Phonon lines

Recoilless Optical Absorption in Alkali Halides. Recently Fitchen et al (JO) have observed zero phonon transitions of color centers in the alkali halides using optical absorption techniques. They have measured the temperature dependence of the intensity of the zero phonon line, and from this have determined the characteristic temperatures for the process. In contrast to the Mossbauer results, they have found characteristic temperatures not too different from the alkali halide Debye temperatures. 

We must recall that that, according to Equation (5.31), the full absorption intensity (the area under the absorption band) is independent of S. Thus, the factor e (which corresponds to /o o) represents the fraction of the absorption intensity taken by the zero-phonon line, while the intensity /o i = e x S represents the fractional intensity related to the 0 1 transition, Iq 2 = x (5 /2) the fractional intensity of the 0 2 transition, and so on. 

EXAMPLE 5.5 Sketch the absorption and emission spectra at OK for bands with zero-phonon line at 600 nm, a coupling with an unique breathing mode of energy 200 cm and a Huang-Rhys parameter ofS = l. 

Figure 5.13 Simulated absorption and emission spectra (at 0 K) for bands with zero-phonon line at 16 666 cm (600 nm), S = 1, and coupling with a phonon of 200 cm . ...

A certain transition metal ion presents two optical absorption bands in a host crystal whose zero-phonon lines are at 600 nm and 700 nm, respectively. The former band has a Huang-Rhys parameter 5 = 4, while for the latter 5 = 0. Assuming coupling with a phonon of 300 cm for the two bands (a) display the 0 K absorption spectrum (absorption versus wavelength) for such a transition metal ion (b) display the emission spectra that you expect to obtain nnder excitation in both absorption bands and (c) explain how you expect these two bands to be affected by a temperature increase. 

A host material is activated with a certain concentration of Ti + ions. The Huang-Rhys parameter for the absorption band of these ions is 5 = 3 and the electronic levels couple with phonons of 150 cm . (a) If the zero-phonon line is at 522 nm, display the 0 K absorption spectrum (optical density versus wavelength) for a sample with an optical density of 0.3 at this wavelength, (b) If this sample is illuminated with the 514 nm line of a 1 mW Ar+ CW laser, estimate the laser power after the beam has crossed the sample, (c) Determine the peak wavelength of the 0 K emission spectrum, (d) If the quantum efficiency is 0.8, determine the power emitted as spontaneons emission. 

Currently worldwide six conventional (generic INll type) high resolution instruments are in operation and one is under construction. Two zero-field instruments that are suitable for polymer investigation are available. In addition, several installations for other purposes such as large angle scattering or phonon line width analysis exist (see Table 2.1 for more details). 

Our study of time-resolved luminescence of diamonds revealed similar behavior (Panczer et al. 2000). Short-decay spectra usually contain N3 luminescence centers (Fig. 4.71d 5.69a,b) with decay time of r = 30-40 ns. Despite such extremely short decay, sometimes the long-delay spectra of the same samples are characterized by zero-phonon lines, which are very close in energy to those in N3 centers. At 77 K Aex = 308 nm excitation decay curve may be adjusted to a sum of two exponents of ti = 4.2 ps and i2 = 38.7 ps (Fig. 5.69c), while at 300 K only the shorter component remains. Under Aex = 384 nm excitation an even longer decay component of 13 = 870 ps may appear (Fig. 5.69d). The first type of long leaved luminescence may be ascribed to the 2.96 eV center, while the second type of delayed N3 luminescence is ascribed to the presence of two metastable states identified as quarfef levels af fhe N3 cenfer. 

In our study we found that H3, H4, S2 and S3 centers are characterized by relatively broad bands with A ax at 520-545 nm, sometimes accompanied by very weak zero-phonon lines at 489 and 523 nm (S2), 498 (S3) and 503 (H3) nm. It is very difficult to distinguish between the centers of this group, especially when they present together. Under pulse laser excitation the decay time differences enable more definite recognition. Different decay components in the green part of the spectrum allow us to establish the presence of H3 (12 ps) and S3 (126 and 213 ps) centers. These broad bands are sometimes accompanied by narrow lines of GR1 center at 794 nm and by system at 700 and 788 nm (Bokii et al. 1986 Davies 1994). The relatively broad fine at 463 nm with a decay time of 312 ps appears which is not described in the hterature (Fig. 4.72). 

The photoluminescence (PL) spectrum in Figure 1.7 shows a number of lines related to nitrogen-bound excitons and free excitons. SiC has an indirect bandgap, thus the exciton-related luminescence is often assisted by a phonon. Bound exciton luminescence without phonon assistance can, however, occur because conservation in momentum can be accomplished with the help of the core or the nucleus of the nitrogen atom. That is why the zero phonon lines of the nitrogen atom are seen, denoted and Q , in the spectrum but not the zero phonon line of the free exciton. 

Chen, Y., and W. A. Sibley, 1969. A study of zero phonon lines in electron-irradiated, neutron-irradiated, and additively colored MgO, Philos. Mag., 20, 217-223. 

Fig. 7. Crossing" of a polariton branch with a second-order phonon line in K3Cu(CN)4. The scattering angles are those inside the sample 31)...

The intraconfigurational transitions of the rare earth ions (4/n) are examples of ions which, even in solids, show sharp lines in their spectra. The width is of the order of wavenumbers and is at 4.2 K usually determined by inhomogeneous broadening. These lines are true zero-phonon lines. The vibronic transitions belonging to these lines are weak and often overlooked. 

If this interaction is very weak, the zero-phonon line dominates in the spectrum (like in the rare earth ions). If the interaction is very strong, the spectra contain only broad bands from which not much information can be obtained. These situations are known as the weak- and strong-coupling case, respectively. Vibrational structure of any importance is usually only observed for the intermediate-coupling case. This is, for example, encountered for transition metal ions and uranate complexes. 

Fig. 4. The emission and excitation spectra of the luminescence of Cs2NaYCl6 Bi3+ at 5 K. The zero-phonon line (0-0) and the progression in the v, mode are indicated at the top. See also Table 1. After A.C. van der Steen, thesis, Utrecht (1980)...

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