Ionization And Dissociation Of N₂ Molecule In Intense Femtosecond Laser Field

If the laser intensity is can parable with atomic and molecular Coulomb potential, atoms or molecules will appear ionization and ionization of molecular ion will be dissociated at the interior of the Coulomb potential. The formation of fragment ions will be expressed as a spectral peak splitting phenomenon in the time of flight mass spectrometry. By studying the spectral peak splitting phe nomenon, under the action of atomic ionization and molecular dissociation of information in intense laser pulse, can be provided, and the experimental basis of molecular dissociation channels under femtosecond pulse control can be achieved.Firstly, the theoretical study of the interaction between atoms and molecules with intense laser pulse has been carried out. The phenomena of ionization and dissociation of atoms under the action of laser field are discussed respectively. The effects of laser intensity and polarization state on the ionization and dissociation of molecules are discussed.Then, to clarify the overall structure and operation principle of time of flight mass spectrometry, including the vacuum in the vacuum target chamber maintenance system and air intake system, inside the vacuum target chamber secondary electric field system, micro channel plate detection system and femtosecond laser system. In addition, the spectral peak calibration method for the time of flight mass spectrometry was also given, and the calibration of the spectral peaks of the residual air in the vacuum chamber was calibrated.On this basis, the use of repetition frequency is 1k Hz, the average laser power for 1.7w, pulse width is 50 fs, Ti: sapphire chirped pulse amplification of femtosecond lasers, and time of flight mass spectrometer, of N₂ molecules in intense femtosecond laser field ionization and dissociation were studied.The effects of femtosecond laser field pulse on the ionization and dissociation of N₂ molecule under intense laser irradiation are studied. In the mass spectrometry, it was found that the N²⁺ molecular ions produced by the ionization of N₂ molecules, and the N³⁺, N²⁺, N⁺ ion fragments produced by the dissociation of N₂. Through the study of the peak of the mass spectra of these four ions, it is found that the N⁺ ion has been found in the mass spectra when the laser intensity is greater than 4×10¹⁴W/cm². With the increase of the laser intensity, the N³⁺ and N²⁺ ions have been observed.The influence of femtosecond laser polarization direction on the ionization and dissociation of N₂ molecules is studied. When the laser polarization direction is parallel to the axial direction of the time of flight mass spectrometer, the ionization and dissociation of N₂ molecule is the most obvious, and the signal of N³⁺, N²⁺ and N⁺ ions in the mass spectra is the strongest. When the laser polarization direction turns 90 degrees, the three kinds of ion signal to the weakest, and even disappear in the noise. In addition, it was found that when the laser polarization direction turns 20 degrees, the N²⁺ molecular ion signal is the strongest. When the laser polarization direction turns 70 degrees, the N²⁺ molecular ion signal is the weakest. In addition, in the time of flight mass spectra, the spectral peaks are separated, which is the phenomenon of Coulomb explosion. When the laser polarization direction is changed, the change of the peak value of the two peaks of N³⁺, N²⁺ and N⁺ ion fragments are compared. It is found that the spectral peaks of N³⁺ and N²⁺ ions are closely related to the polarization direction of laser. When the laser polarization direction around 30 degrees, after the split of the two peak of spectrum is almost the same; when the polarization direction is less than 30 degrees, the back peak of two spectrum peaks on the spectrum peak to peak value is the larger, more than 30 degrees, the opposite is the case. And this is not observed in the N⁺ molecular ion.