The effects of collision energy, vibrational mode, and vibrational angular momentum on energy transfer and dissociation in NO2+-rare gas collisions: an experimental and trajectory study

A combined experimental and trajectory study of vibrationally state-selected NO2+ collisions with Ne, Ar, Kr, and Xe is presented. Ne, Ar, and Kr are similar in that only dissociation to the excited singlet oxygen channel is observed; however, the appearance energies vary by approximately 4 eV between the three rare gases, and the variation is nonmonotonic in rare gas mass. Xe behaves quite differently, allowing efficient access to the ground triplet state dissociation channel. For all four rare gases there are strong effects of NO2+ vibrational excitation that extend over the entire collision energy range, implying that vibration influences the efficiency of collision to internal energy conversion. Bending excitation is more efficient than stretching; however, bending angular momentum partially counters the enhancement. Direct dynamics trajectories for NO2+ + Kr reproduce both the collision energy and vibrational state effects observed experimentally and reveal that intracomplex charge transfer is critical for the efficient energy transfer needed to drive dissociation. The strong vibrational effects can be rationalized in terms of bending, and to a lesser extent, stretching distortion enhancing transition to the Kr+ -NO2 charge state.     

Energy-loss studies on ion-pair formation reactions for the K + NO2 system

The energy loss spectra of K⁺ ions produced in fast potassium atom—NO₂ molecule collisions were studied over the collision energy region 10–40 eV. The energy loss spectra for the K + NO₂ system showed five peaks. The first two peaks at about 3.0 eV are ascribed to the ground-states NO^?₂ with vibrational excitation. The remaining three peaks are ascribed to the electronically excited states of NO^?₂.     

Analytical models for state-specific diatomic dissociation rate coefficients

Two analytical models are presented to approximate the temperature dependent, rotationally-averaged vibrational-state-specific dissociation rate coefficient for collisions between diatomic molecules and rare gas atoms at combustion temperatures. The new models are derived by making simplifying approximations to a more detailed theoretical model recently reported in the literature. For accuracy, the first model requires, for a given collision pair, knowledge of the maximum vibrational quantum number, a single vibrational-rotational energy and an interaction parameter for dissociation, all of which are tabulated in this article for collisions of Ar with p-H₂, O₂, N₂, and CO. This is in contrast to the recently reported theoretical model, which requires knowledge of all vibrational-rotational energies below the dissociation threshold, in addition to the interaction parameter for dissociation. The second model is simpler and more general than the first, but less accurate. To completely specify this model, knowledge of only the hard sphere cross section, and the characteristic temperatures for vibration and dissociation is required. The two analytical models are shown to agree well with the published theoretical values, with the accuracy of each model increasing with increasing temperature. The present models provide an accurate and efficient means of computing thousands or millions of rate coefficients for use in numerical simulations of combustion processes that couple kinetic equations with the governing equations of fluid dynamics. © 1997 John Wiley & Sons, Inc.     

Radiationless transitions between excited singlet states of biacetyl

Emission processes from lower excited states S₁ (fluorescence) and T₁ (phosphorescence) have been studied in the gas and liquid phases when biacetyl is excited into the second singlet state S₂. (In agreement with Kasha's rule no fluorescence is observed from the S₂ state.) In the liquid phase, when biacetyl is excited into the singlet states S₁ and S₂, no difference is observed between these emission processes. This phenomenon certainly results from an efficient nonradiative transition between the second excited singlet state S₂ and the first excited state S₁ with practically no excess vibrational energy. The quantum yield of this transition is almost unity and does not depend on the nature of the solvent. In the gas phase no emission processes are observed when biacetyl is excited into the S₂ state at low pressure (less than 10 mm Hg). High pressure of inert gas is necessary in order to observe these processes. As for excitation into the S₁ state with vibrational energy, loss of vibrational energy through collisions occurs from the S₂ state. The quantum yield of the S₂ → S₁ transition by excitation at 290 nm is estimated around 0.5–0.6 at 6 atm of inert gas (ethane, ethylene, or carbon dioxide).     

Two-photon dissociation of vibrationally excited H2+. Complex quasi-vibrational energy and inhomogeneous differential equation approaches

We present two practical theoretical methods — the complex quasi-vibrational energy and the inhomogeneous differential equation approaches — for numerical computation of multiphoton dissociation cross sections. The methods are applied to the study of the two-photon dissociation of H₂⁺ (1sσg). The cross sections are small for low-lying vibrational states but increase very rapidly with increasing vibrational quantum number, suggesting that experimentally accessible powerful lasers can be used to probe the highly excited vibrational states of the ground electronic state of a homonuclear diatomic molecule.     

A Monte Carlo simulation of energy transfer processes in thermal unimolecular reactions. II. An applications to polyatomic dissociation

The Monte Carlo method has been used to provide a numerical solution to the ro-vibrational master equation for the low pressure unimolecular decomposition of a polyatomic molecule. This type of solution is made possible through the use of a simple exponential transition probability function, that represents the efficiency with which energy transfer takes place between the reactant molecule and an unspecified heat bath gas. The Monte Carlo technique is used to generate random variables that are distributed in a manner prescribed by the transition probability function. In the case of the present simulation, these variables correspond to random energy jumps induced in the molecule through single collision events. In order to account for the energy dependence of the vibrational state densities, we have proposed that vibrational relaxation in the polyatomic takes place from a single vibrational mode. Under equilibrium conditions we are able to show that with this assumption, the Monte Carlo model is capable of reproducing molecular quantities, such as the average vibrational energy per molecule and the vibrational specific heat, that compare favourable with the corresponding values calculated from equilibrium statistical mechanics. The model has been applied to a study of the low pressure unimolecular decomposition of a series of polyatomics. For three of the molecules, CH₄, CD₄, and C₂H₆ the agreement between the calculated and the high temperature experimental rate constants is very good. The calculations indicate that a significant proportion of the molecules that dissociate are rotationally as well as vibrationally excited. Very few of the reactive molecules have a vibrational energy content equal to or greater than E₀, the dissociation energy. The extent of rotational excitation is found to be temperature dependent.     

Hermaphroditism in chemical dynamics: the reaction Ba + Cl2 → BaCl + Cl

The reaction Ba + Cl₂ → BaCl + Cl proceeds through different electronic channels with diametrically opposite collision dynamics: ground state BaCl(X²Σ) is formed via a direct interaction as witnessed by forward scattering and a strongly inverted internal state distribution. Electronically excited BaCl^*(C²Π) is formed via a long-lived collision complex, indicated by a statistical vibrational distribution. A simple RRK argument explains the differences of lifetimes towards unimolecular decomposition of the collision complexes. A lower limit of the BaCl(X²Σ⁺) dissociation energy is placed at 121 kcal/mole.     

Entropy analysis of product energy distributions in nonreactive inelastic collisions

Final state distributions for inelastic collisions of I^*₂ with rare gas atoms are analyzed with the recently introduced surprisal function. Both experimental data and classical trajectory results are compared. Both vibrational and rotational state distributions follow linear surprisal plots, with fairly large values of λ = dj( ?E)/d(?E, indicating relatively weak coupling between internal and translational degrees of freedom. The λ_u and λ_j values appear to be relatively independent of the initial state of the excited molecule and of the collision partner.     

Energy transfer between polyatomic molecules II: Energy transfer quantities and probability density functions in benzene, toluene, p-xylene, and azulene collisions

Collisional energy transfer, CET, is of major importance in chemical, photochemical, and photophysical processes in the gas phase. In Paper I of this series (J. Phys. Chem. B 2005, 109, 8310) we have reported on the mechanism and quantities of CET between an excited benzene and cold benzene and Ar bath. In the present work, we report on CET between excited toluene, p-xylene, and azulene with cold benzene and Ar and on CET of excited benzene with cold toluene, p-xylene, and azulene. We compare our results with those of Paper I and report average vibrational, rotational, and translational energy quantities, , transferred in a single collision. We discuss the effect of internal rotation on CET and the identity of the gateway modes in CET and the relative role of vibrational, rotational, and translational energies in the CET process, all that as a function of temperature and excitation energy. Energy transfer probability density functions, P(E,E'), for the various systems are reported and the shape of the curves for various systems and initial conditions is discussed. The major findings for polyatomic-polyatomic collisions are: CET takes place mainly via vibration-to-vibration energy transfer assisted by overall rotations. Internal free rotors in the excited molecule hinder energy exchange while in the bath molecule they do not. Energy transfer at low temperatures and high temperatures is more efficient than that at intermediate temperatures. Low-frequency modes are the gateway modes for energy transfer. Vibrational temperatures affect energy transfer. The CET probability density function, P(E,E'), is convex at low temperatures and can be concave at high temperatures. A mechanism that explains the high values of and the convex shape of P(E,E') is that in addition to short impulsive collisions there are chattering collisions where energy is transferred in a sequence of short encounters during the lifetime of the collision complex. This also leads to the observed supercollision tail at the down wing of P(E,E'). Polyatomic-Ar collisions show mechanistic similarities to polyatomic-polyatomic collisions, but there are also many dissimilarities: internal rotations do not inhibit energy transfer, P(E,E') is concave at all temperatures, and there is no contribution of chattering collisions.     

高平动能(1.15 eV)CN基的T-V过程的探讨

用193毫微米光解BrCN和ClCN的研究,发现一些很有意义的结果。如在光解BrCN时发现碎片CN X~2∑~+ v″=0的转动能级高达80以上,它可研究高转动能在传能和反应中的作用。研究中发现一些异常现象;Heaven等在扩散分子束中得到BrCN的光解产物CN X~2∑~+,共中v″=0约占80%,v″=1约占20%;而Halpern等在10~(-5)Torr延迟0.2微秒测得的光解产物,v″=0占94%,v″=1占6%。Heaven给出的谱图上出现很高的P_(0,0)支带头,与Halpern结果比较,明显地发生了转动弛豫,达表明实验中有碰撞弛豫。但为什么v″=1的布居不下降,却反而上升?为了弄清达个问题,我们在不同BrCN压办及不同延迟时间下去探测CN X~2∑~+,v″的碰撞弛豫。     

Singlet and Triplet Excited States and Intersystem Crossing in Free‐Base Porphyrin: TDDFT and DFT/MRCI Study

Extensive time-dependent DFT (TDDFT) and DFT/multireference configuration interaction (MRCI) calculations are performed on the singlet and triplet excited states of free-base porphyrin, with emphasis on intersystem crossing processes. The equilibrium geometries, as well as the vertical and adiabatic excitation energies of the lowest singlet and triplet excited states are determined. Single and double proton-transfer reactions in the first excited singlet state are explored. Harmonic vibrational frequencies are calculated at the equilibrium geometries of the ground state and of the lowest singlet and triplet excited states. Furthermore, spin–orbit coupling matrix elements of the lowest singlet and triplet states and their numerical derivatives with respect to nuclear displacements are computed. It is shown that opening of an unprotonated pyrrole ring as well as excited-state single and double proton transfer inside the porphyrin cavity lead to crossings of the potential energy curves of the lowest singlet and triplet excited states. It is also found that displacements along out-of-plane normal modes of the first excited singlet state cause a significant increase of the 2|H_so|S₁>, 1|H_so|S₁>, and 1|H_so|S₀> spin–orbit coupling matrix elements. These phenomena lead to efficient radiationless deactivation of the lowest excited states of free-base porphyrin via intercombination conversion. In particular, the S₁→T₁ population transfer is found to proceed at a rate of ≈10⁷ s⁻¹ in the isolated molecule.     

On a simple reaction mechanism for the collision-induced dissociation He + Li2(B1Πu) → He + Li(2 2S) + Li(2 2P)

A simple reaction mechanism is suggested for the dissociation of electronically excited Li₂(B¹Π_u) in collision with rare-gas atoms. Experimental rate constants for dissociation of Li₂ in specific vibrational—rotational levels (ν1J) show an unexpected behaviour as a function of the initial molecular energy and angular momentum. We propose that raction proceeds by a transition to the ¹Π_g state of Li₂. This may dissociate more readily since it is more weakly bound and has a larger equilibrium distance than the ¹Π_u state.     

Energy transfer in collisions of highly vibrationally excited tetrahedral molecules

Model trajectory calculations of the energy transfer processes in collisions of Ar with highly vibrationally excited CH₄, CD₄, SiH₄ and CF₄ are performed. Special attention is payed to the calculation of the energy transferred to active (vibrational) degrees of freedom. The results support the diffusion model of excitation-dissociation and give the low pressure collision efficiency β_c which qualitatively agrees with experiment in magnitude and temperature dependence.     

Alkylation Effects on the Energy Transfer of Highly Vibrationally Excited Naphthalene

The energy transfer of highly vibrationally excited isomers of dimethylnaphthalene and 2‐ethylnaphthalene in collisions with krypton were investigated using crossed molecular beam/time‐of‐flight mass spectrometer/time‐sliced velocity map ion imaging techniques at a collision energy of approximately 300 cm^?1. Angular‐resolved energy‐transfer distribution functions were obtained directly from the images of inelastic scattering. The results show that alkyl‐substituted naphthalenes transfer more vibrational energy to translational energy than unsubstituted naphthalene. Alkylation enhances the V→T energy transfer in the range ?ΔE_d=?100～?1500 cm^?1 by approximately a factor of 2. However, the maximum values of V→T energy transfer for alkyl‐substituted naphthalenes are about 1500～2000 cm^?1, which is similar to that of naphthalene. The lack of rotation‐like wide‐angle motion of the aromatic ring and no enhancement in very large V→T energy transfer, like supercollisions, indicates that very large V→T energy transfer requires special vibrational motions. This transfer cannot be achieved by the low‐frequency vibrational motions of alkyl groups.     

Dissociation of hydrogen molecules in collisions with light alkali ions. II. Li+-H2

The dissociation of a ground state H₂ molecule in single collisions with a Li⁺ ion has been studied using a time of flight technique over a large range of center of mass scattering angles (30° ? υ ? 180°) and collision energies (16 eV < E_cm < 55.5 ev).The results have been transformed into the center of mass system to obtain inelastic differential cross sections (contour maps). In contrast to most other scattering experiments on collision induced dissociation, the results at high energies (E_cm > 40 eV) cannot be explained by a two-step mechanism. Instead dissociation appears to occur in a time comparable to the collision time. The results are consistent with several collision models. Of these the spectator model in which only one of the atoms of the molecule is struck by the incident ion is favored since it is in good agreement with the differential cross sections for backward scattering.     

Neutral scattering and vibrational excitation in K+Br2 Collisions

Differential cross sections are presented for neutral scattering of K atoms in collisions with Br₂ molecules in the energy range from 20 to 150 eV. In addition energy-loss spectra for the scattered K atoms are shown. The differential cross sections show a large peak near the forward direction. The energy-loss spectra point to considerable vibrational excitation at small angles. The results are attributed to reneutralization from an ion-pair state formed during the collision. In some cases this process can involve three potential surface crossings. The experimental results can be reproduced in simple trajectory calculations on diabatic potential surfaces. The calculations show that the forward scattering is rainbow scattering, caused by the internal motion of the Br₂^? molecular ion during the collision. There is no analog to this rainbow in atom-atom scattering. The internal moti is also responsible for the observed vibrational excitation.     

The born approximation as a simple diagnostic method for direct molecular collisions with applications to Cl + HI and F + H2 reactions

Born approximation computations are presented and discussed for the Cl + HI → I + HCl and F + H₂ → H + HF reactions and their isotopic analogues. Most aspects of the role of reagent energy or the energy disposal in the products previously deduced from experiment or trajectory computations can be accounted for the Born approximation. The procedure used here neglects the interaction between non-bonded atoms. It does thereby provide a very simple computational scheme which requires as input only the spectroscopic constants of the reactants and products. In addition it offers simple qualitative interpretations of the trends in the results. The overall satisfactory agreement between the present results and past studies lends credibility to the basic propensity rule provided by the Born approximation: The most probable transitions are those that minimize the momentum transfer to the nuclei. The principle is discussed with special reference to exothermic (E′_T ? E_T) and endothermic transitions.The computations for Cl + HI indicate a decline of the reaction cross section with increasing kinetic energy and a strong enhancement by HI rotational energy. The surprisal analysis confirms the absence of vibrational population inversion for endothermic transitions. For the F + H₂ (and isotopic variants) reactions, the product-rotational state distribution extends nearly to the energy cut-off. The vibrational state distribution is somewhat different for para- and normal H₂ and, in general, the collision outcome is very sensitive to the initial rotational state of H₂ particularly at low translational energies. The HF/DF branching ratio is F + HD collisions is increasing with increase of the HD rotational state. The vibrational surprisal is essentially isotopically invariant.     

Quantum mechanical calculations of vibrational transition probabilities in collinear atom–triatom collisions

Quantum mechanical calculations have been made of vibrational transition probabilities in the collinear collision of an inert gas atom with either CO₂ or OCS. The dependence of the transition probability on the relative translational energy and the reduced mass is similar to that found for atom–diatom collisions. The transition P_{00 → 10} (excitation of the first stretching mode) is much greater than P_{00 → 01} (the second stretching mode). This is largely due to the difference in frequencies but it has been shown that there is an independent mass factor responsible for this difference.     

C-Br Bond Dissociation Mechanisms of 2-Bromothiophene and 3-Bromothiophene at 267 nm

C-Br bond dissociation mechanisms of 2-bromothiophene and 3-bromothiophene at 267 nm were investigated using ion velocity imaging technique. Translational energy distributions and angular distributions of the photoproducts, Br(²P_3/2) and Br^*(²P_½), were obtained and the possible dissociation channels were analyzed. For these two bromothiophenes, the Br fragments were produced via three channels: (i) the fast predissociation following the intersystem crossing from the excited singlet state to repulsive triplet state; (ii) the hot dissociation on highly vibrational ground state following the internal conversion of the excited singlet state; and (iii) the dissociation following the multiphoton ionization of the parent molecules. Similar channels are involved for photoproduct Br^* of the 2-bromothiophene dissociation at 267 nm; whereas for the photoproduct Br^* of 3-bromothiophene, the dissociation channel via internal conversion from the excited singlet state to highly vibrational ground state became dominating and the fast predissociation channel via the excited triplet state almost disappeared. Informations about the relative contribution, energy disposal, and the anisotropy of each channel were quantitatively given. It was found that with the position of Br atom in thienyl being far from S atom, the relative ratios of products from channels (i) and (ii) decreased obviously and the anisotropies corresponding to each channel became weaker.