The Phase Reconstruction Of A Complex Molecular System In Strong Laser Fields

Phase represents the essential features of any wave-like phenomena as the heart of coherence and interference effects in classical and quantum physics.In atomic and molecular systems,phases define the shape of a wave packet and hence determine its subsequent time evolution.So far,the phase information of an atomic system in strong laser fields can be extracted experimentally.However,how to obtain the phase evolution of molecular system is still an open question.We perform transient-absorption measurement in the dye molecule IR144 in liquid phase.The experimental results reveal that the shape of absorption spectra not only change with the pump-probe time delay,but also depend on the pump-pulse intensities.The aimed sample is assumed to be a multilevel system based on the particle-in-a-box theory,and the experimental results are reproduced by simulation based on the density matrix theory.Relating the absorption spectral shape to the system's dynamic evolution,the artificial phase method is applied to extract the system's phase information,revealing the different dynamic process of excited state system between positive and negative time delay.We find three new characteristics of the complex molecular phase in strong filed: saturation,approximate mirror symmetry and selectivity.This present work not only fills in the blank of the study on the complex molecular phase information and allows us a full understanding of the relationship between complex molecular phase and absorption spectral shape,but also reveals the dynamic evolution of the complex molecular system in strong laser fields.In this thesis,we design and build up the transient-absorption spectral platform based on femtosecond laser equipment,and perform the absorption spectrum measurement in IR144 in the liquid phase.Details as follows.(i)In order to get rid of the nonlinear effect induced by the interaction between the focused laser and cuvette walls,a down-collimator system is applied in the experimental setup to enhance laser intensities,instead of the traditional focused beam.(ii)The step-motor-driver program,developed secondary,is nested with spectrometer program to realize the data automatic acquisition and processing,as well as the on-line monitor.(iii)Assuming the excited molecule as a multilevel system,we reproduce the experimental results based on the density matrix theory.Furthermore,we extract the molecular phase from the absorption spectra ofexcited IR144 system and find three new characteristics about the molecular phase: saturation,approximate mirror symmetry and selectivity.