Transition diatomic molecules

If one of the components of this electronic transition moment is non-zero, the electronic transition is said to be allowed if all components are zero it is said to be forbidden. In the case of diatomic molecules, if the transition is forbidden it is usually not observed unless as a very weak band occurring by magnetic dipole or electric quadnipole interactions. In polyatomic molecules forbidden electronic transitions are still often observed, but they are usually weak in comparison with allowed transitions. 

One of the consequences of this selection rule concerns forbidden electronic transitions. They caimot occur unless accompanied by a change in vibrational quantum number for some antisynnnetric vibration. Forbidden electronic transitions are not observed in diatomic molecules (unless by magnetic dipole or other interactions) because their only vibration is totally synnnetric they have no antisymmetric vibrations to make the transitions allowed. 

Condon E U 1928 Nuclear motion associated with electron transitions in diatomic molecules Phys. Rev. 32 858-72... 

To compare the relative populations of vibrational levels, the intensities of vibrational transitions out of these levels are compared. Figure B2.3.10 displays typical potential energy curves of the ground and an excited electronic state of a diatomic molecule. The intensity of a (v, v ) vibrational transition can be written as... 

If the rotational motion of the molecules is assumed to be entirely unhindered (e.g., by any environment or by collisions with other molecules), it is appropriate to express the time dependence of each of the dipole time correlation functions listed above in terms of a "free rotation" model. For example, when dealing with diatomic molecules, the electronic-vibrational-rotational C(t) appropriate to a specific electronic-vibrational transition becomes ... 

Rule 1 shows that transitions are allowed in heteronuclear diatomic molecules such as CO,... 

Equation (6.8), to (d /dx)g. Figure 6.1 shows how the magnitude /r of the dipole moment varies with intemuclear distance in a typical heteronuclear diatomic molecule. Obviously, /r 0 when r 0 and the nuclei coalesce. For neutral diatomics, /r 0 when r qg because the molecule dissociates into neutral atoms. Therefore, between r = 0 and r = oo there must be a maximum value of /r. Figure 6.1 has been drawn with this maximum at r < Tg, giving a negative slope d/r/dr at r. If the maximum were at r > Tg there would be a positive slope at r. It is possible that the maximum is at r, in which case d/r/dr = 0 at Tg and the Av = transitions, although allowed, would have zero intensity. 

Figure 6.7 Rotational transitions accompanying a vibrational transition in (a) an infrared spectrum and (b) a Raman spectrum of a diatomic molecule...

The rotational selection rule for vibration-rotation Raman transitions in diatomic molecules is... 

As for a diatomic molecule, the general harmonic oscillator selection mle for infrared and Raman vibrational transitions is... 

Promotion of an electron in Hc2 from the (7 15 to a bonding orbital produces some bound states of the molecule of which several have been characterized in emission spectroscopy. For example, the configuration ((J l5 ) ((7 l5 ) ((7 25 ) gives rise to the 2i and bound states. Figure 7.24(a) shows the form of the potential curve for the state. The A-X transition is allowed and gives rise to an intense continuum in emission between 60 nm and 100 nm. This is used as a far-ultraviolet continuum source (see Section 3.4.5) as are the corresponding continua from other noble gas diatomic molecules. 

Figure 7.28 Rotational fine structure of a 77 — electronic or vibronic transition in a diatomic molecule for which > r". The g and u subscripts and s and a labels apply only to a homonuclear molecule...

The g and m subscripts in Figure 7.28 are appropriate only to a homonuclear diatomic molecule. This is the case also for the x and a labels which may result in intensity alternations for J even or odd in the initial state of the transition. Figure 7.28 would apply equally to a A type of transition. 

As for diatomic molecules (Section 7.2.5.2) fhe vibrational (vibronic) transitions accompanying an electronic transition fall into the general categories of progressions and sequences, as illustrated in Figure 7.18. The main differences in a polyatomic molecule are that there are 3A — 6 (or 3A — 5 for a linear molecule) vibrations - not just one - and that some of these lower the symmetry of the molecule as they are non-totally symmetric. 

Indicate which of the following electronic transitions are forbidden in a diatomic molecule, stating which selection mles result in the forbidden character ... 

Unlike nitric oxide, NO, the monomeric radical sulfur nitride, NS, is only known as a short-lived intermediate in the gas phase. Nevertheless the properties of this important diatomic molecule have been thoroughly investigated by a variety of spectroscopic and other physical techniques (Section 5.2.1). The NS molecule is stabilized by coordination to a transition metal and a large number of complexes, primarily with metals from Groups 6, 7, 8 and 9, are known. Several detailed reviews of the topic have been published. ... 

The rotational energy levels of (a) a heavy diatomic molecule and (b) a light diatomic molecule. Note that the energy levels are closer together for the heavy diatomic molecule. Microwaves arc absorbed when transitions take place between neighboring energy levels. 

FIGURE 7.2 Energy curves for a diatomic molecule. Two possible transitions are shown. When an electron has been excited to the point marked A, the molecule may cleave (p. 312). 

Molecules possess discrete levels of rotational and vibrational energy. Transitions between vibrational levels occur by absorption of photons with frequencies v in the infrared range (wavelength 1-1000 p,m, wavenumbers 10,000-10 cm , energy differences 1240-1.24 meV). The C-0 stretch vibration, for example, is at 2143 cm . For small deviations of the atoms in a vibrating diatomic molecule from their equilibrium positions, the potential energy V(r) can be approximated by that of the harmonic oscillator ... 

If we move the chemisorbed molecule closer to the surface, it will feel a strong repulsion and the energy rises. However, if the molecule can respond by changing its electron structure in the interaction with the surface, it may dissociate into two chemisorbed atoms. Again the potential is much more complicated than drawn in Fig. 6.34, since it depends very much on the orientation of the molecule with respect to the atoms in the surface. For a diatomic molecule, we expect the molecule in the transition state for dissociation to bind parallel to the surface. The barriers between the physisorption, associative and dissociative chemisorption are activation barriers for the reaction from gas phase molecule to dissociated atoms and all subsequent reactions. It is important to be able to determine and predict the behavior of these barriers since they have a key impact on if and how and at what rate the reaction proceeds. 

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