Propagation Properties Of Strong Laser Pulses In Atomic And Molecular Media And Resonant X-ray Scattering Spectroscopy

New generations of ultrashort and intense laser pulses as well as high power synchrotron radiation sources and x-ray free-electron lasers evoke people's curiosity and motivate us to study the interaction between strong laser field and the matter, which has promoted a rapid development in nonlinear optics and X-ray spectroscopy. The main aim of this thesis is to explore the micro-mechanisms of the interaction between the intense laser pulses and the media, and to explain and predict new phenomena in nonlinear optics and X-ray region.The objectives of this thesis mainly focus on following aspects: the optical limiting behavior of the organic molecular media, the propagation of X-ray free-electron laser pulses in atomic media, resonant X-ray scattering spectroscopy, and so on. Our studies achieve many novel results in nonlinear optics and X-ray spectroscopy, which is significant not only for the further development of theories but also for the guidance of the experimental work. The main contents and results are summarized as follows:1. TPA induced optical limiting effectBy solving the full-wave Maxwell-Bloch equations with FDTD and predictor-corrector numerical methods, we simulate the propagation of the few-cycle femosecond laser pulses in a strong TPA medium ( 4,4'? bis(dimethylamino) molecules), and investigate the optical limiting behavior of this TPA medium. Our numerical results show that for ultrashort pulses one-step coherent TPA is dominating, and the medium exhibits a good optical limiting performance during pulse propagation. The dynamical TPA cross section of the 4,4'? bis(dimethylamino) molecule is calculated using the output-input peak-intensity relation. It is found that the dynamical TPA cross section is a linearly decreasing function of the incident intensity when the incident intensity is not high enough to induce the TPA saturation. The photoionization has an obvious influence on the TPA process increasing the input pulse intensity. The dynamic optical limiting window becomes wider when the photoiozation rate is larger.2. RSA induced optical limiting effectWe have studied the dynamics of picosecond pulse trains propagating in fullerene C6 0, and given an insight into the mechanism of the optical limiting behavior based on reserve saturable absorption. Due to the short duration of the subspulse, the intersystem crossing (ISC) between the singlet states and triplet states has no time to occur during single short pulse, and the whole system studied can be devided into two independent singlet and triplet subsystems. However, in the time domain between the subpulses, the population of the singlet excited state will relax to the lowest triplet state through the ISC process. Thus two subsystems are coupled with each other. The front subpulses are mainly affected by the linear absorption between singlet states, while the latter subpulses experience the excite-state absorption due to the accumulative nonlinearity. Different absorption mechanisms result in the optical limiting effect. The optical limiting materials should have long excited-state lifetime and a large photon absorption cross section. We suggest a new experimental method to measure the lifetime of the triplet state, in which the lifetime of the triplet state can be determined by scanning the transmittance versus the repetition of the pulse train.3. The dynamics of the XFEL in the atomic mediaWe simulate the propagation of x-ray free-electron lasers (XFELs) through the atomic media (Ar or Mg) for the first time by numerically resolving the Maxwell-Bloch equations. The mechanisms of the interaction between XFEL and the media have been explained in terms of the work function of the XFEL field. The propagation of XFEL field is accompanied by the stimulated resonant Raman scattering (SRRS). The intensity of Stokes field increases during XFEL propagation, and the main reason for the gain of the Stokes field is qualitativleky changed from the amplified spontaneous emission (ASE) to lasing without inversion (LWI). During pulse propagation, the relative Auger yield is suppressed due to the strong stimutated emission. The compression and slowdown of XFELs are observed because of a strong nonlinear interaction between XFELs and the media.4. Resonant X-ray scattering spectroscopyWe explore the resonant X-ray scattering spectroscopy from the polyatomic molecules both theoretically and experimentally in collaboration with MAX-Lab and Elettra (Italy) experimental group in Sweden. Due to the resonant condition, we can obtain stronger singnal of the spectra. Resonant scattering spectroscopies can be used to study the electronic structures and chemical bonding information of the local section by exciting the molecules selectively. First, we develop the theory of resonant X-ray Raman scattering (RXS) in liquid phase and study the RXS spectra from liquid acetone of superhigh resolution at the first time. The static interaction between the molecules in liquid is the major mechanism of the spectral broadening of RIXS. This static broadening has little influence on the REXS band. The 0-0 spectral line of REXS is enhanced strongly due to Thomson scattering. The effect of self-absorption on REXS spectrum is suppressed because of off-resonant absorption by carbon. Second, we study the resonant Auger scattering (RAS) spectra from ethylene molecule. Multimode simulation results are in a good agreement with experimental spectra measured in MAX-Lab laboratory. RAS show much stronger vibrational heating of the ethylene molecule in comparison with the photoelectron spectrum (PES) due to strong multi-mode core-excitation.5. Recoil splitting of X-ray induced optical fluorescenceWe propose a new experimental scheme of detecting the recoil effect, which is called recoil splitting of X-ray induced optical fluorescence or absorption lines. In x-ray regime, the ionized atoms or molecules obtain a large recoil momentum due to the large momentum of the photoelectron. The anisotropy of the recoil velocity distribution of the ions leads to the anisotropy of the subsequent optical fluorescence or absorption. The relevant theory is developed and illustrated with the nitrogen molecule. It is found that the order of the recoil induced Doppler splitting is about 10μeV. In experiment, this phenomenon can be observed using Fourier or laser absorption spectroscopic techniques with the help of power synchrotron radiation sources and x-ray free-electron lasers.This thesis is divided into nine chapters. The first chapter is an overview, which gives a brief review of the history of artificial light sources, the development and application of the ultrashort and intense laser pulses. The developing process of the nonlinear optics and X-ray spectroscopy is also described in this section. In the second and third chapters, the fundamental theories and numerial calculation methods used in our study are outlined. From the fourth chapter to eighth chapter, my own study works are shown here. At last, the conclusions of the contents and novel results, and the prospect of future work are given.