The Ionization And Dissociation Processes Of Polyatomic Molecules With Different Structures In The Femtosecond Laser Field

With the development of modern science and technology, many newstrong field physics processes arise in the interaction between ultrafastintense laser with atoms and molecules, such as High HarmonicGeneration, Above Threshold Ionization and Non-Sequencial DoubleIonization. As the basic strong field physics process, the ionization anddissociation of polyatomic molecules have atttract a lot of experimentaland theoretical attention. In addition to the laser intensity, wavrlength,pulse width and ellipticity which affecting the ionization and dissociationprocesses of polyatomic molecules, molecular structure, size and thecharacters of the electron orbit also affect the ionization probability andthe degree of fragmentation of polyatomic molecules. Compared withatoms and diatomic molecules, the research on the ionization anddissociation of the polyatomic molecules is relatively rare, and theunderstanding of its physical mechanism is still insufficient.In this dissertation, we investigate the ionization and dissocitation ofpolyatomic molecules with similar structure using a time-of flight massspectrum. The difference of ionization and dissociation among thesemolecules is studied under different laser intensity, ellipticity andwavelength(near IR(800nm) and UV(400nm)). We got the following main results:1) A comparative study of the ionizaiton and dissociation of lineartriatomic molecules CO2, CS2and OCS is made under800nm and400nmlinear polarized laser field. We measured the dependence of yield ofparent ions and fragment ions on laser intensity, and analized thedissociation channel of fragment ions by considering the appranceintensity and the laser intensity dependence of the fragment ions yield. Itis found that a characteristic ‘knee’ structure is shown in yield of CO2+2versus laser intensity, combine with the measure of yield versus laserellipticity, we confirm the non-sequencial double ionization in CO2. Inaddition, we also measured the dependence of yield ofON+(N=1,2),CN+(N=1,2) and CO+on laser intensity and ellipticity. Ourresults show that the electron recollision caused by laser field still play animportant role in double ionization, multiple ionization and dissociationof polyatomic molecules.2) We investigated the ionization and dissociation of benzene,toluene and chlorobenzene in800nm and400nm linear polarized laserfield. At low laser intensity, parent ions are dominant for all these threemoleucles in800nm laser field, while fragment ions are400nm laser field.Our results have shown that the resonant photoabsorption of themolecular cations is the main reason of the extensive fragmentation instrong field ionization of polyatomic molecules. And with theincreasement of laser intensity, the degree of the fragmentation of thethree molecules increased obviously no matter what laser wavelength, the infrared800-nm or the ultraviolet400-nm. In addition, we also measuredthe saturation intensity of these three molecules, and the results show thatthe aromatic molecules are more difficult to be ionized than the atomswith the same ionization potential.3) Using800nm laser as a pump light, using400nm laser as the probelight, combined with a time of flight mass spectrum, we investigated thedynamics of cyclopentanone molecular ion. By adjusting the pump andprobe laser intensity, we find that the cyclopentanone molecular ionwhich produced in800nm laser field will resonance absorb a400nmphoton and result in fragment ions. We also measured the yield ofmolecular ion and fragment ions as a function of delay time betweenpump and probe light under different pump and probe laser intensity. Andthe mechanism of fragment ions were discussed. Besides the resonanceabsorption effect in cyclopentanone molecular ion, our results also showthat the dynamic information of molecular ion can be obtained using thisresonance absorption effect combining ultrafast time resolvedpump-probe method.