Electronic absorption bands spectrum

Resonance Raman spectroscopy has been applied to studies of polyenes for the following reasons. The Raman spectrum of a sample can be obtained even at a dilute concentration by the enhancement of scattering intensity, when the excitation laser wavelength is within an electronic absorption band of the sample. Raman spectra can give information about the location of dipole forbidden transitions, vibronic activity and structures of electronically excited states. A brief summary of vibronic theory of resonance Raman scattering is described here. 

Figure 7.32 (a) Broadening of an electronic absorption band of a molecule due to an inhomogeneous environment (b) illustration of a laser-induced photochemical hole burned in the 0-0 A, line of free-base porphyrin in -octane at 2K (c) excitation spectrum of the 0-0 lines of the Sj-So transition of the free base in n-hexane, showing a frequency difference ( 100cm" ) between the two tautomeric forms (1) and (2) of the free base in a single type of site. Irradiation into the line A, transforms it into Aj and vice versa (d) hole burned in line A, at 4.2 K. (After Williams, 1983.)... 

Ethylene glycol is a very viscous liquid and the molecule presents two close OH groups. It has to be noticed that, among all the different solvents studied by pulse radiolysis, the transition energy of the solvated electron absorption band is maximum in liquid ethylene glycol. For these reasons, the electron in EG seems to have a special behaviour and it is of great interest to study the dynamics of the formation of equilibrated solvated electron. Within this context, the present communication deals with the dynamics of solvation in EG of electrons produced by photoionisation of the solvent at 263 nm. The formation of solvated electrons is followed by pump-probe transient absorption spectroscopy in the visible spectral range from 425 to 725 nm and also in near IR. For the first time, the absorption spectrum of the precursor of the equilibrated electron is observed in EG. Our results are shortly compared by those obtained in water and methanol. 

Gaussian curves (normal distribution functions) can sometimes be used to describe the shape of the overall envelope of the many vibrationally induced subbands that make up one electronic absorption band, e.g., for the absorption spectrum of the copper-containing blue protein of Pseudomonas (Fig. 23-8) Gaussian bands are appropriate. They permit resolution of the spectrum into components representing individual electronic transitions. Each transition is described by a peak position, height (molar extinction coefficient), and width (as measured at the halfheight, in cm-1). However, most absorption bands of organic compounds are not symmetric but are skewed... 

Electronic Transitions In an electronic transition an electron is excited from an occupied to an empty molecular orbital (M.O.). The energy of such transitions normally corresponds to photons in the near IR, visible or UV region of the electromagnetic spectrum. Electronic absorption bands give rise to the colours of compounds, including ones without transition metals. 

Electronic absorption bands in the spectrum of PrCls (aq) reproduced with permission from S.A. Cotton, Lanthanides and Actinides, Macmillan (1991) p. 30. 

The molecular cross section of the ordinary Raman effect can be considerably enhanced. If the exciting radiation has a higher frequency, the intensity increases basically by the fourth power of the frequency. Moreover, there is a further increase as electronic absorption bands are approached the pre-resonance and resonance Raman effect (Sections 3.6 and 6.1). Further, the so-called surface-enhanced Raman effect (SERS) increases the molecular cross section. Both effects produce an enhancement of several orders of magnitude (Gerrard, 1991) (see Sec. 6.1). However, these two effects have to be carefully adapted to the specific properties of the investigated molecules. Photochemical decomposition and excitation of fluorescence may make it impossible to record a Raman spectrum. The described techniques may thus be of considerable importance for the solution of special problems, but they are by no means routine techniques to be generally used. 

Notable chemical or physical properties of phosphates are usually not due to the phosphate group itself However, the general stability associated with the strength of the P-O and M-O(P) bonds and the inertness of the phosphate group to chemical attack and redox reactions are important in determining the durability of these materials. Furthermore, a lack of electronic absorption bands in the UV-Visible Near IR regions of the electromagnetic spectrum allows phosphates to be useful optical materials. 

Miscellaneous.—The Schuler band absorption spectrum of ND4, generated photochemically, has been reported by Whittaker e( The response of the signal to pressure and modulation suggests that the ND4 is formed from a FlgND exciplex. The collision-induced relaxation dynamics of UF following excitation in the first electronic absorption band in the near-u.v. has been examined via the resulting red-shifted emission by Castiglione et A... 

As a typical example of polyene spectroscopy, absorption and fluorescence spectra of frons,frans-l,3,5,7-octatetraene in hexane at 23 °C are shown in Figure 3. An absorption band with several peaks is observed in Figure 3a. It should be noted that the positions of electronic absorption bands strongly depend on solvents . This absorption band is dipole allowed, because the molar absorption coefficient of this band is very large (Table 1). This band is attributed to the transition from the 1 kg ground state to the 1 feu excited state (tt-tt transition). Although the absorption peaks are due to vibrational transitions, a precise vibrational analysis cannot be made because of the broad band widths. The position of the observed emission spectrum (Figure 3b) shows a considerable red shift in... 

All the electronic absorption bands of V(N2)g are shifted to higher energies by between 4500 and 8800 cm compared with those of V(CO)g a 10 Dq value of 22 000 cm for derived from the electronic spectrum shows that the N2... 

---