Excitation And Ionization Of An Atom In A Laser Field

Nonlinear phenomena in the interaction between strong field and atom or moleculehave attracted lots of attention. Recently, the strong field has developed fast. Thestrong field plays an important role in many areas, holographic imaging and stereoimaging in medicine and bioengineering, electron microprobe X-ray micro-analysis inbiological science, laser fusion, and so on. However, lots of problems in strong fieldremain unsolved, limited by the costs of the XUV laser source and the speed of CPU.Three hot topics of strong field were studied theoretically in this thesis. We providednew schemes and ideas to solve the problems. This thesis is divided into three parts:First, we present a new scheme to control the harmonic generation process fromaN2molecule with a modulated polarization gating. The polarization gating ismodulated by a weaker linearly polarized pulse. We shorten the attosecond pulseduration by31attoseconds, and enhance the efficiency by one time. Moreover, weinvestigate the dependence of (HHG) on orientation ofN2molecule and find theHHG intensity is enhanced when the angle between the axis of the molecule and theapplied electric field decreases.Second, we provide a path to investigate the carrier envelope phase (CEP) effectin atomic bound-bound transitions. An obvious CEP effect on a two-photonthree-level transition can be observed for a long laser pulse which contains severaltens of optical cycles. The CEP effect originates from the interference between thesum-frequency and the difference-frequency within the two-photon transition. Andthis interference is independent of the pulse duration. Therefore the CEP effect canstay robust even for a long laser pulse. Besides, we theoretically investigate the CEPeffect in a bound-bound transition of an atom in a low-frequency long laser pulseusing the characteristic of small energy difference between two high Rydberg states.We find that the population changes dramatically with the CEP of the low-frequency pulse. This CEP effect is attributed to the interference between the positive-frequencyand negative-frequency components in one-photon transition. These results mayprovide a new method to measure the CEP value of a long laser pulse with lowfrequency.Finally, we develop a program to solve the time dependent Schrodinger equation（TDSE） based on the B-spline and spherical coordinate. We demonstrate how totransform the two electrons TDSE into a coupled differential equations. Using thegraph method we get the transition matrix. We apply the Adams method to solve thesedifferential equations. We compare our results with the literatures related to this workby computing the energy level of He and ionization rate. Then, we calculate theionization yield as a function of the full width at half-maximum (FWHM) durationwith different initial states, finding the ionization yield originated from the initial state1s2p is higher than the initial state1s2s. We successfully convert the programinto Graphic Processing Unit (GPU) parallel code, for saving the time. The computingspeed of GPU is nearly150times faster than CPU.