Predissociative state

Dimers. It is well known that H2 pairs form bound states which are called van der Waals molecules. The discussions above based on the isotropic interaction approximation have shown that for the (H2)2 dimer a single vibrational state, the ground state (n = 0), exists which has two rotational levels f = 0 and 1). If the van der Waals molecule rotates faster ( > 1), centrifugal forces tear the molecule apart so that bound states no longer exist. However, two prominent predissociating states exist which may be considered rotational dimer states in the continuum (/ = 2 and 3). The effect of the anisotropy of the interaction is to split these levels into a number of sublevels. 

Ricks and Barrow (830) have obtained the predissociation limit from a rotational analysis of the emission and absorption bands of the B X3TU system of S2 vapor. The limit is at 35,999 2.5 cm corresponding to the products S(3P2) + S(3I ). The predissociating state (similar to R in Fig. II 5) is identified as the lu state. 

The question arises how does one distinguish experimentally between these two types of photodissociation This question can be answered from consideration of the absorption spectrum. The predissociative state is bound, and, therefore, is characterized by a set of discrete levels. The indirect channel implies the appearance of resonant structure in the photodissociation cross section as a function of the frequency of the incident radiation. Hence, discrete structure in the absorption spectrum indicates the indirect nature of the photodissociation. For example, analysis of the absorption spectrum of C2N2 leads to the conclusion that the process C2N2 (C- -IIu)+ hv -+ CNCX rtj +CN(A II) at V = 164 nm is an indirect photodissociation process (8). 

As mentioned in the previous section, indirect photodissociation is a two-step process. The predissociative state undergoes a radiationless transition to the final state of photofragments. The radiationless transition can be caused by a time-independent term of the Hamiltonian (see, e.g., ref. 15), then the transition occurs between states with the same energy. 

The transition from the quasidiscrete predissociative state (Q) to the final dissociative (D) state is described by the expression (see e.g., ref. 15),... 

Vibrational Predissociation, in this section we discuss the case of a transition from a predissociative state to the photofragment state that occurs on a single adiabatic pes. Such processes cannot occur for diatomic molecules, but they can be observed for polyatomic systems. The transition is caused by intramolecular energy transfer, that is, by internal redistribution of vibrational energy. 

According to eq. 45, the amplitude for the transition from the predissociative state nv to the photofragment state n v is... 

The experimental results for v = 7 [37] showed that the lowest ( 15) rotational levels exhibited single exponential decay with a decay constant that was essentially independent of J. These levels were then assumed to be stable and unaffected by the predissociation. For much higher initial rotational states, J > 28, the observed lifetime was dramatically shortened. A very rapid initial decay was observed followed after a few microseconds by a slower decay. On increasing the pressure, the initial fast decay was hardly affected but the intensity of the longer-lived decay component increased as more molecules were transferred by rotational relaxation out of the initially formed predissociated state into lower-lying stable states. 

The important conclusions of these studies are that, even at low pressures, collisions can drastically modify fluorescence decay curves. Rotational and/or vibrational energy transfer can stabilize an initially formed predissociated state by downward relaxation and conversely can destabilize a stable state by upward energy transfer into an unstable part of the energy level manifold. 

It is reasonable to assume that at the phase transition solid crystal 1 —> solid crystal 2 that is close to the phase transition solid crystal —> liquid crystal the intradimer hydrogen bonds are in a predissociate state. This state can be... 

The nature of vibrationally and rotationally predissociating states of atom-diatom Van der Waals molecules and the fundamental considerations governing their predissociation are discussed. Particular attention is focussed on the influence of the potential energy surface and the information about it which might be extracted from accurate measurements of predissociation lifetimes. Most of the results discussed pertain to the molecular hydrogen-inert gas systems, and details of previously unpublished three-dimensional potential energy surfaces for diatomic hydrogen with krypton and xenon are presented. 

Figure 11. Comparison of calculated (SFCCCC) and experimental widths for rotationally predissociating states of Ar-HD(v=l,j=2). Reproduced with permission from Ref. 20. Copyright 1983, North-Holland Physics Publishing ...

Another route for accessing the ground potential energy surface at energies at or even above dissociation58 59 is via a nonradiative transition from an excited electronic state. One would then expect fluctuations in the lifetimes of the predissociating states much as the transition strengths fluctuate for the... 

Spectral intensities, lifetimes of excited and/or predissociating states, and other transition rates can all be computed as (squares) of off-diagonal matrix elements. Our purpose here is different than the usual approach. We seek to characterize not individual rates but the overall distribution of rates and the... 

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