Vibrationally state-selective photoassociation by infrared sub-picosecond laserr pulses: model simulations for O + H → OH(ν)

The quantum dynamics of a photoassociation reaction in the electronic ground state controlled by an infrared picosecond laser pulse is investigated. The association reaction O + H → OH(ν) is simulated by representative wavepackets. The OH molecule to be formed is modeled as a non-rotating Morse oscillator. It is shown that the initial free continuum state of O + H can be transferred selectively into a specified vibrational bound state by interaction with an infrared laser pulse. Optimal design of the laser control field leads to high association probability with very high vibrational state selectivity.     

用质谱和光电子能谱研究飞秒强场中的电离动力学

报导了用自制飞秒激光器通过飞秒多光子电离质谱和光电子能谱对飞秒强激光场与分子（氨、苯）相互作用的研究。飞秒激光脉宽约100fs,二倍频中心波长407.5nm,聚焦后脉冲功率密度达到1012W/cm2。氨的光电子能谱显示了（2+2）REMPI和（2+2）+1ATI、（2+2）+2ATI三组电子峰,每组峰又包括伸缩振动v1的带系,ATI峰的振动布居出现反转。随着光强增加,谱峰加宽而且振动能级出现平移。这些强场效应可用PonderomotivePotential解释。苯的飞秒质谱图与纳秒情况不同,分子离子为主,碎片峰很少。     

Acetylene-vinylidene isomerization in ultrashort intense laser fields studied by triple ion-coincidence momentum imaging

The isomerization of acetylene via hydrogen migration in intense laser fields (8 x 10(14) W/cm2) has been investigated by coincidence momentum imaging of the three-body Coulomb explosion process, C2H2 (3+)-->H+ + C+ + CH+. When ultrashort (9 fs) laser pulses are used, the angle between the momenta of C+ and H+ fragments exhibits a sharp distribution peaked at a small angle ( approximately 20 degrees ), showing that the hydrogen atom remains near the original carbon site in the acetylene configuration. On the other hand, a significantly broad distribution extending to larger momentum angles ( approximately 120 degrees ) is observed when the pulse duration is increased to 35 fs, indicating that the ultrafast isomerization to vinylidene is induced in the longer laser pulse.     

N4+的生成与检测

用从Nd:YAG激光器发出的波长532nm的激光(能量约为10mJ/pulse)溅射由BN、ZrN粉末分别压成的直径13mm、厚度约2～5mm的固体压片样品,生成了含氮团簇N4+。实验中,用高纯氮气为载气,辅助含氮团簇的生成并冷却激光溅射所产生的团簇。反射式飞行时间质谱仪监测反应过程证实N4+的存在,且其存在时间超过200μs。     

Determination of trace elements in steel by laser ablation inductively coupled plasma mass spectrometry.

A rapid quantitative analysis of the trace elements in steel by laser ablation inductively coupled plasma mass spectrometry is described. The conditions for laser ablation and normalization methods to improve the analytical precision are given. The optimum conditions for laser ablation were achieved when the ion yield was a maximum at the focus position in the fixed Q pulse mode, and above the focus position in the Q-switched pulse mode. It was found that the fixed Q pulse mode was most suitable for the determination of trace metal elements in steel, and that the Q-switched pulse mode was most suitable for both non-metallic elements and elements with a high boiling-point. In order to improve the analytical precision for those elements with a strong background intensity, normalization methods with both the matrix ion, 57Fe+, and 38Ar+ are proposed.     

激光等离子体中超金属氧化物离子LiZnO+的产生

作为一般规律 ,在一些具有热力学稳定性的氧化物分子中 ,氧原子周围共价电子数为 8.自从发现 Li3O和 Li4 O分子以来 ,这类超过 9个或更多共价电子的含金属团簇小分子呈现出来的特殊化学计量比和热力学稳定性引起人们的关注 ,人们称其为超共价分子 (Hypervalent Molecules)或超金属分子(Hypermetallic Molecules) [1～ 4 ] .目前 ,在实验上已发现了 Mn O,Mn S及 Mn C(M=Li,Na,K,Mg;n≥ 3 )等一系列超金属分子 ;在理论上 ,从头计算法已可计算出超金属分子的结构及其稳定性 ,使人们对这些超金属分子的产生与结构有一定的认识 .然而 ,由…     

Effect of laser parameters on ultrafast hydrogen migration in methanol studied by coincidence momentum imaging

The effect of intensity, duration, and polarization of ultrashort laser pulses (795 nm, 40-100 fs, and 0.15-1.5 × 10(15) W/cm(2)) on the hydrogen migration in methanol is systematically investigated using Coulomb explosion coincidence momentum imaging. The ratio of the ion yield obtained for the migration pathway CH(3)OH(2+) → CH(2)(+) + OH(2)(+) with respect to the sum of the yields obtained for the migration pathway and for the nonmigration pathway CH(3)OH(2+) → CH(3)(+) + OH(+) exhibits a small (10-20%) but clear dependence on laser pulse properties, that is, the ratio decreases as the laser peak intensity increases but increases when the pulse duration increases as well as when the laser polarization is changed from linear to circular.     

Open-loop and closed-loop control of dissociative ionization of ethanol in intense laser fields

The relative yield of the C-O bond breaking with respect to the C-C bond breaking in ethanol cation C2H5OH+ is maximized in intense laser fields (10(13)-10(15) Wcm2) by open-loop and closed-loop optimization procedures. In the open-loop optimization, a train of intense laser pulses are synthesized so that the temporal separation between the first and last pulses becomes 800 fs, and the number and width of the pulses within a train are systematically varied. When the duration of 800 fs is filled with laser fields by increasing the number of pulses or by stretching all pulses in a triple pulse train, the relative yield of the C-O bond breaking becomes significantly large. In the closed-loop optimization using a self-learning algorithm, the four dispersion coefficients or the phases of 128 frequency components of an intense laser pulse are adopted as optimized parameters. From these optimization experiments it is revealed that the yield ratio of the C-O bond breaking is maximized as far as the total duration of the intense laser field reaches as long as approximately 1 ps and that the intermittent disappearance of the laser field within a pulse does not affect the relative yields of the bond breaking pathways.     

The role of the laser pulse duration in infrared matrix-assisted laser desorption/ionization mass spectrometry

The role of the laser pulse duration in matrix-assisted laser desorption/ionization mass spectrometry with infrared lasers (IR-MALDI-MS) emitting in the 3 microm wavelength range has been evaluated. Mass spectrometric performance and characteristics of the IR-MALDI process were examined by comparing a wavelength-tuneable mid-infrared optical parametric oscillator (OPO) laser of 6 ns pulse duration, tuned to wavelengths of 2.79 and 2.94 microm, with an Er:YAG laser (lambda = 2.94 microm) with two pulse durations of 100 and 185 ns, and an Er:YSGG laser (lambda = 2.79 microm) with a pulse duration of 75 ns. Threshold fluences for the desorption of cytochrome C ions were determined as a function of the laser pulse duration for various common IR-MALDI matrices. For the majority of these matrices a reduction in threshold fluence by a factor of 1.2-1.9 was found by going from the 75-100 ns long pulses of the Erbium lasers to the short 6 ns OPO pulse. Within the experimental accuracy threshold fluences were equal for the 100 and the 185 ns pulse duration of the Er:YAG laser. Some pronounced pulse duration effects related to the ion formation from a glycerol matrix were also observed. The effect of the laser pulse length on the duration of ion emission was furthermore investigated.     

Controlling the strong field fragmentation of ClCHO+ using two laser pulses –an ab initio molecular dynamics simulation

For a single, intense 7 μm linearly polarized laser pulse, we found that the branching ratio for the fragmentation of ClCHO⁺ → Cl + HCO⁺, H + ClCO⁺, HCl⁺+CO depended strongly on the orientation of the molecule (J. Phys. Chem. Lett. 2012 , 3 2541). The present study explores the possibility of controlling the branching ratio for fragmentation by using two independent pulses with different frequencies, alignment and delay. Born-Oppenheimer molecular dynamics simulations in the laser field were carried out with the B3LYP/6-311G(d,p) level of theory using combinations of 3.5, 7 and 10.5 μm sine squared pulses with field strengths of 0.03 au (peak intensity of 3.15×10¹³ W/cm²) and lengths of 560 fs. A 3.5 μm pulse aligned with the C-H bond and a 10.5 μm pulse perpendicular to the C-H bond produced a larger branching ratio for HCl⁺+CO than a comparable single 7 μm pulse. When the 10.5 μm pulse was delayed by one quarter of the pulse envelope, the branching ratio for the high energy product, (HCl⁺+CO 73%) was a factor of three larger than the low energy product (Cl + HCO⁺, 25%). By contrast, when the 3.5 μm pulse was delayed by one quarter of the pulse envelope, the branching ratio was reversed (HCl⁺+CO 38%; Cl + HCO⁺, 60%). Continuous wavelet analysis was used to follow the interaction of the laser with the various vibrational modes as a function of time. © 2018 Wiley Periodicals, Inc.     

纳秒强激光电离苯产生高价碳离子的波长效应

The photoionization of seeded benzene beam by 25 ns laser pulse at wavelengths of 266,355 and 1064 nm has been studied by the time-of-flight mass spectrometry. The observed mass spectra at 266 nm and 355 nm at intensities of 1010-1011 W/cm2 indicate a multiphoton ionization and dissociation（MPID）process,in which C+,C2Hx+,C3Hx+,C4Hx+ and C6H6+ are main products. While at 1064 nm laser of similar intensities,the domain ion is C4+ which is produced from Coulomb explosion. The longer wavelength facilities the energy absorption rate during inverse bremsstrahlung,which leads to the resulting wavelength dependence of the multicharged atomic ions.     

N2分子在强激光场中的场致电离

研究了在强激光脉冲中各种不同取向的N2分子发生场致电离的电离几率和表观电离效率.用量子化学方法计算了N2+分子离子在各种不同取向上的势能曲线,然后用传递矩阵方法得到了N2分子在不同方向上的电离几率,经过角度平均之后得到了各种取向的所有N2分子的总电离几率,并对计算结果进行了激光时间和空间修正.用800nm和70fs的激光脉冲对N2分子进行了在强激光场中的电离实验,得到了N2分子电离后产生的电子的角度分布图和电离几率随激光功率密度变化的关系曲线.实验结果和理论计算结果符合得很好.     

The Investigation on Ultrafast Pulse Formation in a Tm–Ho-Codoped Mode-Locking Fiber Oscillator

We experimentally investigate the formation of various pulses from a thulium–holmium (Tm–Ho)-codoped nonlinear polarization rotation (NPR) mode-locking fiber oscillator. The ultrafast fiber oscillator can simultaneously operate in the noise-like and soliton mode-locking regimes with two different emission wavelengths located around 1947 and 2010 nm, which are believed to be induced from the laser transition of Tm³⁺ and Ho³⁺ ions respectively. When the noise-like pulse (NLP) and soliton pulse (SP) co-exist inside the laser oscillator, a maximum output power of 295 mW is achieved with a pulse repetition rate of 19.85-MHz, corresponding to a total single pulse energy of 14.86 nJ. By adjusting the wave plates, the fiber oscillator could also deliver the dual-NLPs or dual-SPs at dual wavelengths, or single NLP and single SP at one wavelength. The highest 61-order harmonic soliton pulse and 33.4-nJ-NLP are also realized respectively with proper design of the fiber cavity.     

CS2 +多光子光解产生CS+的动力学研究

The dynamics on the multi-photon dissociation of CS2+ molecular ions to produce CS + ions has been investigated by measuring the CS + photofragment excitation（PHOFEX）spectrum in the wavelength range of 385～435 nm,where the CS2+ molecular ions were prepared purely by［3+1］multiphoton ionization of the neutral CS2molecules at 483.2 nm. With the ～60 ns delay,which is much more than the laser pulse width（～5 ns）,between ionization laser and dissociation laser,the threshold wavelength of dissociation laser to produce CS+ fragment ion from CS2+ molecular ions was obviously observed in the PHOFEX spectrum. The adiabatic appearance potential of the CS+ was determined to be（5.852 ± 0.005）eV above the X 2Σg,3/2（0,0,0）level of CS2+. The product branching ratios,（CS+/S+）,as measured from the PHOFEX spectra,increase from 0 to slightly larger than 1 in the wavenumber range of 47200～50400 cm-1 . The［1+1］dissociation mechanism to get to CS++S from CS2+ was discussed and preliminarily attributed to（i）CS2+（X 2Πg）→ CS2+（A2Πu）through one-photon excitation,（ii）CS2+（A2Πu）→ CS2+（X*）via internal conversion process due to the vibronic coupling between the A and X states,（iii）CS2+（X*）→ CS2+（B 2Σ+u）through the second photon excitation,and（iv）CS2+（B 2Σ+u）→CS +（X 2Σ+）+S（3P）,because of the potential curve crossing with the repulsive 4Σ- state and/or the 2Σ- state correlated with the second dissociation limit. However,when the dissociation laser overlaps the ionization laser in time scale in the laser-molecule interaction zone,the appearance threshold is not available in the PHOFEX spectrum. This fact shows that there are other mixed three-photon paths of［1+1+1'］,［1+1'+1'］,and［1+1'+1］to produce CS+ fragment ion from CS2+ molecular ions besides the above［1+1］dissociation mechanism,that is,CS2+（X 2Πg）→ CS2+（A 2Πu）through one-photon excitation［1］of dissociation laser,CS2+（A 2Πu）→CS2+（X*）via internal conversion process due to the vibronic coupling between the A and X states,CS2+（X*）→ CS2+（B 2Σ +u）through the second photon excitation by dissociation laser［1］or ionization laser［1'］,and third photon excitation by ionization laser［1'］or dissociation laser［1］to reach the adiabatic appearance potential to produce CS+ with the dissociation laser wavelengths longer than 423. 89 nm,at which the［1+1］dissociation mechanism to get to CS+ is unavailable.     

Selective photodissociation of O-H and O-D bonds from ground vibrational state of HOD using simple UV pulses

Selective cleaving of both O-H and O-D bonds in HOD is achieved using reasonably simple UV pulses to excite the HOD molecule in its ground vibrational state to the repulsive first excited A ((1)B(1)) surface. Detailed theoretical analysis of population transfer and flux in the H+O-DH-O+D channels reveals an important preparatory role for the cross-talk between the participating levels and a possible role for the beat structure of the population transfer oscillations in facilitating selective dissociation. Excitation using a 50 fs single color 67,169 cm(-1) laser pulse achieves a branching ratio H+O-DH-O+D=5.64 with 82% flux in the H+O-D channel and 15% in the H-O+D channel. A two color 50 fs laser pulse with frequencies of 54 920 and 52 303 cm(-1) provides a branching ratio of H-O+DH+O-D=2.83 and 52% flux in the H-O+D channel and 18% in the H+O-D channel.     

Yb3+ doped fluorophosphate laser glasses with high gain coefficient and improved laser property

Yb³⁺ doped fluorophosphate glasses with high stimulated emission cross-section, large gain coefficient and low hydroxyl absorption coefficient were prepared by high temperature melting for fiber laser applications, and their spectral, general laser parameters were investigated accordingly by means of fluorescence emission spectrum, decay cure and infrared absorption spectra. Compared with previously reported fluorophosphate glasses, the investigated fluorophosphate glasses have highest grain coefficient and maintain a maximum laser systematical factor over other various types of laser glasses. The introduction of fluorides to fluorophosphate glasses results in the low level of hydroxyl absorption coefficient and concentration. All these advantages might mean that Yb³⁺ doped fluorophosphate glasses are a good candidate as an active laser media for short pulse, high power laser generation used for next generation nuclear fusion.     

State dynamics of acetylene excited to individual rotational level of the V1(2)K1(0,1,2) subbands

The dynamics of the IR emission induced by excitation of the acetylene molecule at the 3(2) Ka2, A1Au<--4(1) la1, X1Sigmag+ transition was investigated. Vibrationally resolved IR emission spectra were recorded at different delay times after the laser excitation pulse. The observed IR emission was assigned to transitions between vibrational levels of the acetylene molecule in the ground state. Values of the relaxation parameters of different vibrational levels of the ground state were obtained. The Ti-->Tj transition was detected by cavity ring-down spectroscopy in the 455 nm spectral range after excitation of the acetylene molecule at the same transition. Rotationally resolved spectra of the respective transition were obtained and analyzed at different delay times after the laser excitation pulse. The dynamics of the S1-->Tx-->T1-->S0 transitions was investigated, and the relaxation parameter values were estimated for the T1 state.     

Fragmentation dynamics of methane by few-cycle femtosecond laser pulses

The fragmentation pattern of CH4 was experimentally studied at an intensity of approximately 10(14) W/cm2 with laser durations varying from 8 to 110 fs. When the laser duration was 8 fs, only the primarily fragmental CH3+ ion was observed in addition to the parent CH4+ ion. When the laser duration was 30 fs, small fragmental CH2+ and H+ ions appeared. When the laser duration was 110 fs, some doubly charged ions were also observed in addition to the abundant singly charged ions. The large mass spectra difference demonstrated that the pulse duration had a strong effect on the fragmentation of the parent ion produced in the single ionization. The effect of laser intensity on the fragmentation of CH4+ was also studied for few-cycle femtosecond laser pulses. The results demonstrated that the first-return recollision between the rescattered electron and the parent ion played a significant role in the fragmentation dynamics of the parent ion. Depending on the ion-electron impact energy, the recollision excited the parent ion to a dissociated state or doubly charged state. The experimentally observed singly charged fragmental ions resulted from the recollision-induced dissociation of CH4+ or the Coulomb explosion of CH(4)2+.     

分子动力学模拟飞秒激光对C60光裂解反应机理的影响

基于半经典分子动力学方法模拟超快激光诱导的C60光裂解反应.选择能量为2.0eV,半峰全宽(FWHM)分别为40和500fs的激光作用于C60分子,调节脉冲强度使其发生裂解反应,比较长短脉宽飞秒激光对C60裂解反应机理的影响.通过分析产物分布、原子平衡指数、温度以及吸收能量(包括动能、势能和电子能量),证实飞秒激光脉冲下C60的光裂解主要由电子激发态控制,非热力学效应在该反应中起重要作用.激光场的作用时间和强度均影响光裂解反应过程,而电子吸收能量饱和后光场强度的作用则变得不明显.     

Dynamics of the dissociation of thes-tetrazine dication produced by one photon excitation of the neutral molecule

¹³C isotope separation by multiphoton dissociation of CHClF₂ was studied by a continuous-discharge CO₂ laserQ-switched at 8 kHz. This laser can easily emit two or more wavelengths with good spatial and temporal overlap. The best irradiation results were obtained by the line pair 9P16 + 9P28. After optimizing also the pressures of CHClF₂ and of buffer gases (He and Ar), the energy density and the gas speed, the process is now much less nonlinear than in previous experiments with a single wavelength. Differences to the TEA laser induced process were observed, and generally the pressure influence on yield and selectivity showed a variety of phenomena. They were interpreted in terms of collisional changes of spectra during the laser pulse.