Initial vibrational excitation

As Fig. 6 demonstrates, in the system H2/Cu the sticking probability is significantly enhanced if the impinging molecules are initially vibrationally excited. In order to quantify the effect the vibrational efficacy is introduced. It is defined as... 

With this simplification, Gray, Rice, and Davis obtained reasonably accurate values for the predissociation rate constant as a function of initial vibrational excitation. The rate constant thus obtained is larger than that from exact trajectory calculations by about a factor of two. By contrast, the RRKM theory would give a rate constant that is about three orders of magnitude larger than is observed. 

Zewail and co-workers [42] made a few measurements of the rate of predissociation of Arl2 but only the ratios of the rate constants for different initial vibrational excitation were reported. The predissociation of Arl2 was also experimentally studied by Levy co-workers [39,40]. The experimental data—for example, the ratio k v = 21)/k(y = 18)—again support the MRRKM theory. In this particular application, the MRRKM calculations were based on a three-dimensional model in which both diatom and fragment orbital angular... 

Figure 19. Reaction probability versus initial vibrational excitation of the molecule for the surface of Schatz and Elgersma (filled circles) and for the interpolated PES (open circles) generated from the first 400 data points of Fig. 16. Error bars represent standard deviations. The interpolated PES was generated for an H, initial vibrational excitation of about 0.085 hartree.

Fig. 7.16 C2-ejection rate and its fitting by the rate formula of the RRK theory. The vertical axis is log(l/r) corresponding to the C2-ejection rate, where T is the time from the initial vibrational excitation to the first C2-evaporation. The values for the hg(l) excitation and ag(l) excitation are denoted by triangles and circles, respectively. The solid line is the rate formula of the RRK theory, Eq.

The enhancement of reactions (79a) and (79b) when Os is vibrationally excited was first observed by Gordon and Lin using a repetitively pulsed COi laser. Since that time they have extended their measurements. With the low Os concentrations and high laser powers in their latest experiments, Gordon and Lin believe that equilibration of the initial vibrational excitation is unimportant so... 

ABSTRACT. After reviewing the time dependent wavepacket method as applied to collision induced dissociation processes,we report accurate quantum results for reactive and non reactive collinear A+BC systems. Both systems display a vibrational enhancement effect in the low energy region. While the non reactive systems exhibit a vibrational inhibition effect at higher energies,a more complex behavior is observed in the reactive case. Below the classical dissociation threshold,the non reactive systems display tunnelling tails which decrease with the initial vibrational excitation of the diatomic molecule. The reactive system displays important quantum effects at energies well above the classical dissociation threshold. 

Selective bond breaking has been demonstrated with HOD by first exciting the fourth overtone (local mode) of the OH bond and then photodissociating the molecule via the A X transition. The A <— X transition is red shifted (hot-band absorption) into the 240-270 nm region and the dissociation of the OH bond, relative to the OD bond, is enhanced by a factor of 15. This type of process is referred to as vibrationally mediated photodissociation and can be a very effective approach, provided the initial vibrational excitation remains localized in one chemical bond for a sufficient length of time to allow further excitation and dissociation. In the case of HOD it is clear that randomization of the vibrational energy is slower than the photodissociation step, and this further emphasizes the direct and impulsive nature of dissociation on the A Bi-state PES. 

H. A hard-sphere model, (a) Develop the hard-sphere model of Problem D in Chapter 5 for a non-reactive A - - BC collinear colUsion. In this model the interaction time is infinitesimally short so that energy transfer is efficient. Hence the model is used to determine the pre-exponential factor for the efficiency of tile collision, (b) The role of the masses. Is it more efficient for A to come from the direction of tiie B atom or from the direction of the C atom (c) Draw a trajectory for a BC molecule that is initially vibrationally excited. Does the outcome depend on tile phase, cf Section 5.2.2, of the initial vibration (d) Under what conditions will A collide more than once with the B atom (e) If you are geometrically minded, show that you can draw a construction that, for a given mass combination, allows you to determine the collision trajectory as a straight line superimposed on your drawing. There can be more than one line. What is tiie physics of the different lines (f) Hint for part (e). Show that certain mass combinations, with a BC molecule that is initially vibrationally cold, will not result in any energy transfer. 

Super collisions, which, for smaller molecules, are also known as balhstic collisions. Fisk and Crim (1977), Flynn et al. (1996). See the Ar -1- Csl example of H.-J. Loesch and D. R. Herschbach, J. Chem. Phys. 87, 2038 (1972) or the Ar -t- KBr results of Fisk et al. Devise a hard-sphere model that will predict a high conversion of the initial vibrational excitation to translation or vice versa for Ar -I- CsF collisions. 

J. Chem. Phys. 98, 8294 (1993) see also Rettner at at. (1996), Darling etal. (2001)]. The initial vibrational excitation of D2 serves to lower the barrier to dissociation. The effect of rotation is in two opposing directions. Dissociation occurs preferentially when the molecule is parallel to the surface, so rotation should hinder dissociation. On the other hand, the higher the initial rotational state, the more energy is brought in by the molecule. The latter effect increases quadraticaiiy with the quantum number and so wins out at the higher j s. 

We apply in Chapter 5 the ABC-Newton machinery to calculating the D-f-H2 (u = 1, j) initial state selected cross sections and rate constants. We find remarkably rapid convergence of the quantum calculations, and for the first time obtain quantitative agreement with experiment for the initial vibrationally excited rate constant... 

In addition, we have studied the initial state selected D-fH2(u = l,j) — DH+H reaction. For the first time, the initial vibrationally excited rate constant K=i T = 310 Jf) agrees quantitatively with experiment. Based on these D+H2 calculations, the H+H2 calculations in Chapter 2, and the recent D+H2 differential cross section calculations of Kuppermann and Wu [1] in their study of the gemetric phase effect, we conclude that many of the quantitative aspects of the H+H2 reaction (and isotopic analogues) on its ground electronic state are well understood. 

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