Mode selective photodissociation

Figure 14. Mode selectivity in photodissociation of V (OCO). The ratio of the reactive (VO + CO) to nonreactive (V + CO2) product is measured at the peaks of the vibronic bands labeled in Fig. 13. The data below 16,600 cm is from bands accessed by one-photon excitation data at higher energy was obtained by vibrationally mediated photodissociation exciting the OCO antisymmetric stretch.

The terms mode-selective and bond-selective dissociation refer to the control of the dissociation products in VMP. The terms are usually used as synonyms although, strictly speaking, the former should refer to selective preexcitation of a vibrational mode and the latter to the resulting selective bond cleavage. Control of the dissociation products in VMP has been extensively reviewed [28-31] and our discussion will focus on molecules studied (or continued to be smdied) after the latest comprehensive review was published [31], An exception will be a short overview on the VMP of water isotopologues since it was the extensive theoretical and experimental investigations of these molecules, in particular H2O and HOD, that opened a new era of detailed smdies of state-to-state photodissociation out of specific rovibrationally excited states of polyatomic molecules. 

Throughout the chapter, we have presented the methodology of VMP and discussed some specific examples, where controlling the photodissociation products ensuing from an electronically excited state is achievable via preexcitation of specific vibrational states in the initial electronic state. This mode selectivity worked so far, with only two exceptions, for molecules not larger than tetratomic. The main reason for this limit is believed to be IVR however, additional work has to be carried out in larger molecules in attempt to find out what are the limits of size and conditions where nonstatistical behavior is still feasible. 

ABSTRACT. Potential energy surfaces (PESs) of several elementary chemical reactions are investigated by ab initio calculations. Topics included are (1 spectroscopy during the K + NaCl and Na + KCl reactions, (2) f + CO charge transfer reaction, (3) photodissociation and (4) two examples of mode-selective reactions. In connection to recent experiments on each subject,... 

In the second part of this chapter (Sect. 3.2), different wave packet propagation phenomena in excited alkali trimers are discussed. The time-resolved pseudorotation of the sodium trimer is presented in Sect. 3.2.2. Last but not least, applying laser pulses of the same wavelength but of different pulse width enables a mode-selective preparation of the trimer, hence controlling its dynamics (Sect. 3.2.4). Wave packet propagation on a repulsive PES (Sect. 3.2.5), studied on the potassium trimer, leads to the phenomena of ultrafast photodissociation, which then is the topic of the subsequent chapter. 

Fig. 11.6 Model of photodissociation of a polyatomic molecule by an intense IR field left, scheme of vibrational-energy acquisition by the molecule in the regions of mode-selective and mode-nonselective excitation right evolution of the fundamental IR absorption band spectrum with increasing vibrational energy of the molecule. Even at the dissociation limit the molecule is capable of absorbing, in a quasi-resonant fashion, IR radiation at a laser frequency i l tuned to the long-wavelength wing of the fimdamental absorption band.

Besides its practical importance, photodissociation — especially of small polyatomic molecules — provides an ideal opportunity for the study of molecular dynamics on a detailed state-to-state level. We associate with molecular dynamics processes such as energy transfer between the various molecular modes, the breaking of chemical bonds and the creation of new ones, transitions between different electronic states etc. One goal of modern physical chemistry is the microscopical understanding of molecular reactivity beyond purely kinetic descriptions (Levine and Bernstein 1987). Because the initial conditions can be well defined (absorption of a single monochromatic photon, preparation of the parent molecule in selected quantum states), photodissociation is ideally suited to address questions which are unprecedented in chemistry. The last decade has witnessed an explosion of new experimental techniques which nowadays makes it possible to tackle questions which before were beyond any practical realization (Ashfold and Baggott 1987). 

The examples indicate that mixing of vibrational levels in the photodissociation process is highly selective and that the amount of energy dumped into the rotational and translational modes is minimal. 

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