By The Isomer Spectrum

Since the natural width cannot be suppressed by the usual spectroscopic methods used for the elimination of other kinds of broadening such as Doppler-broadening, power broadening and pressure broadening, to achieve subnatural linewidth is a long sought goal in laser spectroscopy community. Narducci et al, (Phys. Rev. A 42, 1630) proposed a theoretical model in which a V-type three-level atom is driven by two monochromatic fields, and found out that when the Rabi frequency corresponding to the weak atomic transition is made sufficiently larger than that to the strong transition, the fluorescence linewidths of the strong atomic transition can be greatly narrowed. In this thesis we further extend this model to allow two amplitude-modulated driving fields with common modulation frequency, based on the decomposition of the master equations with harmonic expansions. Our calculations are focused on the subnatural linewidths in the fluorescence and probe absorption spectra corresponding to the strong atomic transition, under the condition that the modulation depth of the driving fields is chosen as 100%, in which the driving fields are effectively bichromatic ones. Our calculations show that both the fluorescence and absorption spectra are periodic functions of the phase delay Φ between the two modulation signals, with a period of Ï€. When Φ= 0, we found that although the fluorescence spectrum turns out to exhibit multi-peak structure, it perfectly inherits the line-narrowing effect which has been found for the monochromatic fields. When 0 <Φ <Ï€ , both spectra demonstrate significant line-splittings, and deep subnatural dip appear on top of each absorption peak for small values of Φ. When the Doppler-broadenings are included, these absorption dips can be observed as subnatural burning holes in an atomic vapor experiment, if a counterpropagating probe beam isutilized. We have also extended the theoretical methods to include both Ξ-type and A-type atoms, under bichromatic, polychromatic and phase-modulated driving fields. Although the detailed calculations in such cases are still under way, the theoretical framework has been completely setup for future studies.The rest of this thesis is concerned with the study of the annealing process of C₆₀ by a molecular dynamics simulation which includes an initially defected C₆₀ isomer and 120 Helium atoms as buffer gas. The relaxation kinetics of C₆₀ are clearly demonstrated by the successive transitions between the adjacent stacks of the Stone-Wales Stack-Map proposed by Austin et al. (Chem. Phys. Lett. 235, 146), and many other transitions to nonclassic isomers that cannot be explained with the classic Stone-Wales rearrangement. The prevalence of the nonclassic C₆₀ isomers, especially those with heptagon defects, is explained by the generalized Stone-Wales rearrangements from a structural analysis, which gives out more than millions of C₆₀ isomers containing heptagon defects. This huge amount of nonclassic isomer is a striking phenomenon since the total number of classic isomers has been found to be only 1812, and most current studies of C₆₀ are usually limited in this rather small range. Based on these arguments, we further proposed a conjecture that the isolation and identification of C₆₀ as isomerically pure Buckminsterfullerene are probably due to the chemical extraction process which may either filter out the other isomers or catalyze the transitions from those isomers to the Buckminsterfullerene.