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Unformatted text preview: 16Feb2011 Chemistry 21b – Spectroscopy Lecture # 19 – Photodissociation Processes & the Reflection Approximation In any environment or experiment where ultraviolet photons are present, photodissociation, typically depicted as XY + h ν → X + Y , (19 . 1) must be considered in addition to the bound state-bound state electronic transitions outlined in Lecture #18. Photodissociation plays a dominant role in the chemistry of diffuse interstellar clouds, in the outer parts of dense molecular clouds, in the atmospheres of planets, and is responsible for many of the molecules and radicals observed in circumstellar envelopes, and in cometary and planetary atmospheres. Photodissociation Mechanisms Photodissociation of a small molecule can proceed in various ways, which are illustrated in Figure 19.1 for the case of a diatomic molecule. For small polyatomic molecules, the processes are similar, but more complicated to illustrate because the potential surfaces are multi-dimensional. The simplest dissociation process is through direct absorption into a repulsive upper state as shown in Figure 19.1a. This absorption may also take place into the repulsive wall of a bound excited electronic state (not shown). As spontaneous emission back to the ground state is relatively slow compared to the time frame for movement along the nuclear coordinate, which occurs on the picosecond time scale, all absorptions therefore lead to dissociation of the molecule. The photodissociation cross section is continuous as a function of photon energy, and its energy dependence is governed to first order by the Franck-Condon principle in that its maximum value is at the vertical excitation energy indicated by the arrow in Figure 19.1a. This is the predominant photodissociation pathway of diatomics such as CH + , NH, HCl, and typically dominates for polyatomics (CH 4 , N 2 O, O 3 , ...). In contrast to direct photodissociation, which involves continuous absorption and can therefore occur over a range of wavelengths, the indirect photodissociation mechanisms each involve discrete transitions to bound vibrational levels of an excited electronic state as a first step. This has profound implications for the transfer of radiation, because line absorption can be saturated much more readily than continuous absorption. In the case of predissociation , illustrated in Figure 19.1b, the bound levels of the excited electronic state are coupled to the vibrational continuum of a third state of different symmetry. The third state usually crosses the excited electronic state within the adiabatic Born-Oppenheimer approximation. The transition to the dissociating state occurs without the emission of radiation, and can in most cases be described by first order perturbation theory. This mechanism is thought to be the predominant way of photodissociating CO. The spectral signature of this process appears as a broadening of the discrete peaks (corresponding to absorption into the bound excited state), due to the interaction with the third state....
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This note was uploaded on 01/03/2012 for the course CH 21b taught by Professor List during the Fall '10 term at Caltech.
- Fall '10