强激光场中H2^＋离解行为的研究

在多电子态模型下，利用二阶劈裂传播子算法计算了Ｈ２＾＋在强激光场中的离散行为。结果发现在高频场时分子的多光子离散碎片是近对称的，而对于低频场，在所考虑的激光强度下Ｈ２＾＋是被隧道离散的，因而其离散碎片Ｈ＾＋分布是反对称的，另外，文中还探讨了三脉冲形状对分子离解几率、离解碎片动力学行为的影响。     

Photoemission and ionization of He+ under simultaneous irradiation of fundamental laser and high-order harmonic pulses

We theoretically study the response of He+ exposed simultaneously to an intense Ti:sapphire laser and its 27th or 13th harmonic pulses. High-order harmonic emission from He+ is enhanced by many orders of magnitude compared with the case of the fundamental pulse alone. Moreover, while an individual 10 fs laser (wavelength lambda(F)=800 nm) or its 27th harmonic pulse with a peak intensity of 3 x 10(14) and 10(13) W/cm(2), respectively, ionizes no more than 5 x 10(-6) of He+, their combined pulse leads to a surprisingly high He2+ yield of 17%.     

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.     

Dissociation and ionization of H+2 by ultrashort intense laser pulses probed by coincidence 3D momentum imaging

Laser-induced dissociation and ionization of H(+)(2) were simultaneously measured using coincidence 3D momentum imaging, allowing direct separation of the two processes, even where the fragment kinetic energy is the same for both processes. The results for 45 and 135 fs 790 nm pulses with an intensity of approximately 2.5 x 10(14) W/cm(2) differ from each other much more than one would expect from previous measurements with longer pulses. Ionization was negligible for the longer pulse and was strongly aligned along the laser polarization for the shorter pulse, but showed no structure in its kinetic energy distribution. In addition, the ionization to dissociation ratio was found to be much smaller than theoretically predicted for H(+)(2).     

Coherent control of photofragment separation in the dissociative ionization of IBr

We have investigated coherent control of the dissociative ionization of IBr using phase-controlled two-color omega+2omega laser pulses with an intensity of 1.0 x 10(12) W/cm and a pulse duration of 130 fs. The directional asymmetries of the photofragment angular distributions showed oscillation behavior dependent on the relative phase difference between the omega and 2omega pulses. The phase dependencies of the directional asymmetries observed for iodine ions and bromine ions were out of phase with each other. This result shows that a phase-controlled omega+2omega optical field can produce molecular orientation in which the optical field discriminates between parallel and antiparallel configuration of molecules that have a permanent dipole.     

Coincident fragment detection in strong field photoionization and dissociation of H2

Electron-ion momentum spectroscopy is used to investigate the correlated electronic and nuclear motion in fragmentation of H2 in 4 x 10(14) W/cm(2), 25 fs laser pulses at 795 nm. Reaction channel dependent photoelectron spectra indicate that besides the main, stepwise H2 ionization H2(+) dissociation mechanism resulting in the products H(1s) + H(+) + e(-) a second new mechanism has to be assumed. The momentum distribution of H(+) ions in the dissociation channels H(1s) + H(+) + e(-) and 2H(+) + 2e(-) is found to be independent of the kinetic energy of the photoelectrons.     

DYNAMICS of H+2 in TWO-COLOR ULTRASHORT LASER PULSE

Numerical simulations of ionization and dissociation processes of hydrogen molecular ion H₂⁺ interacting with two-color intense ( 10¹⁴W/cm²-10¹⁵W/cm²) ultrashort (the duration ≈ 22fs) laser pulse are made. The result shows that the ionization and dissociation processes are strongly dependent upon the relative phase between the two color fields. It means that, in the case of ultrashort pulse, the phase coherence control of ionization and dissociation processes can be realized.     

Laser phase control of high-order harmonic generation at large internuclear distance: the H+ -H2+ system

Exact (Born-Oppenheimer) 3-D numerical solutions of the time-dependent Schr?dinger equation are obtained for the one electron linear H+-H2+ atom-molecule system at large internuclear distance R in interaction with two-cycles intense (I>10(14) W cm(-2)) 800 nm laser pulses. High-order harmonic generation (HHG) spectra are obtained with an energy cutoff larger than the atomic maximum of I(p)+3U(p), where I(p) is the ionization potential and U(p) is the ponderomotive energy. At large R, this extended cutoff is shown to be related to the nature of electron transfer, whose direction is shown to depend critically on the carrier-envelope phase (CEP) of the ultrashort pulse. Constructive and destructive interferences in the HHG spectrum resulting from coherent superpositions of electronic states in the H+-H2+ system are interpreted in terms of multiple electron trajectories extracted from a time profile analysis.     

多周期激光相位对H2+谐波频移的影响

理论研究了多周期激光相位角对H2+谐波频移的影响。结果表明,在多周期激光驱动下H2+谐波光谱在零相位时呈现蓝移现象。随着激光相位增大,谐波光谱由蓝移转向红移。随着激光相位进一步增大,谐波红移减弱。理论分析表明谐波频移是由激光上升和下降区域谐波辐射强度变化引起的。并且谐波辐射强度变化对激光相位比较敏感。     

Yb3+-doped YVO4 crystal for efficient Kerr-lens mode locking in solid-state lasers

We report the first demonstration, to our knowledge, of soft-aperture Kerr-lens mode locking in a diode-pumped femtosecond Yb3+:YVO4 laser. Near-transform-limited pulses as short as 61 fs are generated around a center wavelength of 1050 nm with an output power of 54 mW and a pulse repetition frequency of 104.5 MHz. This is, to our knowledge, the shortest pulse generated directly from an Yb laser having a crystalline host material. The femtosecond operation has a mode-locking threshold at an absorbed pump power of 190 mW. The nonlinear refractive indexes of the Yb3+:YVO4 crystal have been measured to be 19 x 10(-16) cm2/W and 15 x 10(-16) cm2/W for the sigma and pi polarizations, respectively, at 1080 nm.     

Above threshold dissociation of vibrationally cold HD+ molecules

The experimental study of molecular dissociation of H2+ by intense laser pulses is complicated by the fact that the ions are initially produced in a wide range of vibrational states, each of which responds differently to the laser field. An electrostatic storage device has been used to radiatively cool HD+ ions enabling the observation of above threshold dissociation from the ground vibrational state by 40 fs laser pulses at 800 nm. At the highest intensities used, dissociation through the absorption of at least four photons is found to be the dominant process.     

啁啾脉冲对氢分子离子解离动力学的影响

本文通过数值求解一维含时薛定谔方程,理论研究了超短啁啾脉冲（5-fs,790-nm）对处于低振动态氢分子离子解离动力学的影响. 研究过程中有效利用流算符的方法得到不同激光强度下的解离几率及核波包密度分布. 结果表明：低强度的啁啾脉冲可显著增强低振动态氢分子离子的解离过程,使解离过程提前发生并在较短的时间内获得较多的解离产物, 并给出相应的理论解释.     

Semi-classical explanation for the dissociation control of H_2~+

A semi-classical model is utilized to explain the dissociation control of the hydrogen molecular ion(H+2). By analyzing the curve of the dissociation asymmetry parameter as a function of the time delay between the exciting and steering pulses, we find that the dissociation control is dependent not only on the peak intensity and direction of the electric field of the steering pulse, but also on the peak intensity of the exciting pulse.     

Controlling vibrational wave packet motion with intense modulated laser fields

Intense, nonresonant laser fields produce Stark shifts that strongly modify the potential energy surfaces of a molecule. A vibrational wave packet can be guided by this Stark shift if the laser field is appropriately modulated during the wave packet motion. We modulated a 70 fs laser pulse with a period on the time scale of the vibrational motion (approximately 10 fs) by mixing the signal and idler of an optical parametric amplifier. We used ionization of H2 or D2 to launch a vibrational wave packet on the ground state of H2(+) or D2(+). If the laser intensity was high as the wave packet reached its outer turning point, the Stark shift allowed the molecule to dissociate through bond softening. On the other hand, if the field was small at this critical time, little dissociation was measured. By changing the modulation period, we achieved control of the dissociation yield with a contrast of 90%.     

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   

啁啾引入时刻对H_2~+谐波光谱的影响

理论研究了长脉宽激光驱动下啁啾引入时刻对H_2~+高次谐波光谱的影响.结果表明,在长脉宽激光驱动下,H_2~+在激光前半程区域具有较高的谐波辐射效率.因此,在激光前半程引入啁啾调控可获得高强度光谱连续区.此外,在啁啾调控区域引入单极激光场可以进一步获得高能谐波连续区.最后,叠加谐波连续区的谐波,可获得39 as的孤立脉冲.     

Electron localization of linear symmetric molecular ion H_3~(2+)

Electron localization in the dissociation of the symmetric linear molecular ion H_3~(2+) is investigated. The numerical simulation shows that the electron localization distribution is dependent on the central frequency and peak electric field amplitude of the external ultrashort ultraviolet laser pulse. When the electrons of the ground state are excited onto the 2pσ~2Σ_u~+ by a one-photon process, most electrons of the dissociation states are localized at the protons on both sides symmetrically. Almost no electron is stabilized at the middle proton due to the odd symmetry of the wave function. With the increase of the frequency of the external ultraviolet laser pulse, the electron localization ratio of the middle proton increases, for more electrons of the ground state are excited onto the higher 3pσ~2Σ_u~+ ustate. 50.9% electrons of all the dissociation events can be captured by the middle Coulomb potential well through optimizing the central frequency and peak electric field amplitude of the ultraviolet laser pulse. Besides, a direct current(DC) electric field can be utilized to control the electron motions of the dissociation states after the excitation of an ultraviolet laser pulse, and 68.8% electrons of the dissociation states can be controlled into the middle proton.     

Control of electron localization in the dissociation of H2+ and its isotopes with a THz pulse

The molecular dissociation with a two-laser-pulse scheme is theoretically investigated for the hydrogen molecular ion(H+2)and its isotopes(HD+and HT+).The terahertz pulse is used to steer the electron motion after it has been excited by an ultrashort ultraviolet laser pulse and an unprecedented electron localization ratio can be achieved.With the coupled equations,the mass effect of the nuclei on the effective time of the electron localization control is discussed.     

Electron localization of H_2~+ in a dc electric field

A dc electric field is utilized to steer the electron motion after the molecular ion H_2~+ is excited by an ultrashort ultraviolet laser pulse. The numerical simulation shows that the electron localization distribution and the dissociation control ratio are dependent on the polarization direction and amplitude of the dc electric field. Most electrons of the dissociation state move opposite to the dc electric field and stabilize at the dressed-up potential well, for the dressed-down well is occupied by the electrons of the 1 sσ_g state.     

H_2~+分子双H核对高次谐波辐射的贡献

理论研究了H_2~+分子双H核对高次谐波辐射的贡献.结果表明:在少周期激光场下,由于激光场的反对称性,负向H核辐射谐波强度高于正向H核.随着激光脉宽增大,激光波形趋于对称,因此导致双H核辐射谐波的反对称结构减小.谐波辐射的时频分析图显示,当激光场为正向时(E(t)0.0),负向H核辐射谐波强度高于正向H核;当激光场反向时(E(t)0.0),正向H核辐射谐波强度高于负向H核.最后,通过分析含时电子波包及H_2~+的缀饰态给出了电子在双H核之间转移的原因.