Coulomb Explosion Of Molecules Under Femtosecond Laser Fields With Coincidence Momentum Imaging

In recent years,the dynamics of molecular ionization/dissociation in intense laser fields has received extensive attention with the advantage of rapidly developed laser technology.Owing to the characters of ultra-short femtosecond laser pulses and high peak power,the investigation of molecular ultrafast dynamics in intense laser fields has been achieved.COLD Target Recoil-Ion Momentum Spectroscopy?COLTRIMS?realizes coincident measurement,collection of fragments in the solid angle of 4?sr,and reconstruction of ionic or electronical 3D momentum,which is a powerful measurement to explore the reaction mechanism of molecules interacted with femtosecond laser fields.In this thesis,we describe our studies on the Coulomb explosion?CE?dynamics of polyatomic molecule?OCS,NO₂,CH₃Cl?in femtosecond laser fields with the aid of COLTRIMS.First of all,we use coincidence momentum imaging technique to investigate bond-breakage-dependent dissociative multiple ionization?DMI?dynamics of asymmetric molecules in intense femtosecond laser fields with the linearly polarized?LP?at 800 nm.Two groups of CE channels from OCS^q+?q=2,3,4?along C-S or C-O bond breakage have been identified,and their kinetic energy release?KER?distributions have been obtained.Compared the relations between the nuclear distance of CE and the charge states of the parent ion;concerted enhanced multiple ionization and a ladder-climbing-type sequential ionization are respectively proposed for the C-S and C-O bond breakage cases.The potential energy curves?PEC?for those CE channels are calculated with a multistate density-function theory?MSDFT?method to deduce the theoretical KER.The overall good quantitative agreement between the experimental and theoretical results confirms the validity of our proposed mechanism and reveals possible nuclei dynamics during the DMI.In adidition,by comparing study for multiple ionization and molecular configuration of OCS by LP and circularly polarized?CP?laser fields using CE imaging,we find the nonsequential multiple ionization is only confirmed for the charge states of 2 to 4 because the energy for further ionization from the inner orbital is much larger than the maximum recollision energy,3.2 Up.Recollision also affects the yield of channels along C-S and C-O bond breakage;it is possible to adjust the electron recollision by the choice of bond breakage.When the CE occurs at high charge states of the parent ion,the electron recollision can be ignored.We find that the yields are determined not only by the charge states of the parent ion but also by electron density,charge distribution and orbital distribution.At the same time,we studied the three-body CE dynamics of OCS molecules,and found that bond angle corresponding to the CE channel in the LP laser field were greater than that of CP laser field,and with the increase of the charge states of the parent ions,the total KER difference of CE channels between LP and CP increases.The multiple ionization associated with nuclear movement function needs to consider in the CE process,ionization-induced molecular bond stretching and laser-induced molecular bond bending are different by the two kinds of polarized laser field,which leads to the difference of molecular structure configurations.The geometric alignment effect of the molecules in the LP laser field is a factor that affects the molecular configuration.We further investigate the V-shaped molecules NO₂.The NO₂molecule in the ground state absorbs one 400 nm photon to populate on²B₂ excitied state,and then the wave packet moves along the bending coordinate to the stable state.Because of the presence of a single photon resonance in the 400 nm laser field,we chose a comparison experiment with the wavelengths of 400 nm and 800 nm.We found the unimodal and multimodal strucrures of the KER for the NO⁺+O⁺channel.This is because the different population of the electron excited state selectively in different wavelengths leads to different energy spectrum resolution.For the other channels,we found the channel switching and competition.In the study of the three-body CE dynamics of NO₂ molecules,the construction of molecules in different wavelengths is very different by comparing the distribution the Newton Plot and the Dalitz Plot.Through further analysis of the relationship between the bond angles,the results in400 nm wavelength laser fields are smaller than that in 800 nm.By comparing the difference of the momentum,we find that the outer ring of the momentum is the major influential factor for smaller angle,and prove that the absorption of 400 nm photon resonantly excites the molecule to²B₂ state,and through conical intersection,the molecular structure is then transformed into a small angle in the laser field.Finally,we study the ultrafast proton migration and CE dynamics of symmetric top methyl chloride?CH₃Cl?molecules in the intense laser fields.We have experimentally studied the CE of CH₃Cl molecules in 800nm and 400 nm femtosecond laser fields.Eight two-body and five three-body CE channels including four and three proton migration channels have been observed,respectively.In two-body CE channels,the proton migration products from the CE of molecular dication and trication have been identified through the measurement of the KER distributions,the number of pathways corresponding to the channel is the main factor that affects the KER peak width of the proton migration channel.For the direct CE channel CH₃⁺+Cl⁺,we assign the bimodal structure of the KER to the contribution of multiply excited electronic states of CH₃Cl²⁺in the 400 nm laser field.Meanwhile,for the channel CH₃⁺+Cl²⁺,we point out that the different mechanisms respond to the low and high KER peaks.The low KER component of ions may come from the CE process of dication according to the similar KER distribution with channel CH₃⁺+Cl⁺,and the dissociation pathway is the dication CH₃Cl²⁺dissociates to CH₃⁺and Cl⁺in the rising edge of the laser pulse,meanwhile,Cl⁺absorbs extra photons to be ionized to Cl²⁺.The high KER component of ions may come from the direct CE process.We compared the yield of proton migration channel and compared the results with theoretical calculation,found that the main factor which determines the yield is whether the energy of the initial state is higher or lower than the potential of the barriers.The effect of laser field with different wavelengths on parent ion construction is almost indistinguishable in the study of three-body CE dynamics of CH₃Cl.It is further determined that the distribution of energy is only related to the charge state of parent ion,which is independent of the wavelengths and the numbers of proton migration.However,with the increase of the number of the charge states of parent ion and the number of proton migration,the bond angle of the molecular ion decreases,and the geometrical construction changes obviously.Our present work provides further understanding and possible manipulation of proton migration for complex organic halogen molecules in intense laser fields.