Internal collision induced strong-field nonsequential double ionization in molecules.

Using the classical ensemble method, we have investigated the alignment dependence of the correlated electron dynamics in strong-field nonsequential double ionization (NSDI) of diatomic molecules driven by linearly polarized laser pulses. Our numerical results show that the correlated electron pairs are more likely to emit into the same hemisphere (side-by-side emission) for the parallel aligned molecules at the small internuclear distance, in agreement with previous experimental results. Surprisingly, as the internuclear distance increases, this side-by-side emission is more prevalent for the perpendicularly aligned molecules. Back analyzing of the classical trajectories shows that a considerable part of the NSDI events for the parallel aligned molecules at the large internuclear distances occur through an internal collision, not the well-known recollision. In the internal collision induced NSDI, the first electron tunnels through the inner barrier from the up-field core, moves directly towards the other core, and kicks out the second electron. For this type of NSDI events, the electron pairs are more likely to emit into the opposite hemispheres and thus the correlated electron momentum spectrum exhibits a more dominant back-to-back behavior in the parallel aligned molecules.

Quantum dynamics of atomic Rydberg excitation in strong laser fields.

Neutral atoms have been observed to survive intense laser pulses in high Rydberg states with surprisingly large probability. Only with this Rydberg-state excitation (RSE) included is the picture of intense-laser-atom interaction complete. Various mechanisms have been proposed to explain the underlying physics. However, neither one can explain all the features observed in experiments and in time-dependent Schrödinger equation (TDSE) simulations. Here we propose a fully quantum-mechanical model based on the strong-field approximation (SFA). It well reproduces the intensity dependence of RSE obtained by the TDSE, which exhibits a series of modulated peaks. They are due to recapture of the liberated electron and the fact that the pertinent probability strongly depends on the position and the parity of the Rydberg state. We also present measurements of RSE in xenon at 800 nm, which display the peak structure consistent with the calculations.

Dissection of electron correlation in strong-field sequential double ionization using a classical model.

Recent experiments on strong-field sequential double ionization (SDI) have reported several observations which are regarded as evidence of electron correlation, querying the validity of the standard independent electron approximation for SDI. Here we theoretically study SDI with a classical ensemble model. The experimental results are well reproduced with this model. Back tracing of the ionization process shows that these results are ascribed to the subcycle ionization dynamics of the two electrons, not the evidences of the electron correlation in SDI. Thus, the previously reported observations are not enough to claim the breakdown of the independent electron approximation in SDI.

Vibrational excitations in chloromethyl radical formed by the photodissociation of chlorobromomethane.

Using velocity map ion imaging, the photodissociation of chlorobromomethane (CH2BrCl) at 233-234 nm has been studied. The total translational energy distributions and the anisotropy parameters have been determined from the ion images of the photofragments Br ((2)P1/2) (denoted as Br(*)) and Br ((2)P3/2) (denoted as Br) for the dominant CH2BrCl + hv → CH2Cl + Br(*) and CH2BrCl + hv → CH2Cl + Br channels. Using an impulsive model invoking angular momentum conservation, the vibrational energy distributions of the chloromethyl radicals have been derived from the total translational energy distributions for the two channels. The study suggests that there are a number of vibrational modes of the chloromethyl radical to be excited in both of the two photodissociation channels. In the Br* channel, the CH2 s-stretch mode v1 has the most probability of excitation. While in the Br channel, the CH2 scissors mode ν2 is attributed to the highest peak of the vibrational energy curve of the chloromethyl radical. The results further imply that, following absorption of one UV photon of 234 nm, other vibrational modes besides v5 (C-Br stretch mode) are also excited in the parent molecule.

High-harmonic and single attosecond pulse generation using plasmonic field enhancement in ordered arrays of gold nanoparticles with chirped laser pulses.

Coherent XUV sources, which may operate at MHz repetition rate, could find applications in high-precision spectroscopy and for spatio-time-resolved measurements of collective electron dynamics on nanostructured surfaces. We theoretically investigate utilizing the enhanced plasmonic fields in an ordered array of gold nanoparticles for the generation of high-harmonic, extreme-ultraviolet (XUV) radiation. By optimization of the chirp of ultrashort laser pulses incident on the array, our simulations indicate a potential route towards the temporal shaping of the plasmonic near-field and, in turn, the generation of single attosecond pulses. The inherent effects of inhomogeneity of the local fields on the high-harmonic generation are analyzed and discussed. While taking the inhomogeneity into account does not affect the optimal chirp for the generation of a single attosecond pulse, the cut-off energy of the high-harmonic spectrum is enhanced by about a factor of two.

Multistep ion exchange processes of gradient refractive index rod lens.

A mathematical model for research on the refractive index profile (RIP) of multistep ion exchange processes (IEPs) of gradient refractive index rod lenses (GRINs) is established by the different initial condition and boundary condition, based on the Fickian diffusion equation. GRIN rod lenses have been fabricated using the three-step IEPs. Research results indicate that the experimental deviations of refractive index (DRI) are in good agreement with the theoretical data. The DRI of three-step IEPs is superior to the one- and two-step IEPs and smaller than 10(-5).

Gradient refractive index square lenses. II. Imaging.

We establish theoretically the relationship between the lateral magnification m and the object distance L(0) for a gradient refractive index (GRIN) square lens and a GRIN rod lens. Through the linear fit of the object distance and the reciprocal of the magnification for different image locations, we use the imaging method to measure the central refractive index n(0) and focusing constant alpha of the GRIN square lens and the GRIN rod lens. The center refractive indices and focusing constant alpha of the GRIN square lens and the GRIN rod lens are 1.611 and 1.609, 0.0926 and 0.100, respectively. For the GRIN square lens and the GRIN rod lens, the measured central refractive indices are accurate to 10(-3).

Numerical analysis of low chromatic aberration of a gradient refractive index rod lens.

The oxide compositions (OCs) model is established for discussing the chromatic aberrations of a gradient refractive index rod lens. The chromatic aberrations for Na(+)/Li(+), K(+)/Cs(+), and K(+)/Tl(+) ion exchanges are discussed based on the OC model and the Huggins-Sun-Davis (HSD) model. Theoretical results indicate that the function value mainly depends on base glass properties and the nature of exchanging ion pairs, and rarely depends on the quantity of ion exchange. Experimental results show that the chromatic aberrations using the OC model have smaller errors than with the HSD model. The estimating average errors between the OC model and the HSD model are -0.051, -0.0067, and 0.0047 for the K(+)/Tl(+), Li(+)/Na(+), and K(+)/Cs(+) ion exchanges, respectively.

Asymmetric molecular gating for supercontinuous high harmonic generation in the plateau.

Molecular wave packets provide an alternative route to steer the high harmonic generation (HHG). Here we present a new method for isolated attosecond pulse generation from high harmonics in the plateau with asymmetric molecules. It is shown that the photoionization depends on the molecular structure. Through steering the ionization, HHG is controlled with the asymmetric molecule and supercontinuous high harmonics are produced in the plateau, from which an isolated 95-attosecond pulse is generated. In contrast to the cutoff which declines sharply, the plateau shows almost a constant intensity, indicating a higher yield. Moreover, the plateau covers a very broad bandwidth, thus is preferable to produce an isolated attosecond pulse with a rather short duration.

Nondestructive measurement of refractive index profile of gradient refractive index rod lens.

In this paper, a simple nondestructive method is described to obtain the refractive index profile of a gradient refractive index rod lens by means of optical coherence tomography. The approach exploits the fact that optical coherence tomography provides a direct measurement of the optical path of the light traveled through a gradient refractive index rod lens. The refractive index profile for a gradient refractive index rod lens is retrieved by iterative fitting of the optical path calculated by the ray tracing method with that experimentally measured using optical coherence tomography. The measured refractive index profile is in good agreement with theory.

Broadband water window supercontinuum generation with a tailored mid-IR pulse in neutral media.

We propose a new (to our knowledge) scheme to generate a broadband water window supercontinuum in a neutral rare-gas media. Using a phase-stabilized few-cycle mid-IR pulse tailored by a much weaker 800 nm pulse, a supercontinuum from 265 to 425 eV is observed, and the ionization probability is below 0.1%. The nonionized media enable us to adopt the right phase-matching technique to select the short path of the supercontinuum, and an isolated 80 as pulse with a central wavelength of 3.2 nm is straightforwardly obtained.

Gradient refractive index square lenses. I. Fabrication and refractive index distribution.

An equation for the distribution of refractive index of a gradient refractive index square lens has been established, and such a lens has been fabricated using ion exchange. The distributions of refractive indices at different angles of incidence are discussed. Experimental and theoretical data are compared and show good agreement.