Grounded electronic state potential energy surface, vibrational

Studies of this system show a broad range of control over the I to I product ratio. For example, a superposition of ,> and 3> (the first and third vibrational states of the ground-electronic-state potential energy surface) allows an increase of the yield of I from 30%, the value attained by excitation with one frequency, to more than 70%. Furthermore, using a diatomic model for CH3I, BS were able to define conditions which reduce the I yield to zero or increase it fully to one. 

Vibrationally mediated photodissociation (VMP) can be used to measure the vibrational spectra of small ions, such as V (OCO). Vibrationally mediated photodissociation is a double resonance technique in which a molecule first absorbs an IR photon. Vibrationally excited molecules are then selectively photodissociated following absorption of a second photon in the UV or visible [114—120]. With neutral molecules, VMP experiments are usually used to measure the spectroscopy of regions of the excited-state potential energy surface that are not Franck-Condon accessible from the ground state and to see how different vibrations affect the photodissociation dynamics. In order for VMP to work, there must be some wavelength at which vibrationally excited molecules have an electronic transition and photodissociate, while vibrationally unexcited molecules do not. In practice, this means that the ion has to have a... 

Figure 2.1 Schematic representation of the ground and electronic excited potential energy surfaces (PESs) and the corresponding absorption spectra of the parent molecule, resulting from the reflection of different initial wavefunctions on a directly dissociative PES (a) absorption from a vibrationless ground state consists of a broad continuum and (b) absorption from a vibrationally excited state shows that extended regions are accessed, leading to a structured spectrum with intensities of the features being dependent on the Franck-Condon factors. Reproduced with permission from Ref. [34]. Reproduced by permission of lOP Publishing.

In a photodissociation reaction it is usual for the initial state of the molecule to be the ground vibrational state of the ground electronic state. The incident radiation is resonant with an excitation to an electronic state that is dissociative (repulsive potential energy surface) or predissociative (the optically allowed transition is to a bound-state potential energy surface that intersects a repulsive surface). In the Franck-Condon picture, the electrons respond instantaneously to the incident light, while the relatively massive nuclei respond only slowly. Hence, on absorption of a photon the nuclear wave-function retains its shape but is projected up to the dissociative electronic state. In the traditional approach to the calculation of the photodissociation... 

Figure 9.3 Photodissociation Spectra of ABC. A laser pulse transfers the ground state vibrational wavefunction to the repulsive electronically excited potential energy surface. The wavepacket moves and spreads (to, ti, <2, 3 snapshots) on the excited surface, being accelerated in the steepest descent direction (force is negative gradient of the potential) toward A + BCt products (where and f refer respectively to electronic and vibrational excitation). Information about the photodissociation mechanism may be obtained from the (structureless)...

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