Photoionization/Photodissociation And Coulomb Explosion Studies Of Diiodomethane In Intense Femtosecond Laser Field

The interaction of intense laser radiation and molecules has been focused at all times. As for the femtosecond laser, the intensity of electronic field is close to or exceeds the coulomb field between the valence electrons and nucleus in molecules, many interesting phenomenon appear, such as multiphotons ionization; field ionization and coulomb explosion. Otherwise, the fragmentation and ionization mechanism of polyatomic molecules in intense laser field can be varied rather widely by different irradiation parameters: intensity, wavelength, polarization, pulse width et al. Therefore, the influence of laser parameters also attracts a great deal of researchers.In this work, the ionization/dissociation and coulomb explosion process of polyatomic molecules CH2I2 irradiated by intense femtosecond laser has been studied both at different wavelength(400nm&800nm), intensity(ranging from1014 to 1015W/cm2) and polarization(linearly &circularly)using TOF MS.The structure-based model is utilized to derive the modified adiabatic parameter valuesγbased on the calculated molecular electrostatic potential energy surface. It is verified that under the conditions of our experiment, the dominant ionization mechanism of CH2I2 molecules is the field ionization mechanism. Because the time of electrons tunneling barrier is shorter than that of molecular dissociation, it is expected that mechanism of ionization/dissociation in the fs laser field is ionization followed by dissociation. Therefore the cations are still be under the laser field and can further dissociate or coulomb explosion to produce fragmentations. Through the analysis of the mass spectra and the dependence of ions on laser intensity, we conclude that the singly charged fragmental ions had a similar dependence on laser intensity indicated that they come from the same dissociation channel and mainly can be attribute to the direct dissociation of the single charged parent ions. While the peak splitting of the multiply charged ions provides a signature of Coulomb explosion showed that they originated from Coulomb explosion of highly charged molecular ions.By measuring the time differences of the split peaks in the mass spectra, we can obtain the kinetic energy distribution of high charged I atomic ions. It shows that the experimental measured kinetic energies for Im+(m=2,3) is nearly constant with the laser intensity under our conditions. They were larger than Liu group. The formation of atomic ions and the duration of laser pulse et al can both attribute to the different kinetic energies. By rotating the half-wave plate, we could change the linear polarization vector of the laser beam relative to the TOF axis. Thus, we could obtain the angular distribution of the ions by measuring the ion intensity at different laser polarization angles. The singly charged ions were characteristically isotropic, while the high charged atomic ions are highly anisotropic, having a maximum along the laser polarization and a minimum perpendicular to it. The anisotropic angular distributions of atomic ions coming from Coulomb explosion can be attributed into two effects: dynamic alignment and geometric alignment. Using several different methods, we concluded that the geometric alignment mechanism was responsible for the alignment of CH2I2.