Becoming trapped in metastable states

Thermoluminescence (TL) measures the variation in intensity of luminescence of a sample which has been irradiated by UV radiation. X-rays, y-rays or an electron beam as a function of temperature. Electrons excited by the impinging radiation become trapped in metastable states at liquid nitrogen temperatures. These electrons recombine with cations during subsequent heating owing to the enhancement of molecular motion in the sample. Luminescence is observed as... 

An additional delay between excitation and emission is often introduced by metastable electron states known as electron traps, which are filled during excitation. Once an electron has become trapped in such a state it requires certain energy to release it but this can be provided thermally. Emission, which follows the emptying of such traps at a fixed temperature, is known as phosphorescence. 

One of the characteristics of a glassy system is that it can become trapped in any of a large number of metastable states. As the film cooled from 1.1 to 0.3 e/ks, the film thickness h decreased, but the area in the x-y plane remained fixed. This anisotropy led to stress in the film. When the stress exceeded a local yield stress, the... 

A typical image plate phosphor is doped BaFBr. When exposed to x-rays, oxidizes to Eu. Thus produced electrons may either recombine with Eu or they become trapped by F-vacancies in the crystal lattice of BaFBr. The trapped electrons may exist in this metastable state for a long time. They are released when exposed to a visible light and emit blue photons during recombination with Eu ions, e.g. see K. Takahashi, K. Khoda, J. Miyahara, Y. Kanemitsu, K. Amitani, and S. Shionoya, Mechanism of photostimulated luminescence in BaFX Eu (X = Cl, Br) phosphors, J. Luminesc. 31-32,266 (1984). 

In 1984, Decurtins et al. discovered that the compound [Fe(ptz)6](BF4)2 (ptz=l-propyltetrazole) can be converted from the stable LS state to the metastable HS state by irradiation with green light at sufficiently low temperatures [14]. This phenomenon has become known as light-induced excited spin state trapping (LIESST) and is dealt with in detail by A. Hauser in a separate chapter in this series. Later, Hauser reported the reverse-LIESST effect, whereby red light is used to convert the compound back into the LS state [15]. 

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