Metastable dissociation dynamics

Metastable Dissociations Dynamics Kinetic Energy Release... 

Until now, most studies of dissociation dynamics of metastable cluster ions have been made using a double-focusing mass spectrometry method (Lifshitz et al. 1990 Lifshitz and Louage 1989, 1990 Stace 1986). As discussed herein, the novel technique of reflectron time-of-flight mass spectrometry is a valuable alternative approach to more standard methods. With carefully designed experiments, it is possible to derive both kinetic energy releases and decay fractions for... 

Figure 6 (k= rate constant Ej = total ion energy) The PEPICO TOF distribution of C3H3 and C4H ions from energy-selected CgH ions. The asymmetric TOF distributions are a result of the slow reaction of the metastable ions. The solid lines are calculated distributions using the mean ion dissociation rate as an adjustable parameter. Reproduced with permission from Baer T, Willet GD, Smith D and Phillips JS (1979) The dissociation dynamics of internal energy selected CeHg. Journal of Chemical Physics 70 4076-4085. 

Thennal dissociation is not suitable for the generation of beams of oxygen atoms, and RF [18] and microwave [19] discharges have been employed in this case. The first excited electronic state, 0( D), has a different spin multiplicity than the ground 0( P) state and is electronically metastable. The collision dynamics of this very reactive state have also been studied in crossed-beam reactions with a RF discharge source which has been... 

Saue and Jensen used linear response theory within the random phase approximation (RPA) at the Dirac level to obtain static and dynamic dipole polarizabilities for Cu2, Ag2 and Au2 [167]. The isotropic static dipole polarizability shows a similar anomaly compared with atomic gold, that is, Saue and Jensen obtained (nonrelativ-istic values in parentheses) 14.2 for Cu2 (15.1 A ), 17.3 A for Ag2 (20.5 A ), and 12.1 A for Au2 (20.2 A ). They also pointed out that relativistic and nonrelativistic dispersion curves do not resemble one another for Auz [167]. We briefly mention that Au2 is metastable at 5 eV with respect to 2 Au with a barrier to dissociation of 0.3 eV [168, 169]. 

During the last decade knowledge of the ion chemistry of nitro compounds in the gas phase has increased significantly, partly due to the more widespread use of specialized techniques. Thus various ionization methods, in particular electron impact ionization and chemical ionization, have been used extensively. In addition, structure investigations as well as studies on fragmentation pathways have involved metastable ion dissociations, collision activation and neutralization/reionization studies, supplementary to studies carried out in order to disclose the associated reaction energetics and reaction dynamics. In general, the application of stable isotopes plays a crucial role in the in-depth elucidation of the reaction mechanisms. 

DFT calculations [342,343] demonstrate that the only stable molecularly adsorbed N2 state is for an atop vertically oriented molecule with W = 1.1 eV, although a shallow metastable well parallel to the surface also is predicted. Classical calculations on a 6D DFT PES observe dynamic trapping and non-activated dissociation throughout the energy range studied (Et < 1 eV) (Figure 3.36(c)), although the... 

Photo-excited SO2 and SO2 clusters have been observed to undergo a number of excited state and ion-state processes. Ion-state studies have, for example, identified the energy threshold of the ion-state oxygen loss channel of the SO2 monomer and dimer [1], Additionally, studies investigating the metastable decay process of SO2 clusters and mixed S02-water clusters have identified the dissociation pathways and the nature of the charged core of these cationic clusters [2]. The dynamics of oxygen loss of SO2 and SO2 clusters following excitation to the C (2 A ) state, which couples to a repulsive state, have also been studied to determine the influence of the cluster environment on the dissociation process [3]. 

In particular, irregular vibrational spectra with Wignerian level spacing statistics have been observed this last decade for a number of highly excited molecules [3-7]. On the other hand, many recent works have characterized the reactive dynamics in terms of quantum resonances, which allows a rigorous definition of metastable states with finite lifetimes and hence of dissociation rates [4, 8-10]. 

The extensive surface reconstruction in the presence of N has implications for our discussion of the recombination process, since we must consider whether N2 forms from recombination on the unreconstructed Cu(l 1 1) surface or is formed by decomposition of copper nitride islands. In the latter case N recombination may either leave the local Cu atoms in a metastable (100) arrangement or else recombination might be associated with substantial motion of the Cu atoms as they relax from the nitride adsorption geometry. If N recombination occurs at nitride islands then the dynamics of recombinative desorption will sample a phase space which is completely different to that for dissociation on clean flat Cu terraces, making it impossible to relate these two processes by detailed balance. This is the behaviour of H recombination on Si where the large change in the Si equilibrium geometry induced by H adsorption ensures that the adsorption and desorption processes sample very different channels [13]. 

This unusual behavior is interpreted as a reflection of the femtosecond dynamics of the donor molecules and an extremely rapid electron jump within the FET. Because this jump is much faster than the rotation and bending motions of the substituents, the donor presents himself as a dynamic mixture of the conformers. Ionization of these conformers results in the formation of two types of radical cations, one of which is metastable whereas the other one dissociates immediately into radicals and cations. In line with this interpretation, donor molecules with restricted bending motions as well as rigid structures form only one ionization product, which is normally the metastable radical cation. 

In this chapter, we discussed the principle quantum mechanical effects inherent to the dynamics of unimolecular dissociation. The starting point of our analysis is the concept of discrete metastable states (resonances) in the dissociation continuum, introduced in Sect. 2 and then amply illustrated in Sects. 5 and 6. Resonances allow one to treat the spectroscopic and kinetic aspects of unimolecular dissociation on equal grounds — they are spectroscopically measurable states and, at the same time, the states in which a molecule can be temporally trapped so that it can be stabilized in collisions with bath particles. The main property of quantum state-resolved unimolecular dissociation is that the lifetimes and hence the dissociation rates strongly fluctuate from state to state — they are intimately related to the shape of the resonance wave functions in the potential well. These fluctuations are universal in that they are observed in mode-specific, statistical state-specific and mixed systems. Thus, the classical notion of an energy dependent reaction rate is not strictly valid in quantum mechanics Molecules activated with equal amounts of energy but in different resonance states can decay with drastically different rates. 

Molecular dynamics studies of diatomic model detonations were first carried out by Karo and Hardy in 1977 [14]. They were soon followed by other groups [15, 16]. These early studies employed predissociative potentials, in which the reactant dimer molecules are metastable and can dissociate exothermically. More realistic models, combining an endothermic dissociation of reactants with an exothermic formation of product molecules, were introduced by White and colleagues at the Naval Research Laboratory and U.S. Naval Academy, first using a LEPS (London-Eyring-Polanyi-Sato) three-body potential for nitric oxide [17], and later a Tersoff-type bond-order potential [18] for a generic AB model, loosely based on NO [19, 20]. 

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