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Redistribution of vibrational energy in the adenine–uracil base pair is studied when the base pair undergoes an intermolecular interaction with an overtone-bending vibration excited H2O(2 ν bend) molecule. Energy transfer is calculated using the structural information obtained from density functional theory in the solution of the equations of motion. Intermolecular vibrational energy transfer (VET) from H2O(2 ν bend) to the uracil–NH stretching mode is efficient and rapidly followed by intramolecular vibrational energy redistribution (IVR) resulting from coupling between vibrational modes. An important pathway is IVR carrying energy to the NH-stretching mode of the adenine moiety in a subpicosecond scale, the energy build-up being sigmoidal, when H2O interacts with the uracil–NH bond. The majority of intermolecular hydrogen bonds between the base pair and H2O are weakened but unbroken during the ultrafast energy redistribution period. Lifetimes of intermolecular HB are on the order of 0.5 ps. The efficiency of IVR in the base pair is due to near-resonance between coupled CC and CN vibrations. The resonance also exists between the frequencies of H2O bend and NH stretch, thus facilitating VET. When H2O interacts with the NH bond at the adenine end of the base pair, energy flow in the reverse direction to the uracil–NH stretch is negligible, the unidirectionality discussed in terms of the effects of uracil CH stretches. The energy distributed in the CH bonds is found to be significant. The IVR process is found to be nearly temperature independent between 200 and 400 K. [ABSTRACT FROM AUTHOR]

Vibrational energy transfer from DF(1) to toluene is studied by use of the WKB semiclassical procedure in the distorted-wave approximation and both methyl stretches and benzene ring CH stretches are within 200 cm (super -1) of the DF vibrational frequency. It is concluded that as temperature increases, probabilities calculated at long-range decrease, whereas those calculated at short-range increase.

We study the vibrational relaxation and solvation dynamics in size-selected icosahedral Arn(NO-) at 300 K, where NO-(X3Σ-) is in v = 1 and n = 1-12, using a classical dynamics method and an interaction model consisting of detailed host-guest and host-host interactions. Two relaxation time scales are found: (i) the short-time (<200 ps), in which rate is nearly independent of cluster size, and (ii) the ns scale, in which a slow energy transfer process occurs between NO- vibration and argon modes at a rate (∼108 s-1) decreasing slightly from n = 12 to 6 and rapidly from n = 5 to 1 (∼106 s-1). In Ar12(NO-), less than one-quarter of the host atoms sampled evaporate, nearly 60% of evaporation occurring within 200 ps caused by rapid energy transfer from NO- at short time. The fraction of evaporation decreases nearly exponentially with increasing evaporation time, but ∼16% of evaporation still occurs on a time scale longer than 1 ns. Evaporation from one hemisphere of Ar12(NO-) dominates the rest. Final cluster sizes commonly produced from the fragmentation of Ar12(NO-) are n = 6-11 (evaporation of 6-1 atoms) and n = 12 (no evaporation). [ABSTRACT FROM AUTHOR]

Relaxation dynamics of NO-(v=1) in icosahedral (Ar)12NO- clusters are studied using classical dynamics and semiclassical procedures over the temperature range of 100–300 K. The minimum energy of the equilibrium configuration (-9875 cm-1) needed in the study is determined by varying the cluster size z in (Ar)zNO-. NO-(v=1) is embedded in the cluster, which is filled with low frequency motions: 39 cm-1 for the argon modes, 77 cm-1 for the Ar...NO- substructure vibration, 109 cm-1 for the librational frequency of restricted rotation, and 128 cm-1 for oscillatory local translation. Dynamics calculations show that in the early time period (<20 ps), part of the vibrational energy rapidly transfers to rotation, but most energy transfers to Ar atoms on a long time scale (∼1 ns). The long time scale leads to the relaxation rates of 0.403 ns-1 at 100 K and 0.453 ns-1 at 300 K. The rates calculated using analytical formulations vary nearly linearly from 0.288 ns-1 at 100 K to 0.832 ns-1 at 300 K. Although the temperature dependence is stronger in the latter, both approaches give the rates on a nanosecond time scale. The principal energy transfer pathway is from NO- vibration to Ar vibrations via oscillatory local translation, while the NO- rotation is in a librational state. The energy transfer probabilities are two orders of magnitude larger than the vibration-to-translation probabilities in the gas phase collision Ar–NO-(v=1). [ABSTRACT FROM AUTHOR]

The vibrational relaxation of OH(A 2Σ+;v=1) embedded in solid Ar has been studied over 4–80 K. The interaction model is based on OH undergoing local motions in a cage formed by a face-centered cubic stacking where the first shell atoms surround the guest and connect it to the heat bath through 12 ten-atom chains. The motions confined to the cage are the local translation and libration-rotation of OH and internal vibrations in OH...Ar, their energies being close to or a few times the energies of nearby first shell and chain atoms. The cage dynamics are studied by solving the equations of motion for the interaction between OH and first shell atoms, while energy propagation to the bulk phase through lattice chains is treated in the Langevin dynamics. Calculated energy transfer data are used in semiclassical procedure to obtain rate constants. In the early stage of interaction, OH transfers its energy to libration-rotation intramolecularily and then to the vibrations of the first shell and chain atoms on the time scale of several picoseconds. Libration-to-rotational transitions dispense the vibrational energy in small packages comparable to the lattice frequencies for ready flow. Energy propagation from the chains to the heat bath takes place on a long time scale of 10 ns or longer. Over the solid argon temperature range, the rate constant is on the order of 106 s-1 and varies weakly with temperature. [ABSTRACT FROM AUTHOR]

The collision-induced reaction of Xe+ with HCl has been studied by use of classical dynamics procedures at collision energies 2–20 eV using empirical potential parameters. The principal reaction pathway on the potential energy surface is the formation of XeH+ with the maximum reaction cross section, 1.2 Å2, occurring at E=9 eV. At lower energies, the cross section for the charge transfer process Xe++HCl→Xe+HCl+ is comparable to that for XeH+ formation, but at higher energies, it is larger by a factor of 2. The cross section of the XeCl+ formation is an order of magnitude smaller than that of XeH+. For both XeH+ and XeCl+ formations, the reaction threshold is ≈2 eV. The XeH+ formation takes place immediately following the turning point in a direct-mode mechanism, whereas an indirect-mode mechanism operates in the formation of XeCl+. Both XeH+ and XeCl+ formations come mainly from the perpendicular configuration, Xe+...HCl, at the turning point. Product vibrational excitation is found to be strong in both XeH+ and XeCl+. [ABSTRACT FROM AUTHOR]

The vibrational relaxation of hydroxyl radicals in the A 2Σ+ (v=1) state has been studied using the semiclassical perturbation treatment at cryogenic temperatures. The radical is considered to be trapped in a closest packed cage composed of the 12 nearest argon atoms and undergoes local translation and hindered rotation around the cage center. The primary relaxation pathway is towards local translation, followed by energy transfer to rotation through hindered-to-free rotational transitions. Free-to-free rotational transitions are found to be unimportant. All pathways are accompanied by the propagation of energy to argon phonon modes. The deexcitation probability of OH(v=1) is 1.3×10-7 and the rate constant is 4.7×105 s-1 between 4 and 10 K. The negligible temperature dependence is attributed to the presence of intermolecular attraction (>kT) in the guest-host encounter, which counteracts the T2 dependence resulting from local translation. Calculated relaxation time scales are much shorter than those of homonuclear molecules, suggesting the importance of the hindered and free motions of OH and strong guest-host interactions. [ABSTRACT FROM AUTHOR]

The collision-induced dissociation of VO+ by Xe has been studied by the use of classical dynamics procedures on London-Eyring-Polanyi-Sato potential-energy surfaces in the collision energy range of 5.0-30 eV. The dissociation threshold behavior and the dependence of reaction cross sections on the collision energy closely follow the observed data with the threshold energy of 6.00 eV. The principal reaction pathway is VO++Xe→V++O+Xe and the minor pathway is VO++Xe→VXe++O. At higher collision energies (E>8.0 eV), the former reaction preferentially occurs near the O-V+…Xe collinear and perpendicular alignments, but the latter only occurs near the perpendicular alignment. At lower energies close to the threshold, the reactions are found to occur near the collinear configuration. No reaction occurs in the collinear alignment V+-O…Xe. The high and low energy-transfer efficiencies of the collinear alignments O-V+…Xe and V+-O…Xe are attributed to the effects of mass distribution. The activation of the VO+ bond toward the dissociation threshold occurs through a translation-to-vibration energy transfer in a strong collision on a time scale of about 50 fs. [ABSTRACT FROM AUTHOR]

The vibrational relaxation of oxygen embedded in an argon cage through vibrational to local translation, rotation, and argon phonon modes has been studied using semiclassical procedures. The collision model is based on the trapped molecule undergoing the restricted motions (local translation and hindered rotation) in a cage formed by its twelve nearest argon neighbors in a face-centered-cubic structure. At 85 K in the liquid argon temperature range, the deexcitation probability of O2(v=1) is 5.8×10-12 and the relaxation rate constant with the collision frequency from local translation is 23 s-1. The rate constant decreases to 5.1 s-1 at 50 K and to 0.016 s-1 at 10 K in the solid argon temperature range. Transfer of the vibrational energy to local translation, rotation (both hindered and free), and argon phonon modes is the relaxation pathway for the trapped oxygen molecule. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The family of solid solutions of the type β-K2(Bi1-zSbz)8Se13 (0

The collision-induced intramolecular energy flow and C-H bond dissociation in toluene have been studied using classical dynamics procedures. The molecule initially contains high amounts of vibrational excitation in the methyl C-H stretch and the nearby benzene ring C-H stretch and it is in interaction with Ar. The two excited C-H stretches are coupled to each other through two C-C stretching, two H-C-C bending and one C-C-C bending modes, all of which are initially in the ground state. At 300 K, the energy lost by the excited molecule upon collision is not large and it increases slowly with increasing total vibrational energy content between 10 000 and 40 000 cm[sup -1]. Above the energy content of 40 000 cm[sup -1], energy loss increases rapidly. Near 65 000 cm[sup -1] energy loss takes a maximum value of about 1000 cm[sup -1]. The temperature dependence of energy loss is weak between 200 and 400 K. When the energy content is sufficiently high, either or both C-H bonds can dissociate, producing free radicals, C[sub 6]H[sub 5]CH[sub 2], C[sub 6]H[sub 4]CH[sub 3], or C[sub 6]H[sub 4]CH[sub 2].. The ring C-H dissociation occurs almost entirely in a direct-mode mechanism on a subpicosecond time scale. Nearly half of methyl group C-H dissociation events occur on a subpicosecond time scale and the rest through a complex-mode collision in which bond dissociation occurs several picoseconds after the initial impact. In the complex-mode collision, Ar binds to the radical forming a weakly bound benzyl...Ar complex. In both dissociative and nondissociative events, intramolecular energy flow is efficient, taking place upon the initial impact on a subpicosecond time scale. [ABSTRACT FROM AUTHOR]

Raman spectroscopy has been used to assess the concentration dependence of optical phonons in In[sub 1-x]Ga[sub x]As epilayers grown by chemical beam epitaxy on InP(100). The alloy composition was varied from x=0.325 to x=0.55 to cover the technologically important and physically interesting range where the strain changes sign at x=0.468 from negative to positive. The Raman spectra were curve resolved to reveal the GaAs-like longitudinal optic (LO), disorder induced, InAs-like LO, and InAs-like transverse optic phonons. An examination of the concentration dependence of the phonon frequencies showed that the GaAs-like LO mode varied as ω(cm[sup -1])=252.77+58.643x-50.108x[sup 2] for 0.325≤x≤0.55. A comparison of these results with previous infrared and Raman work on In[sub 1-x]Ga[sub x]As has revealed that the concentration dependence of the optical phonon frequencies in the unstrained system is still not accurately known. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The probability of OH (OD) formation and energy deposit of the reaction exothermicity in the newly formed OH (OD) is calculated, particularly in its vibrational motion, in the gas-surface reactions O(g) + H(ad)/Si-->OH(g) +Si and O(g) = D(ad)/Si -->OD(g) + Si. The dependence of reaction probabilities and energy distributions on surface temperature is discovered to be weak, in the range of 0-600 K.

This study on the self-relaxation of vibrationally excited H2O molecules is based on a model in which an excited molecule and its collision partner undergo a long-lived collision in the presence of strong molecular attraction. Transition probabilities are obtained by semiclassical procedures using ladder operators. The study shows that large-impact parameter collisions are responsible for the negative temperature dependence of the VV transition probabilities in H2O(001)→H2O(020), H2O(020)→H2O(010), and H2O(010)→H2O(000). These energy transfer processes occur intramolecularly, and the transfer of the vibrational energy mismatch that is released by the VV process to the low frequency oxygen–oxygen motion in the complex is found to be very efficient, whereas energy transfer to the hindered rotational motions of the excited molecule is inefficient. The model predicts VV probabilities to decrease on deuteration. [ABSTRACT FROM AUTHOR]

The collisional dissociation of the H-surface bond and the formation of the H–O2 bond in the O2(gas)/H(ads) collision taking place on a tungsten surface have been studied by classical trajectory methods over the collision energy range of 0.1–2.0 eV. The effects of the interactions between the H atom and higher-order neighbors of the center metal atom are important in the collisional dissociation of adatoms. This many-body interaction leads to an oscillatory dependence of the adatom dissociation probability on the collision energy. The attractive well depth of the O2(gas)/H(ads) interaction is varied between 0.202 to 4.624 eV. At an intermediate range of well depth, energy preferentially transfers into the adatom bond and leads to a large dissociation probability. As well depth increases, energy transfer to O2 becomes significant, thus causing the accumulation of a smaller amount of energy in the adatom bond, so adatom dissociation is less effective. [ABSTRACT FROM AUTHOR]

The dependence of vibrational energy transfer and bond dissociation on the initial vibrational energy and initial phases in the collision of two oxygen molecules O2(0)+O2(v) has been studied using classical trajectory procedures. When the initial vibrational energy of O2(v) is not large, the energy gained by O2(0) is mainly through the V→V pathway, but as the initial excitation increases the contribution of translational motion becomes important. For the highly excited O2(v), most trajectories lead to T→V energy transfer, and for such molecules dissociation probabilities are found to increase linearly with the initial excitation. The dissociation of O2(v) is sensitively dependent on the initial vibrational phase. For v=1, the resonant energy exchange probability is a linear function of the collision energy. [ABSTRACT FROM AUTHOR]

The vibrational relaxation of the stretching levels of H2O in the H2O+Ar collision is studied using a model of vibration-to-rotation (VR) energy transfer in the temperature range of 200–1000 K. The dominant relaxation pathway from the rapidly equilibrating ν1 and ν3 levels is deexcitation to the 2ν2 level followed by the 020→010 and 010→000 relaxation processes. At 300 K, the probabilities of 001→020, 020→010, and 010→000 are 2.2×10-3, 2.6×10-3, and 1.4×10-3, respectively. In the model the energy release ΔE of each deexcitation process is preferentially removed by rotation. Transition probabilities calculated by a semiclassical procedure indicate that when ΔE is large, the VR mechanism is of major importance at lower temperatures. The VR model correctly predicts both the temperature dependence and magnitude of the 020→010 probability over the temperature range of 200–1000 K. A model which assumes the removal of the energy release by translation seriously underestimates the probability of this process, especially at lower temperatures, and predicts a temperature dependence which is too steep. The contribution of translational motion to the relaxation process becomes important at higher collision energies, especially for 001→020, where ΔE is small. Calculations are extended to the D2O+Ar collision in order to study isotope effects. [ABSTRACT FROM AUTHOR]

We report trajectory calculations on intramolecular energy flow and bond dissociation of a model system OC···Pt···CO in a collision energy range, where the energy transfer to the complex localizes in one of the two Pt···CO bonds. The collision model is an Ar atom incident on the left-hand-side terminal atom of the complex containing a vibrationally excited ligand (OC*···Pt···CO or OC···Pt···CO*) in a collinear configuration. In this energy range, essentially all of the energy transferred to the complex localizes in the right-hand-side Pt···CO bond for a time corresponding to many vibrational periods before dissociation or intramolecular energy flow occurs. The existence of this energy localization time zone and its effect on intramolecular dynamics are discussed in detail at various collision energies. Energy localization leads to long delay for bond dissociation and strong dependence of bond dissociation on the initial excitation site. Time delay for the dissociation of the left-hand-side Pt···CO bond is particularly long, and this inefficient dissociation process is discussed in terms of intramolecular recrossing of the vibrational energy across the heavy mass barrier. At the threshold, dissociation always occurs at the side where the excited ligand is present: Ar+OC*···Pt···CO→Ar+OC*+Pt···CO or Ar+OC···Pt···CO* →Ar+OC···Pt+CO*. [ABSTRACT FROM AUTHOR]

A classical trajectory study of energy flow between two metal-molecule bonds separated by a heavy metal atom in a collisionally perturbed complex O 3/4 C···M···C 3/4 O is presented. The left M···CO bond is excited by collision and subsequent flow of excitation is followed by classical mechanics. In the heavy metal system, the behavior of energy flow blockage by the metal atom prevails at certain total energies of the complex. The blockage causes a long time delay for energy flow between two M···CO bonds and bond dissociation. Such energy flow blockage is found to be absent in the light metal system, where the amount of energy transfer to the complex is much larger. The energy flow patterns associated with the blockage are discussed in detail. Specific systems considered are the collisional excitation of OC···Pt···CO and OC···Ni···CO by Ar. [ABSTRACT FROM AUTHOR]

Classical trajectory calculations and analysis of collision-induced energy flow near the fragmentation threshold in a symmetric van der Waals complex of the type BL···X2···BR, in which two weak bonds are separated by a molecular unit, are presented. The collision of an incident atom with BL results in transfer of a large fraction of translational energy to the complex and most of the energy is found to be localized on the right-hand side bond X2···BR. Though the amount of energy transferred to the complex greatly exceeds the sum of van der Waals bond strengths, the total fragmentation does not occur until internal resonances between BL···X2 and X2···BR bonds cease. Between the fragmentation thresholds of X2···BR and BL···X2, there is a tendency for BL···X2 to stabilize through internal resonant exchange of energy. [ABSTRACT FROM AUTHOR]

Intramolecular energy flow and fragmentation processes in a linear van der Waals complex consisting of four molecular units undergoing collision with an incident atom are studied by solving Hamilton’s equations of motion for positions and momenta of all molecular units and van der Waals bonds. Numerical results are obtained for an O2 tetramer, which is in interaction with an argon atom: Ar+(1–2···3–4···5–6···7–8). Energy flow is found to be very efficient, and above the fragmentation threshold most of the energy transferred to the complex through the collision of the first unit (1–2) of the complex with Ar is found to become localized at the terminal van der Waals bond 5–6···7–8, the dissociating mode, thus causing the end molecular unit (7–8) to break off. Short-time dynamics of energy buildup in each molecular unit and van der Waals bond is analyzed in detail. [ABSTRACT FROM AUTHOR]