Tracking Simple Molecular Ionization And Nonadiabatic Processes With Strong-Field Ultrafast Spectroscopy

In the past three decades,significant progress has been made in the study of the interaction between intense femtosecond laser fields and atoms/molecules.A intuitive physical picture based on strong-field tunneling ionization of atoms and molecules has been established,explaining important experimental phenomena such as high-order above-threshold ionization,nonsequential double ionization,and high harmonic generation.These groundbreaking research achievements have directly advanced the development of fields like femtochemistry and attosecond physics.The physical principles and new methods derived from these studies have facilitated the development of high spatiotemporal resolution ion and electron spectroscopy techniques to characterize and manipulate the properties of femtosecond lasers,and to explore the dynamic evolution of molecular structures and electronic states.This has become a significant research area at present.Ultrafast femtosecond lasers play an indispensable role in driving molecular femtosecond dynamics and generating attosecond laser sources.The precise control of femtosecond laser properties,such as pulse duration,phase,and waveform,holds great importance in controlling strong-field ultrafast processes in atoms and molecules.However,the in situ characterization of femtosecond laser phase information faces challenges.The resolution of this issue is expected to provide new avenues through experimental schemes based on strong-field tunneling ionization of atoms and molecules using photoelectron/photoion spectroscopy.Furthermore,when studying strong-field ultrafast processes in multi-atomic molecular systems,it is often necessary to consider the dynamics of multiple electronic states,as well as the vibrational,rotational,and other degrees of freedom,along with their couplings.In particular,molecular nonadiabatic dynamics,which go beyond the Born-Oppenheimer approximation,occur within the femtosecond time scale and present significant challenges.Given these circumstances,the main objective of this research is to develop new high spatiotemporal resolution spectroscopic methods to characterize and manipulate the properties of femtosecond lasers,acquire transient structures of molecular dynamics,and uncover the nonadiabatic dynamic evolution processes and physical mechanisms in multi-atomic molecules,using hydrocarbon molecules as examples.To achieve this goal,we propose and develop novel experimental approaches,including the combination of femtosecond pulse shaping with cold target recoil ion momentum spectroscopy,the extension of laser-induced electron diffraction imaging,and the implementation of various techniques such as strong-field above-threshold ionization electron dynamics twodimensional spectroscopy and time-resolved Coulomb explosion spectroscopy for detection.Based on these efforts,we successfully reconstruct the phase of ultrashort laser pulses in situ using tunneling ionization processes,achieve tomographic imaging of N2+ using elliptically polarized laser-induced electron diffraction,and reveal the significant role of nonadiabatic dynamic mechanisms in the photoionization-induced neutral H2 generation process.Specifically,we first combined a pulse shaping laser system with a cold target recoil ion momentum spectroscopy to study the femtosecond pulse phasedependent strong-field tunneling ionization yield evolution of atomic and molecular pairs(Xe/O2 and Ar/N2).This allowed for the precise in situ measurement of their phase evolution and enabled successful compression and optimization of the laser pulse.Precise control of the spectral phase to achieve pulse compression is a challenging task in the field of ultrashort pulse laser generation,as in situ measurement of ultrafast pulse laser spectral phase poses significant difficulties,and femtosecond lasers inevitably introduce dispersion effects through optical medium.Although pulse shaping techniques have been widely applied for phase control,the relationship between spectral phase and observables lacks a clear physical explanation.By applying a π phase using pulse shaping techniques and scanning the wavelength of this phase step,we induced tunneling ionization in atomic and molecular systems and obtained the tunneling ionization yield spectra and photoelectron momentum spectra dependent on the wavelength of the phase step.We discovered novel oscillatory structures in the tunneling ionization yield spectra.Based on the strong-field tunneling ionization theory,we established the correspondence between the laser spectral phase and the wavelength-dependent tunneling ionization yield with phase steps.Benefiting from the high-order nonlinear effects of tunneling ionization,we achieved precise in situ measurements of weak off-central wavelength phase evolution in laser spectra.Furthermore,based on the measured spectral phase,we selectively controlled the spectral phase using pulse shaping techniques,leading to pulse compression and optimization of the temporal waveform of the femtosecond laser used in the experiments.Finally,we demonstrated that restricting the laser spectral phase within a range of π also enabled pulse compression of broadband few-cycle lasers.Next,we proposed an ultrafast molecular tomography imaging method based on a pump-probe scheme within an optical cycle,namely elliptically polarized laser-induced electron diffraction imaging.This method enables the tomographic imaging of the transient bond length of N2+ by extracting the elastic differential scattering cross-section of N2 molecules with nonadiabatic alignment under different ellipticities.Conventional laser-induced electron diffraction methods typically use linearly polarized laser as the probing laser,resulting in electrons with fixed backscattering angles.This approach makes it challenging to extract the differential scattering cross-section of complex molecules and perform tomographic imaging in the molecular coordinate system.Therefore,we developed the elliptically polarized laser-induced electron diffraction imaging method,which adjusts the ellipticity(0 to 0.3)of the probing laser to track and control the backscattering angle of the rescattered electrons.This angle,often difficult to quantify due to the uncertainty in the initial momentum of tunnel-ionized electrons,was tracked and analyzed using high-energy photoelectron momentum spectra.We successfully achieved control over the backscattering angle of rescattered electrons ranging from 0° to 20° corresponding to different ellipticities.Using the elliptically polarized laser-induced electron diffraction imaging method,we simultaneously extracted the elastic differential scattering cross-section with angular resolution of rescattered electrons for spherically symmetric system(Xe and Ar atoms)and linear N2 molecules.Finally,combining experimental measurements with theoretical calculations,we reconstructed the transient bond length of N2+through tomographic imaging.Lastly,we developed spectroscopic methods for photoelectron energy spectra,two-dimensional photoelectron momentum spectra,and joint spectroscopic analysis of femtosecond time-resolved ion yield and kinetic energy release spectra.These methods allowed for systematic study of the ionization/dissociation ultrafast dynamics and nonadiabatic processes in C2H6 molecules.For certain typical polyatomic molecules like C2H6,which exhibit pronounced multi-orbital effects,ionization of different orbital electrons leads to population in different electronic states.Due to electron-nuclear coupling and nonadiabatic processes,each electronic state contributes to various reaction channels,making the decoupling of this process more complex.Experimentally,by developing decoupling analysis of mixed photoelectron energy spectra and momentum spectra involving multiple states,we identified fingerprint features sensitive to orbital symmetries and electronic state energies.This unveiled the initial population of ion ground and excited states formed through tunneling ionization and Freeman resonance ionization mechanisms,respectively,and provided branching ratios for nonadiabatic reaction channels(C2H6+→ C2H4++H2).Furthermore,during the aforementioned decoupling process,we discovered that the anomalous high production of H2 molecules in the experiment originated from nonadiabatic coupling between the ionic ground and excited states,leading to a hydrogen transfer channel.The formation of neutral H2 molecules is commonly studied theoretically based on adiabatic Born-Oppenheimer approximation.However,the fast motion of the H atom during the reaction process often breaks the Born-Oppenheimer approximation,resulting in nonadiabatic coupling between different electronic states.Through pump-probe techniques,utilizing femtosecond time-resolved spectroscopic information such as the yield of different Coulomb explosion channels and ion kinetic energy,we real-time tracked the nonadiabatic evolution of vibrational wave packets generated after strong-field ionization of C2H6 molecules and observed that the nuclear wave packet reached the conical intersection region at around 1300fs.