拉伸分子高次谐波产生的阿秒控制

通过数值求解一维含时薛定谔方程,本文研究了高频阿秒脉冲对拉伸双原子分子模型与低频飞秒激光脉冲相互作用产生高次谐波谱的影响.研究表明,若阿秒脉冲在低频脉冲的第二个光学周期截止处注入时,只第二个平台截止位置IP 5.6UP(IP为电离势,UP为有质动力能)附近谐波的效率得到显著提高;若阿秒脉冲的包络峰值处于低频脉冲第三个光学周期T/4(为飞秒激光脉冲的光学周期)附近时,则谐波谱出现了明显的整体抬高现象.对此,本文根据拉伸分子谐波谱的四种产生机制及阿秒脉冲在飞秒激光不同相位注入时的电子电离曲线,对这些谐波谱的结构给出了合理解释.     

Attosecond pulse extreme-ultraviolet photoionization in a two-color laser field

Attosecond-pulse extreme-ultraviolet (XUV) photoionization in a two-color laser field is investigated. Attosecond pulse trains with different numbers of pulses are examined, and their strong dependence on photoelectronic spectra is found. Single-color driving-laser-field-assisted attosecond XUV photoionization cannot determine the number of attosecond pulses from the photoelectronic energy spectrum that are detected orthogonally to the beam direction and the electric field vector of the linearly polarized laser field. A two-color-field-assisted XUV photoionization scheme is proposed for directly determining the number of attosecond pulses from a spectrum detected orthogonally.     

Attosecond resolved electron release in two-color near-threshold photoionization of N2

We have simulated two-color photoionization of N(2) by solving the time-dependent Schr?dinger equation with a simple model accounting for the correlated vibronic dynamics of the molecule and of the ion N(2)(+). Our results, in very good agreement with recent experiments [Haessler et al., Phys. Rev. A 80, 011404 (2009)], show how a resonance embedded in the molecular continuum dramatically affects the phases of the two-photon transition amplitudes. In addition, we introduce a formal relation between these measurable phases and the photoelectron release time, opening the way to attosecond time-resolved measurements, equivalent to double-slit experiments in the time domain.     

Double photoionization of rotating linear molecules

This communication presents a theoretical study of the angular distribution of one or both of the two electrons emitted in one-photon, one-step double ionization of a linear molecule. Experiments which do or do not detect spin of the photoelectrons have been considered. Effects of molecular rotation on double photoionization have been studied in both Hund's coupling schemes (a) and (b) by using parity-adapted states. Selection rules obtained in this paper are very different from those derived earlier for single photoionization and for Auger decay following the absorption of a photon in a rotating linear molecule. It is shown that complete specification of the spin-unresolved and of spin-resolved angular distributions of both photoelectrons require, respectively, three and seven parameters which depend, among other things, on their energies as well as directions of emission. The approach developed in this paper has been used to analyze spin-unresolved double photoionization in the shell of the molecule. The angular distribution is quite different depending on whether or not molecular rotation has been taken into account. Also, it is found to change significantly for different rotational transitions. Effects of electron-electron correlation are clearly manifested even in non-coincident, both rotationally resolved--as well as unresolved--double photoionization. Received: 7 August 1998     

Femtosecond control of photoionization and photodissociation of sodium iodine molecules by laser pulse

Wave packet propagation techniques are used to find experimentally reliable laser parameters that yield optimal production.The photoionization and photodissociation dynamics of sodium iodine molecules are interpreted into several channels.Several frequencies are found to be suitable for NaI molecules during the photoionization and dissociation processes.Photon-dressed excited states and electron-dressed ionic continuum states facilitate the search for available laser parameters.     

Attosecond control of ionization by wave-packet interference

A train of attosecond pulses, synchronized to an infrared (IR) laser field, is used to create a series of electron wave packets (EWPs) that are below the ionization threshold in .helium. The ionization probability is found to strongly oscillate with the delay between the IR and attosecond fields twice per IR laser cycle. Calculations that reproduce the experimental results demonstrate that this ionization control results from interference between transiently bound EWPs created by different pulses in the train. In this way, we are able to observe, for the first time, attosecond wave-packet interference in a strongly driven atomic system.     

Attosecond manipulation of ionization and efficient broadband supercontinuum generation in stretched molecules

<正>We theoretically investigate the high-order harmonic generation from stretched molecules in a linearly polarized intense field.By adopting an infrared pulse combined with an ultraviolet(UV) attosecond pulse,the ionization process can be controlled effectively.In this excitation scheme,the harmonic spectrum beyond I_p+3.17U_P is significantly enhanced by two orders,where I_p and U_p=e~2E_0~2/(4m_eω~2) are the ionization and ponderomotive potential,then smooth broadband supercontinuum with the bandwidth of about 120 eV is obtained,which leads to an isolated sub-60- as attosecond pulse with a high signal-noise ratio.Moreover,the bandwidth of the supercontinuum is weakly dependent on the location and pulse duration of the UV pulse.     

Attosecond photoelectron spectroscopy of electron tunneling in a dissociating hydrogen molecular ion

We demonstrate the potential of intense-field pump, attosecond probe photoelectron spectroscopy to monitor electron tunneling between the two protons during dissociative ionization of the hydrogen molecule, with attosecond temporal and Angstrom-scale spatial resolution.     

Ab initio calculations of the photoionization of diatomic molecules

A review is presented of the calculation of photoionization spectra, particularly in the spectral range where electron autoionization of diatomic molecules takes place. In addition to some interesting results obtained over years that compare favourably with experiment, the emphasis here is put on the relation between the methods developed for the calculation of observables associated with the continuum energy spectrum of the electrons and the Alchemy system of programs. This system of programs serves as a basis for initial and intermediate calculations. The examples presented show that diatomic molecules not only in gas phase but also oriented in space or physisorbed at surfaces may be studied readily.     

Synchrotron radiation VUV double photoionization of some small molecules

The VUV double photoionizations of small molecules (NO,CO,CO2 ,CS2 ,OSC and NH3 ) were investigated with photoionization mass spectroscopy using synchrotron radiation. The double ionization energies of molecules were determined with photoionization efficiency spectroscopy. The total energies of these molecules and their parent dications (NO2+ ,CO2+ ,CO2+2 ,CS2+2 ,OSC2+ and NH2+3 ) were calculated using the Gaussian 03 program and Gaussian 2 calculations. Then,the adiabatic double ionization energies of the molecules were predicated by using high accuracy energy mode. The experimental double ionization energies of these small molecules were all in reasonable agreement with their respective calculated adiabatic double ionization energies. The mechanisms of double photoionization of these molecules were discussed based on a comparison of our experimental results with those predicted theoretically. The equilibrium geometries and harmonic vibrational frequencies of molecules and their parent dications were calculated by using the MP2 (full) method. The differences in configurations between these molecules and their parent dications were also discussed on the basis of theoretical calculations.     

Orthogonalized-plane-wave approximation in the calculation of photoionization cross sections of molecules

The physical bases of the OPW approximation in the calculation of molecular photoionization cross sections are considered in terms of the scattering theory, and the ways of improving the calculation within the framework of OPW approximation are discussed.Numerical calculations are done on several molecules by the ordinary OPW method and by its modifications using the bound orbitals obtained by ab initio SCFMO calculations with gaussian basis set.     

Laser photoionization and mass-spectral detection of single naphthalene molecules

Detection of naphthalene molecules at partial pressure of 10^-14 Torr in the photoionization volume of a mass spectrometer and with relative concentration in air of 10^-7%, which corresponds to 4 molecules in the ionization volume, is reported.     

Attosecond spectroscopy for filming the ultrafast movies of atoms,molecules and solids

Three decades ago, a highly nonlinear nonpertubative phenomenon, now well-known as the high harmonic generation (HHG), was discovered when intense laser irradiates gaseous atoms. As the HHG produces broadband coherent radiation, it becomes the most promising source to obtain attosecond pulses. The door to the attosecond science was opened ever since. In this review, we will revisit the incredible adventure to the attoworld. Firstly, the progress of attosecond pulse generation is outlined. Then, we introduce the efforts on imaging the structures or filming the ultrafast dynamics of nuclei and electrons with unprecedented attosecond temporal and Angstrom spatial resolutions, utilizing the obtained attosecond pulses as well as the high harmonic spectrum itself.     

Attosecond twin-pulse control by generalized kinetic heterodyne mixing

Attosecond double-pulse (twin-pulse) production in high-order harmonic generation is manipulated by a combination of two-color and carrier-envelope phase-control methods. As we show in numerical simulations, both relative amplitude and phase of the double pulse can be independently set by making use of multidimensional parameter control. Two technical implementation routes are discussed: kinetic heterodyning using second-harmonic generation and split-spectrum phase-step control.     

General symmetry selection rules for the photoionization of polyatomic molecules

The model for the reproduction of the intensity distributions within ligand-field d-d spectra, presented in the preceding paper, is applied here to 24 chromophores. Eight have approximate tetrahedral geometry, five are trigonal bipyramidal, and eleven are trigonally distorted octahedra. All intensity analyses rest on prior ligand-field analyses which are collected together here, some being reported for the first time. Good reproduction of relative intensities has been achieved for all the four- and five-coordinated complexes and for none of the six-coordinate. It is presumed that the intensities of these latter species are dominated by vibronic mechanisms.     

Laser multiphoton and multistep photoionization of molecules and mass spectrometry

A review of laser-induced photoionization of polyatomic molecules and the application of this process in mass-spectrometry is presented. Combination of laser selective photoionization and mass spectral analysis of fragments as a promising tool for the detection of trace amounts of complex molecules is considered particularly.     

Multiple photoionization of atoms and small molecules by synchrotron radiation

Absorption of one VUV photon by an atom or a molecule can induce the ejection of several electrons through different processes. Such multiple ionization processes, studied by coincidence electron spectroscopy, provide a wealth of information on electron correlations. A magnetic bottle electron time of flight spectrometer implemented on synchrotron radiation centers has allowed the efficient detection in coincidence of two, three and up to five electrons with good energy resolution. The branching ratios of the different processes are easily extracted from the experimental spectra due to the constant transmission of the spectrometer. Multiple Auger decay was observed in rare gases atoms after inner-shell ionization, while core-valence and core-core initial double ionization followed by Auger decay are other pathways to multiple ionization. For molecules, Coulomb explosion with energy released in ionic fragments may occur after multiple ionization, nevertheless, coincidence electron spectroscopy can also provide a clear interpretation for peculiar decay channels in molecules.     

Large molecules reveal a linear length scaling for double photoionization

We have measured the ratio of doubly to singly charged parent ions of benzene, naphthalene, anthracene, and pentacene using monochromatized synchrotron radiation up to 30 eV above the corresponding threshold. Our measurements show a striking similarity between the ratio of doubly charged to all parent ions and the ratio for helium. Moreover, the magnitudes of the ratios for these molecules scale linearly with their lengths with an amazing accuracy. A high ratio, i.e., a high relative double-photoionization probability, makes a molecule an important source of low-energy electrons that can promote radiation damage of biomolecules [B. Bouda?ffa et al., Science 287, 1658 (2000)].     

Nuclear fusion in excited hydrogen molecules

We evaluate the nuclear fusion rates in the excited vibrational states of molecules of hydrogen isotopes. The ground state fusion rate is increased by about eight orders of magnitude but even in the most favorable situation it is out of any possible experimental test. We discuss the effects due to the nuclear potential in different hyperfine states, and the improvements attainable using coherent states and a solid phase.