Theoretical And Experimental Research On Laser Induced Collisional Energy Transfer And Charge Transfer

A laser-induced collision is an optical process in which a laser field is used during the collision between two different particles to induce selective energy transfer from a level in one atomic or molecular species to another level in a different species. It can't occur unless the two mechanisms - collision and radiative interaction - are both present; any of them singly can't induce this interparticle transition. On one hand, energy transfer between different particles is made more quickly and efficiently because of the participation of the laser field; on the other hand, the effect of interparticle collisions can implement transfers which are difficult for single photon excitation, thus can obtain the expected high excited state and consequently short wavelength laser. Therefore, to study laser-induced collision process is of great significance for development of short wavelength laser sources. In addition, the ability to excite the special target level of the chosen particle permits potential applications in controlling pathways of chemical reactions.Laser induced collisions involve laser-induced collisional energy transfer (LICET) and laser-induced/assisted charge transfer (LICT/LACT). In the traditional researches on LICET, nearly all the theoretical models, based on which former numerical calculations are made, have assumed that the relative speed remains unchanged during the collision process, which obviously does not match the practical situation. Moreover, all of the traditional experimental investigations of LICT/LACT are made for mixed metal vapors, with no gas systems been reported. Aiming at these issues, the two important laser-induced collision processes are innovatively studied both theoretically and experimentally.For the theoretical research of LICET, the existing four-level model of LICET is improved considering the velocity distribution of the atoms during the collision. The distribution function of relative speed between two atoms is derived from the Maxwellian velocity distribution function under thermal equilibrium conditions, with the velocity averaged cross section obtained.On this basis, the incompletely studied Eu-Sr, Ba-Sr systems and the proposed Eu-Sr system are numerically calculated. These three systems are chosen because they are representative in that the former two systems satisfy the two limiting cases of LICET, respectively, thus can be reduced to a three-level system from a four-level one by neglecting one of their intermediate states; while the proposed Eu-Sr system can't be reduced to a three-level system because it doesn't satisfy either of the two limiting cases, and must be solved using the four-level model. Laser induced transition probabilities and collision cross sections of the three systems under different temperature and transfer laser intensities are calculated, with the characteristics of their spectrum profile interpreted from the viewpoint of potential curves of the quasimolecules.Our calculations in the weak field obtain the same spectrum profiles as former calculations, in that the spectrums are strongly asymmetrical, i.e. in the antistatic wing on one side of the line peak, the cross section falls rapidly; while in the quasistatic wing on the other side, the cross section is relatively large for a wide range of transfer laser detuning. However, calculations show that the statistic distribution of the relative speed between two atoms has dramatic impacts on the collision cross section, indicating that it is necessary to consider this distribution in the calculations. The LICET spectrum in strong field shows the following features which obviously differs from those in weak field: (a), the spectrum dramatically narrows in the strong field, and the spectral line shape becomes less asymmetric as the laser intensity increases; (b), the peak of the spectrum is shifted from the resonant frequency towards the antistatic region, with the shift increasing approximately linearly with laser intensity; (c), the peak cross section increases laser intensity and shows saturation at a high intensity.For the research of LICT, a novel one-beam Xe~+-N LICT system is proposed in this paper, which has not been reported home and abroad. Because of the special energy structure of this system, only one ~440nm laser beam is needed to complete the whole process from preparation of the initial state Xe~+ to production of N+ through LICT, which totally differs from the usual laser induced collision processes that require two laser frequencies. The corresponding two-level model for LICT is theoretically deduced, with the motion equations for the probability amplitudes obtained, and the expressions of collisional transition probability and cross section offered. On this basis, the LICT process for this Xe~+-N system is numerically calculated, obtaining transition probabilities and cross sections for different transfer laser intensities and detunings. The results for the transfer laser of ~440nm wavelength come to the conclusion that the laser induced collisional cross section is in its horizontally linear region within this wavelength region, and increases approximately linearly with increasing laser intensity, indicating the feasibility of implementing the proposed one-beam LICT process in this system.Combining the molecular beam technique and time-of-flight (TOF) mass spec- trometry, the LICT process in this Xe~+-N system is experimentally sdudied, which is to our knowledge the first report up to now. The initial state Xe~+ of the system is firstly prepared through resonance enhanced multi-photon ionization (REMPI) of atomic Xe by ~440 nm laser, and ionization spectra of Xe are measured. On this basis, the LICT process for this Xe~+-N system is experimentally realized using only one ~440nm dye laser beam. The product ions are detected by using TOF mass spectrometry, with the impacts of source pressure, laser wavelength and intensity on their intensity and yields analyzed. The experimental results indicate that collision cross section remains almost unchanged with laser wavelength and increases approximately linearly with laser intensity in the vicinity of 440nm, which is con- sistent with the calculated results, indicating validity of our theoretical calculations.