Low-Temperature Excitation

In this section we continue to explore the consequences of the existence of the low temperature excitations in amorphous substances, which, as argued in Section III, are really resonances that arise from residual molecular motions otherwise representative of the molecular rearrangements in the material at the temperature of vitrification. We were able to see why these degrees of freedom should exist in glasses and explain their number density and the nearly flat energy spectrum, as well as the universal nature of phonon scattering off these excitations at low T < 1 K). 

Fig. 14. Origin region of the low temperature excitation (luminescence monitored broad band below 20 200 cm 1) and luminescence (excitation at 457.9 nm with an Ar laser) spectra of [lr(ppy)2bpy]PF5 in [Rh(ppy)2bpy] PF6. M, C and D label electronic origins (from Ref. [45])...

Bai et al. (2005) observed a phonon sideband with a frequency shift of 40-50 cm-1 located on the low-energy side of the 5Do <- 7Fo zero-phonon line (ZPL) in the 77 K excitation spectrum of Eu3+ Y203 NTs and NWs. However, vibronic sidebands generally appears at the high-energy side of the ZPL in the low-temperature excitation spectra since the vibronic transition involving the creation of a phonon with the annihilation of a photon is much more favored than the annihilation of a phonon at low temperature. The origin of this anomaly sideband remains unknown. 

One should carefully note the statement and its qualifications Nothing is said about S approaching a value of zero. The reference to energy attends to the fact that at low temperatures excited configurations may accidentally be frozen in. When considering the state of lowest energy we deal with the most stable configuration. For it, the entropy at the absolute zero of temperature has the lowest possible value and is independent of all z or n. 

Similar low-temperature excitations, associated with spin-flips for those ions which have 0 w/2, can be expected in many amorphous alloys where the exchange interaction between the rare earth moments is relatively weak and where the rare... 

Fig. 3, Schematic picture of the different processes occurring in B850 at low temperature excitation into the B850 band creates an initial, nonselective population on exciton levels (1) this population relaxes through the exciton band in about 100 fs (2) the lowest exciton states are mixed with charge-transfer states and these states are populated by means of a slower process occurring within 0.6 ps (3) due to polaron formation, slow motion along the relaxation coordinate takes place on a time scale of about 10 ps (4) stimulated emission from the polaron states is seen as a new band in the red part of TAS (5). At room temperature, thermal excitations do not allow population of charge-transfer states for a sufficient time to relax the population along the relaxation coordinate hence, only stimulated emission from the lowest exciton states is observed (6).

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