Fate of Excited Molecule

Polyatomic molecules have polydimensional surface which is too complex to be represented by Morse diagram. When such a molecule is irradiated by suitable electromagnetic radiation, it gets electronically excited and after it, a number of phenomenon may occurs which is best represented by Jablonski diagram. 

Radiative process shown by straight line Non-radiative process shown by wave lines VC vibrational. cascade IC Internal conversion ISC Intersystem crossing hv radiation h.  

The superscript v indicates vibrationally excited states excited states higher than Sj and are omitted. 

The loss of excess energy by electronically excited molecule to ground the vibratonal levels of the same electronic state, the energy is given in small increments to environment by collision with neighbouring molecules. The radiationless process is known by energy cascade. 

In solution, the excess vibrational energy of S2 electronic state will be rapidly disappeared by radiationless process, vibrational cascade to the solution. 

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The format is as before, with the addition of one small section on the use of lasers in isotope separation and remote sensing of atmospheric pollutants. Emphasis is again placed upon the physical fates of excited molecules, and in particular on studies of isolated molecules. 

Another useful technique for measuring the rates of certain reactions involves measuring the quantum yield as a function of quencher concentration. A plot of the inverse of the quantum yield versus quencher concentration is then made Stern-Volmer plot). Because the quantum yield indicates the fraction of excited molecules that go on to product, it is a function of the rates of the processes that result in other fates for the excited molecule. These processes are described by the rate constants (quenching) and k (other nonproductive decay to ground state). 

The possible fate of excitation energy residing in molecules is also shown in Figure 2. The relaxation of the electron to the initial ground state and accompanying emission of radiation results in the fluorescence spectrum - S0) or phosphorescence spectrum (Tx - S0). In addition to the radiative processes, non-radiative photophysical and photochemical processes can also occur. Internal conversion and intersystem crossing are the non-radiative photophysical processes between electronic states of the same spin multiplicity and different spin multiplicities respectively. 

Although the novelty of observing chemically produced vibrational excitation provided an initial impetus, the main purpose of the studies to date has been to determine in detail the relative proportions of excited molecules in the various energy states, the fraction of the reaction energy that goes into internal excitation, which products are excited, and the fate of the excited molecules. Such data are used as aids in the construction of potential energy surfaces to be used, in turn, to describe the dynamics of the reactions. In short, the studies have been in the hands of kineticists. As interest in the subject has spread, more attention has been paid to applications laser action and the reactions of the excited molecules. 

To further assess the fate of the molecules in the excited state, we attempted to observe the fluorescence signal, but the signal was so weak that a quantitative measurement of the dispersed spectrum was impossible using our existing setup. However, by recording the decay profile, the fluorescence lifetimes were obtained... 

In reality, as indicated in Figure 5.4, there are many excited states around the barrier that can all lead to reaction. According to activated complex theory, the rate is independent of the fate of the molecule once it has passed the barrier. The rate depends only on the concentration of the molecule in the activated state [A and the time necessary for passing the barrier, T/... 

Excited states of hydrocarbon molecules often undergo nondissociative transformation, although dissociative transformation is not unknown. In the liquid phase, these excited states are either formed directly or, more often, indirectly by electron-ion or ion-ion recombination. In the latter case, the ultimate fate (e.g., light emission) will be delayed, which offers an experimental window for discrimination. A similar situation exists in liquid argon (and probably other liquefied rare gases), where it has been estimated that -20% of the excitons obtained under high-energy irradiation are formed directly and the rest by recombination (Kubota et al., 1976). 

It should be emphasized that the data shown in Fig. 3 do not provide information on the fate of the excited or ionized molecule these data constitute a sum over final molecular states. Generally, measurements of excitation and ionization cross sections for fast electrons do not provide information on subsequent target relaxation modes. However, this information is often available from separate measurements that focus on state-selected partial cross sections and molecular fragmentation [19]. 

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