Ground electronic state lifetimes

Just as above, we can derive expressions for any fluorescence lifetime for any number of pathways. In this chapter we limit our discussion to cases where the excited molecules have relaxed to their lowest excited-state vibrational level by internal conversion (ic) before pursuing any other de-excitation pathway (see the Perrin-Jablonski diagram in Fig. 1.4). This means we do not consider coherent effects whereby the molecule decays, or transfers energy, from a higher excited state, or from a non-Boltzmann distribution of vibrational levels, before coming to steady-state equilibrium in its ground electronic state (see Section 1.2.2). Internal conversion only takes a few picoseconds, or less [82-84, 106]. In the case of incoherent decay, the method of excitation does not play a role in the decay by any of the pathways from the excited state the excitation scheme is only peculiar to the method we choose to measure the fluorescence (Sections 1.7-1.11). 

Figure 10. Excited-state population of ethylene as a function of time in femtoseconds (full line). (Results are averaged over 10 initial basis functions selected from the Wigner distribution for the ground state in the harmonic approximation.) Quenching to the ground electronic state begins approximately 50 fs after the electronic excitation, and a Gaussian fit to the AIMS data (dashed line) predicts an excited-state lifetime of 180 fs. (Figure adapted from Ref. 214.)...

A chemical interconversion requiring an intermediate stationary Hamiltonian means that the direct passage from states of a Hamiltonian Hc(i) to quantum states related to Hc(j) has zero probability. The intermediate stationary Hamiltonian Hc(ij) has no ground electronic state. All its quantum states have a finite lifetime in presence of an electromagnetic field. These levels can be accessed from particular molecular species referred to as active precursor and successor complexes (APC and ASC). All these states are accessible since they all belong to the spectra of the total Hamiltonian, so that as soon as those quantum states in the active precursor (successor) complex that have a non zero electric transition moment matrix element with a quantum state of Hc(ij) these latter states will necessarily be populated. The rate at which they are populated is another problem (see below). 

All the methods used to evaporate metals for atom synthesis were developed originally for the deposition of thin metal films. The more important of these techniques are shown schematically in Fig. la-d. Most of the evaporation devices can be scaled to give amounts of metal ranging from a few milligrams per hour for spectroscopic studies to 1-50 gm/hour for preparative synthetic purposes. Evaporation of metals from heated crucibles, boats, or wires (Fig. la-c) generally gives metal atoms in their ground electronic state. Electronic excitation of atoms is possible when metals are vaporized from arcs, by electron bombardment, or with a laser beam (Fig. Id). The lifetime of the excited states of... 

Phosphorescence corresponds to a different relaxation process. After the absorption phase, corresponding to the transfer of one electron into the Si level (singlet state), a spin inversion can occur if vibrational relaxation is slow, leading the electron to a T, state (triplet state) that is slightly more stable. Flence, return to the ground electronic state will be slower because it involves another spin inversion for this electron. For this reason, radiative lifetimes for phosphorescence can be up to 108 times greater than for fluorescence. 

Depending on the kind of the intermediate molecular ion, all resonance processes can be divided into two groups.116 The first group is the so-called shape resonances, where the electron is trapped in a potential well formed in the ground electron state of the molecule by centrifugal or polarization forces. The lifetime of such states is between 10 15 and 10 s. 

Figure 13 shows the dispersed fluorescence spectra for laser excitation directly into the different vibronic lines labelled a-h in Fig. 12. Under the experimental conditions there is no relaxation from these excited vibronic levels within the 20 ns fluorescence lifetime. Figure 12 thus contains the A —> E transitions from the lowest vibrational level of the ground electronic state, and Fig. 13 contains the A E transitions for a number of initial vibronic E levels to the numerous vibrational levels of the ground electronic state.

Fig. 13.1 Relaxation in the X X (ground electronic state) and A n (excite electronic state) vibrational manifolds of the CN radical in Ne host matrix at T = 4 K, following excitation into the third vibrational level ofthe If state. Populations in individual vibrational levels in both electronic states are monitored independently by fluorescence (for the If state) and by laser induced fluorescence (for the X state). The preferred relaxation pathway for energies above the origin of the Tt state is found to be medium assisted internal conversion as indicated by arrows in the left panel. The right panel shows the dynamics of population and subsequent decays of the vibrational levels 6, 5, 4, and 3 of the ground X state. Levels 6 and 5 relax much faster (lifetimes in the order of 1-3 /xs) than levels 4 and 3 (lifetimes in the ms range). For the latter the internal conversion-assisted pathway is closed as seen in the state diagram on the left, so these long lifetimes correspond to pure vibrational transitions. (From V. E. Bondybey and A. Nitzan, Phys. Rev. Lett. 38, 889 (1977).)...

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