Predissociation resonant state

Budde et have recently observed the ultraviolet laser-induced desorption of NO from oxidized Ni(lOO). The 193 nm excitation wavelength used was resonant with gas phase NO transitions to a predissociative upper state. Desorption yields of NO from clean Ni(lOO) were essentially zero. Comparison of TPD results from clean and oxidized nickel surfaces indicated that an oxidized nickel surface could support a weakly bound NO state not found on clean Ni(100). 

Predissociation of van der Waals molecules is ideally suited for the application of the general expressions for the decay of resonance states derived in Section 7.2, especially Equation (7.12) for the dissociation rate. The reason is that the coupling is so small that we can rigorously define accurate zero-order states... 

See Kato and Baba (1995) for a review of predissociation which includes nearly 400 references. This quasi-bound state, AB, is called a resonance or a resonant state. 

The state AB, often called the superexcited state, is an autoionized or resonance state. Autoionization is called preionization by Herzberg (1950). This can be justified by the analogy between preionization and predissociation. In predissociation, the interaction of a discrete state with the vibrational continuum of the nuclei allows this discrete state a finite probability of dissociation. In preionization, it is the mixing of a discrete state with the electronic continuum that provides a finite ionization probability. 

Molecular applications have thus far involved the calculation of the electronic structure and potential energy surfaces of negative ion "compound states" and of "diabatic states" in the continuous spectrum of polyelectronic diatomics and triatomics and of energies and partial widths with interchannel coupling of vibrational shape and predissociating resonances of diatomics. The same principles and methodologies can be applied to many more such cases. 

The infrared and UV spectra of van der Waals molecules do, however display many sharp lines.This indicates that the excited states often have sufficiently long lifetimes to display sharp spectral features, despite the fact that they have more than enough energy to dissociate. In principle every observed spectral line corresponds to a photodissociation process. If the line is sharp the dissociation proceeds through a long lived intermediate resonance state and, in spectroscopic parlance, is termed a predissociation process. In the present brief overview I will discuss the spectra of van der Waals molecules from this view point. The main objective of the chapter will be to outline the different possible treatments of the process and their relationship to each other as well as to collect together a few key references on the theory of these processes. 

If the predissociation line is sharp, indicating only a small probability of the resonance state breaking up, then a perturbation type approach may be used. This approach is very clearly described in Shapiro s paper of 1972 where he introduces and tests out numerical procedures for evaluating the bound-continuum integrals needed in both this approximate perturbation (or so called golden rule) approach and also in the exact theory of photodissociation processes. The basic theory of the golden rule expressions has been presented by Levine.It has also been carefully derived by Beswick and Jortner who have used it in a pioneering study of vibrational predissociation. 

These partial widths give the "final quantum state distributions in that they are proportional to the probability of the resonant state decomposing to yield a particular final state f . The total width (eq. 8) is the sum of all the partial widths (eq. 10). Recently, in an extremely thorough and elegant study, Halberstadt, swick and Janda have applied both the complete photodisocciation theory (eq. 4b) and the golden rule approach to the study of vibrational predissociation in the Ne-C 2 system. 

Figure 9 Resonance hyper Raman spectrum of CH3I vapour excited at 365.95 nm. Reproduced by permission of Elsevier Science from Campbell DJ and Ziegler LD (1993) Resonance hyper-Raman scattering in the VUV. Femtosecond dynamics of the predissociated C state of methyl iodide. Chemical Physics Letters 201 159-165.

Fig. 8 Cumulative sum of resonance states calculated by the Q-matrix and the rotational predissociation model as a function of total energy from Cl + HF(v =0) for (a) J = 0 and (b) J= 1. The thresholds for the opening of the Cl+ HF(v ) r = 1, 2, 3, and F+HCl(v = 0) as well as the position of the adiabatic barrier are also plotted. The zero of energy is set as the asymptote energy of E + HCl(v = 0) and the plotted energy corresponds to 0-40 kcal/mol from the Cl + HF(v = 0). The dashed line contains the contribution of all resonances in the model while the dotted line only considers the A/ = 1 (short lifetime) resonances.

Unfortunately, predissociation of the excited-state limits the resolution of our photodissociation spectrum of FeO. One way to overcome this limitation is by resonance enhanced photodissociation. Molecules are electronically excited to a state that lies below the dissociation limit, and photodissociate after absorption of a second photon. Brucat and co-workers have used this technique to obtain a rotationally resolved spectrum of CoO from which they derived rotational... 

A relaxation process will occur when a compound state of the system with large amplitude of a sparse subsystem component evolves so that the continuum component grows with time. We then say that the dynamic component of this state s wave function decays with time. Familiar examples of such relaxation processes are the a decay of nuclei, the radiative decay of atoms, atomic and molecular autoionization processes, and molecular predissociation. In all these cases a compound state of the physical system decays into a true continuum or into a quasicontinuum, the choice of the description of the dissipative subsystem depending solely on what boundary conditions are applied at large distances from the atom or molecule. The general theory of quantum mechanics leads to the conclusion that there is a set of features common to all compound states of a wide class of systems. For example, the shapes of many resonances are nearly the same, and the rates of decay of many different kinds of metastable states are of the same functional form. 

---