Surface emission excitation spectra

Fig. 3.17 shows the b-emission excitation spectrum, with an a-polarized excitation, in the region E°° + 220 cm "1 and above. We observe a strong and broad peak, attributed here to the a surface component, following refs. 117,67. This peak is strongly asymmetric, broadened on the high-energy side. The value of the component indicated on the maximum (25 523 cm 1, at 222 cm 1 above the b component) may be falsified by the asymmetry of the band. (This will be examined in Section III.B.3 below.) We find that the surface-exciton Davydov splitting is quite comparable with its bulk counterpart (222 cm"1 vs 224 cm-1), to the accuracy of our experiments. Furthermore, the two 0-0... 

Since at low temperature the surface emission is well resolved from the bulk emission, and transition back to the surface is impossible, the excitation spectrum of the surface emission reveals only surface-state structures. This technique allows one to discriminate the surface states which, in other kinds of experiment, are masked by the bulk states. 

This subsection is devoted to the description of the upper excited states appearing in the excitation spectrum of the surface emission. The way this excitation is performed will be examined in Section III.B.3 below, in connection with the theory of Section III.B.l.b. The upper states examined are associated with the first singlet transition, b- and a-polarized, and are of two types purely electronic and vibronic states in an extended sense, as resulting from the coupling of electronic excitations to vibrations or to lattice phonons. 

The electron affinity can also be deduced from the measurement of the spectrum of the photoelectron emission with monochromatic UV light. This technique is ultra-violet (UV) photoelectron emission spectroscopy (or UV photoemission spectroscopy or UPS). The UPS technique involves directing monochromatic UV light to the sample to excite electrons from the valence band into the conduction band of the semiconductor. Since the process occurs near the surface, electrons excited above the vacuum level can be emitted into vacuum. The energy analysis of the photoemitted electrons is the photoemission spectrum. The process is often described in terms of a three step model [8], The first step is the photoexcitation of the valence band electrons into the conduction band, the second step is the transmission to the surface and the third step is the electron emission at the surface. The technique of UPS is probably most often employed to examine the electronic states near the valence band minimum. 

The spectrum of Nph form on aerosil is not resolved. The wide-band fluorescence contribution relative to the molecular emission is large, afterglow is not observed. The wideband excitation spectrum at 400 nm is shifted relatively to that of a molecular form by 10 nm. For zeolites this is mainly CTC and for aerosil a bimolecular associate of Nph. When adsorbing from a vapor phase, the emission spectrum of Nph in a zeolite consists of a continuous structureless band which is a superposition of CTC and dimers adsorbed at the outer surface (Fig. 3a).In the case of co-adsorption of water vapor or hexane the spectrum transforms with the appearance of structured fluorescence and phosphorescence components (Fig.3b). The coadsorbate seems to promote breaking up of dimers and diffusion of molecules in zeolite cages. 

The fluorescence excitation spectrum of acridine in O.IN sulfuric acid has a band characteristic of acridinium cation at about 25000 cm, but on the other hand, acridine in ethanol has no band below 25000 cm" (see Fig. 8). The fluorescence excitation spectra of acridine both on SG200V and SG200A have a band near 25000 cra . These findings indicate that a proton is transferred to acridone from silanol groups in its ground state as a result of adsorption on the silica gel surface. The emission of acridine adsorbed on silica gel are reasonably assigned to the protonated species of acridine. 

Figure 8.30. Fluorescence emission spectrum of 14 pM of ai-acid glycoprotein in the presence of 160 pM of calcofluor recorded at excitation wavelength of 295 nm (a). The figure displays also (dotted lines) the two Gaussian components of the fluorescence spectrum. The maximum of the bands are 349 nm (b) and 324 nm (c). These positions correspond to the emission from surface and hydrophobic Trp residues, respectively. The bandwidth of band (b) is equal to 53 nm, indicating that the structure of the protein surface is disrupted. Spectrum d shows the sum of spectra (a) and (b). Source Albani, J. R., 2001, Carbohydr. Res. 334, 141-151.

Transfer the sample into the fluorimeter cell. Make sure the optical surfaces are clean. Set the excitation wavelength to 320 nm and scan the emission from 400 to 600 nm. The excitation spectrum is obtained by scanning the excitation wavelength from 200 to 370 nm at the fixed emission wavelength of 420 nm. Typical emission and excitation spectra are shown in Rg. 25-1. 

Laser induced fluorescence (LIF) techniques in supersonic free jets can also yield useful information on the potential energy curves of open shell atomic systems. This type of studies has provided high quality data on tbe ground and excited states of NaAr type of molecules. The LIF technique was also successfully applied for probing the potential surfaces of XeF. The B<-X fluorescence excitation spectrum of XeF in a supersonic free jet is sufficiently simplified that rotational analysis and accurate vibrational spacing are readily obtained, overcoming the complexity of gas phase emission spectroscopy, mainly due in this case to isotopic richness of natural Xe. 

At X 20y the flux densities are obtained by integrating high resolution maps over a 1 beam, and thus we hope to have omitted low surface brightness emission excited by the HII regions. Data are from Becklin et al. (1973)5 Gillett and Forrest (1973)5 Werner et al. (1975)5 Harper et al. (1972), Gezari et al. (197 ), Werner et al. (197 )5 Rieke, Low, and Kleinmann (1973). The spectrum (inside a 1 aperture) from the model with lOOy = 0.55 ot r and L = 10 Lq has been plotted over the KL data. 

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