The Study Of Quantum Interference In Molecular Multi-level System

Quantum interference in multi-level system and quantum cryptography have been interesting topics in the field of quantum optics for recent years. Many novel physical phenomena such as coherent population trapping, lasing without inversion, electromagnetically induced transparency and spontaneous emission cancellation based on quantum interference have been discovered. Besides important academic values, the research of quantum interference in multi-level system has also promising applications.In this thesis, we try to theoretically and experimentally investigate quantum interference in molecular multi-level system to find the suitable molecular energy-level schemes that satisfy the interference conditions, and to explore the influences of quantum interference on molecular spectrumThrough a detail analyze and discussion for the various quantum interferences in molecular multi-level system by using full quantum theory, dressed-state theory and semi-classic theory, respectively, we lucubrated the effects of the additional levels to a system under the conditions of quantum interference for the first time And we explained the influences of the energy-level spacing, the laser power, the initial population and the alignment of transition dipole moments on quantum interference In connection with some different energy-level schemes such as Rhomb-type four-level system, Y-type four-level system and five-level system with two intermediate enhancing levels and two close upper levels, we first propose that quantum interference can result in the senous lineshape variations of two-photon transitionsExperimentally, we found the Rhomb-type four-level configuration and special five-level configuration in sodium dimmers that satisfy the interference conditions by applying the equal-frequency Doppler-free two-photon spectroscopy. The laser induced emission and the incoherent exhaustion out of the upper levels were monitored by visible and UV fluorescence, respectively. The recorded spectral signals show the obvious effects of quantum interference and present remarkable different features from that of usual two-photon excitation signals in a typical three-level system, including the serious lineshape variations and the occurrence of the coherent peaks. Such coherent peaks do not correspond to any real energy-level and display the sensitivedependence upon the outside influences. The experiment results show agreements quite well with the theoretical results.The works should be helpful for better understanding of the quantum interference in molecular multi-level system as well as the developments of quantum optics and molecular spectroscopy.