Cloud electrification is one of the oldest unresolved puzzles in the atmospheric sciences.Though many mechanisms for charge separation in clouds have been proposed, a quantitative understanding of their respective contribution in a given meteorological situation is lacking.Here we suggest and analyze a hitherto little discussed process.A qualitative picture at the molecular level of the charge separation mechanism of lightning in a thundercloud is proposed.It is based on two key physical/chemical natural phenomena, namely, internal charge separation of the atmospheric impurities/aerosols inside an atmospheric water cluster/droplet/ice particle and the existence of liquid water layers on rimers (graupels and hailstones) forming a layer of dipoles with H + pointing out from the air-water interface.Charge separation is achieved through strong collisions among ice particles and water droplets with the rimers in the turbulence of the thundercloud.This work would have significant contribution to cloud electrification and lightning formation.

We propose a periodic table for quadrilateral tetra-atomic molecules. Each molecule consists of four atoms, and the geometric shapes of these molecules are studied by ab initio calculations. It is found that by increasing the valence electron numbers, strong periodicity appears with many molecular properties. The periodic table for the quadrilateral tetra-atomic molecules is considered as an extension of Mendeleev's periodic table. It can also be used for estimating the unknown properties of tetra-atomic molecules.

Based upon experimental observation in the laboratory, we propose that ionic wind from corona discharge inside a thundercloud would play an important role in producing a rain gush. A cyclic chain of events inside a super-saturated environment in a thundercloud is proposed, each event enhancing the successive ones until lightning occurs. These successive events are collision between snowflakes and rimers, charge separation, corona discharge, ionic wind originating from the positively and negatively charged masses of cloud, vortex motion and turbulence when mixed with the updraft, more collision, more charge separation, stronger corona discharge, and so on. Meanwhile, avalanche ionization would produce more cloud condensation nuclei resulting more precipitation and hence rimers formation in the super-saturated environment. More collision in the buoyant turbulence would lead to more fusion of droplets and the formation of larger rimers. The cyclic processes would repeat themselves until the electric field between the two oppositely charged masses of cloud was strong enough to induce a lightning breakdown. There would be no more ionic wind, hence, much less buoyant turbulence. The updraft alone would not be sufficiently strong to support larger rimers which would fall down ‘suddenly’ to the earth surface as a rain gush.

Based upon experimental observation in the laboratory, we propose that ionic wind from corona discharge inside a thundercloud would play an important role in producing a rain gush. A cyclic chain of events inside a super-saturated environment in a thundercloud is proposed, each event enhancing the successive ones until lightning occurs. These successive events are collision between snowflakes and rimers, charge separation, corona discharge, avalanche ionization, ionic wind originating from the positively and negatively charged masses of cloud, vortex motion and turbulence when mixed with the updraft, more collision, more charge separation, stronger corona discharge, and so on. Meanwhile, avalanche ionization would produce more CCN (cloud condensation nuclei) resulting in more precipitation and hence rimers formation in the super-saturated environment. More collision in the buoyant turbulence would lead to more fusion of droplets and the formation of larger rimers. The cyclic processes would repeat themselves until the electric field between the two oppositely charged masses of cloud was strong enough to induce a breakdown. The latter would create a sudden short circuit between the two charged masses of cloud neutralizing the charges. There would be no more ionic wind, hence, much less buoyant turbulence. The updraft alone would not be sufficiently strong to support larger rimers which would fall down 'suddenly' to the earth surface as a rain gush.

The excited state properties of a series of ruthenium(II) amine-substituted bipyridyl complexes, [Ru(bpy)n(NNbpy)3−n]2+, were investigated by steady-state and transient absorption spectroscopy, as well as quantum chemical calculations. The steady-state absorption spectra of these complexes in CH3CN show a distinct red-shift of the 1MLCT absorption with increasing numbers of amine substituent, whereas the emission spectra indicate an energy gap order of [Ru(bpy)3]2+>[Ru(bpy)2(NNbpy)]2+>[Ru(NNbpy)3]2+>[Ru(bpy)(NNbpy)2]2+. Nanosecond, femtosecond transient absorption and electrochemical measurements suggest that NNbpy ligand has a strong influence on the electronic and emission properties of these complexes, due to electron-rich amine substituent. We illustrate how the numbers of amine substituent modulate the spectroscopic properties of transition metal complexes, which is related to the design of new electro-active systems with novel photoelectrochemical properties.

Mendeleev periodic table of atoms is one of the most important principles in natural science. However, there is not such a thing for molecules. Here we propose three periodic tables for diatomic molecules, triatomic molecules and AH(3) tetratomic molecules, respectively. The tables not only contain isolated molecules, but also the "virtual" diatomic molecules in polyatomic molecules. The form of these molecular periodic tables is analogous to that of Mendeleev periodic table. In the table, molecules are classified and arranged by their group number G, which is the number of valence electrons, and the periodic number P, which represents the size of molecules. Basic molecular properties, including bond length, binding energy, force constant, ionization potential, spin multiplicity, chemical reactivity, or bond angle, among others. change periodically with the tables. This periodicity originates from the shell-like electronic configurations of molecules. The periodic tables can be used to predict unknown properties of molecules, to understand the role of virtual molecules in polyatomic molecules, and to initiate new research fields such as the periodicity of aromatic compounds, clusters, or nanoparticles.

We experimentally demonstrate the photodissociation process of Br-2 molecules in the intense femtosecond laser field by a dc-sliced ion velocity map imaging technique. We show that four fragment ions Brn+ (n = 1-4) are observed, and their kinetic energy increases while their angular distribution decreases with the increase of the charge number. We prove that the low (or high) charged fragment ions result from the photodissociation of the low (or high) charged parent ions. We explain the changes of the kinetic energy and angular distribution in these fragment ions by considering the potential energy curves of these parent ions that involve both the interaction of the Coulomb repulsive energy and chemical bonding energy. We also explain the experimental observation that the measured kinetic energy release in the experiment is much smaller than the theoretical calculation by enhanced ionization at a critical distance.

Partially deuterated protonated water dimers, H2O·H(+)·D2O, H2O·D(+)·HDO, and HDO·H(+)·HDO, as important intermediates of isotopic labeled reaction of H3O(+) + D2O, undergo direct dissociation and indirect dissociation, i.e., isomerization before the dissociation. With Rice-Ramsperger-Kassel-Marcus theory and ab initio calculations, we have computed their dissociation and isomerization rate constants separately under the harmonic and anharmonic oscillator models. On the basis of the dissociation and isomerization rate constants, branching ratios of two primary products, [HD2O(+)]∕[H2DO(+)], are predicted under various kinetics models with the harmonic or anharmonic approximation included. The feasible kinetics model accounting for experimental results is shown to include anharmonic effect in describing dissociation, while adopting harmonic approximation for isomerization. Thus, the anharmonic effect is found to play important roles affecting the dissociation reaction, while isomerization rates are shown to be insensitive to whether the anharmonic or harmonic oscillator model is being applied.

The Mendeleev periodic table of atoms is one of the most important principles in natural science.However, there is shortage of analog for molecules.Here we propose two periodic tables, one for diatomic molecules and one for triatomic molecules.The form of the molecular periodic tables is analogous to that of Mendeleev periodic table of atoms.In the table, molecules are classified and arranged by their group number G, which is the number of valence electrons, and the periodic number P, which represents the size of the molecules.The basic molecular properties, including bond length, binding energy, force constant, ionization potential, spin multiplicity, chemical reactivity, and bond angle, change periodically within the tables.The periodicities of diatomic and triatomic molecules are thus revealed.We also demonstrate that the periodicity originates from the shell-like electronic configurations of the molecules.The periodic tables not only contain free molecules, but also the "virtual" molecules present in polyatomic molecules.The periodic tables can be used to classify molecules, to predict unknown molecular properties, to understand the role of virtual molecules in polyatomic molecules, and to initiate new research fields, such as the periodicities of aromatic species, clusters, or nanoparticles.The tables should be of interest not only to scientists in a variety of disciplines, but also to undergraduates studying natural sciences.

We report neutral dissociation of simple molecules in strong laser field, experimentally. Moreover, theoretical calculations of potential energy curves justify neutral dissociation through super excited states in all of the investigated gases.

Superexcited states of NO molecule and their neutral dissociation processes have been studied both experimentally and theoretically. Neutral excited N* and O* atoms are detected by fluorescence spectroscopy for the NO molecule upon interaction with 800 nm intense laser radiation of duration 60 fs and intensity 0.2 PW/cm2. Intense laser pulse causes neutral dissociation of superexcited NO molecule by way of multiphoton excitation, which is equivalent to single photon excitation in the extreme-ultraviolet region by synchrotron radiation. Potential energy curves (PECs) are also built using the calculated superexcited state of NO+. In light of the PECs, direct dissociation and pre-dissociation mechanisms are proposed respectively for the neutral dissociation leading to excited fragments N* and O*.

The ion-pair dissociation dynamics of SO(2) → SO(+) (X(2)Π,υ) + O(-) ((2)P(J)) in the excitation energy range 14.87-15.15 eV has been studied using the tunable XUV laser and velocity map imaging method. The O(-) yield spectrum, the translational energy distributions, and the angular distributions of the photofragments have been measured. The electronic structures and selected cuts of potential energy surfaces for the ion-pair states have been calculated by employing the quantum chemistry calculation method at the CASSCF/6-311++g** level. The equilibrium structures of the six ion-pair states all have linear geometries. An orbital correlation diagram was drawn to illustrate the ion-pair dissociation mechanism. Combining the experimental and theoretical results, it is concluded that the ion-pair dissociation takes place mainly via the predissociation of Rydberg states (1)A(1) [(C(2)B(1))4db(1)] and (1)A(1) [(D(2)A(1))4sa(1)]. The experimental results confirm the previous theoretical calculation results about the symmetry assignments for the energy sequence of SO(2)(+) as C((2)B(1)) < D((2)A(1)).

Investigations on the dissociation kinetics of hydrated protonium ions, (H(2)O)(2)H(+) and their deuterated species (D(2)O)(2)D(+), are reported based on the harmonic and anharmonic oscillator model using the transition state theory and ab initio calculations. We find that the dissociation of (H(2)O)(2)H(+) and (D(2)O)(2)D(+) exhibits a distinct threshold behavior due to the existence of activation energies. Moreover, the deviation between the harmonic and anharmonic dissociation rate constants becomes larger in the high energy or temperature range, with the rate constants becoming unreasonably large under the harmonic oscillator model. The isotope effect is found to become more distinct but only in the case of the anharmonic oscillator model. These results show that the anharmonic Rice-Ramsperger-Kassel-Marcus (RRKM) theory can provide a reasonably good description for the dissociation of (H(2)O)(2)H(+) and (D(2)O)(2)D(+). Furthermore, a theoretical model to demonstrate the principle of vibrational predissociation spectroscopy (VPS) is established from the viewpoint of RRKM theory and applied in determining the experimental conditions and understanding the role of the dissociation rate constant k(E) played in the VPS experiment, using (H(2)O)(2)H(+) and (D(2)O)(2)D(+) as examples.

Superexcited states (SESs) of oxygen molecules and their neutral dissociation processes have been studied both experimentally and theoretically using intense femtosecond laser. We find that at the laser intensity of approximately 2 x 10(14) W/cm(2), ultrashort laser pulse causes neutral dissociation of oxygen molecule by way of SESs. The dissociation products are the excited neutral oxygen atoms, which are observed through fluorescence spectroscopy. Laser power dependence of the fluorescence intensity shows that each molecule effectively absorbs an average of ten laser photons. The total energy absorbed is sufficient to stimulate the molecule to many of the SESs. The effect is equivalent to single photon excitation in the extreme-ultraviolet (XUV) region by synchrotron radiation (SR). Morse potential energy curves (PECs) are constructed for the SESs of O(2) molecules. In light of the PECs, predissociation mechanism is proposed for the neutral dissociation. Quasi-classical trajectory (QCT) calculations show that the predissociation time is as short as 100 fs, which is consistent with our experimental measurement using ultrafast pump-probe technique.

Neutral dissociation of oxygen molecule in superexcited states induced by intense femtosecond laser (10 14 W/cm 2 ) has been studied experimentally and theoretically.

Dissociation of molecules in the strong laser beam at the intensity of 1013–1014 W/cm2 is investigated. Experimentally, the fragmentation of neutral molecules, CH4, C2H4, C4H8; and the disintegration of molecular ions, CH 4 + are studied by fluorescence spectroscopy and mass spectroscopy respectively. Some new phenomena, the strong dependence on the laser intensity, the universal dissociation, and the thorough fragmentation, are found in the fragmentation, and cannot be explained by the existing theories, as Coulomb Explosion theory or Re-scattering theory. We have suggested two new theories. The Super-excited State (SES) theory interprets the neutral fragmentation of molecules, which is stimulated to the SES by intense laser pulse. The Morse potential energy surface of the SES shows that either direct dissociation or pre-dissociation can take place in the SESs. Another theory, the theory of Field-assisted Dissociation (FAD) interprets the fragmentation of ionic molecules. According to this theory, the electric field of the laser pulse is involved directly to the dissociation process. QCT calculations for the trajectories moving on the dressed PES are performed. The result shows that the chemical bond which is parallel to the laser field vector undergo dissociation spontaneously. The dissociation takes place around 100 fs, which is in agreement with the ultrafast measurement in the pump-probe experiment.

We have studied the ion-pair dissociation dynamics of HCl --> Cl(-) ((1)S(0)) + H(+) in the 14.41-14.60 eV using tunable XUV laser and the velocity map imaging method. The measured ion-pair yield spectrum has P- and R-branch resolved vibrational structure, which indicates a predissociation mechanism for the ion-pair dissociation. All of the anisotropy parameters for the angular distribution of the fragments have the limiting values of beta = 2, which suggests that the predissociation occurs via (1)Sigma(+) Rydberg states, and is fast in comparison with the rotational period of HCl. To understand the predissociation dynamics, the diabatic potential energy curve of the ion-pair state has been calculated at the MRCI/CAS/vtz level. The experimental and theoretical results obtained in this work have provided a solid foundation for the previously proposed mechanism that the ion-pair dissociation occurs via predissociation of Rydberg states converging to HCl(+) (A(2)Sigma(+)).

We report neutral dissociation of oxygen and methane in femtosecond laser field through superexcited states. Moreover, the lifetime of the superexcited state is measured by pump and probe technique to be about few hundred femtosecond.

Multiphoton excitation of molecule by intense laser field is studied theoretically to explain the recent experiments of neutral fragmentation. Neutral fragments are created from highly excited parent molecule. Excitation mechanism is discussed with the use of Floqet theory.

Multiphoton excitation of molecules by an intense laser field is treated. When the first excitation energy is much larger than both the photon energy and the transition energy between excited states, excited states are strongly coupled with each other by the photon, even though coupling with the ground state is weak. In such a case, a kind of collective excitation takes places, in which the excitation probability of every excited state is almost linear as functions of intensity on a log–log plot, and the slope is independent of the final state. By taking methane and formaldehyde as examples we numerically study this process.