Vibronic sidebands

Alexandrite, the common name for Cr-doped chrysoberyl, is a laser material capable of continuously tunable laser output in the 700-800 nm region. It was established that alexandrite is an intermediate crystal field matrix, thus the non-phonon emitting state is coupled to the 72 relaxed state and behaves as a storage level for the latter. The laser-emitted light is strongly polarized due to its biaxial structure and is characterized by a decay time of 260 ps (Fabeni et al. 1991 Schepler 1984 Suchoki et al. 2002). Two pairs of sharp i -lines are detected connected with Cr " in two different structural positions the first near 680 nm with a decay time of approximately 330 ps is connected with mirror site fluorescence and the second at 690 nm with a much longer decay of approximately 44 ms is connected with inversion symmetry sites (Powell et al. 1985). The group of narrow lines between 640 and 660 nm was connected with an anti-Stokes vibronic sideband of the mirror site fluorescence. 

The photoconductivity and absorption spectra of the multilayer polydiacetylene are shown in Fig. 22 [150]. The continuous and dotted line relate to the blue and red polymer forms respectively. Interpretation is given in terms of a valence to conduction band transition which is buried under the vibronic sidebands of the dominant exciton transition. The associated absorption coefficient follows a law which indicates either an indirect transition or a direct transition between non-parabolic bands. The gap energies are 2.5 eV and 2.6 eV for the two different forms. The transition is three dimensional indicating finite valence and conduction band dispersion in the direction perpendicular to the polymer chain. 

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. 

The absorption intensity of the 0-1 vibronic sideband is entirely borrowed by HT coupling. The observation that this 0-1 transition is of comparable intensity to the 0-0 transition suggests the feasibility of resonance Raman processes in which both the absorption and emission moments are borrowed. This second-order HT coupling displays itself in the appearance of first overtones and combination bands of non-totally symmetric modes. These are observed extensively in the haem proteins 35). 

A successful instance of empirical correlation was found for the electron-vibrational coupling, between 2Eg and 4A2g-states on Mn4+, entering substitutionally for M in the octahedral sites of Cs2MF6 (M = Si, Ge, Ti, Sn, Zr) and M2SiF6 (M = K, Rb, Cs)121 . The Huang-Rhys number S appears in the intensity of the n th vibronic sideband of a progression as exp (— S)Sn/n . From comparison with his emission data at 80 °K Paulusz found a quadratic dependence of S on the estimated Mn—F distance for both Alg and vibrational modes in the former series of hosts and a virtual constancy for die latter series. 

Abstract A brief review of my work with Carl Ballhausen in 1967-1968 and subsequent work. The assignments of the vibronic sidebands in the emission spectra of chromium ammine complexes are given with some comments on the Jahn-Teller effect in the emissive state. Energy transfer and cross relaxation phenomena are discussed and the shell model for this processes in lanthanide elpasolites is presented... 

If the ZPL is weak, then variable temperature studies can identify its location from hot electronic and vibronic structure, with the latter illustrated in Fig. 10. The vibronic structure can be used to identify certain symmetry types of transition, for example Ti-T2 (Table 4). One viewpoint has been put forward that the spectral interpretation is confused by phonon dispersion in the vibronic sidebands of transitions. However, this complexity of the vibronic structure can in fact be utilized to provide a fingerprint to identify the location of the electronic origin. Where possible, it is more certain to compare both emission and absorption (or excitation) spectra for a particular transition in order to locate the electronic origin. Otherwise, transitions from different emissive states (with different state irreps) can be employed to confirm the symmetries and locations of terminal levels. Whereas other neat systems such as PrCl3 show additional features not present in the electronic spectra of the diluted crystal (LaCl3 Pr3+) due to interactions between... 

Some differences in the vibronic sidebands of Cs2NaLnCl6 and Cs2UBr6 are the sharp peak at 290 cm 1 in the former spectrum (Fig. 4c,d) which corresponds to the flat V mode at special points, and the weak feature at 178 cm 1 due to third shell motion. The low-vibrational-energy shoulder on v3 (at 245 cm-1, Fig. 4c,d and 3 in Fig. 12) is characteristic of Cs2NaLnClg systems and could correspond to rlu modes at special points such as X4 , X5- The assumption has been made that the vibronic sideband is one-pho-... 

The absorption and photoluminescence spectra of both materials are shown in Fig. 12.7 (c,d). The luminescence of p-scxiphcnyl is very similar to the spectrum of the spiro-derivative, only the vibronic sidebands are more pronounced. This shows that aside from the improved morphological stability, the spiro-linkage merely leaves the optical properties of the chromophore unaltered. 

The stabilized E A2 emission at high pressure consists of intense zero phonon lines and vibronic sidebands. A high resolution measurement of the zero phonon portion of the spectrum at 70 K is shown in Fig. 25. The zero phonon lines exhibited a blue shift at low pressure followed by a shift reversal at high pressure. The stabilization of sharp zero phonon lines between 5 kbar and 9 kbar... 

As temperature decreases, the aggregation of the polymers increases. This change in the conformational structure is manifested in a permutation of the absorption spectrum, hence thermochromism [1,11,26,27,92,122-124,180-182]. Thermochromism was clearly shown for poly(3-(2-methyl-l-butoxy)-4-methylthiophene) (PMBMT). A thin film of PMBMT, in the presence of acetone vapor, shows a A ax of 502 nm with vibronic sidebands at 534 and 582 nm. Upon heating above the Tg (50°C), the spectrum blueshifts dramatically afford to a A ax of372 nm. This process was irreversible [ 183]. Thermochromism has also been shown in PATs with chiral substituents. Thin films of poly[3-(S)-2-methylbutylthiophene] show a redshift after heating, indicating a conformational transition that provides a structure with more efficient orbital overlap [90]. 

Polymer photophysics is determined by a series of alternating odd (B ) and even (Ag) parity excited states that correspond to one-photon and two-photon allowed transitions, respectively [23]. Optical excitation into either of these states is followed by subpicosecond nonradiative relaxation to the lowest excited state [90]. This relaxation is due to either vibrational cooling within vibronic sidebands of the same electronic state, or phonon-assisted transitions between two different electronic states. In molecular spectroscopy [146], the latter process is termed internal conversion. Internal conversion is usually the fastest relaxation channel that provides efficient nonradiative transfer from a higher excited state into the lowest excited state of the same spin multiplicity. As a result, the vast majority of molecular systems follow Vavilov-Kasha s rule, stating that FT typically occurs from the lowest excited electronic state and its quantum yield is independent of the excitation wavelength [91]. 

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