Ultrafast Rotational And Vibrational Dynamics Of A Cold Molecule And Its Interaction With The Liquid Helium

The ultrashort duration and ultrahigh intensity of femtosecond laser pulse make it a powerful tool for manipulating the dynamics of electrons and nuclei in molecules.The femtosecond duration enables the real-time observation of the ultrafast dynamics in molecules and provides insight into the fundamental physical processes in the light-matter interactions.On the other hand,the extremely high peak intensity can alter the molecular structures and modulate the electron motions,resulting in molecular rotation/alignment,vibrational excitation,isomerization of molecular structure,and further dissociation or Coulomb explosion of the molecules.The ultrafast strong field physics has become the fornties of the morden physics.Two basic dynamics of the molecules in strong laser field are the multi-states excitation of ro-vibrational states,resulting in periodic evolution of the created wave-packets.After the excitation,the wave-packet coherence in the isolated gas-phase molecular system could maintain for a long time,and the quantum revival phenomena can be observed after the conclusion of femtosecond excitation laser pusles.However,most biochemical and chemical reactions take place in the solution environments.The research on molecular vibrational and rotational behavior in solution is significant for one to understand various chemical reaction mechanisms.In a conventional liquid phase environment,the inelastic collision between solute molecules and solvent environment caused by thermal motion lead to the destruction of wavepacket coherence.In contrast to this,the superfluid liquid helium was regarded as a special solvent in which solutes can move without any resistance,namely the superfluidity.Helium nanodroplets can capture different kinds of molecules and reduce the temperature of the molecules to the order of few hundreds millikelvins by evaporation cooling,which provides a new method for studying the ultrafast dynamics of cold molecules and their interaction with the liquid helium.Also,supersonic molecular beam can provide beam source of 10K magnitude,the low temperature molecules source can be used to study the ultrafast dynamic in single molecule systems.This dissertation focuses on the precise measurements and manipulation of the light-induced vibrational and rotational dynamics of low-temperature molecules and their interaction with the liquid helium environment.The primary contents and innovation points are summarized as follows.1.Observation of laser-induced free rotation of cold D2 molecules in helium nanodroplets.When molecules are embedded in a liquid environment,the surrounding solvent molecules can slow down the movement of the solute molecules,i.e.,the solute molecules undergo resistance from solvents.Although helium nanodroplet was found to be a kind of special solvent of superfluidity,previous researches show that in helium nanodroplets,the embedded molecule would be attached with several helium atoms.This makes the molecular rotational dynamics in helium droplet differ from the gas-phase cases.To testify the scenario of of the free rotation of molecule in the superfluid liquid,here the rotational wavepacket of deuterium（D₂）molecules in the superfluid helium nanodroplet environment is launched by a femtosecond laser pulse.By detecting the time-dependent yields of the characteristic cation signal,the experimental results show that the coherence of molecular rotational wave-packets in helium nanodroplets survives more than 500 quantum periods.Also,the D₂ in helium nanodroplets share the same quantum revival period and constant with D₂ in gas phase:B_He=29.9±1.3 cm^-1 and D_He=0.013±0.060 cm^-1,demonstrating the characteristics of free rotation without resistance in superfluid helium nanodroplets.2.Observation of ultrafast decoherence of D2⁺vibrational wave packet in helium nanodroplets.When the molecules embedded in helium nanodroplets are ionized,the interaction between the charged cations and the helium environment is strengthened significantly,which affects the ultrafast vibrational motions of molecules.Therefore,studying the ultrafast vibration dynamics of molecular ions in the helium environment provides a new aspect to understand the interaction in cold complex systems.Here,the vibrational wave-packet dynamics of D₂⁺ions in helium nanodroplets and gas-phase environments were studied in comparision.Few-cycle laser pulses are used to ionize and excite D₂⁺vibrational wave packets,and the yields of the characteristic ion signal （He D⁺）emiting from the helium nanodroplets are measured as the function of the pump-probe time delays.The experimental results show that the vibrational wave packet of the charged ions in the liquid droplet undergoes rapid incoherent dephasing,and the quantum revival signal cannot be observed,while the ion vibrational wave packet in the gas-phase environment can evolve freely and retain the coherence for a long time.The interaction mechanism of D₂⁺cations with the droplet environment and its experiment results are modeled by semi-classical numerical simulation.This study shows that the light charged ion（D₂⁺）has a strong interaction with the helium environment,and reveals the ultrafast vibrational dynamics of the molecular cations in complex low-temperature system.3.Echo in a single vibrationally excited molecule.Echo is a common phenomenon,which exists in many physical systems.When the system is excited by two stimulations with relative time delay of T successively,and waiting for another delay of T for free evolution,the system will reconstruct the initial external response,i.e.,generate the echo signal.The conventional echo usually appears in the ensemble composed of mass number of particles,and the echo phenomenon is regarded as a collective response of the ensemble to external stimulation.Here,by using two femtosecond laser pulses,we manage to excite the vibrational echo wave-packet of isolated single Ar₂⁺in supersonic molecular beam.High-precision time-domain detection technology is hired to reconstruct the ultrafast dynamic of vibrational echo in the single molecular system.The Ar₂ source with low temperature（10K）from supersonic molecular beam can be used to prepare a large number of molecules with the same initial ground state,which guarantees a single molecule detection by repeating measurements.Single molecular wave-packet echo can be used to detect the decoherence process caused by the interaction of various molecular coherent wave packets with the environment,or to study the ultrafast dynamics within macromolecules.