Theoretical Study of Coherent Excitation of Quascontinua
Abstract
In this thesis we study models where a few discrete levels are coupled to a band of closely spaced levels forming a quasicontinuum (QC). These models describe how, for example, in the infrared multiphoton excitation of polyatomic molecules the initial vibrational mode levels are interacting with the dense set of levels formed by the other vibrational modes. Assuming a coherent monochromatic external field and an infinite, uniform, and flat QC we are able to find exact solutions for the Schrodinger equation in four characteristically different problems. These include the coupling of N discrete levels to a real continuum; the coupling of one or two discrete levels to a QC; and a so-called three-region model where discrete levels are coupled to a QC which in turn is coupled to a real continuum. We first discuss some general properties of the case where N levels are coupled to a real continuum. These properties can also be found to some extent in QC solutions and we discuss the main differences and similarities of these two types of solutions. Our solutions for one or two levels coupled to the QC show regularly enacted recurrences which counteract the irreversible decay usually connected with the excitation of dense and infinite level systems. We study the effect of these recurrences on time development and make connections to the excitation dynamics of continuum and few-level systems. For the case where two discrete levels are included we study the interference between recurrences and two-level Rabi oscillations and Fano dynamics. The possibility of population trapping is investigated. We also study in various cases the spectral and temporal properties of the population distribution in the QC. A critical investigation of the various simplifying assumptions which are needed to derive exact solutions is carried out. We study therefore a finite QC and a QC with a structure. The approximation of a sudden turn-on of the external field is evaluated by considering several examples of adiabatic turn-on. We investigate also the time dynamics of the three region model. We have two discrete levels coupled to the QC, which in turn is coupled to the continuum. The continuum is further coupled in a sequential manner to several additional continua. . . . Author's abstract exceeds stipulated maximum length. Discontinued here with permission of author.) UMI.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1984
- Bibcode:
- 1984PhDT........93K
- Keywords:
-
- MULTIPHOTON;
- AUTOIONIZATION;
- THREE-REGION MODEL;
- Physics: Radiation