Ground vibrational stales

I he upper heavy lines are energy levels for the ground vibrational stales of three excited electronic slates. The two lines (>n the left repre.sent the first (.V,) and second (.S>) electronic. singlet slates. The one on the righi (Tj) represents the energy of the first electronic triplet state. As is normally the case, the energy... 

Molecules excited to electronic slates. V, and, V. rapidly lose any excess vibrational energy and relax to Ihc ground vibrational level of that electronic stale. This nonradialional process is lerined vibraiional rclaxulinn. 

It was staled that at room lomperalure (2S°C) the majority of molecules are in the ground vibrational energy level (v -= 0). 

Tabic 6-5. Comparison of (he aK vibrational modes in the ground and excited states. The totally symmetric vibrations of the ground stale measured in tire Raman spectrum excited in pre-resonance conditions 3S] and in the fluorescence spectrum ]62 ate compared with the results of ab initio calculations [131- The corresponding vibrations in the excited stale arc measured in die absorption spectrum. 

One is familiar with the idea of discrete and definite electronic stales in molecules, as revealed by molecular spectroscopy. Each electronic stale possesses a number of vibrational states that are occupied to a great extent near the ground state at normal temperatures. Each vibrational state has, if the stcric conditions are enabling, a number of rotational states associated with it, and for gas molecules both the vibrational and the rotational states can easily be observed and measured spectroscopically. Correspondingly, the distribution of the vibrational states in solids (phonon spectra) is easily measurable. 

Figure 2.2-1 Observation of the excitation of a vibrational state in the electronic ground stale 5o, by a, infrared absorption b, Ramtin scattering c, inelastic neutron scattering d, fliiorescence.

Figurc 6-24 shows that Ihe energy difference between the ground slate and an electronically excited state is large relative to Ihe energy differences betw cen vibrational levels in a given electronic stale (typically, the two differ by a factor of 10 to 100).

Photolysis at 3471 A yields two ground state atoms from the state ri( 1 ). Capelle et al. (186) have measured fluorescence lifetimes and quenching cross sections of vibrational levels from a = 1 to 31 of the B n(0, ) stale of Brj molecules. The lifetimes vary from 1.3 (a = 27) to 0.14 /rsec (a = 17). Lifetimes must be much shorter than the radiative life since in this absorption region (5130 to 6260 A) photodissociation is predominant (571). 

Fig. 2.3. Configurational coordinate diagram (see also text). The ground state (g) has the equilibrium distance Ro the vibrational states v = 0, 1,2 are shown. The exeiied stale (e) has the equilibrium distance R(/ the vibrational states v = 0, 1,2 are shown. The parabola offset is AR(= R,> -R j)...

By using broad-band emission as depicted in Figure 3.1, it is possible to make a tunable laser. This is a four-level laser (level 1 is the lowest vibrational level of the ground state parabola, level 2 are the vibrational levels of the excited state reached after optical absorption, level 3 is the lowest vibrational level of the excited state parabola, and level 4 is one of the vibrational levels of the ground stale parabola reached after emission). Tunability is achieved by selecting level 4. 

---