Photofragmentation of Na2+ in intense femtosecond laser fields: from photodissociation on light-induced potentials to field ionization.

Photofragmentation of Na2 + molecules in well prepared vibrational levels has been studied employing intense ( 10(11)-10(14) W/cm2) and ultrashort (80 fs) 790 nm laser fields. Four fragmentation channels with different released kinetic energies are observed. Depending on the applied laser intensity, the fragmentation of Na2 + is governed by photodissociation on light-induced potentials and field ionization followed by Coulomb explosion. Below 1x10(12) W/cm2, only photodissociation on light-induced potentials is seen. For intermediate laser intensities, field ionization at large internuclear distances competes with photodissociation, thus preventing the observation of above threshold dissociation. Field ionization at small internuclear distances dominates for the highest laser intensities used.     

Enhanced ionization of perpendicularly aligned H2 in intense laser fields

Using three-dimensional classical ensembles, we have investigated the enhancement of double ionization of perpendicularly aligned H₂ molecules by a 800 nm laser pulse with intensity ranging from 1 × 10¹⁴ W/cm² to 6 × 10¹⁴ W/cm². The simulated results show that double ionization probability of H₂ strongly depends on R and reaches a maximum at an intensity independent critical distance R_C ≈ 5 a.u. Furthermore, the enhancement of double ionization is more pronounced in the cases of weaker or stronger fields. These results, a well indication of the influence of molecular structures and laser–molecule interactions on double ionization of diatomic molecules, are analyzed in detail and qualitatively explained based on the field-induced barrier suppression model and back analysis.     

Counterintuitive alignment of H2(+) in intense femtosecond laser fields

The multiphoton ionization of H2 has been studied using laser pulses of 266 nm wavelength, 250 fs duration, and 5x10(13) W/cm(2) peak intensity. Dissociation of H2(+) via one-photon absorption proceeds through two channels with markedly different proton angular distributions. The lower-energy channel (2.6 eV kinetic energy release) is produced in the bond softening mechanism, which generates parallel alignment. The higher-energy channel (3.5 eV) originates from population trapping in a light-induced bound state, where bond hardening generates orthogonal, counterintuitive alignment.     

Time-resolved imaging and manipulation of H2 fragmentation in intense laser fields

We report on the experimental realization of time-resolved coincident Coulomb explosion imaging of H2 fragmentation in 10(14) W/cm(2) laser fields. Combining a high-resolution "reaction microscope" and a fs pump-probe setup, we map the motion of wave packets dissociating via one- or two-photon channels, respectively, and observe a new region of enhanced ionization. The long-term interferometric stability of our system allows us to extend pump-probe experiments into the region of overlapping pulses, which offers new possibilities for the manipulation of ultrafast molecular fragmentation dynamics.     

Dissociation dynamics of H+2 in intense laser fields: investigation of photofragments from single vibrational levels

We present a photodissociation experiment on H+2 with ultrashort laser pulses ( >/=130 fs) at peak intensities of 相似文献   

Asymmetric electron-nuclear dynamics in two-color laser fields: laser phase directional control of photofragments in H+2

Exact non-Born-Oppenheimer numerical solutions of the time-dependent Schrodinger equation for the 1D H+2 molecule in an intense, two-color (omega+2omega) laser field have been obtained. Both electron and proton kinetic energy spectra show spatial, correlated, asymmetric distributions. The calculated spectra exhibit the same unusual correlations as in experiments, in which both positively charged nuclear fragments and negatively charged photoelectrons were preferentially emitted in the same direction. The above asymmetries of photoemission of electrons seen in our quantum simulation are interpreted in the framework of a quasistatic tunneling model.     

Dynamics of H9+ in intense laser pulses

We discuss the response of the H9+ cluster to an excitation by an intense laser beam. Calculations are done within the framework of TDLDA-MD, an approach which has proven to provide a robust tool for investigations in metal cluster dynamics. We first discuss static and low energy properties of H9+ and make comparisons with other existing ab initio results. We then explore the various excitation/deexcitation scenarios as a function of the laser parameters.     

Two-color laser wavelength effect on intense terahertz generation in air

Near-IR femtosecond lasers have been proposed to produce high-field terahertz radiation in the air via the laser-plasma interaction, but the physical mechanism still needs to be further explored. In this work, we theoretically investigate the effect of the two-color laser wavelength on the terahertz generation in the air based on a transient photocurrent model.We show that the long wavelength laser excitation can greatly enhance the terahertz amplitude for a given total laser intensity. Furthermore, we utilize a local current model to illustrate the enhancement mechanism. Our analysis shows that the terahertz amplitude is determined by the superposition of contributions from individual ionization events, and for the long wavelength laser excitation, the electron production concentrates in a few ionization events and acquires the larger drift velocities, which results in the stronger terahertz radiation generation. These results will be very helpful for understanding the terahertz generation process and optimizing the terahertz output.     

Electronic and nuclear responses of fixed-in-space H2S to ultrashort intense laser fields

The Coulomb explosion dynamics of H2S, H2S3+-->H+ +S+ + H+, in ultrashort intense laser fields (12 fs, approximately 2 x 10(14) W/cm2) is studied by the coincidence momentum imaging of the three fragment ions. Different electronic and nuclear responses are identified depending on the direction of laser polarization epsilon in the molecular frame. The dependence can be interpreted in terms of the electronic and bonding characters of charge transfer states of H2S coupled to the electronic ground state.     

Electron density changes and high harmonics generation in H2 molecule under intense laser fields

Under interaction with a high-intensity laser field, the real-time femtosecond dynamics of the electron density in the H₂ molecule has been studied quantum mechanically. For this purpose, a time-dependent generalized nonlinear Schr?dinger equation of motion, developed earlier in our laboratory by combining density functional theory and quantum fluid dynamics in real space, is solved numerically at the equilibrium internuclear distance of the molecule. By employing various time-dependent calculated properties as probes, information and insight are obtained about the phenomena of excitation, ionization, bond-softening, dipole formation and high-harmonics generation. The present approach goes beyond the linear response formalism.     

强激光场中真空极化效应

通过求解狄拉克方程,对强激光场下真空极化问题进行了研究。理论计算结果表明：在仅随时间变化的电场下,要激发狄拉克海中负能级的电子,需要两个阈值条件,即激光场的电场强度大于等于1016 V/cm和激光场的持续时间大于等于10-21 s。前者主要保证负能态电子有足够的能量跃迁到正能态,后者主要是保证电子在跃迁过程中动量亏损得以补偿。     

强激光场中真空极化效应

 通过求解狄拉克方程,对强激光场下真空极化问题进行了研究。理论计算结果表明：在仅随时间变化的电场下,要激发狄拉克海中负能级的电子,需要两个阈值条件,即激光场的电场强度大于等于10¹⁶ V/cm和激光场的持续时间大于等于10^-21 s。前者主要保证负能态电子有足够的能量跃迁到正能态,后者主要是保证电子在跃迁过程中动量亏损得以补偿。     

Xenon clusters in intense VUV laser fields

A simple model is developed that quantitatively describes intense interactions of a vacuum ultraviolet (VUV) laser pulse with a xenon cluster. We find good agreement with a recent experiment [Nature (London) 420, 482 (2002)]]. In particular, the large number of VUV photons absorbed per atom, at intensities significantly below 10(16) W/cm(2), is now understood.