Jin Ming-Xing

Square-Preserving and Symplectic Structure and Scheme for Quantum System

The time-dependent Schrodinger equation is a square-preserving and symplectic (SPS) transformation. The canonical equations of quantum systems are deduced by using eigenfunction expansion. The normal-square of wavefunction of the quantum systems is an invariant integral of the canonical equations and then the symplectic schemes that based on both Cayley transformation and diagonal Pade approximation to exp(x) are also square-preserving. The evaluated example show that the SPS approach is reasonable and effective for solving time-evolution of quantum system.

Ellipticity-dependent ionization/dissociation of carbon dioxide in strong laser fields*

Ionization and dissociation of linear triatomic molecules, carbon dioxide, are studied in 50-fs 800-nm strong laser fields using time-of-flight mass spectrometer. The yields of double charged ions and various fragment ions (CO+, On+, and Cn+ (n = 1, 2)) are measured as a function of ellipticity of laser polarization in the intensity range from 5.0 × 1013 W/cm2 to 6.0 × 1014 W/cm2. The results demonstrate that non-sequential double ionization, which is induced by laser-driven electron recollision, dominates double ionization of CO2 in the strong IR laser field with intensity lower than 2.0 × 1014 W/cm2. The electron recollision could also have contribution in strong-field multiple ionization and formation of fragments of CO2 molecules. The present study indicates that the intensity and ellipticity dependence of ions yields can be used to probe the complex dynamics of strong-field ionization/dissociation of polyatomic molecules.

Ultrafast Photodissociation Dynamics of the F State of Sulfur Dioxide by Femtosecond Time-Resolved Pump-Probe Method

A femtosecond pump-probe method is employed to study the dissociation dynamics of sulfur dioxide. SO2 molecules are excited to the F state by absorbing two photons of 267 nm femtosecond laser pulses, and ionized by 400 nm laser pulses at different delay times between the two lasers. Transients of both parent ions (SO+2) and the fragment ions (SO+, S+ and O+) are observed. The SO+2 transient can be well fitted to a biexponential decay comprising a fast and a slow component of 280 fs and 2.97 ps lifetimes, respectively. The SO+ transient consists of two growth components of 270 fs and 2.50 ps. The results clearly show that the F state of SO2 dissociates along an S-O bond. The transients of S+ and O+, however, have different behavior, which consist of a fast growth and a long decay component. A possible mechanism of the fragment formation is discussed to understand the dissociation dynamics of the F state of SO2.

A Theoretical Strategy to Generate an Isolated 80-Attosecond Pulse

Using a linearly polarized, phase-stabilized 2.66-femtosecond driving pulse of 400 nm central wavelength orthogonally combined with another linearly polarized long pulse of 800 nm central wavelength irradiating jointly on the helium atom, we demonstrate theoretically the generation of a clean isolated 80-attosecond pulse in the spectral region of 93–155 electron volts in a two-dimensional model.

Hexapole State-Selection and Beam Focus of Polar Top Molecules

We express a description of the state-selection role for a polar molecule in a hexapole electrostatic field. By a quantum mechanical treatment of the molecular Stark energy and a classical mechanical treatment for the molecular trajectory in the field, we present the calculated results of the different molecular rotational state selection and beam focus and discuss the influence of the high order Stark effect, the beam speed on the results for the symmetric top molecule CH3CN, CH3I, and the asymmetric top molecule CH2F2 in the hexapole field. The method established and results obtained can be taken as a guide for hexapole state selection and beam focus of polar top molecules.

Hexapole State-Selection and Beam Focus of Linear Triatomic Molecules

The state selection and beam focus of linear triatomic molecules (OCS, HCN, ClCN, BrCN and ICN) with doubling states in a hexapole electric field have been numerically realized. The method is based on a quantum mechanical treatment of the molecular Stark energy and a classical mechanical treatment for the molecular trajectory in the field. In linear molecules with doubling states, the second-order Stark effect can be neglected and the doubling states have the same value of J and M. The influences of the molecular properties, state energies, and the apparatus parameters such as molecular beam temperature and length of the hexapole, on the role of state selection and focus have been discussed. The method established here can be taken as a guide for hexapole experiment of orientation of polar molecules.

Rydberg excitation of neutral nitric oxide molecules in strong UV and near-IR laser fields*

Rydberg state excitations of neutral nitric oxide molecules are studied in strong ultraviolet (UV) and near-infra-red (IR) laser fields using a linear time-of-flight (TOF) mass spectrometer with the pulsed electronic field ionization method. The yield of Rydberg molecules is measured as a function of laser intensity and ellipticity, and the results in UV laser fields are compared with those in near-IR laser fields. The present study provides the first experimental evidence of neutral Rydberg molecules surviving in a strong laser field. The results indicate that a rescattering-after-tunneling process is the main contribution to the formation of Rydberg molecules in strong near-IR laser fields, while multi-photon excitation may play an important role in the strong UV laser fields.

Re-Heating Effect on the Enhancement of Plasma Emission Generated from Fe Under Femtosecond Double-Pulse Laser Irradiation*

In this paper, we present a study on the effect of inter-pulse delay using femtosecond double-pulse laser-induced breakdown spectroscopy in a collinear geometry. The temporal evolution of spectral intensity is performed for the lines of Fe I 423.60 nm, Fe I 425.08 nm and Fe I 427.18 nm. It is found that, by selecting appropriate inter-pulse delay, the signal enhancement can be significantly increased compared with the single-pulse case. A three-fold enhancement in the current experiment is obtained. The plasma temperature and electron density are also investigated based on the theory of Boltzmann plot and Stark broadening. We attribute the main mechanism for emission enhancement to the plasma re-heating effect.

Quasiclassical-Trajectory Investigation on the Isotopic Effect of H(D)+LiF→H(D)F+Li ( v =0–4, j =0) Reaction

A quasi-classical trajectory (QCT) method is employed to investigate the scalar properties and vector correlations of H+LiF→HF+Li and D+LiF→DF+Li reactions. The collision energy (Ecol = 4–25 kcal/mol) and vibrational excitation effects (v = 0–4) are studied by using the Aguado–Paniagua2-potential energy surface (AP2-PES) [J. Chem. Phys. 107 (1997) 10085]. The reaction probability, cross section and rate constant are calculated, which demonstrate obvious energy and vibrational excitation dependences in the probability, cross section, and a high-temperature region of the rate constant. In addition, two product angular distributions P(θr) and P(r) are calculated to facilitate a deeper insight into vector correlations. The H+LiF→HF+Li and D+LiF→DF+Li reactions reveal strong isotopic effects. Moreover, these scalar and vector results of both the reactions show sensitive behaviors to the changes of vibrational levels and the collision energy.

Field-free orientation of diatomic molecule via the linearly polarized resonant pulses

We propose a scheme to coherently control the field-free orientation of NO molecule whose rotational temperature is above 0 K. It is found that the maximum molecular orientation is affected by two factors: one is the sum of the population of M = 0 rotational states and the other is their distribution, however, their distribution plays a much more significant role in molecular orientation than the sum of their population. By adopting a series of linearly polarized pulses resonant with the rotational states, the distribution of M = 0 rotational states is well rearranged. Though the number of pulses used is small, a relatively high orientation degree can be obtained. This scheme provides a promising approach to the achievement of a good orientation effect.