Collisional Energy Transfer Between The Excitation States Of Alkali Molecules And The Ground States Of Alkali Atoms

Over the past several years there has been much interest in the development of alkali dimer lasers both as a took for the spectroscopic study of the alkali molecules and as a source of high-power quasitunable light .In the latter respect ,the heavier alkalis,rubidium and cesium,are of particular interest since the density of molecular energy levels approaches the point where almost continuous tuning may be achieved .Additionally since equivalent transitions lie further to the red in the heavier alkalis,they offer the possibility of high-power quasitunable lasers in the short-wave infrared where at present such sources are scarce and difficult to operate.However,most of the work to data on alkali dimer lasers has concentrated on the lighter alkalis such as sodium.In this thesis we obtain values for predissociation,radiative,and collisional depopulation rates of part excited states of sodium and cesium by the technique of laser induce fluorescence spectroscopy and free-Doppler high resolution two photon laser spectroscopy.The whole thesis consists of four chapters:In the first chapter give a brief overview of the problem about collision between atom and molecule,including the property of collision and the possible process for optic physics, and collision in the alkali dimer system ,and the levels of the ground states and excitation states for alkali atom and molecule .In the second chapter the experiment technique and approaches are discussed,including laser induce fluorescence spectroscopy which the comprehensive adopted for liner laser spectroscopic study,and high resolution two-photon laser spectroscopy without Doppler broadening which belonged to nonlinear laser spectroscopic ,and the theory about the fluorescence rate equation . In the third chapter a set of experiment apparatus and detection devices for researching collisional energy transfer between the excitation states of alkali molecules and the ground state of alkali atoms are introduced,including the cell preparation ,heater, vacuum devices,photon detector,and laser . Besides, we introduce here some basic concepts on classic spectroscopy that is essential to carry out laser spectroscopy .In the last chapter we report three experimental studies of collisional energy transfer between the excitation states of alkali molecules and the ground state of alkali atoms : (1) Collisional excitation transfer between Na2(B1Πu) and Na2(21Σg+): The B1Πu electronic state of Na2 is excited by the 441.6nm He-Cd laser line.The Na atomic transitions and the A1Σu+→X 1Σg+ band of Na2 have been recorded.From the intensities and spectra of the Na and Na2 fluorescence several collisional processes in the excited sodium atom-dimer system have been identified.The Na atomic lines are the result of collisional energy transfer from the Na2(B1Πu) to the Na(3P).Predissociation process may also contribute to atomic fluorescence. The A1Σu+→X 1Σg+ band is interpreted through a populating mechanism involving collisional transfer from the B1Πu to the 21Σg+ followed by radiative transfer to the A1Σ+u state. From the decay constants and fluorescence intensities,the rate coefficient at 360℃ for collisional energy transfer from Na2(B1Πu) to Na2(21Σg +) is 5.7×10-10cm3s-1.The predissociation rate of the B1Πu is 2.7×106S-1.(2) Photodissociation of Cs2　molecules and collisional energy transfer: Population of the 62D state of Cs atoms by photodissociation of Cs2 molecules through the use of the fixed frequency line 441.6nm of a He-Cd+ laser radiation is produced．Combining ratio of atomic to molecular fluorescence,the ratio of collisional rate coefficient to predissociation rate is estimated to be 9.610-18cm3.The branching ratio is defined as the ratio of 6D3/2 to 6D5/2 fluorescence．The branching is determined in the Cs density range between 1 and 91015cm-3．the ratio of the dissociation rates produced in the Cs62D fine-structure states is 0.41．Fine-structure changing cross section has been measured for wing excitation,the res...