Photodissociation predissociative states

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 total width, T, is the sum of partial widths, which can be calculated but not observed separately. Only the total width can be observed experimentally. This width does not depend on whether the line is observed in an absorption, photoionization, photodissociation, or emission spectrum because the width (or the lifetime) is characteristic of a given state (or resonance). In contrast, the peak profile can have different line shapes in different channels the line profile, q, is dependent on the excitation and decay mode (see Sections 7.9 and 8.9). For predissociation into H+CT, the transition moment from the X1E+ state to the 3n (or 3E+) predissociating state is zero, consequently q = oo and the lineshape is Lorentzian. In contrast, the ratio of the two transition moments for transitions to the XE+ continuum of the X2n state and to the (A2E+)1E+ Rydberg states leads to q 0 for the autoionized peaks (see Fig. 8.26) (Lefebvre-Brion and... 

The photodissociation of an adsorbed molecule may occur directly or indirectly. Direct absorption of a photon of sufficient energy results in a Franck-Condon transition from the ground state to an electronically excited repulsive or predissociative state. Indirect photodissociation of adsorbates, involving absorption of photons by the substrate, can take place via two processes. The first one is analogous to the process of sensitized photolysis in gases. The second one, also substrate mediated, implies the phototransfer of an electron from the substrate to an antibonding orbital of the adsorbate, i.e. charge transfer photodissociation. The basic principles of these two excitation mechanisms will be discussed later in this part. 

The photodissociation of an adsorbed molecule may occur directly or indirectly. Direct absorption of a photon of sufficient energy produces a Franck-Con-don transition from the ground to an electronically excited repulsive or predissociative state. When the excited state is repulsive, bond breaking is very fast ( 10 —10 s) and dissociation competes... 

Photodissociation very commonly produces molecular fragments in a variety of electronically excited states. This fact alone does not constitute branching in the sense implied throughout this review Usually each asymptotic exit channel is related uniquely to some repulsive or predissociating state prepaid in the photon absorption act, and the product distribution simply reflects the accessibility of the various excited states from the ground state (Franck-Condon factors and transition moments). 

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... 

Hydroxyl radical (OH) is a key reactive intermediate in combustion and atmospheric chemistry, and it also serves as a prototypic open-shell diatomic system for investigating photodissociation involving multiple potential energy curves and nonadiabatic interactions. Previous theoretical and experimental studies have focused on electronic structures and spectroscopy of OH, especially the A2T,+-X2n band system and the predissociation of rovibrational levels of the M2S+ state,84-93 while there was no experimental work on the photodissociation dynamics to characterize the atomic products. The M2S+ state [asymptotically correlating with the excited-state products 0(1 D) + H(2S)] crosses with three repulsive states [4>J, 2E-, and 4n, correlating with the ground-state fragments 0(3Pj) + H(2S)[ in... 

The photodissociation products of the homonuclear halogens in the visible and ultraviolet are now comparatively well established in view of the detailed spectroscopic studies that have been made. The strongest absorption system observed in this spectral region is associated with a transition to the 3II0u+ state which correlates with X / ) + X(2Pyz). Thus photoexcitation to the continuum associated with this state leads directly to the formation of an excited atom, while excitation to the banded region followed by predissociation will lead only to ground state atoms. 

Fig. V-22. Quantum yield for photodissociation of I2 (the production of I atoms) as a function of wavelength. The quantum yield approaches unity at both ends. The results are explained on the basis of direct dissociation from the 1 Il(l ) repulsive state and of predissociation from the fl3ll(0 ) state, which is t> dependent (a minimum near v = 15, a submaximum near v = 25). From Brewer and Tellinghuisen (147), reprinted by permission. Copyright 1972 by the American Institute of Physics.

Photodissociation cross sections of ions are measured as a function of photon energy. Information is obtained on the photon-absorbing state(s) as well as on the predissociating or repulsive states formed on photon absorption. The method has not yet been employed for detection of metastable states in ion beams. 

The increases that are observed in the anisotropy parameter with increasing wavelength are thought to be due to the non-negligible lifetime of the iBj state, which proves that predissociation must be occurring in the photolysis. There is no indication in the absorption spectra that this is the case, which illustrates how photodissociation dynamics can reveal new details about the upper electronic states of the molecule. 

The Si PES, calculated by Nonella and Huber (1986), has a shallow minimum above the ground-state equilibrium, or expressed differently, a small potential barrier hinders the immediate dissociation of the excited S complex. Although the height of the barrier is less than a tenth of an eV, it drastically affects the dissociation dynamics, even at energies which significantly exceed the barrier. The excited complex lives for about 5-10 internal NO vibrational periods before it breaks apart. The photodissociation of CH3ONO through the Si state exemplifies indirect photodissociation or vibrational predissociation (Chapter 7). 

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