Strong-field Multiphoton Ionization Research Articles

Photoelectron Circular Dichroism in the Spin-Polarized Spectra of Chiral Molecules.

We studied strong-field multiphoton ionization of 1-iodo-2-methylbutane enantiomers with 395nm circularly polarized laser pulses experimentally and theoretically. For randomly oriented molecules, we observe spin polarization up to about 15%, which is independent of the molecular enantiomer. Our experimental findings are explained theoretically as an intricate interplay between three contributions from HOMO, HOMO-1, and HOMO-2, which are formed of 5p-electrons of the iodine atom. For uniaxially oriented molecules, our theory demonstrates even larger spin polarization. Moreover, we predict a sizable enantiosensitive photoelectron circular dichroism of about 10%, which is different for different spin states of photoelectrons.

Spatial separation of 2-propanol monomer and its ionization-fragmentation pathways

The spatial separation of 2-propanol monomer from its clusters in a molecular beam by an electrostatic deflector was demonstrated. Samples of 2-propanol monomer with a purity of 90% and a beam density of 7×106cm−3 were obtained. These samples were utilized to study the femtosecond-laser-induced strong-field multi-photon ionization and fragmentation of 2-propanol using non-resonant 800 nm light with peak intensities of 3–7×1013W/cm2.

Study on photodynamics of furan via strong field multiphoton ionization by velocity map imaging technique

The photodissociation dynamics of furan at 800 nm was studied using velocity map imaging technology coupled with time-of-flight mass spectrometry. Dissociation channels are obtained by analysis of furan multiphoton ionization dissociation under strong field. According to time-of-flight mass spectrometry experiments, C4H4O+, C3H3+, C3H4+, C2H2O+, HCO+ are obtained by multiphoton ionization dissociation of furan. By calculating the speed distribution and kinetic energy distribution using velocity map imaging technology, it can be seen that there are two channels of high energy channel and low energy channel in the process of photodissociation. The anisotropy parameters of furan dissociation for both channels are close to 0 (Isotropy) by analysis of the angular distribution of dissociated fragment ions. The density functional theory was employed to calculate the configuration change of the furan molecule before and after ionization, the energy level intensity of the ionic states and corresponding oscillator strength.

Energy- and Momentum-Resolved Photoelectron Spin Polarization in Multiphoton Ionization of Xe by Circularly Polarized Fields.

We perform a joint experimental and theoretical study on momentum- and energy-resolved photoelectron spin polarization in multiphoton ionization of Xe atoms by circularly polarized fields. We experimentally measure the photoelectron momentum distributions of Xe atoms in circularly polarized near-infrared (800nm) and ultraviolet (400nm) light, respectively. We analyze the momentum- and energy-resolved photoelectron spin polarization by comparing the experimental photoelectron momentum distributions with the simulations, although we cannot derive the spin polarization solely from the experiment. We show that the use of circularly polarized ultraviolet light at 400nm can create better than 90% spin polarization with focal volume effect considered, which enables the separation of the spin states by momentum gating. This paves the way to produce high-degree spin-polarized electron sources from strong-field multiphoton ionization.

Optical control of the vibrational excitation of the polyatomic ions via strong field multi-photon ionization

The dynamics of multi-photon ionization of CH3I under strong field has been studied experimentally by femtosecond photoelectron imaging. Ultrafast optical control of the vibrational excitation in a polyatomic ion by strong field multi-photon ionization is experimentally realized. The present work enhances the intensity of the ionization beam from 1.6×1013 to 2.5×1013 W/cm2. In the order of this higher field, a new energy component is observed and attributed. From the photoelectron imaging, photoelectron kinetic energy distributions and the photoelectron angular distributions are obtained. The discussions of the previous letter are mostly based on the photoelectron kinetic energy information, and the present study emphasizes on the trend of the photoelectron angular anisotropy. More detailed dynamics on vibrational optical control is further explored.

Molecular Dynamics in Strong Laser Fields

One of the goals for strong field physics is imaging the dynamics of the quantum systems, and in case of molecules, there is strong interest in imaging of the electron rearrangement during chemical reactions. Strong field ionization of molecules has many attractive features that make this process a candidate tool for strong field imaging of transient states and chemical reactions. In this paper, we present analysis of ionization dependence on orientation of molecular axis with respect to polarization of the electric field of the laser. By considering several examples of molecules at their equilibrium internuclear distances and an example of the simplest chemical reaction, namely, the dissociation of diatomic molecule, we present how symmetries of the molecular orbitals influence the alignment-dependent ionization and how this dependence can be used to follow molecular dynamics using strong field multiphoton ionization.

Femtosecond Photoelectron Imaging of N2at 410 nm

We experimentally measure the kinetic energy and angular distributions of photoelectrons of <TEX>$N_2$</TEX> as a function of 410 nm femtosecond laser intensity by using velocity map imaging technique. The strong-field multiphoton ionization of molecules shares many of the characteristics with those of atoms. Electron kinetic energies are nearly independent of laser intensities. The independence suggests that the electron peaks in the photoelectron spectrum actually result from a two-step process, indicative of the occurrence of real population in the intermediate states. The relative amplitudes of electron peaks indicate that in the two-step process, nonresonant population transfer dominates for low intensities, while resonant population transfer dominates for higher intensities.

Strong-field photoionization ofO2at intermediate light intensity

We investigated by electron spectroscopy the strong-field multiphoton ionization of O-2 molecules with ultrashort laser pulses in the intensity range between the multiphoton and tunneling regimes. ...

Exploration of strong-field multiphoton double ionization, rescattering, and electron angular distribution of He atoms in intense long-wavelength laser fields: The coupled coherent-state approach

This work was partially supported by the Chemical Sciences, Geosciences and Biosciences Division of the Office of Basic Energy Sciences, Office of Sciences, US Department of Energy and by the US National Science Foundation. J.G. would also like to acknowledge the partial support of the Chinese Scholarship Council (CSC). J.G. and X.L. were partially supported by the National Natural Science Foundation of China under Grant No. 10974068.

Effects of multiple electronic shells on strong-field multiphoton ionization and high-order harmonic generation of diatomic molecules with arbitrary orientation: An all-electron time-dependent density-functional approach

We present a time-dependent density-functional theory approach with proper long-range potential for an ab initio study of the effect of correlated multielectron responses on the multiphoton ionization (MPI) and high-order harmonic generation (HHG) of diatomic molecules ${\text{N}}_{2}$ and ${\text{F}}_{2}$ in intense short laser pulse fields with arbitrary molecular orientation. We show that the contributions of inner molecular orbitals to the total MPI probability can be sufficiently large or even dominant over the highest-occupied molecular orbital, depending on detailed electronic structure and symmetry, laser field intensity, and orientation angle. The multielectron effects in HHG are also very important. They are responsible for enhanced HHG at some orientations of the molecular axis. Even strongly bound electrons may have a significant influence on the HHG process.

Effects of electron structure and multielectron dynamical response on strong-field multiphoton ionization of diatomic molecules with arbitrary orientation: An all-electron time-dependent density-functional-theory approach

We present a time-dependent density-functional-theory approach for the ab initio study of the effect of correlated multielectron responses on the multiphoton ionization (MPI) of diatomic molecules ${\text{N}}_{2}$, ${\text{O}}_{2}$, and ${\text{F}}_{2}$ in intense short laser pulse fields with arbitrary molecular orientation. We show that the contributions of inner molecular orbitals to the total MPI probability can be significant or even dominant over the highest-occupied molecular orbital, depending on detailed electronic structure and symmetry, laser field intensity, and orientation angle.

Comment on ‘Generalized Bessel functions in tunnelling ionization’

In the recent paper Reiss and Krainov (2003 J. Phys. A: Math. Gen. 36 5575) developed two new approximations for the generalized Bessel function that arises in the analytical treatment of strong-field multiphoton ionization theories. We show that their 'tunnelling approximation 1' systematically underestimates the theoretical ionization rate by a factor of 2 or more for typical laser frequencies. More important still, only a few first ATI peaks of the lowest energy may be reproduced quite well using this approximation.

Metastable Triply Charged Diatomic Molecules Produced with Femtosecond Pulses

We show that ultrashort pulse strong field multiphoton ionization efficiently produces metastable highly charged molecules such as metastable diatomic trications ${\mathrm{I}}_{2}^{3+}$, ${\mathrm{Br}}_{2}^{3+}$, and ${\mathrm{Cl}}_{2}^{3+}$. The efficiency of stable trication production decreases rapidly with increasing pulse duration. Weak pre- or post-pulse irradiation also prevents efficient production or survival of trications. We propose strong-field femtosecond Raman spectroscopy to determine trication vibrational frequencies.

Elastic Rescattering in the Strong Field Tunneling Limit.

High precision measurements of helium photoelectron energy and angular distributions for a broad intensity range reflect the change in the continuum dynamics that occurs as the ionization process evolves into the pure tunneling regime. Elastic rescattering of the laser-driven free electron from its parent ion core leaves a distinct signature on the spectra, providing a direct quantitative test of the various theories of strong field multiphoton ionization. We show that it takes a relatively complete semiclassical rescattering model to accurately reproduce the observed distributions.

Atomic-structure effects in strong-field multiphoton detachment and ionization

Above-threshold photoelectron detachment and ionization spectra are investigated theoretically in the tunneling and over-barrier regime as a function of wavelength (⩾1.064 μm) and polarization of the electromagnetic field. It is found that the zeros in the initial-state wave function can drastically affect the shape of the high-energy photoelectron distribution. The phenomenon is not predicted by tunneling and related models and hence can test their validity and reveal whether Keldysh-type theories are in general applicable to strong-field multiphoton dynamics.

Plasma perspective on strong field multiphoton ionization.

During strong-field multiphoton ionization, a wave packet is formed each time the laser field passes its maximum value. Within the first laser period after ionization there is a significant probability that the electron will return to the vicinity of the ion with very high kinetic energy. High-harmonic generation, multiphoton two-electron ejection, and very high energy above-threshold-ionization electrons are all conssequences of this electron-ion interaction. One important parameter which determines the strength of these effects is the rate at which the wave packet spreads in the direction perpendicular to the laser electric field; another is the polarization of the laser. It will be essential for experimentalists to be aware of these crucial parameters in future experiments.

Strong-field high-frequency approximation to the multiphoton ionization of hydrogen

The strong-field multiphoton ionization of atoms is considered and a theoretical approach dealing nonperturbatively with the radiation field formulated. The general computational scheme is the conventional perturbation theory, but the intermediate states are dressed by the field. We present in detail a method to dress the continuum states and to study the dipole transitions within the continuum. In the high-frequency domain, the proposed procedure rapidly converges over a wide range of field intensity and offers an interesting framework for calculating ionization rates for arbitrary numbers of absorbed (above-threshold) photons and field polarization.

PEAK-SWITCHING OF THE PHOTOELECTRON SPECTRUM

The photoelectron spectrum from autoionizing atoms in strong laser field is investigated on the basis of ref [12] . Peak-switching similar to the corresponding effect in strong field multiphoton ionization is found, implying that the second-order ionization processes and high-order photoelectron peaks play essential roles in strong laser-field induced autoionization-These results show that many of previous results need to be revised.

Strong-field multiphoton ionization of hydrogen. TheS-matrix treatment of the elementary process

A theoretical model is presented to treat the elementary act of strong-field multiphoton ionization of atoms when the electron continuum final-state distribution is important. It is based on theS-matrix formalism and treats the final-state electron-radiation interaction in an essentially nonperturbative way. Selected numerical calculations concern the ionization of hydrogen atoms and include differential and total cross-sections of several multiphoton channels as a function of the laser intensity. Good, qualitative agreement with the experimental observations is found for values of the field intensity which are not critical with respect to the simplifications adopted in constructing the theoretical model. It applies particularly to the use of an ideal model for the laser field. Significant departure from observations is instead found when the implications of the ideal laser model play a critical role, as occurs at channel inversion and suppression. It is concluded that the theoretical treatment to be completely satisfactory needs essentially to incorporate a more realistic laser model such as a multimode one.