Spectroscopy for Materials Characterization. Группа авторов
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In agreement with Figure 2.2, they represent the transitions from the lower vibrational level in the excited electronic state to the first and second vibrational levels in the ground electronic state, respectively. The measured values, therefore, identify the fundamental, ω g , and the overtone, 2ω g , frequencies of the nearly equally spaced vibrational levels of the surface‐nonbridging oxygen in the electronic ground state. We note that these sidebands are more and more wider than the ZPL, their FWHM being ∼20 and 40 cm−1 for the first and the second line, respectively; this spreading is due to an inhomogeneous distribution of the vibrational frequency of centers having the same ZPL. From these emission spectra, we also measure the ratio between the integrated intensities of ZPL and the first and second vibrational lines: I 0L /I 1L = 13.5 ± 0.5, I 0L /I 2L = 160 ± 30, the error being mainly due to the inaccuracy in the reference line subtraction to account for the overlapping with nonselectively excited luminescence. Based on the linear electron–phonon coupling, subsequently evidenced by the equal values of vibration frequency both in the ground and in the excited state, we compare these values with the Poisson’s distribution, I kL = exp(−S) L × (S L ) k /k! and extract the partial Huang–Rhys factor S L : I 0L /I 1L = 1/S L yields S L = 0.074 ± 0.003 and I 0L /I 2L = 2/(S L )2 yields S L = 0.11 ± 0.01. We note that the difference between the values of S is larger than the experimental uncertainty; despite this incongruence, these results quantify the low coupling of the electronic transition with the local Si─O• stretching mode, namely, the nearly absent relaxation of the Si─O• bond after excitation.
Figure 2.12 Time‐resolved PL spectrum of the surface‐NBOHC (
We now return to the experimental results on the (
Inhomogeneous broadening measured by ZPL distribution: Finally, we report the study of the inhomogeneous properties of NBOHC at the surface of silica taking advantage of time‐resolved experiments being able to detect ZPL under tunable laser excitation [29]. Figure 2.13a shows a series of time‐resolved emission spectra measured at T = 10 K with the excitation energy stepwise incremented from 1.887 to 2.077 eV (minimum step 0.003 eV); each spectrum is displayed in the vicinity of the excitation energy thus evidencing the ZPL. From these spectra, we plot in Figure 2.13b the distribution of the ZPL intensity. The experimental data are best fitted by a Gaussian curve centered at 1.995 ± 0.003 eV with FWHM of 0.042 ± 0.005 eV (340 ± 40 cm−1) that represents the inhomogeneous distribution w inh(E 00) of the electronic transitions, due to the different local environment surrounding the (
Figure 2.13 Panel (a): Time‐resolved PL spectra of surface‐NBOHC (
We observe that the main experimental outcome is the detectability of the ZPL under site‐selective excitation of inhomogeneously distributed centers, thus allowing the inhomogeneous curve to be drawn directly. The detection of the ZPL is therefore a probe of the silica structure near the NBOHC; this potential is precluded for other defects in silica, because of the stronger phonon coupling of their optical transitions. In those cases, the deconvolution between homogeneous and inhomogeneous broadening can be done only indirectly.
References
1 1 Herzberg, G. (1966). Molecular Spectra and Molecular Structure. New York: Van Nostrand Reinhold.
2 2 Rebane, K.K. (1970). Impurity Spectra of Solids. New York: Plenum Press.
3 3 Stoneham, A.M.