Theoretical Research On Dynamics Process Of Single And Double-resonance Multiphoton Ionization

Resonance Enhanced Multiphoton Ionization (REMPI) technology is one of thesignificant means of studying the interaction of light and matter. The research of thecharacteristic of its dynamic behavior has important theoretical significance andhuge potential application. This thesis establishes the theoretical model ofsingle-resonance multiphoton ionization, double-resonance multiphoton ionizationand cascade type three-level system firstly, and then, based on the density-matrixequations theory of Quantum mechanics, the variations of particle number indifferent energy levels are analyzed. The dynamic behavior (such as absorption,ionization and radiation etc.) of the particles passing through the resonant level isobserved also by MATLAB numerical simulation. The phenomenon ofelectromagnetically induced transparency is studied deeply for a cascade typethree-level system.By solving density-matrix equations that describing the process of the singleresonance multiphoton ionization, we find that both spontaneous radiation andionization probability will increase with the decrease of the laser frequencydetuning. With the increase of laser Rabi frequency, they will increase until get tosaturation. If Rabi frequency continues to increase after saturation, spontaneousradiation will decrease due to energy level splitting. It is firstly discovered that thereexist a phenomenon of absorption transparency in the process of REMPI. Thetransparent depth will decrease with the increase of the ionization rate and the widthwill increase with Rabi frequency.By numerical simulation, it is found that for the process of double-resonancemultiphoton ionization, the population in each energy level will vary periodicallywith the prolong of laser radiation time. The larger the laser pulse duration and Rabifrequency is, the faster the population changes. The match between the parameters of two lasers is closely related to the ionization probability. So in the practicalapplication, it would be better to select the best matching value and the delay timebetween two laser fields according to the experimental conditions.We think the double-color and double-resonance absorption in the process ofREMPI as a cascade three-level system, the influence of coupling and probe field onthe probe absorption is analyzed. It is found that if resonant strong coupling fieldinteract with the system, the absorption of weak probe field will present EITphenomenon, which will change to Aulter-Towns doublet with the increase of theintensity of coupling field. While when off-resonant strong coupling field interactwith the system, the absorption profile will be narrowed. The reason for thenarrowing of spectral linewidth is explained with dressed-state theory. Thedependence of the narrowing linewidth on the physical parameters is presented. Ifthe coupling field is a weak one, the probe absorption will experience thetransformation process from EIA to anomalous dispersion to EIT to normaldispersion and to EIA with the frequency detuning of coupling field changes fromnegative to positive.Above results could provide more valuable reference information for thetheoretical study and practical application of the technique of REMPI.