Spectral hole width

After observing quite a few anomalous properties of optical transitions in glasses and attributing them to the dynamics of TLS [14], the tunneling model was adopted by Reinecke [15] to explain the low-temperature line widths of optical transitions in amorphous solids using the concept of spectral diffusion. This concept had originally been developed for the description of spin resonance experiments [16] and had already been applied to the theoretical treatment of the above mentioned ultrasonic properties of glasses [17]. Soon after this step, the possibility of a connection between thermal and optical properties of amorphous solids was supported by the observation of time dependence of spectral hole widths [18]. 

Meltzer et al. (2001) showed direct evidence for long-range interactions between lanthanide impurity ions in embedded nanocrystals with the TLS s of the matrix based on hole burning data. The samples were prepared as follows monoclinic 23 run Y2C>3 0.1% Eu3+ nanocrystals, produced by condensation after laser evaporation, were dispersed in a siloxane polymer. Fig. 13 shows the temperature dependence of the width of spectral holes (khb), which were burned in the 5Do <- 7Fo transition of Eu3+ ions located on the c sites (582.8 nm)innanociys-tals embedded in polymer, in comparison with the case of the free, as-prepared nanocrystals having the same size. The embedded nanocrystals exhibit a drastic increase in hole width and a very different power-law behavior for the temperature dependence of the hole widths relative to similar isolated nanoparticles. The 7 3 temperature dependence of the hole width... 

The population and reorientational dynamics are not indicated in the figure, but may be also derived from the pump-probe measurements. The population lifetime Ti in the first excited level can be inferred from the excited state absorption monitoring (lie v = I v = 2 transition. The molecular reorientation becomes experimentally accessible, introducing polarization resolution in the probing step. For known structural and population dynamics the temporal evolution of the width of the observed spectral hole also provides information on the dephasing time T2 of the vibrational transition (63). 

The results for various alcohol solutions are compiled in Table 2. The data refer to room temperature except for methanol in C5CI6, which was measured at 333 K. A few spectroscopic parameters are shown in the first four lines. There is evidence for inhomogeneous broadening of the monomeric OH-stretching vibration for the various investigated alcohols with a width of the observed spectral holes as small as 50% of the conventional absorption band (DMEP). The anharmonic shift of the OH mode is indicated in the table to amount to 170 cm-1, independent of solute or solvent within experimental accuracy. The large anharmonicity of the monomeric alcohols is noteworthy. The population lifetime Ti of the mode... 

Similar experiments have also been performed on a higher concentrated mixture of 1.2 M ethanol and CCI4. Again spectral holes could be identified from the transient spectra with a width of 25 cm-1 and lifetime of 1 ps. From transient spectra taken for different excitation frequencies in the OH band, evidence for a faster hole relaxation with increasing red shift (bond strength) is inferred, which is accompanied by the differences in the temporal evolution of the isotropic Gaussian component related to level 2. 

The widths of the spectral holes are comparable to the ones determined for ethanol solutions (78). For the latter samples we infer from model computations a size of the oligomers directly excited by the pump pulse of 4-6 (87), while in the isotopic mixture even longer associates may interact with the pump. Depending on the excitation frequency, a protonated species within an associate of 5-10 d6-ethanol molecules is estimated to be... 

Unie resolved ground state hole spectra of cresyl violet in acetonitrile, methanol, and ethanol at room temperature have been measured in subpicosecond to picosecond time region. The time correlation function of the solvent relaxation expressed by the hole width was obtained. The main part of the correlation function decayed much slower compared with that of the reported correlation function observed in time dependent fluorescence Stokes shift. Some possible mechanisms are proposed for understanding of the time depencences of the spectral broadening under the condition with the distribution of the relaxation times in fluid solution based on the entropy term in the solvent orientation as well as the site dependent response of the solvent. 

Our first steps toward the single-molecule regime arose from work at IBM Research in the early 1980s on persistent spectral hole-burning effects in the optical transitions of impurities in solids (for a review, see [20]). Briefly, if a molecule with a strong zero-phonon transition and minimal Franck-Condon distortion is doped into a solid and cooled to liquid helium temperatures, the optical absorption becomes inhomogeneously broadened (Fig. 2.2A). The width of the lowest electronic transition for any one molecule (homogeneous width, Yjj) becomes very small because few phonons are present, while at the... 

The temperature dependence of hole formation and hole profile is affected by four factors decrease in the Debye-Waller factor, broadening of the hole width, spectral diffusion, and laser-induced hole filling. The first two effects are reversible phenomena and recover at low temperatures. The latter two are irreversible and their influence cannot be eliminated by cooling the sample again. The temperature dependence of the Debye-Waller factor (DiV(T) — S0(T)/S 4)) for TPP/PMMA and TPP/phenoxy resin systems, shown in Table 2.13 by a dotted line, agrees well with the slope of 0 at 4-20 K. The temperature dependence of the Debye-Waller factor is smaller in poly(vinyl alcohol), which shows a higher Es value (23 cm4). Thus, hole formation efficiency is controlled by the temperature dependence of Debye-Waller factor for temperatures below T and, for temperatures above T it is affected mainly by the simultaneous occurrence of spectral diffusion and laser-induced hole filling due to structural relaxation. 

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