Probe And Control Ultrafast Rotational Dynamics Of Molecules

With the rapid development of ultrafast laser and particle detection technology,the precise probe and control of microscopic dynamics at the atomic and molecular level is becoming possible.Ultrafast processes like ionization and dissociation strongly depend on the spatial orientation of molecular axes,while they are usually randomly distributed due to the thermal motion of molecules.The random distribution of the molecular axes introduces detrimental average effect on the detection results.Align or even orient molecular axes in space using ultrashort laser pulses can effectively avoid the average effect and obtain pure molecular response signals.Therefore,the laser-induced molecular alignment and orientation are important for studying many fundamental physical problems,including molecular orbital and structural imaging,chemical reaction control,and nonlinear optical manipulation.The laser-induced molecular alignment process itself also provides an important method for studying the rotational dynamics of molecules.The coherent rotational wave packet excited by the ultrashort laser pulse shows alignment and anti-alignment periodically under field-free condition,which greatly expands the practical application of the molecular alignment.At the same time,the multi-particle coincidence measurement of cold target recoil ion momentum spectroscopy provides a powerful tool for directly observing the spatiotemporal evolution of molecular dynamics.This dissertation focuses on the precise measurement and control of the complex dynamics of the molecular rotational wave packet driven by ultrashort laser pulse.The primary contents and innovation points are summarized below.1)Visualizing molecular unidirectional rotationConventional laser induced molecular alignment can only control the spatial orientation of the molecular axis,but not the direction of molecular rotation.We use two time delayed,polarization crossed femtosecond pulses to further achieve precise manipulation of the angular velocity of the molecule.The unidirectional rotation of the molecules in a clockwise or counterclockwise sense can be selectively excited by applying a second±45°polarized pulse at the alignment or anti-alignment peak caused by the first one.We directly visualized the spatiotemporal evolution of the unidirectional molecular rotational wave packet by self-developed femtosecond Coulomb explosion imaging technology,revealing the dynamics that cannot be resolved by traditional optical methods.2)Molecular alignment echoesA pair of time-delayed laser pulses can result in a series of periodic alignment events.The first two of them immediately following each excitation,and the followings are echoes that appear with a delay equal to that between the exciting pulses.We theoretically and experimentally investigated full and fractional echoes using the Coulomb explosion imaging technique.Three new echo phenomena,i.e.high-order fractional echo,rotated echo and imaginary echo are discovered.High-order fractional echoes are closely related to the principles of free-electron lasers,which provide new visual images for generating high order harmonics.The rotated echo is an echo signal entangled in space and time.By changing the polarization of the second laser pulse,the spatiotemporal characteristics of the rotated echo can be well controlled.It is generally believed that the echo signal can only appear after the excitation pulse,while the imaginary echo appears before the excitation pulse.We successfully observed the imaginary echo at the"negative"time delay by utilizing the quantum mechanical phenomenon of the molecular alignment revival.3)All-optical field-free 3D orientation of asymmetric-top moleculesThe field-free three-dimensional orientation of complex molecules,especially asymmetric-top molecules,has always been the"Holy Grail"in the field of laser induced molecular alignment,which is extremely challenging.We proposed a novel method of using a pair of phase-locked cross-polarized two-color laser pulses to achieve all-optical field-free three-dimensional orientation of asymmetric-top molecules.Experimentally,taking the SO₂ molecule as an example,the fundamental wave（FW）and the second harmonic（SH）of the two-color field are set to orthogonal polarization,and the intensity ratio between the two colors is adjusted to match the polarization component of the SO₂ molecule.The O-axis is successfully aligned along the FW direction and the S-axis is oriented along the SH direction.The spatial orientation of the molecule can be varied by adjusting the relative phase between the two colors.By changing the intensity ratio and polarization between the two colors,we can extend this method to other complex asymmetric-top molecules.