Ultrafast Ionization And Rotational Dynamics Of Molecules In Strong Laser Fields

The investigation of ultrafast molecular dynamics is of great importance towards the understanding of a variety of natural phenomena in physical and chemical sciences.With the rapid development of femtosecond laser systems and precision detection technologies,it is possible now to visualize and steer the motion of molecules in matter as well as the ultrafast dynamics of electrons and nuclei in molecules on a microscopic timescale.When a molecule is exposed to a strong laser field,the electrons in molecule can be freed or excited,and often followed by molecular dissociation,in which the electrons and nuclei exhibit a strong correlation,while the electronic motion on attosecond timescale is much faster than that of the nuclei ranging from femtosecond to picosecond timescale.One of the examples is the absorbed photon energy shared by the electron and nucleus in molecular dissociative ionization process.In addition,a lot of physical processes such as the electron tunneling ionization have a strong dependence on the spatial orientation of molecular axes with respect to the polarization of the incident laser field.Confining molecular axes to a specific direction is significant in unambiguously understanding and control the ultrafast response of the molecules to the laser fields.The field-free molecular alignment,stimulated by the intense femtosecond laser fields,is recognized as a powerful method to control the spatio-temporal distribution of the molecular axis and serves as an important tool for studying the rotational dynamics of complex molecules and ultrafast collisional dissipation of dense gas medium.By using the waveform controlled strong femtosecond laser fields,and the advanced detection technologies of multi-particle coincidence measurement of Cold target recoil ion momentum spectroscopy and time resolved birefringence detection,this thesis investigates the ultrafast Rydberg excitation,dissociation,rotation and collision dynamics of molecules.The main contents are summarized below.1.Dissociative Rydberg excitation of molecule in strong laser fieldThe generation of Rydberg nuclear fragments in molecular dissociative ionization processes are accessed via either the frustrated tunneling ionization or the multiphoton resonant excitation.Different characteristics of these two mechanisms in kinetic energy release spectra are observed and studied in this thesis.First,the dissociative frustrated multiple ionization of HCl molecules in near infrared femtosecond laser fields are experimentally investigated,in which processes the tunneled electrons are recaptured by the outgoing ionic nuclear fragments.Several dissociative frustrated multiple ionization channels by trapping one or two released electrons to highly excited Rydberg states are observed.The formed excited nuclear fragments,depending on their principle quantum numbers,can be either directly detected in the photoion-photoion coincidence spectrum and forming a“knee”structure in the kinetic energy release spectrum,or singly ionized by a weak static electric field of the spectrometer after the strong laser pulse and be identified in the photoelectron-photoion coincidence spectrum.The kinetic energy release(KER)spectrum and momentum angular distribution of dissociative Rydberg excitation channels are similar to that of the corresponding dissociative ionization channels,which results from the electron recapture occurs at the end of the laser pulse,and thus has almost no influence on the nuclear kinetic energy.Second,the multiphoton dissociative Rydberg excitation(DRE)of O₂ molecule in ultraviolet femtosecond laser field,forming an O and an excited O^*fragments,is experimentally observed by studying the photoelectron-photoion coincidence spectrum.The discrete peak positions of the sum kinetic energy releases spectrum of the ejected electron and nuclear fragments of the DRE channel are measured to be similar to that of the dissociative single ionization(DSI)channel.However,their nuclear kinetic energy spectra are distinct which is inconsistent with the frustrated tunneling ionization mechanism.The experimental results indicate that the absorbed photon energy above threshold is mostly deposited to the nuclear kinetic energy in the DRE channel.While the released electron will take away a part of the absorbed photon energy in the corresponding dissociative ionization channel,resulting in a smaller nuclear kinetic energy as compared to the DRE channel.By analyzing the kinetic energies spectra,the multiphoton resonance excitation is recognized as the mechanism in accessing the DRE channel,which generally occurs when the energy gap between the excited and ground states matches the energy of one or sum of several photons.Moreover,different pathways towards the DRE and DSI are identified based on the correlated dynamics of the ejected nuclear fragments and electrons.2.Tunneling-site-sensitive ultrafast dynamics of molecule in strong laser fieldAs compared to atoms,on the one hand,the dissociation accompanying multiple excitation and ionization often occurs in molecules excited by ultrashort laser pulses.On the other hand,these ultrafast processes have a strong dependence on the electron tunneling site in molecular frame.In this thesis,the electron tunneling site sensitive ultrafast dynamic in molecular dissociative ionization processes are experimentally explored.First,molecular bond breaking is one of the key ingredients for light-induced chemical reactions.Here,the asymmetric dissociative double ionization of HCl molecules with respect to the molecular orientation and instantaneous laser field vector are experimentally revealed by employing the angular streaking technique and analyzing the asymmetric sum momentum spectra of the ejected nuclear fragments as a recoil of the ejected electrons in elliptically polarized laser pulse.Three different dissociative pathways involving various molecular orbitals of HCl are distinguished.The experimental results show that the HCl molecule is preferred to be ionized when the laser field vector points from H to Cl,which is not consistent with the expectation by a tunneling model associated with the laser induced Stark shifts of the HCl and HCl⁺.Our results indicate that the asymmetric dissociative double ionization of HCl molecules are mainly governed by the profiles of multiple molecular orbitals.For complex triatomic molecules,the dissociative double ionization of N₂O molecule,including denitrogenation and deoxygenation channels,is experimentally investigated by using the similar procedure.Different pathways towards the denitrogenation and deoxygenation channels,involving various bound and repulsive states by removing electrons from different molecular orbitals,are uncovered by examining the momentum angular distributions and asymmetric sum momentum spectra.The experimental results demonstrate that the asymmetric dissociative double ionization of N₂O molecules are governed by the detailed potential energy curves,the profiles of the molecular orbitals,and the electron localization-assisted enhanced ionization of the stretched molecules.Second,the strong laser field tunneling ionization induced transient valence charge localization and its ultrafast intramolecular motion are explored and visualized.Taking the dissociative single ionization of HCl as a research system,when the valence electron tunnels out with an exit near H(or Cl),an initial optical-field-induced transient charge is localized on H(or Cl)site and subject to subsequent rearrangements.The asymmetric transient charge localization,depending on the electron tunneling site in the molecular frame,is encoded in the asymmetric momentum of the outgoing proton fragments acquired from the strong laser field.Combining the angular streaking technique and axial recoil approximation,the electron tunneling site dependent asymmetric momentum angular distribution of outgoing proton fragments is clearly observed,which allows one to reveal the asymmetric initial transient charge localization.3.Ultrafast rotational dynamics of asymmetric-top acetone moleculeControlling molecular axes to a specific direction is a long-standing goal in ultrafast physics,which enables to distinguish the molecular orientation effect on the measured results.Impulsive alignment induced by ultrafast laser pulses with field-free revivals is considered as a powerful method to control the spatio-temporal distribution of the molecular axes.Field-free molecular alignment is also recognized as an important tool for studying basic physical processes such as rotational dynamics of complex molecules and collisional dissipation.However,the achievement of molecular alignment for non-linear molecules is a much more challenging task,especially in the case of asymmetric-top molecules.In this thesis,the ultrafast rotational dynamics of asymmetric-top acetone molecule in gas phase in intense femtosecond laser fields is investigated by using molecular alignment and rotational echoes.Field-free alignment revivals of acetone molecule in gas phase created by a linearly polarized laser field are observed in the experiment.By combining the measurements and calculations and comparing with the CO₂ molecule,the observed very weak alignment revivals are attributed to large molecular asymmetry,ultrafast collisional decay and multiphoton resonant dissociation.The field-free alignment degree of acetone is improved by using rotational alignment echoes excited by a pair of time delayed intense femtosecond laser pulses.As compared to the linear molecules,the fractional alignment echoes are observed in the sensitive birefringence measurement,which can be used to study the nonlinearity of complex molecular system.Moreover,the third-harmonic generation from a circularly-polarized fundamental laser pulse in a sample of aligned acetone molecules is demonstrated.The experimental findings are fully supported and reproduced by classical molecular dynamics simulations and quantum calculations.4.Ultrafast collisional dissipation probed by molecular rotational alignment echoThe measurement of the molecular rotational dynamics in dense gas media,where interactions between atoms and/or molecules cause a rapid relaxation of the system,have attracted much attention in the past few years.In this thesis,the ultrafast collisional dynamics of molecules in dissipative media are explored by using molecular rotational alignment echoes.First,the ultrafast collisional dissipation of the linear CO₂ and the symmetric-top C₂H₆ molecules in pure gas and diluted in helium at high pressure(for which field-free alignment revivals are unobservable)are successfully studied by using the molecular rotational alignment echoes produced in the first few picoseconds after the laser interaction(before the appearance of the first alignment revival).The reduced amplitude of the rotational alignment echoes with increasing the time delay between the two pump laser pulses,reflecting the collisional relaxation of the system,are clearly observed by probing the transient birefringence of the medium.The measured decay time constants of the rotational alignment echoes are in good agreement with the purely classical molecular dynamics simulations.Our results demonstrate that alignment echoes enable the observation of extremely fast collisional dissipation occurring in few picoseconds for linear and symmetric-top molecules,and potentially for asymmetric-top molecules and liquid.Second,nonsecular dynamics in ultrafast relaxation induced by thermal collisions are successfully unveiled by using rotational alignment echoes.The rotational decoherence of N₂O molecules diluted in helium mixtures created by two successive nonresonant intense femtosecond laser pulses are probed by analyzing the reduction of amplitude of the density-normalized alignment echoes with increasing the gas density at fixed time delay.The decay time constants at different times ranging from 2.8 to 17.6ps are measured.By interrogating the system at the early stage of its collisional relaxation,a significant variation of the collisional decay time constant with the time of appearance of the rotational alignment echoes is observed,featuring a decoherence process that is well reproduced by the nonsecular equations for modeling molecular collisions.Our results indicate that the transfers among coherences as well as between populations and coherences in the nonsecular regime reduce the decoherence rate of the system lasting for a few picoseconds.