Fragmentation of CO in Femtosecond Laser Fields

Fragmentation of CO in a linearly polarized femtosecond laser field within the intensity order of 10^14 W.cm^-2 at 820nm is investigated experimentally by using velocity mapping technique. According to the observed kinetic energy and angular distributions of different charged fragment ions, fragmentation channels of CO are proposed. The angular distributions provide helpful information for assigning the dissociation channels.     

Semiclassical Approach to Non-Sequential Double Ionization of Diatomic Molecules in Strong Laser Field

We develop a semiclassical model to describe the non-sequential double ionization of diatomic molecules in an intense linearly polarized field, achieving insight into the two-electron correlation effect in the ionization dynamics. Compared to the experimental data of nitrogen molecules, our model shows a good agreement in the tunnelling regime and a qualitative agreement in the over-barrier regime. We find that the classical collisional trajectories are the main source of the double ionization in the tunnelling regime. As a prediction of our theory, we also calculate the double ionization ratios of H2^2＋/H2^＋ for hydrogen molecules and predict a ratio less than that of nitrogen molecules.     

Structural Deformation of CO22+ in Intense Femtosecond Laser Fields

The angular distributions of CO^＋ from the dissociation of CO2^2＋ and CO2^＋ in intense femtosecond laser fields （45 fs, about 5 × 10^15 W/cm^2） are studied at a laser wavelength of 800nm based on the time-of-flight mass spectra of CO^＋ fragment ions. The experimental results show that structural deformation occurs in the charge state of CO2^2＋ and the CO^＋ maintains linear geometrical structure.     

Absence of Charge-Resonance-Enhanced Ionization in Attosecond Pulse Photoionization： Numerical Result on One-Dimensional H2^＋

We present a numerical result of photoionization rate for the one-dimensional molecular hydrogen ion model exposed to intense light of 1 × 10^16-2×10^16 W/cm^2, 55-as pulse duration, and 800nm wavelength. In contrast to the previous calculation result of charge-resonance-enhanced ionization for lower intensity and much longer pulse, our result exhibits an ionization saturation. The numerical results are interpreted in the field-dressed potential picture as over-the-barrier liberation of electrons. This extremely short pulsewidth and relatively high field phenomenon requests experimental demonstration.     

Extreme Ultraviolet Pulse Shaping with Aligned Molecules

We theoretically propose a simple scheme to shape the extreme ultraviolet pulse on the attosecond timescale with the aligned molecule. This may offer a new way to manipulate and control photon emission on the attosecond timescale.     

Double-Exponentially Decayed Photoionization in CREI Effect： Numerical Experiment on 3D H2^＋

On the platform of the 3D H2^＋ system, we perform a numerical simulation of its photoionization rate under excitation of weak to intense laser intensities with varying pulse durations and wavelengths. A novel method is proposed for calculating the photoionization rate： a double exponential decay of ionization probability is best suited for fitting this rate. Confirmation of the well-documented charge-resonance-enhanced ionization （CREI） effect at medium laser intensity and finding of ionization saturation at high light intensity corroborate the robustness of the suggested double-exponential decay process. Surveying the spatial and temporal variations of electron wavefunctions uncovers a mechanism for the double-exponentially decayed photoionization probability as onset of electron ionization along extra degree of freedom. Henceforth, the new method makes clear the origins of peak features in photoionization rate versus internuclear separation. It is believed that this multi-exponentially decayed ionization mechanism is applicable to systems with more degrees of motion.     

异核双原子和三原子分子在红外强激光场中的电离抑制

Ionization is the fundamental process in interaction of atoms/molecules with femtosecond strong laser fields. Comparing to atoms, molecules exhibit peculiar behaviors in strong-field ionization because of their diverse geometric structures, molecular electronic orbitals as well as extra nuclear degrees of freedom. In this study, we investigate strong field single and double ionization of carbon monoxide (CO) and carbon dioxide (CO₂) in linearly polarized 50-fs, 800-nm laser fields with peak intensity in the range of 2×10¹³ W/cm² to 2×10¹⁴ W/cm² using time-of-flight mass spectrometer. By comparing the ionization yields with that of the companion atom krypton (Kr), which has similar ionization potential to the molecules, we investigate the effect of molecular electronic orbitals on the strong-field ionization. The results show that comparing to Kr, no significant suppression is observed in single ionization of both molecules and in non-sequential double ionization (NSDI) of CO, while the NSDI probability of CO₂ is strongly suppressed. Based on our results and previous studies on homonuclear diatomic molecules (N₂ and O₂), the mechanism of different suppression effect is discussed. It is indicated that the different structure of the highest occupied molecular orbitals of CO and CO₂ leads to distinct behaviors in two-center interference by the electronic wave-packet and angular distributions of the ionized electrons, resulting in different suppression effect in strong-field ionization.     

Classical Effects of Carrier--Envelope Phase on Nonsequential Double Ionization

A classical microcanonical 1＋1-dimensional model is used to investigate the ion momentum distributions in nonsequential double ionization with linearly polarized few-cycle pulses. We find that the ion momentum distribution has a strong dependence on the carrier-envelope phase of the few-cycle pulse, which is consistent with the experimental results qualitatively. Back analysis shows that the ionization probability of the first electron at different phases and its returning kinetic energy play the main role on the ion momentum distributions.     

Gap Caused by Strong Pairing in the Ladder Model of DNA Molecules

By directly diagonalizing the Hamiltonian of the ladder model of deoxyribonucleic acid （DNA） molecules, the density of states is obtained. It is found that DNA behaves as a conductor when the interchain hopping is smaller than twice the intrachain one, otherwise, DNA behaves as a semiconductor.     

Theory of Tunnel Ionization in Complex Systems

We present a detailed derivation of the Ammosov-Delone-Krainov theory of tunnel ionization in complex systems （CS-ADK）. A few mistakes in the previous works have been found and corrected. The theory is then applied to CO2, showing that CS-ADK yields better agreement with experiment than the molecular ADK theory.     

High-Order Above-Threshold Ionization of H2+ in Intense Laser Field

The nonperturbative quantum electrodynamies method proposed by Fu et al. [Phys. Rev. A 75 （2007） 063419] is employed to study the high-order above-threshold ionization （ATI） of a diatomic molecule. Based on this frequency-domain theory, the high-order ATI process can be regarded as ATI followed by laser-assisted collision, where the total transition amplitude is the coherent summation of the contributions from each ATI channel. The angular-resolved ATI spectrum, which agrees with the results by Becket et al. based on the time-domain method, is obtained by this frequency domain theory. Furthermore, it is demonstrated that the interference characteristics representing the molecular structure in the ATI spectrum originates from the recollision of the electron with two-centre ion in each ATI channel.     

Ionization of fast rydberg atoms in longitudinal and transverse electric fields

The main properties of longitudinal and transverse electric field ionizers for fast Rydberg atoms n=21–40 have been investigated. The dispersion and the background due to collisional processes between fast atoms and residual gas molecules have been measured and calculated. The kinetic energy spread of ions formed by field ionization of Rydberg atoms and their trajectories have been calculated. The potassium beam energy was 3.9 keV.     

Laser pulse sequence effects on selective ionization in three-level systems driven by coherent radiation fields

Using the time-dependent Schrödinger equation we investigate the effect of different pulse sequences of two lasers on selective ionization in three-level systems driven by coherent radiation fields. It is assumed that inhomogeneous broadening as well as the radiative decay can be neglected during the laser pulses. We demonstrate to what an extent the selectivity given by the absorption or relaxation process can be increased by coherent interaction during the first step and different timing of the incoherent interaction during the second step of a two-step photoionization. A comparison is made with the rate-equation results. We could show that utilization of coherent excitation for high ionization selectivity is most efficient if the two laser pulses do not overlap.     

Time-resolved Stark effect in rapidly varying fields

The cycle-averaged ac Stark effect associated with the ^[ A _]2Σ⁺v^′=2?^[ X _]2Π1/2v^′=0 two-photon absorption of NO at intensities between 7.7 and 15.2 TW cm^-2 has been characterized in real time through a synergic combination of bichromatic laser experiments and quantum-dynamics calculations. Measurements of the fluorescence emitted by the Rydberg ^[ A _]2Σ⁺v^′=2 level as a function of time between Stark and probe components of a bichromatic field exhibit a characteristic evolution in temporal peak structure with Stark-field intensity, which is interpreted in terms of a time-dependent Floquet analysis of the laser–matter interaction. The experimental observations are consistent with a dynamic Stark shift of Δε_s(ω₁,ω₂)≤0.23 eV of the optical transition at these intensities. Received: 18 January 2002 / Revised version: 6 March 2002 / Published online: 24 April 2002     

Enhancement of Bichromatic High-Order Harmonic Generation by a Strong Laser Field and Its Third Harmonic

We investigate high-order harmonic generation （HHG） in a linearly polarized bichromatic field composed of a fundamental laser field with frequency w and an additional laser field with frequency 3w. The numerical results show that it is possible to enhance the intensity of most high harmonics in orders of magnitude. A most striking feature in the enhancement is that the intensity of several special high harmonics is practically impaired as compared with that in the monochromatic case. The qualitative explanation to the great enhancement is that the additional high-frequency field can provide new transition paths for electrons to reach the continuum. The relative phase between the fundamental field and its third harmonic field also affects the intensity of high-order harmonics near the cutoff efficiently.     

Measurement of Carrier-Envelope Phase and Field Strength of a Few-Cycle Pulse by Non-sequential Double Ionization

We propose a method to measure the carrier-envelop phase （CEP） and the intensity of a few-cycle pulse by controlling the non-sequentiai double ionization （NSDI） process. By using an additional static electric field, we can change the momentum distribution of the double-charged ions parallel to the laser polarization from an asymmetrical double-hump structure to a nearly symmetrical one. It is found that the ratio between the strength of the static electric field and that of the laser field is sensitive to the CEP but robust against the intensity fluctuation. Therefore we can determine the OEP of a few-cycle pulse precisely by measuring the static electric field. Fhrthermore, if the CEP of the few-cycle pulse is fixed at a certain value, we can also calibrate the intensity of the laser pulse by the static electric field.     

Simple and efficient photo-ionization loading of ions for precision ion-trapping experiments

We report a simple and efficient method to load a Paul trap with Ca⁺ ions. A beam of neutral atomic calcium is ionized in a two-step photo-ionization process using uv-diode lasers near 423 nm and 390 nm. Photo-ionization of a calcium beam for loading a Paul trap has first been demonstrated by Kjaergaard et al. The advantages of our method are the use of cheap and easily handled diode-laser systems and the large cross section for field ionization when exciting high-lying Rydberg states. Finally, we discuss the advantages of photo-ionization for ion generation compared to loading by electron bombardment. Received: 24 August 2001 / Revised version: 16 October 2001 / Published online: 23 November 2001     

Three-dimensional tomographic reconstruction of ultrashort free electron wave packets

We present a tomographic technique based on Photoelectron Angular Distributions (PADs) measured by Velocity-Map-Imaging (VMI) to reconstruct the three-dimensional shape of ultrashort free electron wave packets obtained from 1+2 Resonance Enhanced Multi-Photon Ionization (REMPI) of potassium atoms. To this end the laser pulse is rotated about its propagation direction and a set of PADs are recorded at different rotation angles. The tomographic reconstruction technique is described and results for linear and elliptical polarization are presented. Results for linearly polarized light producing cylindrically symmetric electron wave packets confirm the validity of our method whereas elliptically polarized light serves as a prototype for polarization-shaped laser pulses.     

Photoelectron angular distributions from strong-field coherent electronic excitation

Photoelectron Angular Distributions (PADs) resulting from nonperturbative excitation of potassium atoms using shaped femtosecond laser pulses are presented. We study control exerted by (1) the polarization of an unshaped, i.e., a bandwidth-limited light pulse, (2) shaped linearly polarized light, and (3) a combination of both degrees of freedom, i.e., polarization-shaped laser pulses. A theoretical approach to describe PADs from nonperturbative Resonance Enhanced Multi-Photon Ionization (REMPI) with ultrashort polarization-shaped laser pulses is presented and compared to experimental results. Applications of this technique to the generation and observation of atomic ring currents are discussed.     

One- and Two-Dimensional Arrays of Double-Well Optical Traps for Cold Atoms or Molecules

We propose a novel scheme to form one- and two-dimensional arrays of double-well optical dipole traps for cold atoms (or molecules) by using an optical system composed of a binary π-phase grating and a lens illuminated by a plane light wave, and study the relationship between the maximum intensity Imax of each optical well (or the maximum trapping potential Umax for ^85Rb atoms) and the relative aperture β (= α/f) of the lens. We also calculate the intensity gradients of each optical well and their curvatures, and estimate the spontaneous photon-scattering rate of trapped atom in each well, including Rayleigh and Raman scattering rates. Our study shows that the proposed 1D and 2D arrays of double-well traps can be used to prepare 1D and 2D novel optical lattices with cold atoms (or molecules), or form an all-optically integrated atom optical chip, or even to realize an array of all-optical double-well atomic (or molecular) Bose-Einstein condensates by optical-potential evaporative cooling, and so on.