M. Yu. Ivanov

Time-Resolved Holography with Photoelectrons

The intefererence pattern produced by photoelectrons provides holographic snapshots of the photoionization process. Ionization is the dominant response of atoms and molecules to intense laser fields and is at the basis of several important techniques, such as the generation of attosecond pulses that allow the measurement of electron motion in real time. We present experiments in which metastable xenon atoms were ionized with intense 7-micrometer laser pulses from a free-electron laser. Holographic structures were observed that record underlying electron dynamics on a sublaser-cycle time scale, enabling photoelectron spectroscopy with a time resolution of almost two orders of magnitude higher than the duration of the ionizing pulse.

A semiclassical approach to intense-field above-threshold dissociation in the long wavelength limit. II. Conservation principles and coherence in surface hopping

This paper is a companion to our recently published semiclassical formalism for treating time-dependent Hamiltonians [J. Chem. Phys. 105, 4094 (1996)], which was applied to study the dissociation of diatomic ions in intense laser fields. Here two fundamental issues concerning this formalism are discussed in depth: conservation principles and coherence. For time-dependent Hamiltonians, the conservation principle to apply during a trajectory hop depends upon the physical origin of the electronic transition, with total energy conservation and nuclear momentum conservation representing the two limiting cases. It is shown that applying an inappropriate scheme leads to unphysical features in the kinetic energy of the dissociation products. A method is introduced that smoothly bridges the two limiting cases and applies the physically justified conservation scheme at all times. It is also shown that the semiclassical formalism can predict erroneous results if the electronic amplitudes for well-separated hops are ...

A SEMICLASSICAL APPROACH TO INTENSE-FIELD ABOVE-THRESHOLD DISSOCIATION IN THE LONG WAVELENGTH LIMIT

A new semiclassical formalism has been developed to treat Hamiltonians having explicit time dependence, with particular application to the dissociation of diatomic ions in intense laser fields. Based on this formalism, a hopping algorithm is presented which specifies how classical trajectories should be moved between coupled electronic surfaces. The theory is laid out in a rigorous, general form and an analysis is also presented for the case where only two electronic surfaces are strongly coupled. In addition, valuable physical insight into the hopping process is obtained by considering the theory in a number of physically relevant limiting cases. From this insight a number of guidelines are proposed which detail the manner in which trajectory hopping should be implemented when time‐dependent potential energy surfaces are present, including the effects of phase coherence and conservation principles.

Effect of multiple conduction bands on high-harmonic emission from dielectrics

We find that, for sufficiently strong mid-IR fields, transitions between different conduction bands play an important role in the generation of high-order harmonics in a dielectric. The transitions make a significant contribution to the harmonic signal, and they can create a single effective band for the motion of an electron wave packet. We show how high harmonic spectra produced during the interaction of ultrashort laser pulses with periodic solids provide a spectroscopic tool for understanding the effective band structure that controls electron dynamics in these media.

Coulomb–laser coupling in laser-assisted photoionization and molecular tomography

Using the example of laser-assisted photoionization, we analyse the interplay of an intense laser field and the atomic/molecular potential during the electron motion after ionization. We give conditions to determine when the electrons oscillations in the strong laser field are approximately decoupled from its acceleration in the ionic potential, and when they are not. Excellent agreement between analytical and numerical results allows us to assess the recipes for analysing interference structures in high harmonic generation in molecules.

Electron wavepacket control with elliptically polarized laser light in high harmonic generation from aligned molecules

We study experimentally and theoretically the high harmonic emission from aligned samples of nitrogen and carbon dioxide, in an elliptically polarized laser field. The ellipticity induces a lateral shift of the recombining electron wavepacket in the generation process. We show that this effect, which is well known from high harmonic generation (HHG) in atoms, can be useful to maintain the plane wave approximation in the case of HHG from molecules whose orbitals contain nodal planes. The study of the harmonic signal as a function of molecular alignment also reveals the role of the ellipticity on the recollision angle of the electron wavepacket, which can be used to accurately track the position of resonances in harmonic spectra.

Ultrafast multiphoton forest fires and fractals in clusters and dielectrics

We describe the interaction of ultrashort infrared laser pulses with clusters and dielectrics. Rapid ionization occurs on a sub-laser wavelength scale below the conventional breakdown threshold. It starts with the formation of nano-droplets of plasma which grow like forest fires, without any need for heating of the electrons promoted to the conduction band. The dimensionality of the damaged area can be fractal and changes during the laser pulse. This mechanism is operative in both rare gas clusters and dielectrics interacting with ultrashort, moderately intense laser pulses which include only several periods of the driving field, so that the traditional avalanche mechanisms have no time to develop.

The use of intense-field ionization in time-resolved measurements

Abstract We show that intense radiation introduces a length-scale in molecules which translates into a dramatic enhancement of the rate of multiple ionization of molecules at characteristic internuclear separations. This feature is common to a wide range of systems and is insensitive to variations in frequency and intensity. The electronic properties responsible for this effect are explored. We propose the use of the sensitivity of intense-field ionization to the molecular configuration as a probe in time-resolved studies of dissociative dynamics.

Angular trapping and rotational dissociation of a diatomic molecule in an optical centrifuge

We perform a detailed quantum study of forced molecular rotation in an optical centrifuge, recently proposed by J. Karczmarek [Phys, Rev. Lett. 82, 3420 (1999)]. The approach uses strong nonresonant laser fields with chirped frequency to induce efficient rotational excitation of anisotropic molecules via a sequence of Raman transitions. Quantum calculations firstly of angular confinement (angular trapping) of a molecule in the early stages of the centrifuge evolution and secondly of the resulting rotational dissociation process are carried out herein. The trapping calculations include both angular degrees of freedom while the dissociation calculations include one vibrational and one rotation degree of freedom. Diatomic Cl2 is used as a test case. An extension of the scheme outlined by Karczmarek et al. is proposed as a method of producing molecules in a single selected J=Jz level.

Selective dissociation of the stronger bond in HCN using an optical centrifuge

Using the example of the HCN molecule, we study theoretically the possibility of selectively breaking the stronger bond in a triatomic molecule by rotationally accelerating it in an optical centrifuge using a combination of two oppositely chirped and counter-rotating strong laser fields. In our simulation the resultant field forces rotational acceleration of the HCN molecule to a point where the centrifugal force between the two heavy atoms (C and N) exceeds the strength of their (triple) bond. The effects of bending, rovibrational coupling, and the Coriolis force, which conspire to prevent the molecule from rotational dissociation into HC+N, can be efficiently counteracted by simple optimization of the frequency chirp.