Mechanisms of vibrational excitation

Comparison of the collision numbers given above for vibration-vibration transfer with those for vibration-translation, given in Section 4, shows that in many cases vibration-vibration transfer between two resonant or near-resonant modes is much more efficient than vibration-translation transfer from either. This applies equally to homomolecular and heteromolecular collisions, and carries the interesting consequence that the quickest route for vibrational excitation of upper levels from the ground level by homomolecular collisions is an initial vibration-translation excitation to the v = 1 level, followed by successive vibration-vibration transfers to higher levels. Because of the selection rule, Av = 1, 

The slowest process will be the vibration-translation activation to the (v = 1) level, which will be rate-determining, and the subsequent vibration-vibration transfers will occur at increasingly fast rates with increasing vibrational quantum number. (For harmonic oscillators 1 = n(m+ l) 1 .) Shock-tube experi- 

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The width and shape of the energy loss peaks in HREELS are usually completely determined by the relatively poor instrumental resolution. This means that no information can be obtained from HREELS about such interesting chemical physics questions as vibrational energy transfer, since the influence of the time scale and mechanism of vibrational excitations at surfaces on the lifetimes, and therefore the line widths and shapes, is swamped. (Adsorbates on surfaces have intrinsic vibra-... 

It is obvious from the above results that adsorption of acetic acid, and, of course, presumably other carboxylic acids, is different in detail from one metal oxide to another and is perhaps also somewhat a function of whether adsorption occurs from gas or solution phase. However, in all cases acetate ions are formed and differences presumably reflect more subtle features of surface structure and chemistry. In general, there seems to be a correspondence between the frequencies reported by IR and IETS for IR active modes although intensity patterns are not similar, as one should expect based on the different mechanisms of vibrational excitation. Further work is obviously needed to define the differences between the two spectroscopies more exactly. 

The most clear demonstration of the predictions of a resonant mechanism of vibrational excitation was provided by Hanh et al. [9]. The authors find a decrease in the conductance associated with the onset of activation of an 0-0 stretch mode, for O2 on Ag(llO). Such reversed behavior follows predictions made by Persson et al. [10] for those systems with narrow molecular resonances around the Fermi level (Ep). The theoretical fundaments of these and related issues will be discussed later in this chapter. 

Connection with vibrational lifetime on surfaces. The decay of molecular vibrations in the excitation of the electron-hole pairs of metallic surfaces have been identified with the mechanisms of vibration excitation by tunneling electrons [42]. Intuitively this may seem so. Indeed, an excited vibration may couple to the surface electronic excitations through the same electron-vibration matrix elements of Eqs. (2) and (4). The surface... 

Figure 3. Schematic diagram indicating mechanism of vibrational excitation in electron encounter. An electron possessing energy E0 inadequate to cause an electronic excitation of the molecule M interacts with M, at that lower energy, to form an intermediate [ M e], a virtual negative ion, of fairly tong duration which dissociates to yield a vibrationally excited intermediate M and an electron. The possibility of some light emission (hv) is indicated. At the end of the process, the electron possesses energy Ef-hv and the vibrational energy of M is...

An extensive review of the literature reveals that the only studies of vibrational effects in insertion chemistry have focused on reactions of 0(1D)175-177 and C(1D)177,178 with H2. Since there is no potential energy barrier to insertion in these systems, reaction proceeds readily even for unexcited reactants.179 Since the efficiency of vibrational excitation was 20% in both studies, due to the large cross-sections for ground state reactions, only small changes were observed in the experimental signal. From an analysis of the product distributions, it was concluded that while H2(v = 0) primarily reacted via an insertion mechanism, direct abstraction seemed to become important for = 1). For 0(1D), this is similar to behavior at elevated collision energies.180... 

Perhaps the first evidence for the breakdown of the Born-Oppenheimer approximation for adsorbates at metal surfaces arose from the study of infrared reflection-absorption line-widths of adsorbates on metals, a topic that has been reviewed by Hoffmann.17 In the simplest case, one considers the mechanism of vibrational relaxation operative for a diatomic molecule that has absorbed an infrared photon exciting it to its first vibrationally-excited state. Although the interpretation of spectral line-broadening experiments is always fraught with problems associated with distinguishing... 

In addition, this mechanism was criticized on the basis of the Kassel-Rice theory of vibrationally excited ground states some time earlier by Zimmerman et alS3i)... 

The validity of Johnston s interpretation of the experimental facts in terms of the simple unimolecular dissociation (1) has been questioned by Lindars and Hinshelwood120 and by Reuben and Linnett121. These workers maintain that isothermal plots of k versus p are not smooth curves, but consist of a number of straight lines linked by markedly curved portions. To explain such behaviour they incorporate into their mechanism a collision-induced crossover of vibrationally excited N20 (XS) to repulsive 3II and 3E states. While we incline towards the simpler view held by Johnston105 and others106-116, we feel that this feature of the decomposition kinetics merits further investigation. 

V. S. Letokhov My answer to Prof. Quack is that it is indeed difficult to predict theoretically the effect of intense femtosecond IR pulses on the IVR rate of polyatomic molecules, which is important for the transfer of vibrationally excited molecules from low-lying states to the vibrational quasi-continuum. We are developing the relevant theoretical mechanisms of IR MP E/D of polyatomics since the discovery of this effect for isotopic molecules BC13 and SF6 in 1974-1975.1 hope that it will become more realistic to study experimentally the influence of intense IR pulses on IVR due to the great progress of femtosecond laser technology. 

I might point out that in 1979 I made a simple model prediction that ionization at high intensities will always become the fastest process (compared to dissociation or isomerization reaction) [4, 5], In this early work we considered the possible mechanisms of vibrational preionization, perhaps field assisted, as well as direct electronic excitation and field ionization [4, 5]. Infrared laser ionization with CO2 lasers was recently shown to occur in diatomic molecules by Dietrich and Corkum [6], and Professor Letokhov has right now informed me about previous work in his group on this problem (experiments in the 1980s). We have also some evidence from recent experiments in our group that ionization with CO2 lasers may occur more easily than usually anticipated. 

More recent studies have made possible direct determination of the rate constants k7a and k7b. McNeal and Cook47 have followed the concentration of 02(1A9) in a discharge-flow system by the photoionization technique (Sect. III-E). In the presence of ozone, 02(1A,) decays by a predominantly first-order mechanism, so that, presumably, the second-order pooling process (11) does not contribute significantly to the loss of 02(1A9). If it is assumed that the only loss process for Oa(1A9) is reaction with ozone in reaction (7a), then k7a lies between 1 x 106 and 2 x 106 liter mole-1 sec-1. This value is considerably lower than the rate constant measured by Mathias and Schiff87 if Oa in reaction (7) is largely O Aj). McNeal and Cook consider the possibility that their photoionization current decreases by less than that expected on the basis of [02(1A9)J decay, as a result of the formation of vibrationally excited oxygen in reaction (15d)... 

In this section we will explain the essential mechanism of vibrational predissociation by virtue of a linear atom-diatom complex such as Ar H2. Figure 12.1 illustrates the corresponding Jacobi coordinates, t In particular, we consider the excitation from the vibrational ground state of H2 to the first excited state as illustrated in Figure 12.2. The close-coupling approach in the diabatic representation, summarized in Section 3.1, provides a convenient basis for the description of this elementary process. For simplicity of presentation we assume that the coupling between the van der Waals coordinate R and the vibrational coordinate r is so weak that it suffices to include only the two lowest vibrational states, n = 0 and n = 1, in expansion (3.4) for the total wavefunction,... 

The general theory for the absorption of light and its extension to photodissociation is outlined in Chapter 2. Chapters 3-5 summarize the basic theoretical tools, namely the time-independent and the time-dependent quantum mechanical theories as well as the classical trajectory picture of photodissociation. The two fundamental types of photofragmentation — direct and indirect photodissociation — will be elucidated in Chapters 6 and 7, and in Chapter 8 I will focus attention on some intermediate cases, which are neither truly direct nor indirect. Chapters 9-11 consider in detail the internal quantum state distributions of the fragment molecules which contain a wealth of information on the dissociation dynamics. Some related and more advanced topics such as the dissociation of van der Waals molecules, dissociation of vibrationally excited molecules, emission during dissociation, and nonadiabatic effects are discussed in Chapters 12-15. Finally, we consider briefly in Chapter 16 the most recent class of experiments, i.e., the photodissociation with laser pulses in the femtosecond range, which allows the study of the evolution of the molecular system in real time. 

In the case of vibrational excitation of NH3 at a metal surface a very different dependence has been observed [119]. In this case, the vibrational excitation could be attributed to mechanical excitation of the NH3 umbrella mode in the collision with the surface. Finally, it is worth mentioning that at much higher energies, way into the domain of tens of eV s, mechanical excitation will lead to molecular vibrational excitation and dissociation for all molecules, see e.g. [120]. 

Quantum mechanical and classical calculations have been performed [245] for H + Cl2 on a recently optimised extended LEPS surface [204]. The quantum mechanical results were transformed to three dimensions by the information theoretic procedure and are in good agreement with the distributions determined in the chemiluminescence experiments. However, three-dimensional trajectory calculations on the surface consistently underestimate (FR) at thermal energies and it is concluded that the LEPS surface which was optimised using one-dimensional calculations does not possess the angular dependence of the true three-dimensional surface. This appears to result from the lack of flexibility of the LEPS form. Trajectory studies [196] for H + Cl2 on another LEPS surface find a similar disposal of the enhanced reagent energy as was found for H + F2. The effect of vibrational excitation of the Cl2 on the detailed form of the product vibrational and rotational state distributions was described in Sect. 2.3. 

The chemiluminescent reaction A1 + 03 yields AlO (A, B) with preferential population of high vibrational levels in both states [415, 416]. In contrast, a Boltzmann vibrational state distribution is observed [415] for the A10 (B) product from A1 + N20, suggesting a different reaction mechanism in this case. An electron-jump mechanism operates for A1 + 03 giving the observed preferential population of AlO (A) [417] with a high degree of vibrational excitation. For A1 + N20, reaction takes place at shorter range, allowing production of the B state of AlO. ... 

Once we go beyond the band edge, the situation is quite different. As we see in the top panel, beyond 150 cm-1 the absolute scale of the friction is two orders of magnitude smaller than it is inside the band, but the friction there is well represented by the IP theory. Now it is the collective, linear INM theory that makes far too small a contribution. Even at this early stage we can conclude that the mechanism of vibrational energy relaxation must switch when we cross the band edge, and that the new mechanism must involve exciting highly localized solvent motion mediated in some essential... 

Indirect transfer occurs by a two-part mechanism, as shown in Fig. 18. First a vibrational excitation decays by generating phonons. The phonons then produce vibrational excitation on other molecules by multiphonon up-pumping. Indirect transfer will not occur unless the density of vibrational excitations is large enough to produce a real increase in the bath temperature. 

In a model proposed by Lewis [228] the effect of the excited state of retinal on the conformational state of the protein is considered to be the first step of the excitation mechanism. Charge redistribution in the retinal by excitation with light would have the consequence of vibrationally exciting and perturbing the ground state conformation of the protein, i.e., excited retinal would induce transient charge density assisted bond rearrangements (e.g., proton translocation). Subsequently, retinal would assume such an isomeric and conformational state so as to stabilize maximally the new protein structure established. In this model, 11-m to trans isomerization would not be involved in the primary process, but would serve to provide irreversibility for efficient quantum detection. It was also proposed that either the 9-m-retinal (in isorhodopsin) or the 11-m-retinal (in rhodopsin) could yield the same, common... 

The quenching of 0(XZ)) by N2, reaction [4], can be a very efficient source of vibrationally excited nitrogen [260] even though the curve-crossing mechanism may not permit more than one or two quanta of N2 vibrational energy per Of1/)) quench [258, 259]. Hunten and McElroy [56] have pointed out that whereas 0(XZ>) is readily quenched by energy transfer to N2, O S) is not, even though it is nearly resonant with v = 16. 

Neutron [102] and electron [70,103] irradiated TCNQ salts were investigated by IR spectroscopy. The spectra of irradiated samples gradually become weaker when the dose is increased the broad lines change shape and decrease in intensity some even vanish (Fig. 18). It was shown [103] that the vibrational features connected with the totally symmetric modes of the TCNQ are particularly sensitive to structural disorder. In some cases, for example, in MTPP(TCNQ)2 irradiated by electrons, the disappearance of distinct doublets of activated ag modes was noticed. Different mechanisms of the excitation of both the narrow and wide components explain their different dose and temperature dependences [70]. The changes of band intensity in the UV-VIS region imitate the electrical conductivity as a function of dose. 

The corresponding values for cyclohexanone appear to be independent of exciting wavelength. Lee and coworkers (47,48 have interpreted the sharp decrease in for cyclobutanone to be due to a competing predissociation mechanism from vibrationally excited S. This facile predissociation is described as a-cleavage of the vibrationally and electronically excited ketone to form an acyl alkyl biradical which subsequently decomposes to products. Interestingly, the rate of this cleavage must be extremely... 

Increasing theoretical and experimental attention has been paid to the vibrational relaxation and predissociation of vibrationally excited vdW molecules 195.203-205) Efficient vibrational relaxation takes place through the collisional mechanism ... 

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