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The nuclear dynamics of polyatomic molecules in intense laser fields (∼1 PW/cm^2) is studied through the momentum imaging of the fragment ions produced through Coulomb explosion. Characteristic nuclear dynamics that occur on the multidimensional potential energy surfaces in intense laser fields, such as sequential and concerted bond-breaking and hydrogen migration, are elucidated from momentum correlations among the fragment ions.

Ionization of organic molecules irradiated with intense infrared femtosecond pulses is explained from the perspective of intact molecular ion formation. Although fragmentation is more suppressed in general by femtosecond pulse excitation than pico- and nanosecond pulse excitations, molecules are still often heavily fragmented. Among the excitation parameters affecting ionization and fragmentation processes, excitation wavelengths and pulse durations at a fixed laser intensity have been found to drastically change ionization patterns. Intact molecular ions are produced when the wavelengths are non-resonant with the electronic levels of cations, whereas fragmentation proceeds to a large extent when the wave length is resonant with the electronic transitions. An ultimately short pulse presumably leads to the formation of fragment-free ions. Time-of-flight spectra of femtosecond pulse ionization of cyclohexadiene isomers, 2,3-dimethyl-1,3-butadiene, naphthalene, anthracene, and dioxin are presented. The great advantage of femtosecond laser mass spectrometry (FLMS) for the intact molecular ion formation is shown by referring briefly to the results obtained for dioxin. Some details of the experimental methods, such as a method of estimating irradiation intensity, are described.

We discuss the role of strongly driven electrons in the generation of attosecond laser pulses and in time-resolved experiments that can be performed using attosecond lasers. We show that the formation of attosecond laser pulses in high harmonic generation is an inevitable consequency of the physics of atoms in strong laser fields, combining the sub-cycle dependence of atomic ionization rates with controlled acceleration of the ionized electrons in the oscillatory field of the laser. Molecules undergoing dynamic alignment and/or enhanced ionization, and explosions of large rare gas clusters in an intense laser field are discussed in the context of attosecond electron dynamics.

Laser-induced double and multiple ionization of atoms may proceed either sequentially or nonsequentially in one coherent process. For near-infrared lasers of moderate intensity (10^14 to 10^15 W/cm^2), the physical mechanism of the coherent process is inelastic rescattering of a first-ionized electron off its parent ion. The quantum-mechanical S -matrix description of this process is reviewed. Momentum distributions of the ejected electrons and the doubly charged ion are calculated and compared with the experimental data. Their shape is found to be determined by the effective electron-electron interaction, by which the recolliding first electron ejects the up to this time bound second electron. The significance of the finalstate electron-electron interaction is assessed. The underlying classical dynamics are elucidated. Recent experiments with ultra-short phase-stabilized laser pulses are discussed. Nonsequential multiple ionization is modeled by assuming that the returning electron thermalizes with a certain number of bound electrons, and the corresponding thermalization time is estimated.

An effective scheme for the laser control of wavepacket dynamics applicable to systems with many degrees of freedom is discussed. It is demonstrated that specially designed quadratically chirped pulses can be used to achieve fast and near-complete excitation of the wavepacket without significantly distorting its shape. The parameters of the laser pulse can be estimated analytically from the Zhu-Nakamura (ZN) theory of nonadiabatic transitions. The scheme is applicable to various processes, such as simple electronic excitations, pump-dumps, and selective bond-breaking, and, taking diatomic and triatomic molecules as examples, it is actually shown to work well.

An efficient semiclassical optimal control theory for controlling wavepacket dynamics on a single adiabatic potential energy surface applicable to systems with many degrees of freedom is discussed in detail. The approach combines the advantages of various formulations of the optimal control theory: quantum and classical on the one hand and global and local on the other. The efficiency and reliability of the method are demonstrated, using systems with two and four dimensions as examples.

We demonstrated accurate pulse shaping of amplified femtosecond laser pulses by modulating the amplitude and the phase prior to regenerative chirped-pulse amplification. Based on a spectral interferometer, the pulse shaper was adjusted to compensate for a transfer function of the amplifier consisting of the gain narrowing and self-phase modulation. We performed experiments aiming at selective bond breaking in such as a simple C-C-O structure by utilizing the comprehensive pulse shaping technology, which can produce almost arbitrarily shaped 800-nm laser pulses in the amplitude and the phase at peak intensities up to ∼4 × 10^15 W/cm^2 range.

High-order harmonic generation in a long gas jet is controlled in the space and time domains using chirped and self-guided femtosecond laser pulses. Since high-order harmonic generation is intrinsically connected to the ionization process of harmonic generation medium, ionization effects on high-order harmonic generation should be properly understood and taken into account. Here, we present a method to control high-order harmonic generation process by controlling the propagation mode of intense femtosecond laser pulses through the ionizing medium. Experimental results and theoretical analysis show that self-guided and chirped laser pulses can optimize high-order harmonics for achieving high brightness, low beam divergence, and narrow spectral bandwidth.

The focusability of intense coherent radiation generated by high-order harmonic conversion is investigated. A variety of focusing mirrors are tested and used in the focusing experiments. We demonstrate the focusing of an extremeultraviolet/soft X-ray beam to a 1 µm spot size with a peak intensity as high as 1 × 10^14 W/cm^2, which is sufficient to stimulate nonlinear optical phenomena in the short wavelength region.

X-ray line emissions from ultrashort high-intensity laser-produced plasma were studied in order to clarify the physics of energy transport associated with the generation of ultrashort X-ray pulses for use in various applications. This article reviews two topics. The first is the application of Kα spectroscopy to the study of energy transport in laser-produced plasma. The second topic is the application of X-ray polarization spectroscopy to measurements of the anisotropy of hot electrons generated with ultrashort high-intensity laser pulses.