Experimental Study On Ionization Of C₆₀ Molecules Under Intense Laser Field

Since the C₆₀ molecule has extremely high stability and symmetrical structure,it has been frequently selected as a model system to explore the excitation and ionization mechanism of macromolecules in a strong pulsed laser field in the past decades.The strong field excitation and ionization of C₆₀ molecules are different from that of single atoms and simple small molecule systems.Due to the existence of the multielectron activation effect and the strong electron-vibrational energy level coupling,even the laser intensity is lower than the saturation ionization intensity,other decay channels of C₆₀ molecules(such as fragmentation channels that are easy to detect in the experiment)are opened.In this case,in order to better understand the strong field excitation and ionization mechanism of such a complex molecule,it is extremely important and necessary to accurately measure its ionization and fragmentation in a wide range of laser intensities to cross its true saturation ionization threshold.Duringthe past decades,in the experiments of strong field ionization of C₆₀,traditional time-of-flight mass spectrometer which used MCPs to record charged particles was often employed to obtain laser intensity-dependent ionization and fragmentation rates,and to extract the saturation ionization and fragmentation threshold information of C₆₀ molecules.However,this kind of traditional spectrometer has suffered thermal noise from MCP detectors and disturbance from other ions with the same flight time,leading to poor signal-to-noise ratio,which made it difficult to measure the intensity dependent rates of ionization and fragmentation in very low intensity region.Consequently,the extracted threshold of saturation ionization and fragmentation could be overestimated in pervious studies.This thesis proposes a new method for measuring the intensity dependent strong field ionization and fragmentation of C₆₀ molecules based on a femtosecond laser system(wavelength of 798 nm and pulse width of 25 fs).This method uses photoion velocity imaging technology to record time sliced photoion images under different light intensities,and its photoion detection scheme uses a combination of MCP,fluorescent screen and ICCD instead of simple MCP detector in the traditional time-of-flight mass spectrometer.We demonstrated that the photoion detection scheme has better signal-to-noise ratio and a unique advantage in identifying the photoion species.Based on this method and using the strictly collimated C₆₀molecular beam,we made systematic measurements on the ionization and fragmentation rates of C₆₀ molecules in the light intensity range of(0.3-6.7)?10¹³W/cm².It is demonstrated that this method allows us to extend the experimental measurement of the intensity dependence of the ionization and fragmentation of C₆₀molecules to a lower light intensity region that was never been touched by previous studies.The saturation intensity and the fragmentation threshold values corresponding to the single ionization,double ionization and triple ionization of C₆₀molecules determined in the present work are much lower than those of the previous experiments,however,could be well described by the theoretical predictions based on the conducting sphere model.The experimental results obtained in this thesis provide some new insights for understanding the strong field excitation,ionization and fragmentation mechanisms of C₆₀ molecules.At the same time,the new method proposed for studying ionization and fragmentation of C₆₀ molecules in this thesis could benefit the experimental investigation of the ionization and fragmentation of big molecules in strong laser field.