Predissociating resonance

The physical significance of Eq. (53) is clear. At an isolated resonance the excitation and dissociation processes decouple, all memory of the two excitation pathways is lost by the time the molecule falls apart, and the associated phase vanishes. The structure described by Eq. (53) was observed in the channel phase for the dissociation of HI in the vicinity of the (isolated) 5sg resonance. The simplest model depicting this class of problems is shown schematically in Fig. 5d, corresponding to an isolated predissociation resonance. Figures 5e and 5f extend the sketches of Figs. 5c and 5d, respectively, to account qualitatively for overlapping resonances. 

Complex-Coordinate Coupled-Channel Methods for Predissociating Resonances in van der Waals Molecules... 

The above-mentioned three-step transformation procedure allows us to expand the scope of the method of complex scaling to virtually any system. The method has been applied successfully to the study of predissociating resonances of several vdW molecules. Including Ar-H, Ar-HD, Ar-N and Ar-HCl etc.. Involving either piece-wise or numerical potentials. 

N. Lipkin, N. Moiseyev, and C. Leforestier, A three-dimensional study of NelCl predissociation resonances by the complex scaled discrete variable representation method, J. Chem. Phys. 95 1888 (1993). 

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 combination of the Azimuthal method of Section He with close-coupling expansions in the vy=0, vy=l and v q=1 vibrations enables vibrational predissociation resonances to be predicted. Por the reasons given above these predicted resonances are quite broad for the Ne complex( -- 10" cm ) but are much narrower for Ar(- 10" cm" ). This prediction remains to be verified in experiments. Furthermore, methods similar to those described for Ne-HF, but with the inclusion of Ae sudden approximation, have been used very recently to predict the full infrared spectrum for Ne-C2H4[71]. 

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... 

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). 

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. 

Perhaps the most pertinent observation to make at this point is that the process by which the molecule with energy spectrum hypothesized decays is simply a form of predissociation. There is one difference between the process we consider and the usual case of predissociation from a single zero-order molecular energy level. Because the exact resonant level is represented as a linear combination of the zero-order localized level, the... 

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). 

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