Theoretical study of the visible photodissociation spectrum of Ar3

The six A′ potential energy surfaces were computed by a DIM-like method involving a valence-bond quasidiabatic basis. Transition dipole moments were also determined using a similar method. The 4D dynamics of this system (restricted to a molecular plane fixed in space) was obtained with the HWD method (hemiquantal dynamics with the whole DIM basis) and the visible photoabsorption spectrum was determined with the help of a 1D full quantum mechanical program applied to each normal mode. The photoabsorption spectrum of Ar₃⁺ was calculated in the range 440–710 nm. It corresponds to photodissociation since the excited Ar₃⁺ ions almost all dissociate into the Ar⁺ +Ar+Ar channel by a rapid explosion of the cluster, and only very few into Ar₂⁺ +Ar. It is dominated by a transition to the second excited state along with a symmetric stretching motion. We found a prominent 80 nm wide peak centered at 530 nm, with a maximum cross section of 4.2 × 10⁻¹⁶cm². The Ar₂⁺ formation results from a transition to the first excited state under low-energy laser excitation and is controlled by non-adiabatic transitions.     

Simulation of the photodissociation of Ar 3 +

A simulation of the photodissociation of Ar ₃ ⁺ is performed in the range 200–650nm. The approach relies on Diatomic in Molecules electronic modelling, Molecular Dynamics and the Hemiquantal method. The results are used to interpret the experimental abundance of slow ions in terms of the various absorption bands. Kinetic energy distributions of all fragments are analysed at 530 and 280nm for two internal energies. The time evolution of the cluster is discussed and a concerted one-step mechanism emerges involving charge fluctuations. The simulation only yields fast-fast and fastslow photoneutral-photoneutral coincidences.     

Theoretical study of the ArH+ photodissociation

The multireference Spin-Orbit (SO) Configuration Interaction (CI) method in its Lambda-S Contracted SO-CI (LSC-SO-CI) version is employed to calculate potential energy curves for the ground and low-lying excited states of the ArH(+) cation. For the first time, electric dipole moments are also computed in the approach, including SO coupling for transitions to the states responsible for the first absorption continuum (A-band) of ArH(+). On this basis, the partial and total absorption spectra in this energy range are obtained. It is shown that absorption in the A-band is dominated by the parallel A(1)Sigma(+)<--X(1)Sigma(+) transition. In the low-energy part of the band (<95 x 10(3) cm(-1)) the absorption is caused by the perpendicular B(1)Pi<--X(1)Sigma(+) excitation, but transitions to the b(3)Pi(0(+),1) states are also not negligible. The branching ratio Gamma for the final photodissociation products is calculated and it is shown to increase smoothly from 0 in the red tail of the band to 1 at E>or= 10(5) cm(-1). The latter value corresponds to the exclusive formation of the spin-excited Ar(+)((2)P(1/2)) ions, and thus leads to the inverse population of the Ar(+)((2)P(1/2)-(2)P(3/2)) ion states.     

Photofragmentation of cluster ions: the visible photoabsorption spectrum of Ar2N 2 +

The absolute cross section for photodissociation of Ar₂N ₂ ⁺ was measured as a function of wavelength in the 470–550 nm range. A structureless broad band was observed; the cross section has a maximum of ～ 210 × 10^?18 cm² at ～ 500 nm. The measurement of the photofragment time-of-flight spectrum shows that(1) N ₂ ⁺ , Ar⁺ and Ar ₂ ⁺ are produced in the photodissociation of Ar₂N ₂ ⁺ in the wavelength range studied, and that(2) the observed visible absorption band is ascribable to a parallel-type transition of Ar₂N ₂ ⁺ , which possibly retains a linear geometry.     

Vibrational spectrum of Ar3(+) and relative importance of linear and perpendicular isomers in its photodissociation

The photodissociation dynamics of the argon ionized trimer Ar(3)(+) is revisited in the light of recent experimental results of Lepe?re et al. [J. Chem. Phys. 134, 194301 (2009)], which show that the fragment with little kinetic energy is always a neutral one, thus the available energy is shared by a neutral and ionic fragments as in Ar(2)(+). We show that these results can be interpreted as the photodissociation of the linear isomer of the system. We perform a 3D quantum computation of the vibrational spectrum of the system and study the relative populations of the linear (trimer-core) and perpendicular (dimer-core) isomers. We then show that the charge initially located on the central atom in the ground electronic state of the linear isomer migrates toward the extreme ones in the photoexcitation process such that photodissociation of the linear isomer produces a neutral central atom at rest in agreement with measured product state distributions.     

Infrared spectrum of ArHD: Simulation through the accurate calculation of photodissociation cross sections

The absolute values of the photodissociation cross sections and lineshapes for 29 sets of predissociative transitions of the ArHD van der Waals complex have been computed. These lineshapes are summed with the correct statistical weighting and compared with the experimentally observed spectrum of McKellar.     

Wave packet study of the UV photodissociation of the Ar2HBr complex

The photodissociation dynamics of the Ar2HBr van der Waals molecule is studied using the multiconfiguration time-dependent Hartree method. Standard Jacobian coordinates are used to describe the molecule. Two four-dimensional calculations are carried out where the rotation of the Ar2 molecule and, in addition, either the vibration of Ar2-Br or that of Ar2 are frozen. The time-evolution of the probability density in the different modes and the calculation of the dissociative flux show that the dissociating hydrogen atom preferentially moves out of the plane defined by Ar2 and Br. A comprehensive study of the cage effect in the process is presented.     

Ab initio study of the KrH+ photodissociation

The multireference spin-orbit configuration interaction method is employed to calculate potential energy curves for the ground and low-lying excited states of the KrH(+) cation. For the first time, the spin-orbit interaction is taken into account and electric dipole moments are computed for transitions to the states responsible for the first absorption continuum (A band) of KrH(+). On this basis, the partial and total absorption spectra in this energy range are obtained. It is shown that the A-band absorption is dominated by the parallel A (1)Sigma(+)<--X (1)Sigma(+) transition. In the low-energy part of the band (<83x10(3) cm(-1)) the absorption is mainly caused by the spin-forbidden b (3)Pi(0(+) )<--X (1)Sigma(+) excitation, while perpendicular transitions to the B (1)Pi and b (3)Pi(1) states are significantly weaker. The branching ratio Gamma for the photodissociation products is calculated and it is shown to increase smoothly from 0 in the red tail of the band to 1 at E>or=90x10(3) cm(-1). The latter value corresponds to the exclusive formation of the spin-excited Kr(+)((2)P(12)) ions, which may be used to obtain laser generation on the Kr(+)((2)P(12)-(2)P(32)) transition.     

Theoretical study of AlC3+

A theoretical study of the AlC₃⁺ species has been carried out. Predictions have been made for some of the molecular properties (geometries, dipole moments, and harmonic vibrational frequencies) which could help in their possible experimental detection. In addition, a topological analysis of the electron density and its associated Laplacian has also been carried out. The global ground state is predicted to be a linear species with ¹Σ electronic state, but a rhombic four‐membered ring (¹A₁) lies close in energy. It seems that both isomers could be accessible to experimental detection. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Quasiclassical trajectory study of the collision-induced dissociation of CH3SH+ + Ar

Quasiclassical trajectory calculations were carried out to study the dynamics of energy transfer and collision-induced dissociation (CID) of CH(3)SH(+) + Ar at collision energies ranging from 4.34 to 34.7 eV. The relative abundances calculated for the most relevant product ions are found to be in good agreement with experiment, except for the lowest energies investigated. In general, the dissociation to form CH(3)(+) + SH is the dominant channel, even though it is not among the energetically favored reaction pathways. The results corroborate that this selective dissociation observed upon collisional activation arises from a more efficient translational to vibrational energy transfer for the low-frequency C-S stretching mode than for the high-frequency C-H stretching modes, together with weak couplings between the low- and high-frequency modes of vibration. The calculations suggest that CID takes place preferentially by a direct CH(3)(+) + SH detachment, and more efficiently when the Ar atom collides with the methyl group-side of CH(3)SH(+).     

水分子激光光解的动力学理论研究

通过对水分子在激光作用下选择性解离的动力学过程进行计算分析,得到了产率随实验参数(相差和相对强度)的变化图和与其相应的等高线,讨论了解离能量和初态对产率的影响.     

Theoretical study of the CH2 + O photodissociation of formaldehyde adsorbed on the Ag(111) surface

Configuration interaction calculations of the ground and excited states of the H2CO molecule adsorbed on the Ag(111) surface have been carried out to study the photoinduced dissociation process leading to polymerization of formaldehyde. The metal-adsorbate system has been described by the embedded cluster and multireference configuration interaction methods. The pi electron-attachment H2CO- and n-pi* internally excited H2CO* states have been considered as possible intermediates. The calculations have shown that H2CO* is only very weakly bound on Ag(111), and thus that the dissociation of adsorbed formaldehyde due to internal excitation is unlikely. By contrast, the H2CO- anion is strongly bound to Ag(111) and gains additional vibrational energy along the C-O stretch coordinate via Franck-Condon excitation from the neutral molecule. Computed energy variations of adsorbed H2CO and H2CO- at different key geometries along the pathway for C-O bond cleavage make evident, however, that complete dissociation is very difficult to attain on the potential energy surface of either of these states. Instead, reneutralization of the vibrationally excited anion by electron transfer back to the substrate is the most promising means of breaking the C-O bond, with subsequent formation of the coadsorbed O and CH2 fragments. Furthermore, it has been demonstrated that the most stable state for both dissociation fragments on Ag(111) is a closed-shell singlet, with binding energies relative to the gas-phase products of approximately 3.2 and approximately 1.3 eV for O and CH2, respectively. Further details of the reaction mechanism for the photoinduced C-O bond cleavage of H2CO on the Ag(111) surface are also given.     

Theoretical study of photodissociation dynamics on the lowest-lying Rydberg state of ketene

In the present study, an attempt is made to reveal the main mechanism of photodissociation on the lowest-lying Rydberg state (1)B(1) of ketene, referred to as the second singlet excited state S(2), by means of the complete active space self-consistent field and the second-order multiconfigurational perturbation theory methods. The located S(2)S(1)T(1) three-surface intersection plays an important role in the dissociation process. It is shown that the intersection permits an efficient internal conversion from S(2) to S(1) state, but prohibits the intersystem crossing from S(2) to T(1) state because of the small spin-orbital coupling value of 0.136 cm(-1). The main photodissociation process could be described as follows: after one photon absorption to the S(2) state, ketene preferentially relaxes to the minimum S(2)C(2v), and undergoes a transition state S(2)TS with small potential barrier along the C(s)-I (out-of-plane bent) symmetry, and passes through the S(2)S(1)T(1) intersection to reach S(1) surface, then arrives at the transition state S(1)TS along the minimum energy path. As is well known, S(1)-->S(0) internal conversion around the Franck-Condon region is expected to be very efficient, and eventually the hot S(0) molecule has accumulated enough energy to yield the CH(2) (a (1)A(1)) and CO (X (1)Sigma(+)) products.     

Theoretical study on the OH + CH3NHCOOCH3 reaction

The multiple-channel reactions OH + CH3NHC(O)OCH3 --> products are investigated by direct dynamics method. The optimized geometries, frequencies, and minimum energy path are all obtained at the MP2/6-311+G(d,p) level, and energetic information is further refined by the BMC-CCSD (single-point) method. The rate constants for every reaction channels, R1, R2, R3, and R4, are calculated by canonical variational transition state theory with small-curvature tunneling correction over the temperature range 200-1000 K. The total rate constants are in good agreement with the available experimental data and the two-parameter expression k(T) = 3.95 x 10(-12) exp(15.41/T) cm3 molecule(-1) s(-1) over the temperature range 200-1000 K is given. Our calculations indicate that hydrogen abstraction channels R1 and R2 are the major channels due to the smaller barrier height among four channels considered, and the other two channels to yield CH3NC(O)OCH3 + H2O and CH3NHC(O)(OH)OCH3 + H2O are minor channels over the whole temperature range.     

Theoretical study on the Br + CH3SCH3 reaction

The multiple-channel reactions Br + CH(3)SCH(3) --> products are investigated by direct dynamics method. The optimized geometries, frequencies, and minimum energy path are all obtained at the MP2/6-31+G(d,p) level, and energetic information is further refined by the G3(MP2) (single-point) theory. The rate constants for every reaction channels, Br + CH(3)SCH(3) --> CH(3)SCH(2) + HBr (R1), Br + CH(3)SCH(3) --> CH(3)SBr + CH(3) (R2), and Br + CH(3)SCH(3) -->CH(3)S + CH(3)Br (R3), are calculated by canonical variational transition state theory with small-curvature tunneling correction over the temperature range 200-3000 K. The total rate constants are in good agreement with the available experimental data, and the two-parameter expression k(T) = 2.68 x 10(-12) exp(-1235.24/T) cm(3)/(molecule s) over the temperature range 200-3000 K is given. Our calculations indicate that hydrogen abstraction channel is the major channel due to the smallest barrier height among three channels considered, and the other two channels to yield CH(3)SBr + CH(3) and CH(3)S + CH(3)Br are minor channels over the whole temperature range.     

Theoretical study on the structure and UV–visible spectrum of pyridine–chloranil charge-transfer complex

The ground-state structure of the charge-transfer complex formed by pyridine (Py) as electron donor and chloranil (CA) as acceptor has been studied by full geometry optimization at the MP2 and DFT levels of theory. Binding energies were calculated and counterpoise corrections were used to correct the BSSE. Both MP2 and DFT indicate that the pyridine binds with chloranil to form an inclined T-shape structure, with the pyridine plane perpendicular to the chloranil. The CP and ZPE corrected binding energies were calculated to be 14.21 kJ/mol by PBEPBE/6-31G(d) and 23.21 kJ/mol by MP2/6-31G(d). The charge distribution of the ground state Py–CA complex was evaluated with the natural population analysis, showing a net charge transfer from Py to CA. Analysis of the frontier molecular orbitals reveals a σ–π interaction between CA and Py, and the binding is reinforced by the attraction of the O₇ atom of CA with the H₂₃ atom of Py. TD-DFT calculations have been performed to analyze the UV–visible spectrum of Py–CA complex, revealing both the charge transfer transitions and the weak symmetry-relieved chloranil π–π^* transition in the UV–visible region.     

Ar2H+ 分子振动光谱的理论计算

基于近期由本组提供的Ar2H+分子的基态势能面,应用含时波包演化方法,计算了总角动量J=0时的振动光谱,并对其中的一些谱峰进行了指认.与现有的ab initio结果进行比较,这个新势能面包含了关于Ar2H+基态的比较正确的信息.     

The visible spectrum of jet-cooled CF3NO

The electronic origin of the à A″ ← X̃ A′ transition of trifluoronitrosomethane (CF₃NO) has been reassigned to the very weak feature near 717.9 nm in the fluorescence excitation spectrum of the jet-cooled molecule. The prominent torsional progression has been reinpreted and the height of the threefold torsional barier in the Ã( n,π^* ) state has been revised to 601.5 ± 10 cm⁻¹.     

Theoretical study of the effect of surface density on the dynamics of Ar + alkanethiolate self-assembled monolayer collisions

We present a classical-trajectory study of energy transfer in collisions of Ar atoms with alkanethiolate self-assembled monolayers (SAMs) of different densities. The density of the SAMs is varied by changing the distance between the alkanethiolate chains in the organic monolayers. Our calculations indicate that SAMs with smaller packing densities absorb more energy from the impinging Ar atoms, in agreement with recent molecular-beam scattering experiments. We find that energy transfer is enhanced by a decrease in the SAM density because (1) less dense SAMs increase the probability of multiple encounters between Ar and the SAM, (2) the vibrational frequencies of large-amplitude motions of the SAM chains decrease for less dense SAMs, which makes energy transfer more efficient in single-encounter collisions, and (3) increases in the distance between chains promote surface penetration of the Ar atom. Analysis of angular distributions reveals that the polar-angle distributions do not have a cosine shape in trapping-desorption processes involving penetration of the Ar atom into the alkanethiolate self-assembled monolayers. Instead, there is a preference for Ar atoms that penetrate the surface to desorb along the chain-tilt direction.