The Dynamics of Molecular Photo-Dissociation

The Dynamics of Molecular Photo-Dissociation

Copyright: © 2014 |Pages: 44
DOI: 10.4018/978-1-4666-4687-2.ch002


Such quantum spectroscopy tools as absorption cross sections, autocorrelation functions, Fourier transformations, and Born-Oppenheimer approximations are employed for the phenomenological and analytical characterization of the molecular evolution under light interaction. Events such as photo-dissociation, photo-association, and molecular fragmentations with specialization to collinear triatomic molecular A+BC states are considered in this characterization.
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2.2. Types Of Photo-Dissociation

Dissociation energies vary from a few thousandths of an electron-Volt (978-1-4666-4687-2.ch002.m01) for physically bound van der Waals molecules to several 978-1-4666-4687-2.ch002.m02 for chemically bonded molecules.

As a molecular mechanism of dissociation dynamics under external influence (Figure 1), the photon excites the molecule from the ground to a higher electronic state; the dissociation of the excited complex 978-1-4666-4687-2.ch002.m03 only occurs if the potential of the upper electronic state is repulsive along the intermolecular coordinate 978-1-4666-4687-2.ch002.m04. Accordingly, part of the photon energy 978-1-4666-4687-2.ch002.m05 is consumed to break the A – B bond and the excess energy (Schinke 1993)

Figure 1.

Diatomic potential curves linked from the ground (AB) and for excited (AB)* and dissociation (A+B) states under external photonic influence that respects the inter-atomic separation distance; adapted and redrawn from Schinke (1993).

is spanned between the translational energy 978-1-4666-4687-2.ch002.m07 and the internal energy 978-1-4666-4687-2.ch002.m08 of the product atoms or molecules (including vibrational, rotational and electronic energy).

In addition, the dissociation energy 978-1-4666-4687-2.ch002.m09 is measured starting from the Zero Point levels of the parent molecule and up to the Zero Point levels of the products. As such, the photon creates in general a Single Quantum State in the upper electronic manifold as a consequence of UV photodissociation, which is usually carried out by a long light pulse of low intensity and narrow bandwidth. The corresponding energy follows the equation 978-1-4666-4687-2.ch002.m10 (978-1-4666-4687-2.ch002.m11 is the energy of the parent molecule). Multiphoton dissociation represents another type of dissociation, and it occurs in the electrostatic ground state, as illustrated below (Figure 2). Because the exact number of absorbed photons cannot be controlled, an ensemble of quantum states may be created by the laser above the dissociation threshold, and these states would have the distribution of energies 978-1-4666-4687-2.ch002.m12. In consequence, the multiphoton dissociation becomes the result of the averaging over many quantum states (Schinke 1993).

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