Dynamic and geometric alignment of molecules induced by intense laser fields

We present a method to discuss simultaneously the relative importance of molecular dynamic and geometric alignment induced by intense laser fields in theoretical view. This method divides the process of molecular alignment into three steps, which are tightly correlated with that of molecular multielectron dissociative ionization and Coulomb explosion. A fourth-order Runge-Kutta algorithm and a developed counting approach are used to calculate the angular distribution of molecules in the first and second steps of molecular alignment. The last step is described by a field-ionization, Coulomb explosion model. The angular distribution of molecules at the critical distance originated from geometric alignment is obtained by calculating the volume of shells associated with a series of particular angle. The final angular distributions of molecules are obtained by properly weighting the results of three steps. The numerical results of distinguishing between dynamic and geometric alignment for certain conditions are presented and discussed. Our computational results show that the alignment mechanism, which dominates the observed anisotropy of angular distributions of ionic fragments for a given condition, is determined by the dependences of the extent of dynamic and geometric alignment on laser parameters and molecular parameters.     

Femtosecond laser-induced dynamic alignment in Coulomb explosion of CO: Roles of laser wavelength, intensity and pulse duration

Dependences of dynamic alignment of CO molecules induced by intense femtosecond laser fields on laser wavelength, intensity and pulse duration are investigated by numerical simulations. A counting approach and a fourth-order Runge-Kutta algorithm are used to calculate the angular distribution and the time evolution of molecules. A two-step Coulomb explosion model of diatomic molecules in intense laser fields is used to determine the instant that CO molecular dynamic alignment is over. Our calculating results show that the linear polarizability and the damping force play an important role in the angular rotation of CO molecule in conditions of 800 nm laser wavelength and 10¹⁵ W/cm² laser intensity. The contributions of the second-order field-induced dipole moment and the higher-order correction term to molecular rotation acceleration comparing to the linear polarizability and damping force are negligible. The extent of dynamic alignment of CO molecules reduces with the increasing of laser intensity. The dynamic alignment time of CO molecules is tightly connected to the laser pulse duration. The angular distributions of CO molecules as the laser pulse length varied from 50 to 250 fs at laser intensity of 3×10¹⁴ W/cm² are shown and discussed.     

Nonsequential double ionization of the aligned hydrogen molecule in strong field

This paper studies the nonsequential double ionization(NSDI) process of diatomic molecules aligned parallel and perpendicular to an intense linearly polarized laser field by using a three-dimensional semiclassical model.With this model,it achieves insight into the ion momentum distribution under the combined influence of a two-centre Coulomb potential and an intense laser field,and this result shows the significant influence of molecular alignment on the ratio between double and single ionization rate.Careful investigations show that the NSDI process for different alignment molecules has a close relation to the laser intensity and the different bounding electron distribution has a significant influence on the final ion momentum distribution.     

Laser-induced Alignment and Coulomb Explosion of CO2

实验研究了CO₂分子在飞秒强激光脉冲作用下的动力学过程,包括分子取向,隧穿电离和库仑爆炸,激光强度从1×10¹³W/cm²变化到6×10¹⁴W/cm². 当激光强度小于分子的电离阈值时,CO₂分子的非绝热转动激发形成一个相干转动波包,波包演化导致分子沿激光电场方向取向. 激光脉冲结束后,分子取向可以周期性地再现,利用另一束激光可以对取向结构进一步进行修饰. 当激光强度大于分子     

Numerical Simulations of femtosecond-laser-induced dynamic alignment of molecules in the high-frequency off-resonance regime

The motion equation for ? between the molecular axis and laser polarization direction in a high-frequency off-resonance femtosecond laser field is deduced while simultaneously examining the effects of a permanent dipole moment and field-induced polarizability and hyperpolarizability to molecular rotation. Femtosecond-laser-induced dynamic alignment of CO, N₂, and Br₂ molecules are investigated by numerically solving the obtained rotation equation for the angle ?. The effects of the molecular permanent dipole moment and the field-induced polarizability and hyperpolarizability on the degree of alignment are presented at different intensities. Our computational results show that the dynamic alignment of molecules is primarily determined by field-induced polarizability and the second hyperpolarizability for the laser intensity range from 5 × 10¹⁴ to 5 × 10¹⁶ W/cm². The contributions of higher order correction terms to molecular alignment can usually be neglected. The polarizability-field interaction makes the angular distributions of a molecule have a maximum along the polarization axis and a minimum perpendicular to it. The role of the second hyperpolarizability keeps the molecular counts maximum along the laser polarization direction but minimum at an angle of 45° between the molecular axis and the polarization direction. There is also a second maximum of molecular counts perpendicular to the polarization axis. For CO, N₂, and Br₂ molecules, the dependences of laser-induced dynamic alignment on laser intensity exhibit completely different characteristics.     

Angular distributions of CH3I fragment ions under the irradiation of single pulse and trains of ultrashort laser pulses

The angular distribution of CH₃I is investigated experimentally using a single Fourier transform-limited laser pulse and a pulse train, where a 90-fs 800-nm linearly polarized laser field with a moderate intensity of 2.8×10¹³ W/cm² is used. The dynamic alignment is demonstrated in a single pulse experiment. Moreover, a pulse train is used to optimize the molecular alignment, and the alignment degree is almost identical to that with the single pulse. The results are analysed by using chirped femtosecond laser pulses, and it demonstrates that the structure of pulse train rather than its effective duration is crucial to the molecular alignment.     

Role of highest occupied molecular orbitals in molecular field-free alignment by a femtosecond pulse

This paper studies the molecular rotational excitation and field-free spatial alignment in a nonresonant intense laser field numerically and analytically by using the time-dependent Schr?dinger equation. The broad rotational wave packets excited by the femtosecond pulse are defined in conjugate angle space, and their coefficients are obtained by solving a set of coupled linear equations. Both single molecule orientation angles and an ensemble of O₂ and CO molecule angular distributions are calculated in detail. The numerical results show that, for single molecule highest occupied molecular orbital (HOMO) symmetry σ tends to have a molecular orientation along the laser polarization direction and the permanent dipole moment diminishes the mean of the orientation angles; for an ensemble of molecules, angular distributions provide more complex and additional information at times where there are no revivals in the single molecule plot. In particular, at the revival peak instant, with the increase of temperature of the molecular ensemble, the anisotropic angular distributions with respect to the laser polarization direction of the π _g orbital gradually transform to the symmetrical distributions regarding the laser polarization vector and for two HOMO configurations angular distributions of all directions are confined within a smaller angle when the temperature of the molecular ensemble is higher.     

Active manipulation of the selective alignment by two laser pulses

This paper solves numerically the full time-dependent Schrdinger equation based on the rigid rotor model, and proposes a novel strategy to determine the optimal time delay of the two laser pulses to manipulate the molecular selective alignment. The results illustrate that the molecular alignment generated by the first pulse can be suppressed or enhanced selectively, the relative populations of even and odd rotational states in the final rotational wave packet can be manipulated selectively by precisely inserting the peak of the second laser pulse at the time when the slope for the alignment parameter by the first laser locates a local maximum for the even rotational states and a local minimum for the odds, and vice versa. The selective alignment can be further optimised by selecting the intensity ratio of the two laser pulses on the condition that the total laser intensity and pulse duration are kept constant.     

Photoelectron angular distributions of molecules in bichromatic laser fields of circular polarization

Photoelectron angular distributions (PADs) from above-threshold ionization of O₂ and N₂ molecules irradiated by a bichromatic laser field of circular polarization are studied. The bichromatic laser field is specially modulated such that it can be used to mimic a sequence of one-cycle laser pulses. The PADs are greatly affected by the molecular alignment, the symmetry of the initial electronic distribution, and the carrier-envelope phase of the laser pulses. Generally, the PADs do not show any symmetry, and become symmetric about an axis only when the symmetric axis of laser field coincides with the symmetric axis of molecules. This study shows that the few-cycle laser pulses can be used to steer the photoelectrons and perform the selective ionization of molecules.     

Multiorbital tunneling ionization of the CO molecule

We coincidently measure the molecular-frame photoelectron angular distribution and the ion sum-momentum distribution of single and double ionization of CO molecules by using circularly and elliptically polarized femtosecond laser pulses, respectively. The orientation dependent ionization rates for various kinetic energy releases allow us to individually identify the ionizations of multiple orbitals, ranging from the highest occupied to the next two lower-lying molecular orbitals for various channels observed in our experiments. Not only the emission of a single electron, but also the sequential tunneling dynamics of two electrons from multiple orbitals are traced step by step. Our results confirm that the shape of the ionizing orbitals determine the strong laser field tunneling ionization in the CO molecule, whereas the linear Stark effect plays a minor role.     

分子的飞秒强光电离

文章以一个实验者的角度，介绍了分子的飞秒强光电离的研究现状。文章从对比飞秒激光电离质谱与纳秒激光电离质谱开始，接着介绍分子在激光场作用下的可能电离机理，着重描述了几个处理分子场致电离的理论模型和实验验证，最后对飞秒激光导致的分子在激光脉冲作用后取向研究进行了简单介绍。     

Angular momentum distribution in strong-field frustrated tunneling ionization

Using the classical-trajectory Monte Carlo model, we have theoretically studied the angular momentum distribution of frustrated tunneling ionization(FTI) of atoms in strong laser fields. Our results show that the angular momentum distribution of the FTI events exhibits a double-hump structure. With this classical model, we back traced the tunneling coordinates, i.e., the tunneling time and initial transverse momentum at tunneling ionization. It is shown that for the events tunneling ionized at the rising edge of the electric field,the final angular momentum exhibits a strong dependence on the initial transverse momentum at tunneling.While for the events ionized at the falling edge, there is a relatively harder recollision between the returning electron and the parent ion, leading to the angular momentum losing the correlation with the initial transverse momentum. Our study suggests that the angular momentum of the FTI events could be manipulated by controlling the initial coordinates of the tunneling ionization.     

Ionic Angular Distributions Induced by Strong-Field Ionization of Tri-Atomic Molecules

Angular distributions of fragment ions from ionization of several tri-atomic molecules(CO_2, OCS, N_2O and NO_2) by strong 800-nm laser fields are investigated via a time-of-flight mass spectrometer. Anisotropic angular distributions of fragment ions, especially those of atomic ions, are observed for all of the molecules studied.These anisotropic angular distributions are mainly due to the geometric alignment of molecules in the strong field ionization. Distinct different patterns in ionic angular distributions for different molecules are observed. It is indicated that both molecular geometric structure and ionization channels have effects on the angular distributions of strong field ionization/fragmentation.     

Quantum interference of multi-orbital effects in high-harmonic spectra from aligned carbon dioxide and nitrous oxide

We investigate experimentally multi-orbital effects in high-order harmonic generation(HHG) from aligned CO_2 and N_2O molecules by intense femtosecond laser fields with linear and elliptical polarizations.For either of the aligned molecules, a minimum in the harmonic spectrum is observed, the position of which shifts to lower-order harmonics when decreasing the intensity or increasing the ellipticity of the driving laser.This indicates that the minimum originates from the dynamic interference of different channels, of which the tunneling ionization and recombination are contributed via different molecular orbitals.The results show that both the highest occupied molecular orbital(HOMO) and low-lying HOMO-2 in CO_2(or HOMO-1 in N_2O) contribute to the molecular HHG in both linearly and elliptically polarized strong laser fields.Our study would pave a way for understanding multi-electron dynamics from polyatomic molecules irradiated by strong laser fields.     

Experimental observation of revival structures in picosecond laser-induced alignment of I2

We report the experimental observation of revival structures in the alignment of a ground-state rotational wave packet following nonresonant excitation of I2 molecules by an intense picosecond laser pulse. The revivals appear at characteristic time delays following the excitation by the pump laser pulse, and show a significant narrowing of the angular distribution during a few picoseconds. The interaction with the pump laser also leads to a steady-state alignment of the molecule, due to rotational pumping.     

Role of rotational state-selected for nonadiabatic alignment: OCS molecules in femtosecond laser fields

Nonadiabatic alignment by intense nonresonant laser fields is a versatile technique to manipulate the spatial direction of molecules. By solving the time-dependent Schrödinger equation numerically the degree of alignment of the molecules initially in different rotational state are calculated and the results show that the degree of alignment strongly depends on the initial rotational state. Thus, the present study indicates that, for obtaining a high degree of alignment for molecules, appropriate selection of molecular rotational states is necessary.     

Alignment-dependent strong field ionization of molecules

We demonstrate a method to measure strong field laser ionization of aligned molecules. The method employs a macroscopic field-free dynamic alignment, which occurs during revivals of rotational wave packets produced by a femtosecond laser pulse. We investigate the dependence of strong field ionization of N2 on molecular orientation. We determine that N2 molecules are four times more likely to ionize when aligned parallel to the field than when aligned perpendicular to it.     

Ionization effects on field-free molecular alignment observed with high-order harmonic generation

In this paper, high-order harmonic generation (HHG) is demonstrated to provide a sensitive way for studying the dynamic process in the field-free molecular alignment. The dependence of the harmonic yield and the degree of alignment on the intensity of aligning pulse is observed in impulsively aligned CO₂ molecules. A good agreement is found between the experimental and calculated results. At low pump intensities, the harmonic yield increases monotonously until the ionization induced refractive index gradient and neutral molecule depletion are significant. The results show that the optimum intensity is around 8.95 × 10¹³ W/cm²; this value is related to the molecular ionization potential.     

Classical theory of the molecule alignment in a laser field

In the classical mechanics framework the solution is given for the problem of the two-atomic molecule alignment in a field of linearly polarised laser pulse. The numerical results are presented for the Cl₂ molecule, together with the analytical results in the limit of a very short and strong laser pulse. The width of the angular distribution for molecular axes in field is demonstrated to depend on the radiation intensity as S ^?1/4, the alignment along the polarisation direction is possible as well as in the transversal direction. The analysis of some experimental data is given.     

纳米级润滑膜分子排列取向的拉曼光谱表征技术

利用激光拉曼散射技术,对剪切作用下受限于钢球与石英盘之间的纳米级液晶5 CB的分子排列取向进行研究. 结果表明,在特定的实验条件下,可以得到高信噪比的纳米级润滑膜的拉曼散射信号(20∶1). 同时发现,当激光偏振方向与剪切运动方向平行(垂直)时,所得拉曼信号强度达到最大(小)值,表明纳米级液晶5 CB分子在剪切诱导作用下,沿剪切运动方向趋于定向排列. 另外,当钢球与石英盘之间的剪切速度逐渐增大时,受限的纳米级液晶5 CB的拉曼信号强度也逐渐增大. 最后,利用根据相对光强干涉原理研制的纳米膜厚测量仪对纳米级 关键词： 薄膜润滑 分子排列取向 拉曼散射