Vibronic progression

The b-polarized absorption bands (see Fig. 6-3b) at 20945 cm 1, 22250 cm-1 and 23535 cm-1 are assigned to a vibronic progression built on the bu Davydov component with the totally symmetric mode at 1275 cm-1. 

The absoiption spectra of these three materials are shown in the bottom panel of Figure 9-16. From these spectra it becomes clear that the m-LPPP shows the longest effective conjugation length 23 the best resolution of vibronic progression, and the steepest onset of absorption [231. Therefore, one would assume the m-LPPP to be a material of the highest chemical definition. This is indeed con-... 

Two Raman lines, either of which may be the Cr—Cr mode, have been observed at 340 and 400 cm-1 for [Cr2(mhp)4]. The former is preferred197 since the vibronic progression is 320 cm-1, and this is lower than earlier proposed.1 9 Others believe that this is a Cr—O or Cr—N totally symmetric stretch.198 The 22500 cm-1 band and the shoulder at 29 600 cm-1 are metal-dependent since they are intensified, and move together and to lower frequency from Cr to Mo to W. The shoulder at 29 600 cm-1 is assigned to the d—>x (Fli— 1E) transition.197... 

A second feature of note is the weak spectroscopic feature consisting of a vibronic progression in the low frequency mode built on one quantum of the high frequency mode (Figs. 3b and 3c). Vibronic bands from the high frequency... 

The physical insight obtained from the time-domain point of view allows simple qualitative predictions about the widths of the progressions to be made. Two potential surfaces (such as the coupled and uncoupled surfaces in Fig. 2) can be compared in terms of the slope of steepest descent at the position of the initial wavepacket at t = 0. The steeper the slope, the faster the initial decrease of < (j) 10(t) > in the time domain and the broader the spectrum in the frequency domain. This insight is important in developing a strategy to fit experimental spectra. In the example above, a vibronic progression in an experimental spectrum that is broader than that expected from a harmonic potential surface (Fig. 3a) requires that the wavepacket be displaced in the positive Qx direction in the coupled potential surface. 

The simplest form of coupling between two normal coordinates is linear, i.e. (k y)QxQy This form of the coupling is not able to account for the quantitative vibronic intensities in the emission spectrum of Me2Au(hfacac). However, spectra resulting from linearly-coupled potential surfaces also show series of vibronic progressions that are not replicas of each other. A brief examination of the effects of linear coupling are included here in order to illustrate what spectral features can arise. 

The peaks of a vibronic progression will only be resolved when 2Avi/2 < Avh when 2Avi/2 > hv, the vibronic components will be unresolved, but the spectral band may not have a Gaussian shape. 

Figure 1.14. The 260-nm absorption band of benzene (by permission from Callomon et al., 1966). The vibronic progression based on the totally symmetric breathing mode v = 920 cm is labeled A,. Aj,. . . , and the hot band is marked H.

The excited state properties of trans-W(N2)2(dppe)2 were the focus of several recent spectroscopic [57-59] and photochemical studies [60-64]. The lowest energy excited state was assigned to a spin forbidden transition from a metal to dppe charge transfer (MLCT) excited state. All of the reported studies of the luminescence were carried out in glassy media (2-MeTHF). At temperatures between 8 and 80K, the spectra contain one apparent vibronic progression with a spacing of about 500cm [59]. 

The Missing Mode Effect (MIME) is a regularly spaced vibronic progression in the luminescence spectrum which does not correspond to any ground state normal mode vibration. How can the regularly spaced bands in the emission spectrum not correspond to a normal mode which is displaced in the excited state We call this effect the Missing Mode Effect or MIME because the mode which appears to be present in the emission spectrum is actually missing from the molecule. 

The fluorescence spectrum of the Chlorella chloroplast is relatively flat between 715 and 740 nm compared with that of Zea mays, which often shows an increase in fluorescence intensity from 715 to 740 nm. The situation suggests that the far-red component of the Zea mays is substantially influenced by PSI whilst the far-red component ofthe Chlorella is dominated by a vibronic progression ofthe chlorophyll fluorescence of PSII. It is then reasonable that the red and far-red fluorescence images are relatively similar to one another. 

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