Ionization And Dissociation Behavior Of Formic Acid Molecules In Intense Laser Fields

Ionization and dissociation behavior of theoretical and experimental research has recently become a hot issue, especially for the theory of sequential multi-photon double ionization and non-sequential double ionization which has been more mature than before. After the research of some universal diatomic molecules, we hope to begin with the research regarding on the polyatomic molecule. As for formic acid, it is a multi-atom molecule, but the molecular structure is not very complicated, so it can be a good research bridge between the diatomic molecule and polyatomic molecule.In this thesis, we focus on the research of ionization dissociation behavior of the formic acid molecule in the laser of 400nm and 800 nm, and the intensities region of 80-240 TW /cm2. In this study, we use three-dimensional velocity map imaging device to measure the coincidence of double ionization dissociation of formic acid molecule. We got photoion-photoion coincidence map, and by processing experimental data, we obtained time of flight mass spectra, the energy distribution, momentum distribution and angular momentum distribution.By analyzing the model of Coulomb explosion of molecule in the laser field and the mechanism of ionization dissociation, and by understanding of double ionization dissociation mechanism of molecule, then with a variety of image we got through data processing, we mainly get the following information:By calculating the KELDYSH factor of Formic acid molecule's ionization in 400nm and 800nm laser field, formic acid molecules by this two wavelengthsand six kinds of light intensity, the results show that the multi-photon ionization is the main mechanisms. Only in the wavelength of 800nm and light power of 80mw, the KELDYSH coefficient is close to 1, what we can conclude is that both the multi-photon ionization and field induce ionization are the main mechanism.We know from the photoion-photoion coincidence map, in the circularly polarized laser pulse and linearly polarized laser pulse, the formic acid molecule dissociation channels do not change. So we can know that double ionization of formic acid mainly is sequential ionization. Through the analysis of kinetic energy of ion pair of formic acid and the Coulomb potential model of formic acid, when we put the experimental measurement of ion kinetic energy values into the Coulomb potential model to calculate the bond length, this bond length we got is greater than the theoretical calculation of bond length. Thus we conclude that the main ionization of formic acid is enhanced resonant ionization.By analysis of three-dimensional angular distribution of momentum of formic acid ion pair, we can obtain that the angular distribution of these ions is mainly along the direction of laser polarization, and the distribution did not change with the light intensity, we can get that the intense laser field have a certain alignment effect to formic acid's ionization.All in all, what this thesis mainly concerns is double-ionization dissociation behavior of the formic acid molecules in the laser field, and we analyze secondary ionization mechanism of formic acid, ionization and dissociation mechanisms, the distribution of kinetic energy and momentum, and three-dimensional angular distribution of momentum. I hope the work will be helpful with the future research of more complex polyatomic ionization dissociation behavior.