2004 Vol. 17, No. 3

Review
In the past fifteen years,free radical reactions in the gaseous phase have been widely studied by timeresolved fourier transform spectroscopy. Sloan et al. first investigated O(1D)reactions and then Leone and Hancock groups studied O(3P)reactions. Later,Kong,Zhu et al. investigated small radical,including CH,CH2,CH3,C2H3,C2H5,C2H,C3H3 and C3H5,reacting with the molecules of O2,NO,N2O,NO2 . For each reaction,nascent products and primary channels are observed in the IR spectrum. Combining with ab initio theoretical investigations,the reaction mechanisms are almost understood.
Letter
The electron momentum spectroscopy of the inner valence orbitals 3a1 and 2b2 of methylene fluoride was studied by electron momentum spectroscopy(EMS). The experiment was performed using a high resolution(ΔE=1.15 eV FWHM,Δp=0. 1 a. u.)(e,2e)EMS spectrometer. The experimental momentum profiles of these two orbitals are compared with those calculated by Hartree-Fork method and Density Functional Theory.
Article
The method established previously for studying the etching rates of micro-scale silicon and silica was used to study the etching process of silicon and silica on the Si(100)surface. Photolithography was used to pattern a positive photoresist mask to confine the etching area,and the atomic force microscopy was used to probe the etched surface. The lateral etching rate of silicon or silica on the silicon surface was defined,and the lateral and longitudinal etching rates of silicon and silica on the Si(100)surface in 40% ammonium fluoride aqueous solution were measured. The effect of the dissolved oxygen on the etching rates was studied by bubbling the solution with high purity nitrogen. The lateral and longitudinal etching rates of silicon and silica on the(100)surface increase with temperatures except for the lateral etching rate of silica in a N2 -bubbled solution which probably reaches the limit of diffusion controlled reaction. The etching rates of silicon and thermal silica on the Si(100)surface show remarkable difference with that on the Si(111)surface in both air-saturated and N2 -bubbled solutions. The apparent activation energies for the silicon and silica etching processing in ammonium fluoride solution were obtained from the etching rates at different temperatures in the range 20. 6-34. 1℃. The similarity of the apparent activation energies for the etching processing of silicon and silica on the(100)surface to that on the(111)surface probably suggests that the rate-determined-step is the same in both cases. A lot of gas bubbles are seen to aggregate on the surface in silicon dissolution process at 38. 2℃,and it is found that the gas bubbles have great influence on the silicon etching rate. The formation of bubbles accelerates the silicon dissolution at the beginning but blocks the etching as the bubbles gradually aggregate on the surface.
The N-methylformamide(NMF)-water clusters were studied by ab inito calculations at MP2/6-31+G** and MP2 / 6-311 ++ G(d,p)levels. The equilibrium geometries and the dissociation channels and dissociation energies of both neutral and ionic NMF-H2O clusters are presented. For N-methylformamide,cis-form has lower energy than trans-form. In NMFH+,the proton prefers to link with the O atom of N-methylformamide. The results show that both cis- and trans- form of NMF can form a linear hydrogen bond with water. Although the energy of trans-NMF is higher than cis-NMF,trans-form exits more stably because it can form a double hydrogen bond with water. After the ionization of the NMF-H2O cluster,both the cis- and the trans-form will produce protonated products.
The density functional(B3P86)method has been used to optimize the structure of Ni2 molecule. Results show that the ground state for Ni2 molecule is 5 multiple state,not 1 multiple state and 3 multiple state the literatures concluded. That shows the spin polarization effect of Ni2 molecule of transition metal elements for the first time. They take 1 multiple state and 3 multiple state as the ground state of the Ni2 molecule because the minimal energy value of 1 multiple state,3 multiple state and 5 multiple state of Ni2 molecule are very close to each other. Meanwhile,we have not found out any spin pollution and the ground state wave function doesn't mingle with wave function with higher energy state. The result shows that the ground state for Ni2 molecule is 5 multiple state,which shows the spin polarization effect of Ni2 molecule of transition metal elements. That is,there exist 4 parallel spin electrons,at this time;the number of the non-conjugated electron is the most. These electrons occupy different spacious tracks so that the energy of Ni2 molecule reduces to the minimum. It shows that the effect of parallel spin of Ni2 molecule is larger than the effect of the conjugated molecule. It is obviously related to the effect of electron d delocalization. The Murrell-Sorbie potential function with the parameters for ground state for Ni2 molecule are also derived. Dissociation energy of the ground state Ni2 molecule is 1.835 eV,the equilibrium bond length is 0.2243 nm,and the vibration frequency is 262. 35 cm-1 . The force constants f2,f3 and f4 are 1.1901 aJ / nm2,-5.8723 aJ/nm3,21.2505 aJ/nm4 respectively.
The optical properties of metal nanoparticles are quite different from those of the bulk materials mainly due to the collective oscillations of their conduction electrons known as the surface plasmon resonance(SPR),which is strongly dependent on the particle shape and size,and the dielectric properties of the local environment where the nanoparticles are embedded in. Based on the discrete dipole approximation(DDA)method,we studied the optical properties of silver nanorods with different aspect ratios in some special dielectric environment including air,water,acetone,methylene chloride and pyridine. The DDA simulation of the ultraviolet-visible(UV-Vis)extinction spectra of silver nanorods with varying aspect ratios shows the plasmons absorption splits into two bands corresponding to the oscillation of the free electrons along and perpendicular to the long axis of the rods. The transverse mode shows almost a fixed resonance at about 350 nm while the resonance of the longitudinal mode is red-shifted and strongly depends on the aspect ratio of the nanorods. An empirical formula was given to predict the peak position of the longitudinal palsmon band of the silver nanorods with different aspect ratios in the air. The calculation result also shows the maximum of the longitudinal plasmon band of a silver nanorod with a fixed aspect ratio depends on the medium dielectric constant in a linear way. The TEM image and corresponding UV-Vis extinction spectrum of silver nanosphere and nanorods synthesized by our lab are in good agreement with the DDA simulation results.
The solvent effect on the photoinduced charge transfer between N,N-dimethylaniline and quinone has been investigated. Geometries of the isolated donor and acceptor have been optimized using B3LYP/6-31G** method. Two relative stable conformers P and T of the complex have been obtained with the symmetry of Cs. Excited states of the complex have been calculated at the level of CIS/6-31+G** in the gas phase. The results indicate that the complete charge separation within the complex occurs in the third excited singlet state of P,but only partial charge transfer for the same excited state of T. At the same basis level,excited states of the complex in the solution have been investigated. Comparing the results in the solution with the gas-phase ones,the charge separation of the third excited singlet states of both conformers is found fairly well. The absorption spectrum in the solution has a large red shift.
Molecular vibration is correlated to the motion of a pendulum and the lowly excited states are corresponding to that of the pendulum around its stable fixed point while the highly excited states are to the unstable fixed point. Specifically,the transitional state due to internal rotation is also corresponding to the unstable fixed point of the pendulum. As for a perturbed pendulum,chaos occurs first around its unstable fixed point,which is a reasonable consequence that the highly excited state and the transitional state are full with intrinsic chaotic motion. With this conjecture,HCN,its isomer HNC and the excited delocalized transitional state due to the internal rotation of H-C around the skeleton of C-N are interpreted with Morse oscillators in resonance. It is stressed that the delocalized transitional state is in multiple resonances between the H - C stretch and the bending due to that the classical bending frequency is lowered as the transitional state is approached. Multiple resonances,or the overlapping of resonances,lead to chaos as noted by Chirikov. Hence,the delocalzed transitional state can be in a chaotic state. Besides,the internal rotational state of HCP due to H atom is analyzed by this physical picture. For this purpose,an algebraic Hamiltonian for HCN,its isomer HNC and the delocalized transitional state is proposed with its coefficients elucidated by fitting with the quantal levels adopted from the literature by the quantum mechanical algorithm. The result shows that both the transitional state due to the internal rotation of H atom and the highly excited states of HCN and HNC are full of multiple resonances. Therefore,chaos is expected for these systems. Finally,all these ideas are compounded by a proposed model for unfolding the characteristics of chaos in the molecular system.
The Ω = 0,1,2 levels of the Na2 2 3Πg and 3 3Πg states in the energy region between 34860 cm-1 above the Te of the ground state and the 3s+3d atomic limit have been probed by pulsed laser perturbation facilitated optical-optical double resonance(PFOODR)spectroscopy. Strong mixing of these two states has been observed. The Tv and Bv values are given for the Ω = 0 component without deperturbation.
The(9,0)band of the[14. 6]2Δ5/2 - X2Δ5/2 transition of NiI in the visible region has been studied at high resolution using laser vaporization / reaction supersonic free jet expansion and laser induced fluorescence spectroscopy. Spectrum taken at a resolution of about 80 MHz shows the resolved hyperfine structure which is caused by an unpaired electron in the excited state interacting with the large magnetic moment of the I nucleus with nuclear spin I = 5/2. The rapid decrease in hyperfine width of the low J lines in the v=9 level suggests that the hyperfine coupling in the[14. 6]2Δ5/2 state conforms to the Hund's Case aβ coupling scheme. Accurate rotational and hyperfine parameters for the[14. 6]2Δ5/2 state have been obtained,which indicate that the v=9 level is perturbed.
A further study on the temporal behavior of collision-induced transition amplitude and phase angle within CO(A~e)mixed states is carried out based on the first order Born approximation of time dependent perturbation theory for He or HCl as the collision partners. The calculation shows that the transition amplitudes appear some damped oscillation with the period reciprocally proportional to the energy gap between the initial and final states while the damping time is determined by the inter-molecular potential of the collision system. For a given impact parameter and collisional velocity,the transition of CO from an initial rotational state J to the final state J' may proceed through many channels with different quantum numbers M of CO and different J and M quantum numbers of the collision partner. So the experimental transition amplitudes with its real and imaginary parts are the average of all the possible channels. It is interesting to note that in the case of He as the collision partner,all the transition amplitude vectors for both singlet and triplet channels are located in the 1st quadrant of the complex plane,whereas for HCl as the collision partner the transition vectors of singlet and triplet channels are located respectively in the 4th and 1st quadrants. These results well explain the experimental fact that the interference phase angle is smaller than 90o for He as the collision partner,whereas it is larger than 90o for HCl as the collision partner.
The vacuum ultraviolet(VUV)pulsed field ionization-photoelectron( PFI-PE)spectrum for trichloroethene(ClCH=CCl2)has been measured in the energy range of 76400-79650 cm-1 . The vibrational bands resolved in the VUV-PFI-PE spectrum are assigned based on ab initio vibrational frequencies and calculated Franck-Condon factors for the ionization transitions,yielding eleven vibrational frequencies for ClCH=CCl2+:v1+=148 cm-1,v2+= 80 cm-1,v3+=286 cm-1,v4+=402 cm-1,v5+= 472 cm-1,v6+=660 cm-1,v7+=875 cm-1,v8+=990 cm-1,v9+=1038 cm-1,v10+=1267 cm-1,and v11+=1408 cm-1. These measurements along with the frequency v12+=3073 cm-1 determined in the recent VUV-infrared photo-induced ionization study have provided the complete set of twelve experimental vibrational frequencies for ClCH = CCl2+ in its ground electronic state. On the basis of the spectral simulation of the origin VUV-PFI-PE vibrational band,we have determined the IE(ClCH=CCl2)to be(76441.7±2.0)cm-1((9.4776±0.0002)eV).
Nano-cystal Bi12 TiO20 powders have been prepared by the chemical solution decomposition( CSD)method using bismuth nitrate and titanium butoxide as the starting precusors. The powders were characterized by the thermogravimetry and differential thermal analysis(TG-DTA),X-ray diffractometry(XRD)and transmission electron microscopy(TEM). The formation mechanism of Bi12TiO20 naocrystal has been analyzed. The UV-Vis diffuse reflectance spectra of the prepared Bi12TiO20 powders were measured to determine their optical absorption characteristic. The results showed that pure nano-crystal Bi12TiO20 powders could be easily prepared from stoichiometric precursor solutions at the annealing temperature 1550℃. A broad optical absorption band between 560-385 nm (about 2. 2-3. 22 eV)was observed for the prepared nano-crystal Bi12TiO20 powders.
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.
Using the surfactant(PVA)in the preparation process,the nano-iron oxide / Bentonites were prepared through a reaction between a solution of iron salt and a dispersion of Bentonite clay. The X-ray diffraction(XRD)results reveal that the catalysts mainly consist of α-Fe2O3 . The photo-catalytic activity of iron oxide / Bentonites was examined in the photo-assisted degradation of an organic azo-dye Orange Ⅱ. It is found that the photo-catalytic activity of the catalysts is much higher than that of α-Fe2O3 . The experimental results(using the different catalysts in which the quantities of Fe2O3 are equal)demonstrate that the photo-catalytic activity of catalysts is as follows:Fe-A > Fe-B > Fe-C > Fe-D. In addition,the research shows that Orange Ⅱ degradation ratio of heterogeneous photo-Fenton process is higher than that of homogeneous one.
A home-made magnetic-bottle time-of-flight anion photoelectron spectrometer(PES)for the investigation of binary metal cluster geometry and electron structure is described. The photoelectron spectrometer is installed near the first space focus of home-made reflectron time of flight mass spectrometer(RTOFMS),coupled with laser ablation,pulse supersonic molecular carrier gas cluster source. The magnetic-bottle photoelectron spectrometer's resolution is about 0. 1 eV for 1 eV photoelectrons. The adiabatic electron affinity energies of neutral clusters and some features relative to their excited states can be obtained from the spectra,i. e. ,from the anion's spectra,not only the features of the anion but also the neutral clusters' features can be investigated. The detailed design,construction,and operation of the new apparatus are presented. And studied PbM-(M = Cu,Ag,Au)binary metal cluster anions,the results give clear diagram about their structures and the bond interactions. The adiabatic electron affinity energies obtained by the photoelectron spectrometer agree well with the calculated results using relativistic density functional theory(DFT)method. It show that this anion photoelectron spectrometer can be well used in studying binary metal cluster anions in the experiment condition.
The reaction mechanism of C2(a 3Πu)+ NO is investigated at the level of G2(CC,MP2). The equilibrium geometries,harmonic frequencies and energy of various stationary points on the potential energy surfaces have been calculated in the lowest doublet states. It is found that there are two reaction mechanisms:one is CCON mechanism that begins from O atom of NO attacks C2 and the intermediate is CCON;the other is called CCNO mechanism for its intermediate is CCNO formed by N atom of NO attacks C2 . In the same time,the five possible ground product pathways corresponding to these two mechanisms for this reaction are analysed and concluded that the pathway that O atom of NO attacks C2 to produce the major products CN+CO via CCNO mechanism is the most favorable pathway.
The character of tunable Vacuum-Ultraviolet(VUV)laser generated by two-photon resonant four wave difference frequency mixing in Xenon was studied. The intense VUV laser was generated in the wavelength range of 151-171 nm using 6P[1/2,0]level of Xe atom as the two-photon resonant state. The pulse intensity of VUV laser was estimated to be 0.2 μJ,and the conversion efficiency relative to the wavelength-fixed laser was determined to be 0.1%. The line width of VUV laser was found to be 0.3 cm-1 from the laser-induced fluorescence spectrum of A-X(0,0)rotational line profiles of jet-cooled CO,which was much broader than those of the two employed dye lasers(0. 1 cm-1),mainly due to the saturation broadening of Xe level by intense laser field. The dependencies of VUV intensity on Xe pressure and two dye laser intensities were also investigated in this experiment.
The semirigid vibrating rotor target(SVRT)model is applied for the reaction F+CH2D2→CH2D/CHD2+DF/HF. The time-dependent wave packet approach is also used in the calculation. Reaction probabilities,crosssections,and rate constants are calculated for the title reaction from the ground state of the reagent on the modified J1(MJ1)potential energy surface(PES)for both channels. Numerical calculation shows the oscillatory structures in the energy dependence of the calculated reaction probability. Those structures are generally associated with broad dynamical resonances. They are smooth in the energy dependence of integral cross-sections due to summation over partial waves. The calculated rate constants are in good agreement with the experimental measurement.
A novel experimental technique has been developed to measure the attributes of product pair correlation of bimolecular reactions under the crossed molecular beam condition. The first system that we picked is F + CD4/CHD3 / CH4 reactions. By combining a crossed molecular beam method with a time-sliced ion velocity imaging technique,the product state-resolved pair-correlated differential cross sections were revealed directly from the measurements. Several facets of the product pair correlation have been explored. The dependence on the collisional energy has been elucidated. The pair-correlated angular distributions show strong dependences on the HF/DF vibrational quantum numbers,and weaker yet not negligible dependences on the methyl radical vibrational quantum numbers. For the F + CH4 reaction at collisional energies close to the reaction threshold,the first experimental evidences of a reactive resonance in a polyatomic reaction were discovered. The product pair-correlated information helps us to unravel the complexity of polyatomic reactions and offers the important link between A + BC type of reactions and more general polyatomic reactions.
Near-ultraviolet absorption spectrum of the B 2A" 0 ← X 2A" band of the vinoxy radical(CH2CHO)is recorded by cavity ringdown spectroscopy(CRDS). The absorption spectrum shows a series of vibronic bands starting from 28786 cm - 1 and an increasing broad background towards higher photon energy. The CRDS absorption spectrum is similar to an early low-resolution absorption spectrum;and the vibronic peak positions match well with those in the laser-induced fluorescence and photofragment yield spectra.
A simplified formulation for treating the linear and nonlinear spectroscopy of ordered molecular systems is presented,in order to help experimentalists to have an explicit physical picture and quantitative tool on using linear and nonlinear spectroscopy to study molecules in ordered molecular systems. This formulation is expended from our recent quantitative orientational and polarization treatment on second-order nonlinear spectroscopic techniques in interface studies,namely,the Second Harmonic Generation(SHG)and Sum Frequency Generation-Vibrational Polarization Spectroscopy(SFG-VPS). The key to this formulation is to simplify the effective linear or nonlinear molecular susceptibility and construct the general orientational functional with a clear approach to calculate the orientational and intensity parameters from the experimental parameters,which determines the orientational and polarization behavior of the general orientational functional in a particular experimental configuration. Also discussed are the advantages of coherent spectroscopic techniques over incoherent ones for the accurate measurement of orientation and ordering of ordered molecular system.