Advances in Multi-Photon Processes and Spectroscopy

The following sections are included:

• Preface

• Contents

The following sections are included:

• Introduction
  • Nonlinear phenomena in XUV/soft X-ray region for ultrafast optics
  • Autocorrelation measurement

• Generation of Intense Harmonic Fields
  • Single atom response
  • Propagation of the harmonic fields with pumping laser field: Phase matching
  • Development of intense high-order harmonic generator

• Two-Photon Double Ionization

• Measurement of Attosecond Pulse Train with Two-Photon ATI

• Interferometric Autocorrelation of APT with Two-Photon Coulomb Explosion
  • Similarity of APT with mode-locked laser pulses
  • Why do we need interferometric autocorrelation?
  • Two-photon Coulomb explosion
  • Interferometric autocorrelation

• Summary and Prospects

• Acknowledgements

• References

The following sections are included:

• Introduction

• Theory of High-Order Harmonic Generation
  • Basic theory
  • Three-step model
  • The strong-field approximation
  • Odd and even harmonics

• Influence of Molecular Structure on HHG
  • Ionization step
  • Recombination step

• Dynamical Effects

• Conclusions

• Acknowledgments

• References

A sample of aligned or oriented molecules is an ideal anisotropic quantum system. In this chapter, focuses are placed on the present status of molecular orientation based on laser technologies and applications with a sample of aligned molecules. After the introduction, the principle of molecular orientation with combined electrostatic and intense, nonresonant laser fields is outlined. In Secs. 3.1 and 3.2, it is shown how to achieve and observe one- and three-dimensional molecular orientation, respectively, with combined electrostatic and pulsed, nonresonant laser fields. In order to demonstrate the efficacy of aligned molecules, two representative works from our laboratory are presented in Sec. 4. In fact, a sample of aligned molecules is used to investigate multiphoton ionization processes in I₂ molecules with time-dependent polarization pulses (Sec. 4.1) and high-order harmonic generation from N₂, O₂, and CO₂ molecules (Sec. 4.2). Finally, some future subjects are presented in Sec. 5.

Nonlinear spectroscopy, both second harmonic and sum frequency generation — SHG and SFG — have proven to be powerful techniques for probing a variety of interfaces from the very dynamic, high vapor pressure liquid–air surface to buried interfaces between hydrophobic and hydrophilic phases to irregular and amorphous solid surfaces. With the advent of off-the-shelf laser systems, it has become easier and easier to collect nonlinear spectra. The major impediments to wide spread usage of nonlinear spectroscopy are the challenges in interpretation of the spectra produced. This work begins with an introduction to nonlinear spectroscopy based on an optical-geometrical view of the interaction between the probe beams and molecules in the interfacial region. The introduction serves as a basis for exploration of recent developments and current issues. Two case studies are included: examination of ions at the aqueous interface including evidence for H₃O⁺ at the interface and investigation of molecular interactions on nonmetallic, nanostructured interfaces.

In this work, we have studied the changes in the Four-Wave Mixing spectroscopic signal response in the frequency space by propagation of the electromagnetic fields and intramolecular coupling effects. Firstly, in a propagation treatment at all orders in the pump field and first order in the probe and signal fields, we obtain a description of the Four-Wave Mixing signal. Secondly, for the inclusion of the intramolecular effects, the model is a generalization of the conventional two-level approach in that it includes a simplified description of the molecular structure. The Born–Oppenheimer electronic energy curves for this simplified molecular model consist of two intercrossing harmonic oscillators potential with different force constants. The transition and permanent dipole moments are critical quantities for this analysis in the frequency space of the local macroscopic polarization in the adiabatic representation when the rotating wave approximation is not considered.

Theoretical studies on laser control of molecular chirality, which have been carried out in our laboratory, are presented. Two fundamental issues about molecular chirality transformation in a racemic mixture are described. The first issue is that enantiomers should be pre-oriented to control chiral transformation by applying laser fields within the dipole approximation, and the second issue is that pure enantiomers can be selectively prepared from a preoriented racemic mixture by taking into account the photon polarization direction of the laser field. Based on these two issues, new control schemes for molecular chirality in a racemic mixture are presented. These control schemes include analytical treatment of preparation of enantiomers in the ground electronic state in an oriented racemic mixture, pump–dump control via an electronic excited state, control of molecular chirality in a randomly oriented racemic mixture using three polarization components of electric fields, stimulated Raman adiabatic passage method, and sequential pump–dump control of chirality transformation competing with photodissociation in an electronic excited state. For the three polarization components of laser fields, two polarization components are used for orientation of randomly oriented chiral molecules and the other component is used for chirality transformation of the oriented chiral molecules.