Ultrafast Dissociation Dynamics Of N₂O Cations In Strong Laser Fields

The research of ultrafast dissociation dynamics of molecular ions has great significance in atmospheric physics,interstellar physics and femtosecond chemistry.Thus,the real-time measurement of ultrafast evolution of molecular structure has attracted extensive attentions from researchers.The femtosecond laser has become one of the most important technical tools for studying ultrafast dissociation dynamics of molecular ions because of its properties of the ultra-short pulse duration and extremely intense peak power.However,the dynamic behavior of molecular ions under the femtosecond laser field is very complicated because the inter affect from different dissociation channels.The COLd-Target-Recoil-Ion-Momentum Spectroscopy?COLTRIMS?has the ability of complete differential coincidence momenta measurement,and it can record the accurate experimental data of all products from various dissociation channels and assign the specific reaction channels.Therefore,the time-resolved Coulomb explosion imaging based on pump-probe and coincidence measurement has been used to imaging the ultrafast evolution of molecular structures and tracking the ultrafast dissociation dynamics of molecular ions.It is well known that N₂O⁺is the intermediate product of the important reaction O⁺+N₂?NO⁺+N in the upper ionosphere of our earth.The dissociation and predissociation dynamics of N₂O⁺have been extensively studied both experimentally and theoretically.In this dissertation,we study the ultrafast structure evolutions and kinetic energy correlation of singly and doubly ionized N₂O molecules by using pump-probe techniques combined with coincidence measurement.Furthermore,the corresponding ultrafast electron distributions during the two-body dissociation process of N₂O⁺have been tracked by the ion momentum distribution and the electron momentum distribution.In the third chapter,we describe the study for the ultrafast dissociation of N₂O⁺and N₂O²⁺by using the same experimental mthods.We employed a linearly polarized femtosecond laser as the pump to ionize N₂O molecules into the N₂O⁺and N₂O²⁺,and then bring another elliptically polarized probing laser to further ionize the N₂O⁺and N₂O²⁺to N₂O³⁺,which is unstable and will breaks up into N⁺+N⁺+O⁺.The valence state of ions can be distinguished directly from the measured momentum distribution of these product ions by the elliptically polarized probe laser,giving the time-dependent kinetic energy from the dissociation of N₂O⁺and N₂O²⁺channels.The fast evolution of KER spectra observed indicates that the dissociation most probably occurs from B₂?47?state,and the analysis of the time-dependent Dalitz distributions reveals that significant bending takes place prior to the dissociation of N₂O⁺on the B₂?47?state.The time resolved kinetic energy correlation maps show that the concerted fragmentation dominates at short delay time while the contribution of sequential fragmentation increases with time delays.From these measurements,two two-body?N₂⁺+O⁺and N⁺+NO⁺?and two three-body?N+N⁺+O⁺and N⁺+N⁺+O?fragmentation pathways of N₂O²⁺are assigned.The different ion-ion energy correlations and fragmentation processes from two-body and three-body dissociation are observed by measuring Dalitz distribution of ions under different delay time and different kinetic energy.It is believed that the time-dependent coincidence measurements will open up avenues for the study of not only the static structure but also dynamic information of nonstationary states of even more complicated molecules.In the fourth chapter,we focus on the study for the evolution of the ultrafast electron distribution during the ultrafast dissociation process of N₂O⁺.The pump laser ionizes the molecules to N₂O⁺and the probe laser further ionizes the ions to N₂O²⁺.The time dependent kinetic energy release of NO⁺+N⁺and N₂⁺+O⁺channels are obtained through scanning the time delay between pumping and probing lasers.The time dependent electron momenta equals to the ion momenta distribution because of the conservation of momentum between ions and electrons.According to strong field molecular ionization model,the electron momentum distribution is related to their distribution on the molecular orbitals.The experimental results show that the electron momentum distribution gradually changes from a single-peak to a double-peak structure with increasing delay time.This double-peak structure mainly appears at large delay time?300-500 fs?,which comes from the ionization after the molecular ion dissociates into single atoms,and whereas the single-peak structure mainly comes from the ionization of molecular orbitals.The observation also indicates the asymmetry of the molecular orbitals changes with the delay time.It is believed that the time-dependent coincidence measurements combined with ultrafast femtosecond laser can observe the ultrafast electron evolution in the process of molecular dissociation in real time.