Monitoring The Molecular Rotation And Nuclear Motion Via High-order Harmonic Spectroscopy

Studying the ultrafast dynamics of molecules provides crucial insights into chemical reactions,guiding advancements in chemical production and drug design.When matter interacts with femtosecond laser pulses,it generates high-order harmonics,which involve three distinct electron processes: ionization,acceleration,and recollision under strong laser fields.This unique feature enables high-order harmonic spectroscopy to probe molecular rotation,nuclear motion,and electronic motion with sub-femtosecond temporal resolution and subangstrom spatial resolution.In high-order harmonic experiments of molecules,molecular alignment serves as a crucial link between the molecular and the laboratory frame.However,it is impossible to achieve perfect molecular alignment in experiments,and the resulting high-order harmonic spectra are always the coherent superposition of the single-molecule radiation within the molecular axis distribution range.Consequently,the information carried by the high-order harmonics is blurred by the averaging effect.Therefore,to achieve singlemolecule ultrafast measurements based on high-order harmonics,this paper proposes two methods for accurately measuring molecular alignment and decoupling the harmonic signal of aligned molecules to obtain molecular nuclear motion processes at the single-molecule level.The specific details are as follows:(1)A method for full-optical measurement of molecular rotational dynamics based on angle-resolved high harmonic generation is proposed.By measuring the angular distribution of high-order harmonics at different delays,the real-time spatial distribution of molecular rotational wave packets is achieved.Since the experimentally measured high-order harmonics are coupled with the rotational response of the molecule and its inner electronic response,the simulated annealing algorithm from machine learning is adopted to deconvolute the experimental high-order harmonic signals to achieve the measurement of the amplitudes and phases of the molecular coherent rotational states after laser excitation.The evolution of the coherent rotational state amplitudes and phases clearly reflects the stimulated Raman transitions that occur during the molecule-laser interaction,as well as the field-free evolution processes experienced after the interaction.The coherent contributions from different rotational states are superimposed to further achieve a femtosecond time-resolved ”movie”of molecular rotation.This method combines machine learning algorithms with high-order harmonic measurements,laying a foundation for the disentanglement of single-molecule information in subsequent research.(2)We propose a method for in-situ measurement of molecular rotational temperature and pump intensity via high-order harmonic spectroscopy.Through theoretical analysis of high-order harmonic signals emitted by aligned molecules with various rotational temperatures and pump intensity,we found that the arising time of the local extrema at 1/2 and 1rotational revivals have opposite dependencies of rotational temperature and pump intensity,while the arising time of local extrema at 1/4 rotation revival are independent of rotational temperature and pump beam intensity.In the experiment,we adjusted the nozzle pressure and the distance between the nozzle and the laser axis to measure the corresponding highorder harmonic signals.By correcting the time delay based on the arising time of local extreme values at 1/4 rotation period and measuring the molecular rotational temperature and pump intensity based on the arising time of local extrema at 1/2 rotation period,we successfully measured the corresponding rotational temperature and pump intensity under different conditions.The results indicated that the molecular rotational temperature decreases with increasing pressure and diffusion distance.Such method has a certain degree of flexibility in practical use,as it only requires the occurrence of two arising times of the local extrema that have opposite dependencies of rotational temperature and pump intensity.An ultrafast measurement of the nuclear motion processes of hydrogen(deuterium)ammonia molecules with high spatiotemporal resolution is achieved.In the experiment,we measured the high-order harmonic spectra from aligned hydrogen(deuterium)ammonia molecules,and ascertaining the molecular alignment via the method outlined in(2).The projections onto convex sets algorithm was subsequently utilized to decouple the high-order harmonic spectra,facilitating the reconstruction of the angle-resolved single-molecule induced dipole moment.Our results evince that the structural rearrangements induced by the molecular nuclear motion have a profound impact on the angle-resolved amplitude of the induced dipole moment,which,in turn,furnishes the requisite groundwork for extracting the structural parameters from the reconstructed induced dipole moment.Subsequently,we derived a complete nuclear motion process of hydrogen(deuterium)ammonia molecules at laser fields of 800 and 1300 nanometers from the reconstructed induced dipole moment.This result differs from previous studies of nuclear motion measurements via high-order harmonic spectroscopy which relied on isotopic molecule,thereby presenting a novel approach for detecting the molecular nuclear motion of a more intricate molecule.