Real-time probing of vectorial properties of ultra-fast photodissociations in liquids

Polarization-resolved femtosecond experiments are shown to yield valuable insight into transition-state dynamics associated with ultra-fast photodissociation reactions of triatomic species in liquid solution. After the fundamental act of bond fission is completed, such experiments reveal rotational dynamics of the fragments, which can be used to extract information regarding fragment rotational excitation in the asymptotic limit of the reaction. Received: 4 October 1999 / Published online: 30 June 2000     

乙醛光解动力学的离子速度成像

利用时间切片离子速度成像技术在275～321 nm能量范围内重新研究了乙醛自由基通道CH₃+HCO的光解动力学. 通过共振增强多光子电离的方法探测甲基碎片. 对甲基的伞形振动基态和激发态(v₂=0和1)进行了影像探测. 乙醛通过T₁电子态系间窜越到S₁电子态的解离产物具有很高的动能释放和很低的内能激发,碎片的振动能和转动能随激发能量的增加而增加. 乙醛T₁电子态的势垒高度经测量高于基电子态3.881±0.006 eV.     

NO rotational orientation following 308 nm photodissociation of NO2

The rotational angular momentum orientation and alignment of the NO fragments generated via linearly polarized 308 nm photodissociation of NO2 has been determined using laser induced fluorescence. By observing the dependence of the photofragment NO Doppler-resolved transition line shapes on experimental geometry, it has proved possible to determine multipole moments of the photofragment angular momentum distribution up to, and including, rank 3. The implications of the results for the mechanism of the dissociation are considered.     

Analytical efficiency of the molecular rotational quantum echo

The sensitivity of the delay time of the molecular rotational quantum echo to change in the rotational constants of molecules was investigated with a model of the rotational quantum echo considered as an optically induced quantum synchronization of the Fourier rotational modes of an asymmetric molecular top. The J-echo and the K-echo were investigated for the A- and C-configurations of rotation of a top molecule. The contour maps of the dependence of the sensitivity of the delay time of these types of echo to changes in the rotational constants on the parameters of the molecular tops have been obtained for all the rotational constants. They show that the delay time of the rotational quantum echo is greatly dependent on the configuration of rotation of the top molecule and on the rotational constants. It is shown that the optical probing of the rotational quantum echo can also be performed by the optical rotation of the investigated molecule, and in this case the rotational force is dependent on the intramolecular orientation of the magnetic dipole moment. It was determined that a high sensitivity of the delay time of the echo to changes in the rotational constants can be responsible for the broadening of the echo pulse and even for its disappearance. Institute of Molecular and Atomic Physics, National Academy of Sciences of Belarus, 70 F. Skorina Ave., Minsk, 220072, Belarus; e-mail: lssm@imaph.bas-net.by. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 6, pp. 758–764, November–December, 1999.     

甲基自由基在212.5 nm的光分解:母体内能态的激发对产物态分布的影响

利用里德堡氢原子飞渡时间谱的方法,进行了甲基自由基在212.5nm 的光解动力学的实验研究.甲基自由基来自于碘甲烷分子在266nm 的光分解.实验测量了不同振动态的甲基自由基通过3s 电子激发态解离的产物 CH_2自由基的平动能分布和角分布,并从平动能分布得到了 CH_2自由基的转动布居.实验同时详细地研究了母体分子甲基自由基的振动和转动激发对光解动力学的影响.实验结果表明母体分乙甲基自由基的伞形振动对于产物的转动激发和角分布均有较大的影响,而母体分子的转动激发只对产物的角分布有较明显的影响.     

氨分子在000和201带的选择光离解

本文利用脉冲紫外激光(UV)选择激发氨分子到?¹A″₂电子激发态的两个最低振动能级ν′₂=0和ν′₂=1(ν₂振动),然后检测新生态H原子的飞行谱(TOF),研究了氨分子的光碎片动力学。光谱证实了最近所测的离解能D₀⁰(H-NH₂)=4.645eV;绝大多数生成的NH₂(X²B₁)基处于非振动激发,但是具有围绕a惯性轴的高度转动激发。通过NH₃(?)的ν′₂=1光离解产生的NH₂(X)基具有较高的内部激发,并且显示了在N=K_a转动能级上的反转布居。 关键词：     

225∽260 nm波段溴化氰光解动力学系统性研究

本文报道了在自行搭建的时间切片离子速度成像装置上进行的225∽260 nm波段内溴化氰光解动力学的研究. 在该波段内选取了若干溴原子(²P_3/2或²P_1/2)的共振线对产物溴原子的共振电离并采集其切片影像,得到了光解产物的总平动能谱,进而获得了产物氰基的振转态布居等信息. 本文发现了在Br^*通道,产物氰基的内能激发比Br通道低;Br和Br^*通道产物氰基振动的最高布居分别为v=0和1. 另外,还发现对于Br通道,溴化氰分子在长波处与短波处解离时,氰基产物的振转激发差异很大,这揭示了其显著不同的光解动力学.     

HOBr/DOBr态到态光解动力学的量子波包研究

本文基于高水平的多参考组态相互作用计算,报导了HOBr的最低三个单重态的三维势能面. 并且对该分子的光解离过程用实波包传播的方法进行了量子动力学研究,不仅计算结果得到了吸收光谱,而且还产生了光解产物的内态和角度分布. 结果与在紫外区域测得的HOBr的总吸收截面定量吻合,并很好的重现了实验上在266 nm处观察到的振动冷和转动热的光解OH/OD产物. 此外,预测了OH/OD的反冲各向异性参数接近平行跃迁的极限值,表明在266 nm处从基态(1¹A'')到2¹A''态的面内跃迁是一个快速的解离过程. 这与实验上从测量转动取向得出的结论一致. 然而,想要实现与实验上的定量一致,还需要考虑自旋和电子角动量的影响.     

硝基甲烷与硝基乙烷266 nm光解动力学

利用单光子激光诱导荧光技术，在硝基甲烷与硝基乙烷气相条件下266 nm光解过程中，对初生态的解离产物OH进行了测量．结果表明，两个解离过程中产生的初生态OH都没有振动激发．硝基甲烷光解过程中产生的OH的转动分布可以用玻尔兹曼温度简单的表示，相对于自旋轨道分裂的2|3=2与2|1=2两个态，其转动温度分别为2045§150与1923§150 K．硝基乙烷光解过程中产生的OH不是简单的玻尔兹曼分布，因此不能用玻尔兹曼温度来描述，这一结果 意味着硝基甲烷与硝基乙烷在生成OH的过程中经历了不同的解离过程．硝基甲烷     

Rotational-coherence molecular laser isotope separation

We have proposed a laser isotope separation method, utilizing rotational coherence of a simple molecule. In the scheme, photoexcited molecules are isotopically separated by difference of rotational period between them. To illustrate this method, two-pulse photodissociation of mixed ⁷⁹Br₂/⁸¹Br₂ isotopes has been investigated theoretically. The photodissociation probabilities of ⁷⁹Br₂ and ⁸¹Br₂ have been calculated as functions of time delay between the photoexcitation and dissociation laser pulses. We have demonstrated that isotope enrichment factor of ⁷⁹Br relative to ⁸¹Br can be changed from 0.34 to 1.8, by simply changing the time delay only by 0.2 ns. Additionally, we have shown that this method is effective for heavy isotopes, based on mass dependence of the isotope enrichment factor.     

Analysis of femtosecond laser ionization/dissociation of polyatomic molecule C6H10O from one-colour pump-probe measurement

This paper reports that a one-colour fs pump-probe measurement has been carried out for studying photoionization/photodissociation of cyclohexanone (C6H10O) in intense laser field. Two of the fragments from cyclohexanone, C2H3+ and C3H3+ , are studied under 800 nm laser pump-probe and the results obtained show similar time evolutions. It proposes a feasible model for analysing the experimental observations of the one-colour fs pump-probe measurement. The results demonstrate that as an intermediate product, the excited molecular parent ions play a very important role in photionization/photodissociation processes in intense laser field.     

The photodissociation of D2O by vacuum ultraviolet excitation sources

The photodissociation of D₂O was carried out at excitation wavelengths of 1216, 1236, and 1302 Å. The distribution of the excess energy to rotational and vibrational degrees of freedom of the product OD was determined by an investigation of the OD ²Σ⁺ emission. The population of the highest allowed rotational levels was observed with results similar to those obtained for H₂O. It has been found that the two spin components of the higher rotational levels were unequally populated. An attempt is made to account for this anomalous population distribution.     

State selective predissociation spectroscopy of hydrogen chloride ions (HCl+) via the A2Σ+ ← X2Π3/2 transition

The photodissociation of hydrogen chloride ions (HCl⁺) has been investigated through the A²Σ⁺ (ν′ = 6, 7 and 8) ← X ²Π_3/2 (ν″ = 0) transition. The spectra reveal that state selective photodissociation with complete resolution of the spin, orbital, and rotational angular momentum is possible in the A ²Σ⁺ (ν′ = 6) state. The analysis of these spectra yields the rotational and the spin-rotation coupling constant of the A ²Σ⁺ (ν′ = 6) state. The lifetime of HCl⁺ decreases significantly with increasing vibrational excitation in the ²Σ⁺ state. Within the experimental error limits no J dependence of the lifetime is observerd. The state selective photodissociation of the HCl⁺ ions is also shown to be a very sensitive probe for unexpected parity transitions in the 2 + 1 REMPI formation of the HCl⁺ ions in the X 2Π_3/2 (ν″ = 0) state.     

Coherent electromagnetic radiation from the fragments of nuclear fission

The electromagnetic radiation from two vibrating fission fragments is classically treated. The rotational and translational motion of the fragments during emission and the interference of the radiation from the two individual fragments are taken into account. It is shown that the angular distribution of the radiation contains valuable information on the fission process and that the spectral distribution depends sensitively on the damping of the shape vibration.Dedicated to Professor P. Armbruster and P. Kienle on the occasion of their 60th birthday     

The Mo−OH proton of the low-pH form of sulfite oxidase: Comparison of the hyperfine interactions obtained from pulsed ENDOR, CW-EPR and ESEEM measurements

Pulsed electron nuclear double resonance (ENDOR) spectra have been obtained for the exchangeable Mo-OH proton of the low-pH form of native chicken liver sulfite oxidase (SO) and recombinant human SO for the first time. The spectra of the two enzymes are very similar, indicating a similar binding geometry of the hydroxyl ligand to the Mo center. The isotropic hyperfine interaction (hfi) constant for the proton of the OH ligand in both enzymes is about 26 MHz. The anisotropic components of the hfi obtained from the pulsed ENDOR spectra are about 1.6–1.8 times larger than those obtained by continuous-wave electron paramagnetic resonance and electron spin echo envelope modulation. These hfi differences are explained by a rotational disorder of the Mo-OH group. A similar rotational disorder of the coordinated exchangeable ligand has been found previously for the high-pH and phosphate-inhibited forms of SO.     

低振动激发HeI2分子振动预离解寿命和终转动态分布

用含时黄金规则波包法，对ＨｅＩ２分子在低初始振动激发（ｖ＜１２）态下振动预离解动力学作了全维量子力学计算。所预言的总衰变宽度和寿命与谱线宽和皮秒时间分解的实时态－态测量外推数据符合得相当好。计算的总衰变宽度对初始振动态ｖ是敏感的并呈现一种非线性关系。结果表明低振动激发ＨｅＩ２分子衰变模式仍应是量子力学的。除终态相互作用对决定终转动分布有重要作用以外，首次发现，低振动激发态的初始特性也能显著影响终转动态分布。用Ｉ２的转动常数对ｖ的关系合理地解释了这个独特现象     

2-溴丁烷光解离的离子速度成像方法

用离子速度成像结合共振增强多光子电离技术研究了2-溴丁烷在233.62和233.95 nm的光解离动力学. 从离子速度图像确定了光解产物Br和Br*碎片的速度分布和角度分布. Br和Br¤速度分布中包含的两个高斯分量是来自于2-溴丁烷在约234 nm处的两个独立的光解离反应通道. 高能的高斯分量对应于沿着C-Br伸缩模式的直接解离,低能的高斯分量对应于弯曲振动和C?Br伸缩振动耦合形成的排斥模式的解离. 讨论了2-溴丁烷光解离中³Q₀、³Q     

Imaging the translational dynamics of CF3 in liquid helium droplets

The translational dynamics of CF3 in liquid helium have been investigated by photodissociating CF3I dissolved in helium droplets consisting of several thousands of atoms. The velocity distribution of CF3 fragments that have escaped from the droplets has been determined using ion imaging techniques and is found to be considerably shifted to lower speeds with respect to the photodissociation of gas phase CF3I. The fragments furthermore show a speed dependent angular distribution that is isotropic for the slowest and approaches the gas phase distribution for the faster fragments. These distributions point to a nonthermal escape process in which, at least for the speeds relevant for the present experiment, the kinetic energy and momentum transfer from the fragments to the solvent appears to be governed by binary collisions with the individual helium atoms.     

Production of cold CN molecules by photodissociating ICN precursors in brute-force field

We theoretically investigate the production of cold CN molecules by photodissociating ICN precursors in a brute-force field. The energy shifts and adiabatic orientation of the rotational ICN precursors are first investigated as a function of the external field strength. The dynamical photofragmentation of ICN precursors is numerically simulated for cases with and without orienting field. The CN products are compared in terms of their velocity distributions. A small portion of the CN fragments are recoiled to near zero speed in the lab frame by appropriately selecting the photo energy for dissociation. With a precursor ICN molecular beam of ～ 1.5 K in rotational temperature, the production of low speed CN fragments can be improved by more than 5 times when an orienting electrical field of 100 k V/cm is present. The corresponding production rate for decelerated fragments with speeds ≤ 50 m/s is simulated to be about ～2.1×10~(-4) and CN number densities of 10~(8) –10~(10) cm~(-3) can be reached with precursor ICN densities of ～10~(12) –10~(14) cm~(-3) from supersonic expansion.     

Decay and fission of oriented nuclei

The angular distributions of fragments originating from the binary decay of oriented spherical and deformed nuclei are investigated with allowance for correct transformation properties of wave functions under time inversion. It is shown that, as in the case of protonic decay, the adiabatic approximation for collective rotational degrees of freedom of the systems under investigation is inapplicable in describing the angular distributions of fragments of the deep-subbarrier alpha and cluster decays of nuclei. It is demonstrated that this approximation is justified in describing spontaneous and induced low-energy nuclear fission. The dependence of partial fission widths on the orientation of intrinsic axes, spins, and projections of spins and relative orbital angular momenta of fission fragments is analyzed by using the formalism of the unified theory of nuclear reactions and the theory of open Fermi systems. It is shown that the adiabatic approximation leads to the coherent interference between the wave functions for the relative motion of fragments, whereby the universal angular distributions of fission fragments of oriented nuclei is formed. Deviations from the A. Bohr formula are investigated for these distributions.