Electronically excited molecules rotational

Using a tunable laser as a probe they have observed that CN(X2E) radicals produced at this wavelength are vibrationally and rotationally excited. The rotational distribution follows the Boltzmann law, indicating that dissociation is not immediate but occurs after many vibrations of the electronically excited molecule. Thus, the distribution of the rotational population reflects (lie statistical nature of the dissociation processes. The distribution of the excess energy beyond that required to break the C—C bond is 54% in electronic, 20% in translational, 14% in vibrational, and 11% in rotational energies. See also p. 87. 

The photochemist is little interested in rotational energy, mainly because it is an unimportant fraction of the total energy possessed by an electronically excited molecule under most experimental conditions. The vibrational energy is quite another matter, and we must now examine the possibilities which may arise after the electronic energy of a diatomic molecule is increased. 

Analysis of the fluorescence from electronically excited molecules in a conventional static gas system21 provides a way of investigating vibrational relaxation of such molecules, and is also a means of studying selection rules for rotational relaxation22. It is now well established that multiple quantum rotational jumps can occur with high probability (see Section 6). 

The electronically excited molecules and atoms generally are not very stable, and soon lose their extra energy. They mostly do this by passing the energy on to other molecules by interaction with vibrating and rotating atoms bound within those molecules. For molecules in solution, for example, the energy transfer is... 

The essential distinction between thermal and photochemical reactions now needs to be explored more fully. Thermal energy may be distributed about all the modes of excitation in a species in a molecule these modes will include translational, rotational, and vibrational excitation, as well as electronic excitation. However, for species in thermal equilibrium with their surroundings, the Boltzmann distribution law is obeyed. If we take a typical energy of an electronically excited state equivalent in thermal units to 250 kj mol-1, at room temperature a fraction of the species of just 4 x 10 6 would be excited. To achieve a concentration of only 1% of the excited species would require a temperature of around 6800 °C in the event most molecular species would undergo rapid thermal decomposition from the ground electronic state and it would not be possible to produce appreciable concentrations of electronically excited molecules. In contrast, if molecules absorb radiation at a wavelength of about 500 nm as a result of an electronic transition, then electronic excitation certainly must occur, and the concentration... 

If an electronic transition band has well-resolved vibrational and rotational structure, it is possible to prepare electronically excited molecules in a specified single rotational and vibronic level (SRVL) using finely tuned monochromatic radiation. Obviously, tunable dye lasers are ideally suited for this task (see ref. 135). They have been used in numerous recent... 

Since an electronically-excited molecule can make a transition to several different vibrational states, and since, in addition, changes in rotational energy occur, the electronic spectrum of a molecule will involve a series of bands. If the electronic spectrum is observed in the vapor phase, the bands will consist of a series of sharp lines, which result from the closely-spaced rotational levels (see Figure 2.4). These sharp lines are known as the fine structure of the bands. In the liquid state or in solution the molecules cannot rotate freely. In these states the fine structure is considerably blurred. 

Very little is known about the nature of rotational energy transfer in a collision between an electronically excited molecule and a ground-state atom or molecule. In the few reported studies the experimental method is fundamentally the same as that described at the beginning of Section III.A. An initial rotational distribution is established by narrow-band excitation. The fluorescence emission contour is recorded twice, under collision-free and thermal equilibrium conditions, and then again under conditions such that there is one collision during the lifetime of the excited state. The differences in the rotational contours of the three emission spectra are then used to infer the pathway of rotational energy transfer, and the rate of that transfer. Some examples of the emission spectra recorded under these conditions are shown in Fig. 22. Because of the small spacings between the rotational levels of polyatomic molecules most excitation sources prepare nonthermal superpositions of rotational states rather than pure rotational states, and this complicates interpretation of the observations. 

INVOLVING VIBRATIONALLY, ROTATIONALLY, AND ELECTRONICALLY EXCITED MOLECULES... 

It seems to follow from the experimental data on vibrational relaxation of electronically excited molecules that, as a rule, the vibrational (and rotational) relaxation of these, molecules is faster than that of the same molecules in the ground electronic state. There are several reasons for it. 

Now let s allow the excited molecule to rotate randomly so that the transition dipole explores all possible orientations. If the sample is excited with a single short flash at time t = 0, the initial fluorescence anisotropy immediately after the flash (ro) will be 2/5 (or the value given by Eq. (5.68) if internal conversion to a different electronic state occurs very rapidly), but the anisotropy will decay to zero as the excited molecules rotate into new orientations (Fig. 5.13). By examining the decay kinetics of the anisotropy, we can learn how rapidly the molecule rotates and whether the rotational motions are isotropic or anisotropic. 

The greatest difficulties appear in studying elementary processes if reactants are atoms, radicals, and rotationally, vibrationally, or electronically excited molecules. Such species very rapidly react with others and, hence, their lifetimes are short. Shortlived reactive species are conventionally named active species. 

These results do not agree with experimental results. At room temperature, while the translational motion of diatomic molecules may be treated classically, the rotation and vibration have quantum attributes. In addition, quantum mechanically one should also consider the electronic degrees of freedom. However, typical electronic excitation energies are very large compared to k T (they are of the order of a few electronvolts, and 1 eV corresponds to 10 000 K). Such internal degrees of freedom are considered frozen, and an electronic cloud in a diatomic molecule is assumed to be in its ground state f with degeneracy g. The two nuclei A and... 

In (a), an ion and a gas atom approach each other with a total kinetic energy of KE, + KEj. After collision (b), the atom and ion follow new trajectories. If the sum of KE, + KEj is equal to KE3 + KE4, the collision is elastic. In an inelastic collision (b), the sums of kinetic energies are not equal, and the difference appears as an excess of internal energy in the ion and gas molecule. If the collision gas is atomic, there can be no rotational and no vibrational energy in the atom, but there is a possibility of electronic excitation. Since most collision gases are helium or argon, almost all of the excess of internal energy appears in the ion. 

There exist a series of beautiful spectroscopy experiments that have been carried out over a number of years in the Lineberger (1), Brauman (2), and Beauchamp (3) laboratories in which electronically stable negative molecular ions prepared in excited vibrational-rotational states are observed to eject their extra electron. For the anions considered in those experiments, it is unlikely that the anion and neutral-molecule potential energy surfaces undergo crossings at geometries accessed by their vibrational motions in these experiments, so it is believed that the mechanism of electron ejection must involve vibration-rotation... 

In this case the excited molecules produced on interaction with radiation undergo spin reversal to yield a triplet state with a much longer lifetime than that of the singlet excited state. One or more jt-bonds are broken in the triplet state since one of the n-electrons affected is in an antibonding n molecular orbital. This means that the o-bond is free to rotate and cis and trans isomers can be formed next to each other on recombination of the double bond. 

---