Correlated Electron-Nuclear Dynamics Of Molecules In Ultrafast Strong Laser Pulses

Molecules are composed by electrons and nuclei,of which motions are of fundamental importance in determining the chemical reactions.When molecules are exposed to ultrashort strong laser pulses,the bound electrons may absorb multiple photons from the laser field and eventually escape to the continuum or populate onto highly excited Rydberg states,leading to the photoionization or excitation of the molecules.In general,the electronic motion is accompanied by ultrafast nuclear motions.Owing to the lighter mass of the electron,the timescales of the electronic motion at sub-femtosecond or attosecond are much shorter than that of the nuclear motion which typically ranges from tens to hundreds of femtoseconds or even longer.Therefore,the dynamics of the electrons and nuclei of a molecule in the strong laser fields are usually separated according to the Born-Oppenheimer approximation.However,the electrons and nuclei are intrinsically coupled in a molecule and should be considered on an equal footing in strong laser fields.The correlated electron-nuclear dynamics,in particular,the electron-nuclear energy sharing of the absorbed photons,govern the succeeding photon-induced molecular dynamics and thus the fate of the molecule,including the asymmetric dissociative ionization,the above-threshold dissociation,the Rydberg excitation in dissociative ionization of the molecules,and molecular isomerization,and so on.Investigation of the ultrafast molecular dyanmics in strong laser fields from the aspect of electron-nuclear correlation is of great significance to reveal how the molecule absorbs photon energy,to understand the energy transfer between the electron and nuclei,and to realize the precise control of the molecular structure and material properties.By using the waveform-controlled ultrafast strong laser pulses and the multi-particle coincidence measurement of the cold target recoil ion momentum spectroscopy?COLTRIMS?,this dissertation focuses on the study of correlated electron-nuclear dynamics of molecules exposed to strong laser fields,aiming in revealing the underlying mechanism of the correlated photon energy sharing between the electrons and nuclei of the molecules and investigating the roles of the electron-nuclear energy sharing in the succeeding dynamics of the molecules,including the asymmetric dissociative single ionization,and Rydberg states excitation in frustrated double ionization of molecules.The main content and innovation of the results are summarized below.1.Investigating the correlated electron-nuclear photon energy sharing in strong field ionization of multielectron molecules.The photon energy absorption and deposition stands as the primary stage of light-matter interaction in determining the molecular photochemical reactions.Upon the essential question of how the molecule absorbs the photon energy and partitions the energy between electrons and nuclei of the molecular system,we experimentally explored the electron-nuclear photon energy sharing in above-threshold dissociative single ionization of multielectron molecules.Using CO as a prototype,vibrational-and orbital-resolved electron-nuclear sharing of the photon energy is observed for the first time.The population of numerous vibrational states of the molecular cation as the energy reservoir in the ionization process is revealed as the underlying mechanism for the electron-nuclear photon energy sharing dynamics.The participation of the multiple orbitals and the coupling of various electronic states in the strong-field ionization and dissociation processes alter the photon energy partition law between the electrons and nuclei of the multielectron molecule.The powerful electron-nuclear joint energy spectrum?JES?allows to reveal the fingerprints of the multi-orbital effect and dissociation pathway of the nuclear fragment in the molecular process and thus pave the way to understand the more complex molecular dissociative ionization dynamics.2.Revealing the photon-number-resolved directional dissociative single ionization of H2 molecules from the aspect of electron-nuclear correlation.Driven by a phase-controlled,linearly polarized two-color femtosecond laser pulse,it was experimentally demonstrated that the directional dissociative single ionization of H₂molecule depends on the total number of photons absorbed by the molecules where the photon number can unambiguously be counted in the electron-nuclear JES.We retrieved the accessibility of various dissociation pathways and their interference-induced asymmetric electron localization as a function of the total number of photon absorbed by the molecules.Since the asymmetric proton emissions are induced by the interference of various pathways with opposite parities,the ultimate asymmetry are thus related to the photon number-dependent accessibility of the involved interfering pathways.Our results revealed the important role of electron-nuclear correlation in the directional bond breaking of molecules.3.Probing and controlling of the ultrafst dynamics of Rydberg state excitation in strong-field dissociative frustrated double ionization of molecules.For molecules exposed to strong laser fields,the excited Rydberg fragments can be formed in the molecular dissociative frustrated double ionization?FDI?,in which process one the released electron is either recaptured or directly populated onto the Rydberg orbitals of the dissociating ionic core.Based on the developed technique of multi-particle?electron,ion,excited neutral Rydberg atom?coincidence measurement,we carried out the following studies by focusing on the underlying mechanism of the dissociative FDI of hydrogen molecules.Visualizing and steering electron recapture dynamics in dissociative FDI of molecules.We experimentally real-time visualize the dissociative FDI of D₂ molecule,i.e.,D₂?D⁺+D^*+e^-,by using few-cycle laser pulses?⁷ fs?in a pump-probe scheme.The laser-created ionic D⁺,freed electron as well as the excited neutral Rydberg atoms D^*ejected from the same molecule are measured in coincidence in a reaction microscope of COLTRIMS apparatus.Three internuclear distance of the stretching molecular ion are recognized to enhance the dissociative FDI at different instants.By further taking advantage of the angular streaking of an elliptically polarized few-cycle laser pulses,we monitor the momentum distributions of the detected freed electron as a function of the pump-probe time delay,which allows to deduce that the electron released in the second ionization step is preferred to be recaptured.Meanwhile,based on the knowledge of the dynamics of the dissociative FDI of homonuclear molecule H₂ and heteronuclear molecule of CO,we realize the steering of the recapture of the tunneling ionized electron to a desired outgoing ionic cores by finely adjusting the phase of a two-color laser pulse.Our results suggest that the Rydberg atom is favored to emit to the direction of the maximum of the asymmetric optical field,which open the possibility to selectively excite the neutral fragments ejected from a breaking molecule by using waveform-controlled ultrafast laser pulses.Electron-nuclear multiphoton-route to Rydberg fragments of molecules.Based on the electron-nuclear JES,we explored the electron-nuclear correlated multiphoton-route to Rydberg fragments in dissociative FDI of H₂ molecules driven by intense ultraviolet laser pulses.Ruled by the ac-Stark effect,the internuclear distance for resonant Rydberg excitation increase with the laser intensity.It alters the photon energy partition between the ejected electrons and nuclei and thus leads to distinct nuclear kinetic energy spectra of the dissociative FDI channel.As the laser intensity increases up to a certain value,the nuclear energy spectrum of the dissociative FDI channel resemble that of the double ionization channel which is generally explained by the electron recapture picture.The measurements indicate that the multiphoton excitation is general and can also explain the experimental observations of the Rydberg excitation driven by different laser intensities and wavelengths.Our findings show that the electron-nuclear correlation is crucial to explore the underlying dynamics of the Rydberg excitation.The full understanding of the physical mechanism makes it possible to produce Rydberg states with desired characteristics,and thus create the coherent quantum systems for various applications.