Electronic absorption spectra excitation, collisional

While collision-induced transitions in excited electronic states can be monitored through the satellite lines in the fluorescence spectrum (Sect. 8.2.2), inelastic collisional transfer in electronic ground states of molecules can be studied by changes in the absorption spectrum. This technique is particularly advantageous if the radiative lifetimes of the investigated rotational-vibrational levels are so long that fluorescence detection fails because of intensity problems. 

Examples of such continuous absorption and emission line profiles are the optical dye spectra in organic solvents, such as the spectrum of Rho-damine 6G shown in Fig. 3.26, together with a schematic level diagram [3.40]. The optically pumped level Ei is collisionally deactivated by radiationless transitions to the lowest vibrational level Em of the excited electronic state. The fluorescence starts therefore from Em instead of Ei and ends on various vibrational levels of the electronic ground state (Fig. 3.26a). The emission spectrum is therefore shifted to larger wavelengths compared with the absorption spectrum (Fig. 3.26b). 

In the bulk, the low concentration of ground-state pairs excludes their observation by absorption. The formation of the excited-state complex, termed exciplex, is a collisional process electronic excitation of either the acceptor or the donor leads to the formation of a locally excited state (for instance, in hydrocarbon molecules, it is a nn state). During the lifetime of this state, a collision with the other partner (which is in the ground state) leads to the formation of the exciplex. This mechanism is compatible with the fact that the absorption and fluorescence excitation spectra of the system are identical with those obtained by superimposing the spectra of the individual components. At the same time, the fluorescence emission spectrum changes drastically—a broad band, red shifted with respect to the bare molecule s emission spectrum, appears. It is usually devoid of vibrational structure, and is shifted to longer wavelengths as the solvent polarity increases [1],... 

In addition to LIF resonant two-photon ionization (Sect. 1.4) can also be used for the sensitive detection of collision-induced rotational transitions. This method represents an efficient alternative to LIF for those electronic states that do not emit detectable fluorescence because there are no allowed optical transitions into lower states. An illustrative example is the detailed investigation of inelastic collisions between excited N2 molecules and different collision partners [995]. A vibration-rotation level (v, J ) in the a Jig state of N2 is selectively populated by two-photon absorption (Fig. 8.10). The collision-induced transitions to other levels v + An, / + AJ) are monitored by resonant two-photon ionization (REMPI, Sect. 1.2) with a pulsed dye laser. The achievable good signal-to-noise ratio is demonstrated by the collisional satellite spectrum in Fig. 8.10b, where the optically pumped level was v = 2, J = 7). This level is ionized by the P(l) parent line in the spectrum, which has the signal height 7.25 on the scale of Fig. 8.10b. 

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