Lasers radiative deactivation

The possibility of deactivation of vibrationally excited molecules by spontaneous radiation is always present for infrared-active vibrational modes, but this is usually much slower than collisional deactivation and plays no significant role (this is obviously not the case for infrared gas lasers). CO is a particular exception in possessing an infrared-active vibration of high frequency (2144 cm-1). The probability of spontaneous emission depends on the cube of the frequency, so that the radiative life decreases as the third power of the frequency, and is, of course, independent of both pressure and temperature the collisional life, in contrast, increases exponentially with the frequency. Reference to the vibrational relaxation times given in Table 2, where CO has the highest vibrational frequency and shortest radiative lifetime of the polar molecules listed, shows that most vibrational relaxation times are much shorter than the 3 x 104 /isec radiative lifetime of CO. For CO itself radiative deactivation only becomes important at lower temperatures, where collisional deactivation is very slow indeed, and the specific heat contribution of vibrational energy is infinitesimal. Radiative processes do play an important role in reactions in the upper atmosphere, where collision rates are extremely slow. 

The reaction dynamics of few excited complexes are known however, the opportunities provided by pulsed lasers promise to make this research area one of major emphasis of mechanistic studies. Such methods are necessary because few transition-metal complexes exist as electronically excited states in RT solutions with lifetimes exceeding 1 fjis, and many are shorter lived. Several competing processes lead to ES decay nonra-diative deactivation to the ground state (GS), radiative deactivation (i.e., emission) to the GS, unimolecular reaction to products (such as ligand substitutions or redox decomposition) or bimolecular electron transfer or energy transfer with another species, Q, in solution. These processes are indicated in Eqs. (a)-(e) for a hypothetical complex [MLJ" + ... 

I 1.6.3. Quasi-Stationary Inversion in Collisional Gas-Discharge Lasers, Excitation by Long-Lifetime Particles and Radiative Deactivation... 

Plasma He-Cd lasers operate at high pressures with inversion based on radiative deactivation of the lower working level. When excitation of plasma laser is provided by electron beams or by charged products of nuclear reactions, effective lasing has been achieved on the following electronic transitions of the singly charged cadmium ions >5/2 ... 

Fluorescence lifetime studies have been also performed in the NdAl2 Cl6,H 3 vapor complex(es) (Krupke 1976b, Jacobs et al. 1977). The Nd F /2 state was populated via a rapid non radiative process subsequent to optical excitation at 531 nm performed by means of a Nd rYAG laser. The authors then monitored fluorescence intensities from the Nd + F3/2 —> I9/2 and F3/2 —> " ll 1/2 transitions and determined fluorescence lifetimes they discussed several mechanisms accounting for non-radiative deactivation and for the observed temperature dependence of the measured radiative lifetimes. 

The effective lifetimes of all these excited states are determined by radiative as well as collisional deactivation, and which contribution is the more significant depends on pressure and transition probability. The simultaneous recording of the absorption and fluorescence spectra yields information about the ratio of radiative to collisioninduced nonradiative decays. This ratio is proportional to the quotient of total fluorescence from the excited level to total absorbed laser light. Such experiments have been started by Ronn oif... 

The extremely narrowband emission of a laser allows the specific excitation of molecular states. The non-Boltzmann distribution produced by the excitation process is quickly destroyed by radiation processes and collisional deactivation. The relative contribution of these different deactivation channels depends on the nature of the level excited as shown in Fig. 3. In the microwave region where rotational levels are excited, the radiative life time is very long compared to the very efficient rotational relaxation processes (R—R rotation—rotation transfer and R—T rotation—translation transfer). Therefore, the absorbed radiation energy is transformed within a few gas kinetic collisions into translational energy. The situation is similar for... 

The following estimation illustrates under which conditions this maximum ion rate can be realized Typical cross sections for photoionization are Oki 10 cm. If radiative decay is the only deactivation mechanism of the excited level k), we have Rk = Ak 10 s In order to achieve nLjCTH > Ak, we need a photon flux nLj > 10 cm s of the ionizing laser. With pulsed lasers this condition can be met readily. 

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