NO2分子里德堡态的光学-光学双共振多光子离化谱

采用光学-光学双共振多光子离化（Optical-optical double-resonant multiphoton ionization，简称OODR-MPI）技术详细研究了NO2分子里德堡态的能级结构。结果显示，用不同强度的激光作抽运光，尽管NO2分子的OODR-MPI谱均由规则的光谱序列组成，但NO2分子的离化通道存在很大区别，当抽运光强度较高时，NO2分子通过(3+1+1)双共振多光子过程离化；而当抽运光强度较低时，NO2分子通过(1+2+1)双共振多光子过程离化，两种离化机制的最终共振能级属于不同的里德堡态。     

飞秒时间分辨实验中相关函数和时间零点的确定

结合飞秒激光在研究分子激发态弛豫动力学中的应用,介绍了几种飞秒时间分辨实验中确定泵浦激光脉冲与探测激光脉冲的相关函数和时间零点的方法.对于波长在可见波段的泵浦和探测激光脉冲,我们可以利用非线性光学的技术手段,探测泵浦光与探测光的和频光的强度随二者间的时间延迟的变化来确定相关函数和时间零点;对于波长在紫外甚至更短的波段的泵浦和探测激光脉冲,由于单脉冲能量比较低,目前还很难利用技术手段来测定泵浦激光与探测激光的相关函数及时间零点,可以利用某些原子气体(如Xe)或某些具有短寿命态的分子作平行实验进行间接测量.     

Dynamics mechanism of optical-optical double-resonant multiphoton ionization of nitrogen dioxide

The optical-optical double-resonant multiphoton ionization (OODR-MPI) spectrum of NO2 molecule in the 460-605-nm wavelength region of the probe photon is presented. The mechanism of the OODR-MPI of NO2 molecule is analyzed. The results show that the resonant features can be assigned to the transitions from the first 3sσg Rydberg intermediate resonant state to the final npσu Rydberg series. The ionization pathway is NO2 (X2A1) 3hv1→ 3sσghv2→npσu hv2 or autoionization→NO2+ + e. It is found that the converging potential of the npσu Rydberg series and the quantum defect of np orbit about NO2 are (78803 ± 14) cm-1 and 0.652 ± 0.014, respectively. The bending vibration frequency of 5pσu state is determined also.     

Third-harmonic and sum-frequency generation in a quadratically nonlinear polymer by time-ordered ultrashort laser pulses

Third-harmonic and sum-frequency generation in quadratically nonlinear azopolymer films is experimentally studied using femtosecond chromium forsterite laser pulses. A noncollinear geometry of sum-frequency and third-harmonic generation developed and implemented in this work allows the influence of the time ordering of ultrashort laser pump pulses on nonlinear-optical phenomena to be experimentally observed. Femtosecond laser pulses induce transitions of azopolymer molecules to an electronically excited state and produce vibrational wave packets, leading to an asymmetry in the dependence of the efficiency of second-and third-order nonlinear-optical processes on the delay time between the pump pulses.     

飞秒时间分辨实验中泵浦-探测交叉相关函数的测量和时间零点的确定

飞秒激光技术的出现使得实时探测与跟踪激发态超快弛豫动力学过程成为可能,并能够给出激发态动力学过程清晰的物理图像。而在飞秒时间分辨实验中,泵浦-探测相关函数和时间零点直接影响实验结果的可靠性和准确性。本文结合飞秒激光在分子激发态超快动力学过程中的应用进展,介绍了根据实验条件和要求,在具体实验过程中泵浦-探测相关函数测量和时间零点确定的几种方法。实验中选择可见光作为泵浦光和探测光时,可以通过测定随泵浦-探测时间延迟变化的泵浦激光与探测激光的和频/差频光强来确定泵浦探测交叉相关函数和时间零点;而选择中心波长在紫外甚至真空紫外的激光脉冲作为泵浦光或探测光时,泵浦-探测交叉相关函数通常采用校正的方法测量。     

ANALYSIS OF F′0+u ION-PAIR STATE OF IODINE

The optical-optical double resonant multiphoton ionization (OODR-MPI) technique and the fluorescent excitation spectroscopy technique have been applied to the study of the F′0⁺_u ion-pair state of iodine. This paper presents OODR-MPI spectrum and fluorescent excitation spectrum of I₂ in the region of 54000-55300cm^-1 by the three-photon resonant, two-photon ionization (1+2+2) and (1+(1+1)+2) processes.     

Ionization dynamics of dense matter generated by intense ultrashort X‐ray pulses

We investigate a pump‐probe X‐ray Thomson scattering (XRTS) experiment that might be carried out at a free electron laser facility to study warm‐to‐hot states of dense matter. Ultrashort and intense X‐ray pulses with different energies (1,560–1,830 eV) heat a 1 µm thick Al target isochorically and create homogeneous and uncompressed warm‐to‐hot states of dense matter. A second pulse with variable delay probes this heated state via XRTS. The X‐ray laser–target interaction is modelled within radiation‐hydrodynamic simulations applying the HELIOS‐CR code. The HELIOS‐CR results qualitatively agree with Monte‐Carlo simulations, where the laser pulse absorption is simulated based on a uniform random sequence of events. The electron feature in the simultaneously observed X‐ray scattering spectrum is a function of the degree of ionization and the target temperature. Therefore, the temporal evolution of the plasmon peak measures the ionization dynamics on ultra‐short time scales. The XRTS spectrum is calculated based on the Chihara formula utilizing the Born‐Mermin approximation for the free electron dynamic structure factor. The proposed experiment will reveal important details of the ionization dynamics on ultra‐short time scales as well as of the relaxation on ps time scales.     

飞秒激光参数对非绝热耦合分子态布居的影响

利用含时波包法研究强飞秒泵浦-探测激光场中激光脉宽、波长和场强对非绝热耦合NaI分子各态布居的影响.波包在势能面上做周期性运动,周期约为1 000 fs.延时为200 fs时,波包第一次到达交叉区域分裂成两部分.波包在交叉区域的分裂情况影响各态布居.脉宽增长,NaI分子的激发概率增大,而解离概率减小.泵浦波长为共振波长318 nm时,激发概率最大.泵浦波长增长,NaI分子的解离概率减小.泵浦场强增大,激发概率增大,但解离概率不变.探测激光波长和场强不影响NaI分子各态布居分布.调节激光场参数可实现对波包运动的控制从而控制态布居的选择性分布.研究结果为实验上实现分子的光控制过程提供参考.     

Clocking ultrafast wave packet dynamics in molecules through UV-induced symmetry breaking

We investigate the use of UV-pump-UV-probe schemes to trace the evolution of nuclear wave packets in excited molecular states by analyzing the asymmetry of the electron angular distributions resulting from dissociative ionization. The asymmetry results from the coherent superposition of gerade and ungerade states of the remaining molecular ion in the region where the nuclear wave packet launched by the pump pulse in the neutral molecule is located. Hence, the variation of this asymmetry with the time delay between the pump and the probe pulses parallels that of the moving wave packet and, consequently, can be used to clock its field-free evolution. The performance of this method is illustrated for the H(2) molecule.     

Ultrafast dynamics of o-fluorophenol studied with femtosecond time-resolved photoelectron and photoion spectroscopy

The ultrafast dynamics of o-fluorophenol via the excited states has been studied by femtosecond time-resolved photoelectron imaging. The photoion and photoelectron spectra taken with a time delay between 267 nm pump laser and 800 nm probe laser provide a longer-lived S1 electronic state of about ns timescale. In comparison,the spectra obtained by exciting the S2 state with femtosecond laser pulses at 400 nm and ionizing with pulses at 800 nm suggest that the S2 state has an ultrashort lifetime about 102 fs and reflects the internal conversion dynamics of the S2 state to the S1 state.     

Role of excited states in coherent harmonic bremsstrahlung in a photoionized plasma

It is established for a photoionized plasma formed in the barrier-suppression ionization regime that the preliminary population of the excited states of the ionizing atoms plays an important role. It is established that in this case an anomalously strong (several orders of magnitude) increase occurs in the efficiency of generation of the harmonics of the pump radiation. It is shown that a relative decrease of the harmonics generation efficiency occurs with time as a result of collisions of the electrons produced by ionization.     

Laser ion kinetics: set-up and application of a single-shot femtosecond pump–probe technique

In this contribution the first measurements of a single-shot femtosecond laser pump–probe technique are reported. The technique is based on counter-propagating femtosecond laser pulses in a supersonic beam of a low density of sample molecules and simultaneous probe detection by ion or fragment-ion formation through a reflectron time-of-flight mass spectrometer. It will be shown that the range of the pump–probe delays covers the time span between 100 fs and 10 ps depending on the pulse width of the laser used and the stability of the volt ages of the mass spectrometer. The application of this technique to organometallic compounds as well as to medium-sized organic molecules reveals some insight into the electron-transfer process during ionization through a 1+1 multi-photon absorption procedure. Furthermore it is demonstrated that this technique is also applicable to the investigation of ultra-fast isomerization and fragmentation processes. As an example the results of the processes within iron pentacarbonyl and substituted benzalacetones are reported. Received: 22 October 1999 / Published online: 24 July 2000     

低强度周期量级脉冲驱动排列分子的非次序双电离

本文利用三维经典系综模型研究了低强度周期量级脉冲驱动排列分子的非次序双电离. 结果表明, 电子对的关联特性强烈地依赖于分子的排列方向和激光脉冲的载波包络相位; 垂直分子反关联电子对的比例总是高于平行分子反关联电子对的比例; 当载波包络相位从0到π 逐渐增加时, 反关联电子对的数目先增加再减少; 对于平行分子, 电子对的释放总是以正关联为主; 而垂直分子的主导关联模式则依赖于激光脉冲的载波包络相位, 当载波包络相位为0.3π-0.7π之间时, 电子对以反关联释放为主, 其他相位下以正关联为主. 本文利用分子势能曲线和电子返回能量很好地解释了电子关联特性对分子排列方向和载波包络相位的依赖关系.     

用非简并泵浦探测方法研究C70/甲苯溶液的激发态吸收动力学

通过非简并的时间分辨泵浦探测技术对C70富勒烯/甲苯溶液的非线性光学性质进行了研究。在532 nm波长的皮秒激光脉冲泵浦下,用600 nm的皮秒激光脉冲测量了C70富勒烯的激发态非线性吸收动力学响应。利用五能级模型对实验结果进行了数值模拟,得到了非简并情况下C70富勒烯分子的光物理参数。实验结果表明:C70富勒烯在简并和非简并情况下的激发态寿命没有变化,但非简并情况下的单重态吸收截面要比简并情况下单重态吸收截面小。     

热散焦效应对甲烷后向受激喇曼散射的影响

采用单纵模Nd:YAG二倍频激光泵浦,研究了CH4气体中的受激喇曼散射．实验发现,当激光的重复频率为2 Hz,1．1 MPa CH4中,泵浦能量为95 mJ时,后向一级斯托克斯(BS1)的量子转化效率可达73％,并且由于激光脉冲的张弛振荡,BS1的脉宽被压窄到1．2 ns,峰值功率达到了泵浦光功率的2．7倍,其光束质量大大优于泵浦光．在同样条件下,而重复频率为10 Hz时,BS1的量子转化效率降低为36％,但对光束质量影响不大,这是因为BS1呈现为与泵浦光波前翻转的复制光波,可以补偿热畸变．研究结果表明,如果设计一个CH4／He混合气体的带封闭循环冷却系统的高性能630 nm喇曼激光器,可能有其实际应用价值．     

Measuring picosecond excited‐state lifetimes at synchrotron sources

A new analysis method for the short excited‐state lifetime measurement of photosensitive species in crystals is described. Based on photocrystallographic techniques, this method is an alternative to spectroscopic methods and is also valid for non‐luminescent excited species. Two different approaches are described depending on the magnitude of the lifetime τ. For very short lifetimes below the width of the synchrotron pulse, an estimated τ can be obtained from the occurrence of the maximal system response as a function of the pump–probe delay time Δt. More precise estimates for both short and longer lifetimes can be achieved by a refinement of a model of the response as a function of the pump–probe delay time. The method also offers the possibility of the structure determination of excited species with lifetimes in the 40–100 ps range.     

DYNAMICS OF IONIZATION-ENHANCED SPECTRAL EXPANSION IN WATER INDUCED BY AN INTENSE FEMTOSECOND LASER BEAM

The dynamic process of white-continuum generation in water was investigated by the pump－probe technique with a femtosecond intense laser at 805nm. The spectrum width of the probe beam was broadened at the blue side and varied with different delay times. This blueshift was attributed to the ionization-enhanced optical nonlinearity, in which both the multi-photon ionization and avalanche ionization had an effect.     

Plasma formation dynamics within a water microdroplet on femtosecond time scales

We demonstrate that a highly confined plasma inclusion is formed inside a micrometric water droplet that is excited by a femtosecond laser. An 800-nm laser pulse generates a plasma inclusion by laser-induced breakdown that is subsequently probed by the elastic scattering of a second (400-nm) time-delayed ultrashort pulse. For a 25-microm-radius droplet and an incident intensity of 7 x 10(12) W/cm2 the radius of the highly localized plasma is 1.9 +/- 0.4 microm. We probed the plasma formation dynamics on femtosecond time scales by varying the delay between the pump and the probe pulses. Good agreement with numerical simulations of the process was found.     

利用双色激光感生光栅光谱法测定^127I的核自旋

本文利用ＮｄＹＡＧ激光倍频光５３２ｎｍ激光分成强度大致相等的两束激光作为泵浦光以２．７３°交叉于碘分子样品室中，两束泵浦光发生干涉在碘分子中选择激发形成了空间正弦分布的激光态分子光栅和基态耗尽型光栅。另一束窄线宽染料激光作为探索光射入到激光感生光栅上，沿着布拉格衍射的方向接收信号光。利用泵浦共振为３２－０Ｒ（５５），探索共振为１３－０Ｐ（５５），Ｒ（５５）和泵浦共振为３２－０Ｐ（５２），探索共振为１３－０Ｐ（５２），Ｒ（５２）的碘分子双色激光感生光栅光谱谱线强度测定了１２７Ｉ的核自旋，测定值为５／２。