Photodissociation of diatomic molecules

The simplest case of photodissociation involves electronic excitation from the ground state of a molecule to a purely repulsive state. In effect, the bonding in the molecule is annulled by promotion of an electron from a bonding or non-bonding orbital into an antibonding orbital. The molecule then dissociates along a repulsive potential surface on the femtosecond time-scale  

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The concepts of laser chemistry are developed along the lines of unimolecular reactions, or, in other words, dissociative processes in the most common sense. The discussion evolves from the photodissociation of diatomic molecules through triatomic species up to larger polyatomic entities (Chapters 15 to 17). Suitable coverage is also given to multiphoton and photoionization processes, which involve the subtle inclusion of intermediate and continuum states (Chapter 18). The part on unimolecular reactions concludes with a discus-... 

Figure 15.3 The effect of molecular orientation, with respect to the transition moment ft and the electric vector f on theexcitation and recoil direction forthe photodissociation of diatomic molecules. In (a) the transition momentandelectricvectorare parallel, whichistheoptimum forexcitation, whereas in (b) they are perpendicular, result-ingin zero excitati on...

We next study how the results of wavepacket dynamics can be tracked experimentally as a real-time history of chemical or physical events. Femtosecond time-resolved spectroscopy enables us to observe nuclear dynamics and to chart the path of chemical reactions in real time, and has been exploited in numerous applications ranging from fundamental studies of real-time motion in the photodissociation of diatomic molecules to stud-... 

Let us consider some experimental studies. Developing practically the idea proposed as early as 1968 by van Brunt and Zare (see [176, 376] for a review), a number of authors [232, 328, 383] investigated fluorescence polarization on atomic photofragments excited in the process of photodissociation of the simplest diatomic molecules. Observation of linear polarization (of the order of V 0.05) in the fluorescence of Na(2P3/2) atoms after the photodissociation of Na2 molecules is reported in [328]. Detailed studies of photodissociation of K2 through the transition —> BlHu... 

Most of the theoretical papers dealing with the photodissociation of polyatomic molecules are included in Table 9 under specific headings. Lee et introduced the multidimensional reflection (MR) approximation to replace the quasi-diatomic model often used in the theoretical descriptions of polyatomic molecule photodissociation. They utilized the results of the MR approximation to examine the dependence of the extinction coefficient on i max— V, where is the frequency of maximum absorption, to obtain the slope and orientation of the co-ordinate of steepest descent on the upper state surface and to explain the dependence of the absorption cross-section from initially excited vibrational states on the orientation of this co-ordinate. 

Another equation, analogous to Eq. (7.2.13), may be derived for experiments in which a beam of linearly polarized light (propagating along X, polarized along Z ) is used to photodissociate a diatomic molecule. For this scheme, the photofragment ejection direction O, is defined relative to the linear polarization direction, Z, and the resultant photofragment distribution is described by... 

Note added in proof a theoretical analysis of the polarization of fluorescence from the diatomic fragments formed in the photodissociation of triatomic molecules has been developed by Macpherson, Simons and Zare ). 

Since nuiny processses demonstrate substantial quantum effects of tunneling, wave packet break-up and interference, and, obviously, discrete energy spectra, symmetry induced selection rules, etc., it is clearly desirable to develop meAods by which more complex dynamical problems can be solved quantum mechanically both accurately and efficiently. There is a reciprocity between the number of particles which can be treated quantum mechanically and die number of states of impcxtance. Thus the ground states of many electron systems can be determined as can the bound state (and continuum) dynamics of diatomic molecules. Our focus in this manuscript will be on nuclear dynamics of few particle systems which are not restricted to small amplitude motion. This can encompass vibrational states and isomerizations of triatomic molecules, photodissociation and exchange reactions of triatomic systems, some atom-surface collisions, etc. 

So far, the explanation was for an example of diatomic molecules. For the polyatomic molecules composed of n-pieces of atoms, the idea of the correspondence of the appearance of the absorption spectrum and the photo-excitation is the same, but the potential surfaces are in n 1 dimensions. Since photodissociation can occur with multiple processes such as ABC AB + C, A + BC, multi-dimensional potential surfaces for each dissociating inter-atomic distance must be considered. 

Figure C3.5.7. Possible modes of vibrational wavepacket (smootli Gaussian curve) motion for a highly vibrationally excited diatomic molecule produced by photodissociation of a linear triatomic such as Hglj, from [8].

In spite of the fact that in alkali vapors, which contain about 1 % diatomic alkali-molecules at a total vapor-pressure of 10 torr, the atoms cannot absorb laser lines (because there is no proper resonance transition), atomic fluorescence lines have been observed 04) upon irradiating the vapor cell with laser light. The atomic excited states can be produced either by collision-induced dissociation of excited molecules or by photodissociation from excited molecular states by a second photon. The latter process is not improbable, because of the large light intensities in the exciting laser beam. These questions will hopefully be solved by the investigations currently being performed in our laboratory. 

The examples gathered here deal primarily with relatively simple organic reactions, though the photodissociation of one inorganic diatomic molecule is included as an historically important paradigmatic process. The examples cover but a fraction of the field, but may serve to illustrate the powers and limitations of the area at its present state of development. 

Unlike the case of simple diatomic molecules, the reaction coordinate in polyatomic molecules does not simply correspond to the change of a particular chemical bond. Therefore, it is not yet clear for polyatomic molecules how the observed wavepacket motion is related to the reaction coordinate. Study of such a coherent vibration in ultrafast reacting system is expected to give us a clue to reveal its significance in chemical reactions. In this study, we employed two-color pump-probe spectroscopy with ultrashort pulses in the 10-fs regime, and investigated the coherent nuclear motion of solution-phase molecules that undergo photodissociation and intramolecular proton transfer in the excited state. 

Photoionization is a special case of a two-body dissociation where Me Ma. thus, the photoelectron receives essentially all of the KER and the atom or molecule (e.g., from a photodissociation event) essentially retains its initial velocity as it is ionized. For photodissociation of a homonuclear diatomic the KER is equally shared between the two product atoms (A and A ). The sum of the internal energies in the product atoms can be determined from the measured KER (l/2mAVA + l/2mA.VA) and from... 

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