A Study Of The Modulations Of The Short-pulse Laser And Time-dependent Field To Atomic Recurrence Spectra

Due to its clear physical picture and wide applications, the closed-orbit theory has been extensively used to explain the oscillating phenomena of the complex spectra of the highly excited Rydberg atoms and study the dynamics of Rydbery wave packet and become an important link to connect the classical and quantum theory.In recent years, along with the fast development of scientific experiment and practical application, much attention has been paid to the photoabsorption characters of atoms in time-dependent fields. Different from static external fields, the time-dependent fields have more complex influence on the recurrence spectra. For example, the quantum control of the Guanssian short-pulse laser on the Rydberg wave packet. In experiment, the Rydberg state can be obtained by the continuum laser exciting the atoms, while the Rydberg wave packet can be obtained when short-pulse laser excites the atoms. The Rydberg wave packet had been prepared such that it is radically localized with external field and observed that it travels like a classical particle along the classical trajectory in short time and may split and recombine coherently during its evolution in long time. Auto-correlation function is a characteristic quantity that can reflect the properties of dynamics of Rydberg wave packet and is correspondent with oscillator strength density so that can be calculated on the base of the closed-orbit theory.In this submitted thesis works, applying the closed-orbit theory and time dependent perturbation method to study the influences of the short pulse laser and the time-dependent field on the recurrence spectra of atoms, respectively. The main works are as follows:(1) Applying time-dependent perturbation theory and rotation wave approximation, the formula of the auto-correlation of the double-pulse is derived. It shows that auto-correlation function can be written as a sum of modified Gaussians. Each modified Gaussian term comes from a parent oscillation in the oscillator strength density that can be calculated using procedure in closed-orbit theory, which is characterized by a set of parameters including its center position, height, width, delayed time and other properties. And then the dependences of auto-correlation function on duration time of the pulse, time delay, and relative phase are briefly discussed.(2) Using the time-dependent semiclassical closed-orbit theory, we calculate the recurrence spectra of the Li Rydberg atoms in static electric field plus a weak oscillating electric (radio frequency)field and analyze the influences of the oscillating electric field on the recurrence spectra.We extend the application scope of the closed-orbit theory from the time-independent systems to the time-dependent systems. In the previous researches, The applied external fields that people mostly adopted include the static electric field,magnetic field or the coexistence of the electric and magnetic fields, but have done a few calculation on the time-dependent systems. When the atom is placed in time-dependent fields, as the Hamiltonian of the system is no longer a conservation, the outgoing angles, returning angles, periods and classical actions of the closed orbits will be changed considerably, producing new modulation phenomena then and therefore increasing the complexity of the problem.We calculate and analyze the recurrence spectra of the Li Rydberg atom in static electric field plus a weak oscillating electric field. The results show that when there is only static external field, there exist many strong recurrence in the absorption spectra, while an oscillating electric field parallel to the external field is added, the recurrence spectra is weakened according to a complicated pattern. If the period of the radio frequency field can be compared with the period of the classical closed orbit, the recurrence will be reduced compactly. Moreover the periodic orbits are sensitive to the periodic field, recurrences with period near integer multiples of the period of the oscillating field survive, while those that recurrence deviate from the above frequencies are weakened or eliminated. These irregular phenomena are caused by the effect of the perturbation to the closed orbits and the modulation of the Bessel function to the recurrence spectra.This thesis is divided five chapters. The first chapter is summarization, which chiefly introduces the development of semiclassical closed-orbit theory. In the second chapter, the formula of the auto-correlation of the double-pulse is derived, applying closed-orbit theory and time-dependent perturbation theory; the dependence of auto-correlation function on the parameters of the double-laser pulses is discussed. The concepts of scaled law and recurrence spectroscopy are presented in the third chapter. In the next chapter, we extend the closed-orbit theory from the time-independent system to the time-dependent system and calculate the recurrence spectra of the Li Rydberg atom in static electric field plus a weak oscillating electric field, the effects of the oscillating field on the recurrence spectra are analyzed in detail. At last, we make an inspection to the challenge and the application of this theory.