Energy distributions of CO produced in an acetone-containing discharge

Tunable infrared diode laser absorption (TDLAS) and Fourier transform infrared absorption spectroscopies (FTIR) have been utilized to characterize the translational, rotational and vibrational distributions of CO in an acetone/argon DC plasma at total pressures ranging from 4 to 5 Torr and currents of 0.1–0.3 A. A broad vibrational distribution of CO was observed with gradually decreasing intensities from the fundamental band to v=12←11. When nitrogen was added to the plasma, the distribution is narrower, due to the efficient energy transfer between CO and N₂ molecules. The measured translational temperature in such plasmas ranged from 400–550 K. The rotational distribution can generally be fitted to a Boltzmann distribution within each vibrational level although the rotational temperature is highest for the lowest vibrational quantum number.     

Production of vibrationally excited hydrogen molecules by atom recombination on Cu and W materials

We have measured vibrational population of H(2) and D(2) molecules produced by atom (H or D) recombination on tungsten and copper material. The vibrational spectroscopy, based on the properties of dissociative electron attachment to hydrogen molecule, was used. The vibrationally excited molecules were produced by atom recombination in a cell where the studied sample is exposed to hydrogen atoms, from hot tungsten filament. Vibrational populations were obtained for the studied materials, which can be well described by the Boltzmann distribution, with specific vibrational temperatures for each material. The experimentally obtained vibrational populations for copper approximately agree with the theoretical predictions, whereas the experimentally obtained vibrational temperature for tungsten is higher and thus showing a considerable overpopulation of highly excited vibrational states than predicted. We propose that the origin of this higher excitation is related to the existence of high hydrogen surface coverage on tungsten, where hydrogen is occupying binding sites with different desorption energies. In order to obtain an insight into the recombination mechanism with more than one binding site per unit cell, a Monte Carlo simulation was performed, where it was assumed that the main production of molecules proceeds through the hot-atom recombination with an adsorbed atom. The results show that the recombination proceeds mainly through the weak binding sites, once they are occupied.     

Electron energy distribution functions and dissociation kinetics of HCl in non-equilibrium plasmas

Electron energy distribution functions (edf) have been calculated by numerically solving the Boltzmann equation coupled to a system of vibrational master equations which simulates both the vibrational relaxation due to e—V (electron—vibration), V—V (vibration—vibration) and V—T (vibration—translation) energy exchanges and the dissociation process. The calculated edf strongly depend on the vibrational nonequilibrium present in the gas phase, even though the atoms coming from the dissociation process tend to destroy the vibrational energy content of the molecules. Vibrationally excited molecules determine a joint vibro—electronic mechanism in the dissociation of HCl. This mechanism is the more efficient the smaller the gas temperature and pressure. Finally the contribution of a purely vibrational mechanism in the dissociation of HCl is presented and discussed.     

混合量子-经典动力学方法研究反胶团水池中I2分子的振动频率位移

结合Monte Carlo模拟技术, 提出了一种反胶团溶液的快速数学建模新方法. 利用量子-经典动力学模拟方法, 考察了I2分子受限于两个不同尺寸的反胶团水池中振动频率的诱导位移及谱分布. 结果表明, 相比于体相水, 受限于反胶团水池中I2分子的诱导位移表现为蓝移, 且蓝移大小随水池尺寸变化不大. 通过对I2分子与周围环境相互作用的分解分析, 得到了水池水、表面活性剂以及有机溶剂分子对I2分子振动频率诱导位移的瞬态贡献, 揭示了I2分子振动弛豫的微观作用机制. 此外, 对于受限水池中水分子的诱导贡献及空间分布的研究表明, I2分子振动频率位移的诱导贡献主要来自于第一溶剂层, 它是由4个水分子蓝移贡献和2个水分子红移贡献组成.     

On the optical probing of excited molecules that have undergone radiationless transitions

An explanation is given for the experimentally observed red-shift of the absorption band of molecules, in a molecular beam, that have undergone radiationless transition. A theoretical analysis shows that the effect originates from a decrease of the frequencies of the vibrational modes under the electronic transition. The value of the experimentally observed red-shift is shown to be difficult to use for discriminations between the selective excitation of a few vibrational modes and a random distribution of the energy of excitation among all the vibrational modes of a molecule. The latter seems more probable.     

Structural and dynamic features of water and amorphous ice

Structural properties and microscopic dynamics of water and amorphous ice have been studied by the molecular dynamics method. It has been found that the distribution function of the tetrahedricity parameter exhibits two ranges, which correspond to local molecular formations with low and high degrees of tetrahedricity. The number of molecular clusters with a high degree of tetrahedricity grows as temperature decreases. It has been shown that the vibrational density of states comprises two vibrational modes. A low-frequency vibrational mode strongly depends on pressure and is almost independent of temperature, while a high-frequency mode is relevant to the pressure-independent heat motion of molecules. The geometric criterion of hydrogen bonds has been used to evaluate their continuous lifetime as depending on temperature for molecules with different coordination values. The average lifetime of a hydrogen bond substantially depends on the coordination of molecules, with the temperature dependence of the coordination obeying the activation dynamics.     

Vibrational relaxation dynamics of I35Cl(B, v') induced by low-temperature collisions with He atoms

Using laser-induced fluorescence and two-laser, pump-probe spectroscopy, collision-induced vibrational relaxation is observed to compete with the dissociation of electronically excited ICl in a helium carrier gas expansion. By thoroughly characterizing the expansion properties, we observe that collisions of ICl(B, v'= 3) molecules with He atoms in the expansion induce vibrational relaxation of the initially prepared dihalogen down to rotor states in the next lower ICl(B,v'= 2) level on timescales that compete with the rate for non-adiabatic transfer from the B state to the Z1 state. The resulting ICl(B,v'= 2,j') product rotational distribution, along with the analogous ICl(B,v'= 1,j') distribution formed by collisional relaxation of molecules in the long-lived ICl(B,v'= 2) level are compared to ICl(B,v'= 2,j') products formed by vibrational predissociation of He...ICl complexes prepared in different intermolecular vibrational levels within the He + ICl(B,v'= 3) potential. No evidence is observed for resonance-enhanced collisional cross sections, even at the low temperatures achieved, T < 1.0 K.     

A DFT study on the vibrational spectroscopy of protoporphyrin IX

Infrared and Raman spectra of protoporphyrin IX were recorded. DFT quantum chemical calculations were performed. Optimised molecular geometry, electric charge distribution, vibrational force constants were computed. The normal coordinate analysis and the scaling of the force constants yielded all the necessary data for the simulation of the infrared and Raman spectra and the potential energy distribution calculations. The result was the interpretation of all vibrational modes of the molecule. Conclusions were drawn from the difficulties arisen during the assignment of the vibrational spectra of such large molecules.     

3D Generalized langevin equation approach to gas–surface reactive scattering: model H+H→H2/Si(100)-(2×1)

Classical trajectory calculation has been performed for the H+H→H₂/Si(100)-(2×1) reaction by the 3D Generalized Langevin Equation (GLE) approach. The implementation of the 3D GLE approach to the H+H→H₂/Si(100)-(2×1) reaction is presented. Reaction probabilities are calculated for given surface temperatures and given collision energies. We also calculated vibrational and rotational distributions of product H₂ molecules from the reaction. About 80% of the product hydrogen molecules are in the ground vibrational state and the remaining 20% of the products are in the excited states. The rotational state shows non-Boltzmann distribution which can be seen in the direct collision process. Vibrational and Rotational distributions are strongly related to the impact parameter. The vibrational distribution is correlated with the x-component of the impact parameter and reflects the dimer nature of the silicon (2×1) surface. Details of the dynamics involving vibrational and rotational transitions are discussed.     

Transfer of electron excitation energy to vibrational degrees of freedom in complex molecules. Part I

Energy transfer is discussed for paraffin molecules excited by electrons of energy about 100 ev; the nonadiabatic transient wave function for the positive ion of a long molecule is derived and is used to examine the distribution of vibrational energy along the chain.     

Determination of intermolecular energy transfer by time resolved CARS measurements

Energy transfer in methyl isocyanide following i.r. multiphoton excitation has been examined with a CARS probe. Results show that CARS is an effective technique both for measuring the fraction of molecules receiving energy from the laser and for monitoring intermolecular energy transfer following laser irradiation. In addition, the Raman transition at 2169 cm⁻¹ shows a vibrational progression due to hot bands of the ν₈ bend. Since the ν₈, bend is the only low energy mode of the molecule, it is an effective vibrational “thermometer”. Utilizing this thermometer, results show that the vibrational state distribution does not fit a Boltzmann distribution for 7 μs following i.r. multiphoton excitation.     

多原子分子受激光激励后振动分布的模型计算

以SF_6分子为例, 进行了多原子分予受脉冲红外激光激励之后, 其瞬态振动布居的模型计算。本模型可从实验测得的红外荧光或吸收光潜, 反演出与这些光谱相应的振动分布。反之亦然。与同类模型相比, 本模型的计算精度高, 可精确地生成振动跃迁几率矩阵, 且可计算受激光激励后分子在任一时刻的振动分布。用此模型, 首次获得SF_6分子低温条件下(20 K)的可转动分辨的吸收光谱和荧光光谱。     

Three-dimensional quantum dynamics study of vibrational predissociation of HeI_2 van der Waals molecule for low vibrational excitation using the time-dependent wave packet method

Three-dimensional quantum mechanical calculations for vibrational predissociation of HeI2(B) van der Waals molecules are presented using the time-dependent wave packet technique within the golden rule approxima tion.The total and partial decay widths,lifetimes,rates and their dependence on initial vibrational states were obtained for HeI2 at low initial vibrational excited levels.Our calculations show that the calculated tota decay widths,lifetimes and rates agree well with those extrapolated from experimental data available The predicted total decay widths as a function of initial vibrational states exhibit highly nonlinear behavior.The very short propagation time (less.than 1 ps) required in the golden rule wave packet calculation is determined by the duration time of the final state inter-action between the fragments on the vibrationally deexcited adiabatic potential surface.The final state interaction between the fragments is shown to play an important role in determining the final rotational distri     

A pure-dephasing model for overtone lineshapes

A model is developed for calculatinq the pure-dephasing (T₂) contribution to overtone lineshapes in isolated molecules. The linewidth is attributed to time-dependent fluctuations in the frequency of the overtone transition, resulting from the vibrational motions in the molecule, and no transfer of energy (intramolecular vibrational distribution) occurs in this picture. Numerical calculations for 2,3,5,6-paradeuterobenzene are presented.     

Electronic spectroscopy of molecules in superfluid helium nanodroplets: an excellent sensor for intramolecular charge redistribution

Electronic spectra of molecules doped into superfluid (4)He nanodroplets reveal important details of the microsolvation in superfluid helium. The vibrational fine structure in the electronic spectra of phthalocyanine derivatives and pyrromethene dye molecules doped into superfluid helium droplets have been investigated. Together with previous studies on anthracene derivatives [J. Chem. Phys.2010, 133, 114505] and 3-hydroxyflavone [J. Chem. Phys.2009, 131, 194307], the line shapes vary between two limiting cases, namely, sharp Lorentzians and nonresolved vibrational fine structure. All different spectral signatures are initiated by the same effect, namely, the change of the electron density distribution initiated by the electronic excitation. This change can be quantified by the difference of the electrostatic moments of the molecule in the electronic ground state and the corresponding Franck-Condon point in the excited state. According to the experimental data, electronic spectroscopy suffers from drastic line broadening when accompanied by significant changes of the charge distribution, in particular, changes of the dipole moment. Vice versa, the vibrational fine structure in electronic spectra of molecules doped into helium droplets is highly sensitive to changes of the electron density distribution.     

On the vibrational relaxation of diatomic molecules at intermediate excitation

The theory of quasistationary vibrational distributions of diatomic molecules at low gas temperatures is developed for the case of intermediate excitation temperatures when the flow of quanta is nondiffusional in Treanor's part of the distribution. In certain conditions the distribution contains a region of inversion which is followed by a plateau.     

The determination of the populations of vibrational levels of molecules from electron diffraction data

The application of modern gas-phase electron diffraction for directly determining the populations of vibrational levels is demonstrated for the molecule I₂. It is concluded that, under the conditions of the conventional electron diffraction experiment, a thermal equilibrium can reliably be assumed for a gas jet but at a temperature which can be markedly lower than that of the nozzle. The diffraction results favour a Boltzmann distribution of molecules over the vibrational states.     

Ab initio and experimental studies on structure and vibrational spectra of some partially reduced benzo[c]phenanthrenes

A systematic study has been conducted on the conformation, electronic structure and vibrational spectra of benzo[c]phenanthrene and some of its partially reduced derivatives by experimental infrared spectroscopic and quantum chemical techniques. Electrostatic potential surfaces have been mapped over the electron density isosurfaces to obtain information about the size, shape, charge density distribution and chemical reactivity of the molecules. Possibility of hydrogen-hydrogen bonding has been explored in all the molecules. Partial reduction of the aromatic rings in benzo[c]phenanthrene leads to considerable molecular distortion with the approximate mean angle between the terminal rings increasing from 27.3 degrees to 46.0 degrees . The distortion is unequally distributed near the aromatic and saturated rings; the latter absorbs most of strain due to flexibility of the rings. A complete vibrational analysis of the experimental infrared spectra has been reported on the basis of frequency and intensity of the vibrational bands and potential energy distribution over the internal coordinates and characteristic bands have been identified.     

Deactivation dynamics of vibrationally excited nitrogen molecules by nitrogen atoms. Effects on non-equilibrium vibrational distribution and dissociation rates of nitrogen under electrical discharges

The non-equilibrium vibrational distribution and the dissociation kinetics of N₂ in electrical discharges has been calculated by solving an appropriate vibrational master equation. Attention has been devoted to the role of nitrogen atoms in deactivation the vibrational distribution. To this end, a complete set of V—T (vibration—translation) deactivation rates of vibrationally excited molecules by nitrogen atoms has been calculated using a quasiclassical trajectory technique. The results show that nitrogen atoms formed by electron impact are able to deactive the high vibrational levels (v > 25) of N₂ strongly affecting the heavy particle dissociation kinetics of N₂.     

Three-dimensional quantum dynamics study of vibrational predissociation of HeI2 van der Waals molecule for low vibrational excitation using the time-dependent wave packet method

Three-dimensional quantum mechanical calculations for vibrational predissociation of He1₂(B) van der Waals molecules are presented using the time-dependent wave packet technique within the golden rule approximation. The total and partial decay widths, lifetimes, rates and their dependence on initial vibrational states were obtained for HeI₂ at low initial vibrational excited levels. Our calculations show that the calculated total decay widths, lifetimes and rates agree well with those extrapolated from experimental data available. The predicted total decay widths as a function of initial vibrational states exhibit highly nonlinear behavior. The very short propagation time (less than 1 ps) required in the golden rule wave packet calculation is determined by the duration time of the final state interaction between the fragments on the vibrationally deexcited adiabatic potential surface. The final state interaction between the fragments is shown to play an important role in determining the final rotational distribution. This interpretation clearly explains the dynamical effect that the final rotational distribution shifts to the lower rotational energy levels as the initial vibrational quantum numberu increases.