Absorption multiphonon

In Figure 5.16(a), the maximum of the absorption spectrum (at 0 K) corresponds to the line AB, the maximum overlap of the vibrational wavefunctions. This transition terminates in the vibrational level corresponding to point B, which is below the crossover point, X. This proces s is followed by a fast down-relaxation by multiphonon emission to the point C, from which the emission originates. Thus, the emission spectrum has its maximum at an energy corresponding to the line CD. Finally, another multiphonon emission process takes place by down-relaxation from D to the departing point A. 

When the frequency of a laser falls fully into an absorption band, multiple phonon processes start to appear. Leite et al 2° ) observed /7 h order ( = 1, 2. 9) Raman scattering in CdS under conditions of resonance between the laser frequency and the band gap or the associated exciton states. The scattered light spectrum shows a mixture of fluorescent emission and Raman scattering. Klein and Porto 207) associated the multiphonon resonance Raman effect with the fluorescent emission spectrum, and suggested a possible theoretical approach to this effect. 

G. Spitzer, Multiphonon Lattice Absorption D. L. Slierwall and R F. Potter, Emittance Studies H- R- Philipp and H. Ehrenveich, Ultraviolet Optical Properties M. Cardona, Optical Absorption above the Fundamental Edge . J. Johnson, Absorption near the Fundamental Edge... 

Bendow, B., 1978. Multiphonon infrared absorption in the highly transparent frequency regime of solids, Solid State Phys., 33, 249-316. 

For ZBLAN, the IR edge is in the 5-8 m range with a 50% transmission located at 7.1 /on (1400 cm-1) for a 2 mm-thick sample. This is to be compared with silica glass whose transparency starts decreasing at 3 pm. The IR edge is due to multiphonon absorption processes related to the fundamental vibration frequencies... 

One of the main spectroscopic properties that differentiate fluoride glasses from silica-based glasses is the low multiphonon emission rate. These non-radiative relaxations that may strongly compete with radiative processes in rare-earth ions are nearly three orders of magnitude lower in ZBLAN glass than in silicate, as shown in Fig. 2. This property is directly related to the fundamental vibration modes of the host and, therefore, varies basically in the same manner as the infrared absorption edge. 

The energies of thermal emission, observed by DLTS and by optical absorption and luminescence agree to within 0.1 eV. The Stokes shift is therefore very small and unable to account for the capture cross-section within the multiphonon model. The mechanism of non-radiative capture at defects remains puzzling. 

Figure 11.02 Absorption and scattering losses for a range of optical materials plotted on a log-log scale which gives straight tines for Rayleigh scattering but a slight curvature to the multiphonon edge (After France ct al., 1986)...

To obtain further insight into the meaning of the inelastic neutron spectra, it is necessary to have specific theoretical models with which to compare the experimental results. In the harmonic approximation it is possible to calculate the incoherent inelastic neutron spectrum i.e., the neutron scattering cross section for the absorption or emission of a specific number of phonons can be obtained with the exact formulation of Zemach and Glauber.481 A full multiphonon inelastic spectrum can be evaluated by use of Fourier transform techniques.482 The availability of the normal-mode analysis for the BPTI136 has made possible detailed one-phonon calculations483 for this system the one-phonon spectrum arises from transitions between adjacent vibrational levels and is the dominant contribution to the scattering at low frequencies for typical experimental conditions.483 The calculated one-phonon neutron en-... 

Various broadband sources employed to optically pump Ha include tungsten, mercury, xenon, and krypton lamps. The last source provides an especially good spectral match to the near-infrared absorption bands of Nd3+ in YAG. To reduce lattice heating resulting from the multiphonon emission decay to the F3/2 state, semiconductor diodes and laser sources at 0.8-0.9 ym nave pumped Nd lasers (58). Sun-pumped Nd and chromium-sensitized Nd lasers have been demonstrated and considered for space applications (59). Lasing of Nd3+ by electron beam excitation has also been reported (bO). 

Ptutoyujum. (Am +). The energy level scheme and possible lasing transitions for Pu + are very similar to those of Np +. Prospective transitions include 6Hg/2+6H5/2, 9/2 7/2, and h7/2 Hc/2 For efficient fluorescence and laser action from either tne °Hg/2 or j/2 states, hosts should have low phonon frequencies to reduce nonradiative decay by multiphonon processes. Depending upon the host and the exact positions of higher-lying states, excited-state absorption may reduce or prevent net gain. 

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