2002 Vol. 15, No. 3

Letter
The prospects of control chemical reaction in high-intensity laser field are talked about here, and some experimental and theoretical designs are reviewed and discussed also.
The progress in the quantum coherent controlling of chemical reactions is briefly reviewed. The controlling was firstly realized by modulating the phasa angle between one-photon and three-photon excitation of some molecules. The fluorescence intensities of atom in gaseous phase or molecules in liquid phase,as well as the dissociation branching of molecules were also controlled by self-optimizing the phase and the amplitude of an ultrafast laser pules. Some new possible propress of coherent control may appearin strong laser field,in condense phase or on catalysts.
Article
Ba+HF reaction is an interesting example whether for the experiment or theoretical research. But no potential energy surface for this reaction emerges for a long time because of the difficulty of the exact ab initio caused by the large mass number of the Ba atom. The first quasiclassical trajectory calculation for the exothermic reaction Ba+HF (v,J) → BaF(v′,J′)+H is carried out based on a constructed extended-LEPS PES. The calculated BaF product state distributions agree closely with experiment for both Ba+HF(v=0) and Ba+HF(v=1) . The reaction mechanism is described as following two pathways, abstraction and insertion, and the small increment of the barrier height along with the attacking deviation from the linear direction (Ba-F-H) is the main cause of this competition. Low collision energy favors abstraction whereas high vibration excitation leads to insertion.
A dynamic model of collisional quantum interference (CQI) on rotational energy transfer in an atom-diatom system has been described by Sun et al., based on the first order Born approximation of time dependent perturbation theory and long-range interaction potential. The experiment CO A 1Π~e 3Σ- collision with He in a static cell has been simulated successfully. To obtain directly the relationship between interference angle and relative velocity from experiment, an experiment is proposed by Sha, using molecular beam and velocity imaging technology. As a theoretical study of the relationship between interference angle and relative velocity, the interference angles are calculated with different relative velocities, and the changing tendency is obtained. It is revelatory for the molecular beam experiment to measure directly the interference angles by controlling the relative velocities.
The multiphoton ionization of pyridine clusters was studied by use of 400 nm femtosecond laser and TOF MS. A series of protonated and unprotonated pyridine cluster ions were detected,which indicates that the proton transfer can also occur in weak hydrogen bonded clusters . The analysis of both ion peak width and the ion intensity ' s change with the gas resource pressure suggests that the protonated cluster ions are coming from the fragment of larger cluster ions and the relevant unprotonated cluster ions are the direct ionization products of neutral clusters. ab initio calculation results indicate the pyridine clusters are formed by weak hydrogen bonds C-H.N and the dissociation of cluster ions prefers to produce protonated products.
Photodissociation of NO2 in its second absorption band(B2B2 state)has been studied by monitoring the nascent NO X2Πproduct using the single-photon laser-induced fluorescence(LIF)technique. The rotational distributions of the nascent NO photofragment after photodissociation of NO2 at B2B2 state have been firstly reported. In v" = 1,the rotational distributions are bimodal. We proposed that there are two dissociation mechanisms that govern the product internal distributions and the photodissociation mechanism.
Based on the global potential energy surface(PES)of Ar2H+ ground state provided by our group recently,we calculated the vibrational spectra with total angular momentum J = 0 by time-dependent wave packet method and some of the spectra peaks have been assigned. The comparison between some properties gained from this PES with ab inito results showed that this new established PES contains accurate information and it can be used for further dynamics studies.
Amorphous Ni-V2O5 composite film cathodes with excellent electrochemical properties were successfully prepared by 355 nm pulsed laser reactive deposition for the first time. The optimum experimental conditions for the film deposition are:a laser fluency of 2 J/cm2,a temperature of substrate at 300℃,oxygen gas pressure 14 Pa,deposition duration 0 . 5 h and stainless steel used as a substrate material. X-ray diffraction (XRD)and scanning electron microscopy(SEM)analyses indicated that the Ni-V2O5 composite films deposited on stainless steel substrate are amorphous. Among the three composite films with different Ni-V2O5 molar ratio (x= 0.1,0.3,0.5),Ni0.3V2O5 (x= 0.3)composite film electrode exhibited best electrochemical performance. This film electrode retained a capacity of 200 mAh/g at a high rate of 20 C with excellent reversibility,upon cycling with no obvious fading over more than 1000 cycles. The improved specific capacity,rate capability and cycleability of amorphous Ni-V2O5 film electrodes demonstrate that these film electrodes could be used as a promising cathode material for all-solid-state thin film lithium ion rechargeable batteries.
Expressions used for extracting the alignment parameter of photofragment from the laser-induced fluorescence(LIF)intensity are derived by employing the density matrix approach. The alignment of photofragment is described by molecular state multipoles. Both lasers used to dissociate parent molecule and to excite photofragment are linearly polarized lights. And detection fluorescence is an unpolarized light . The LIF intensity is a function of the initial molecular state multipoles,the line strength factor and the dissociation-excitation geometrical factor. The alignment parameter of photofragment can be found by measuring the polarization ritio of fluorescence and calculating the dynamic factors.
Dispersed laser induced fluorescence spectra of gas-phase CBr2 and Br2 produced by the reaction of F atom with CH2Br2 in the microwave discharge flow tube apparatus have been measured. The dispersed fluorescence spectrum excited at 571.9 nm was assigned to A(0,13,0)→X(0,v2",0)( v2"=16)transitions of CBr2,the fundamental vibrational frequencies of bending mode of gas - phase CBr2 in electronic ground state was firstly derived from the spectrum to be v2"=215 cm-1 . The dispersed fluorescence spectrum excited at 575. 3 nm was assigned to B3Π+u →X1Σ+g transitions of Br2 . The generation of CBr2 and Br2 species is believed to be the second or third step reaction of F atom with CH2Br2 .
68 electronic states of 7Li2 observed and/or calculated have been relabeled according to the dominant hydrogenic nlλ characters of the Rydberg orbitals and classified as core-penetrating vs core-nonpenetrating states. The perturbations between doubly excited valence states and Rydberg states are discussed. The significance of this labeling is also discussed.
Various analytical physical models are presented to extract the photodissociation dynamics information from the data obtained in the femtosecond pump-probe experiment. The single- and double-component models are employed to explain the single- and double-channel dissociation of parent molecules. Another single-component model for fragment dissociation or deexcitation is also presented. All cases are explanatorily demonstrated on the pump-probe experimental data.
A (VUV+UV)-REMPI spectroscopy scheme was exploited to detect the methyl radical (CH3). To demonstrate its high sensitivity, a crossed-beam scattering experiment was conducted with CH3(CD3) as the reaction product from F+CH4(CD4). The absolute detection limit was estimated to be about 107/cm3. From the observed spectra, the spectroscopic constants of the two intermediate Rydberg states, 3d 2E″ and 3d 2A1′, were also refined.
The microsopic rate constants of N(4S)CH2X(X=F,Cl)reaction have been calculated by RRKM theory . The barrierless dissociation was handled with the loosing transition state model. The results reveal that when activated internal energy is low(280.29 kJ/mol),the major products of N(4S)+CH2F reaction are NCHF+H and H2CN+F,with corresponding branching fraction of 59.2% and 37.4%,respectively . On the other hand,when internal energy is 267.78 kJ / mol,the major products H2CN+Cl give a 90.3% yield of the N(4S)+CH2Cl reaction. When the internal energy is ultra high(500.00 kJ / mol),the main reaction channel is still NCHF+H pathway for the N(4S)+CH2F reaction,while it changes into NCHCl + H for the reaction of N(4S)+CH2Cl,which gives a 51.5% yield,the proportion of H2CN+Cl is reduced to 40.4% .
Hydroformylation of 1-hexene over a series of silica supported noble metal catalysts,HRhCO(TPP)3,HRhCO(TPPTS)3/SiO2 and TPPTS-Rh/SiO2 catalyst were investigated. The results show that the selectivity towards aldehydes and the ratio of liner to branched aldehyde( n/b)over Rh-TPPTS / SiO2 catalyst are near to those of HRhCO(TPPTS)3/SiO2,but much higher than to those of Rh/SiO2 . Under the higher pressure of 7.0 MPa,the TOF of aldehydes on Rh-TPPTS / SiO2 catalyst increase greatly up to 0.0692 S-1 . A possible of catalyst model of Ligands containing lone-pair electron tightly chemically connected to the highly dispersed Rh particles and formation of supported aqueous-film heterogeneous catalyst possessing of homogeneous catalytic performance has been tentatively proposed.
193.3 nm photodissociation dynamics of jet-cooled 1-propanol and 2-propanol has been examined by using high-n Rydberg-atom time-of-flight (HRTOF) technique. Isotope labeling study indicates that O-H bond fission is the primary H-atom production channel. Center-of-mass (CM) product translational energy release of this channel is large, with 〈fT〉= 0.76 for H+1-propoxy and 0.78 for H+2-propoxy. Maximum CM translational energy release yields an upper limit of the O-H bond dissociation energy: (432±2)kJ/mol in 1-propanol and (433±2)kJ/mol in 2-propanol. H-atom product angular distribution is anisotropic (with β≈-0.79 for 1-propanol and -0.77 for 2-propanol), indicating a short excited-state lifetime. The 193.3 nm H-atom dissociation of both 1-propanol and 2-propanol is prompt and occurs on a repulsive excited-state potential energy surface.
Using our recently developed quantum dissipation theory, we study the influence of dissipation on the population transfer efficiency between the stimulated Raman adiabatic passage (STIRAP) and the pump-dump passage in a simple three-level Λ system. By comparison, STIRAP is found to be relatively insensitive to the relaxation and fluctuation of the intermediate state. Numerical results also demonstrate the applicability of the new quantum dissipation theory for studying the correlated driving-dissipation dynamics problems.