Structure-induced absorption

A comparison of the absorption edge of catalyst A with and without MgO shows considerable differences (Figure 12). Though the oxidation state of V is essentially the same, the edge fine structure induced by MgO indicates that the local environment about V had been altered significantly. As with the cyclic microunit studies, the... 

An additional piece of information can be obtained by studying a synthetic compound derived from the GFP chromophore (1-28) fluorescing at room temperature. In Fig. 3a we show the chemical structure of the compound that we studied in dioxan solution by pump-probe spectroscopy. If we look at the differential transmission spectra displayed in Fig. 3b, we observed two important features a stimulated emission centered at 508 nm and a huge and broad induced absorption band (580-700 nm). Both contributions appear within our temporal resolution and display a linear behavior as a function of the pump intensity in the low fluences limit (<1 mJ/cm2). We note that the stimulated emission red shifts with two characteristic time-scales (500 fs and 10 ps) as expected in the case of solvation dynamics. We conclude that in the absence of ESPT this chromophore has the same qualitative dynamical behavior that we attribute to the relaxed anionic form. 

Induced spectra actually consist of contributions arising from free-to-free, free-to-bound, bound-to-bound, and bound-to-free transitions. At temperatures much greater than the well depth of the intermolecular potential, kT e, the observed induced absorption is nearly fully due to free-to-free transitions as Welsh and associates suggest, but individual dimer lines or bands may still be quite prominent unless pressure broadening and perhaps other processes (like ternary interactions) have obliterated such structures. However, at lower temperatures, kT collision-induced is a poor choice. 

Sample cells. Variable temperature. Temperature control has been essential in much of the collision-induced absorption studies. Temperature variation accesses different parts of the intermolecular interaction potential and redistributes the relative importance of overlap and multipolar induction. Furthermore, at low temperatures, collision-induced line shapes are relatively sharp induced lines may be resolved at low temperatures whose structures may be masked at higher temperatures. 

On pp. 31 Iff., a preliminary discussion of the symmetry of induced line profiles was given. The spectral lines encountered in collision-induced absorption show a striking asymmetry which is described roughly by a Boltzmann factor, Eq. 6.59. However, it is clear that at any fixed frequency shift, the intensity ratio of red and blue wings is not always given exactly by a Boltzmann factor, for example if dimer structures of like pairs shape the profile, or more generally in the vibrational bands. We will next consider the latter case in some detail. 

Fig. 7.3. Upper figure Emission spectrum of Jupiter in the far infrared two diffuse, dark fringes are seen at the H2 Sb(0) and Sb(l) rotational transition frequencies, caused by collision-induced absorption in the upper, cool regions. The lower figure presents an enlarged portion which shows the dimer structures near the So(0) transition frequency [150].

Trafton has shown in 1964 that the opacity in the far infrared of the atmospheres of the outer planets is due to the rototranslational band of H2-H2 and H2-He pairs [393], It is now clear that collision-induced absorption plays a major role in the thermal balance and atmospheric structure of the major planets. The Voyager emission spectra of Jupiter and Saturn show dark fringes in the vicinity of the So(0) and So(l) lines of H2, Fig. 7.3, which are due to collision-induced absorption in the upper,... 

Interesting ripples were first seen in the interaction-induced absorption spectra of compressed nitrogen [107] and later also of oxygen and in various gas mixtures in the infrared [73, 78, 84]. It has been suggested that these weak, roughly sinusoidal structures that are superimposed with the quasi-continuous, induced background arise possibly from line mixing, due to the anisotropy of... 

Interaction-induced absorption by the vibrational or rotational motion of an atom, ion, or molecule trapped within a Ceo cage, so-called endohedral buckmin-sterfullerene, has excited considerable interest, especially in astrophysics. The induced bands of such species are unusual in the sense that they are discrete, not continuous they may also be quite intense [127]. Other carbon structures, such as endohedral carbon nanotubes, giant fullerenes, etc., should have similar induced band spectra [128], but current theoretical and computational research is very much in flux while little seems to be presently known from actual spectroscopic measurements of such induced bands. 

In native VCPO, the position and intensity of cofactor-induced absorption are sensitive to pH change at pH 5, the band is blue-shifted (Xmax 308 nm), and the intensity is decreased to e = 2.0 mM-1 cm-1. The exact correlation between the above-mentioned spectral features and the known X-ray data are complex, but a further UV-VIS analysis of several mutants of this enzyme showed the importance of several structural elements. A His496Ala mutant showed no optical spectrum (Hemrika et al. 1999), whereas crystal structure of this mutant showed the presence of tetrahedral vanadate in the active site (Macedo-Ribeiro et al. 1999), indicating that the V— N bond is crucial for the observed spectrum. The effect of other mutations can be seen in Table 1.1. 

In summary, semiconductor polymers such as polyacetylene and polythiophene have experimentally demonstrated nonlinear optical processes (photo-induced absorption, photo-induced bleaching and photo-luminescence) with characteristic time scales in the picosecond range or faster. These phenomena are intrinsic and originate from the instability of these conjugated polymers toward structural distortion. 

The extended conjugation in the molecule due to the phenanthrolin structure induces an absorption in the visible range the solution coloration is orange. 

We have applied the ultrafast confocal microscope to map excited state dynamics in thin films of poly(9,9-dioctylfluorene) (PFO, see chemical structure in figure 2(a)), blended with polymethylmethacrylate (PMMA, 10% wt. PFO in PMMA). PFO is a blue-emitting polymer, with an absorption maximum at 385 nm (see Fig. 2(a)), while PMMA is transparent at our pump wavelength and it does not interact with PFO [6] so that it is optically inert. Figure 2(b) shows the macroscopic AT/T spectrum of PFO measured at x = 1 ps at 570 nm probe wavelength we observe a photo-induced absorption (PA) due to photo-generated polarons [7],... 

Fig. 39.9(b) shows the collision-induced absorption spectra owing to the 6sS ls% transition of the collision complex. The 6S-7S atomic line was calculated at 607 nm in comparison with 540 nm of the experimental spectrum. The induced absorption peak separated by about 30 nm in the experimental spectrum was reproduced in the theoretical one separated by 23 nm. This peak is due to the potential minimum of the 7s2 state. The vibrational structure of the experimental spectrum indicates an existence of the attractive well in the excited state, which certainly exists in the potential curve shown in Fig. 39.7. 

A. Kuhn, S. Steuerwald, K. Bergmann, Coherent population transfer in NO with pulsed lasers the consequences of hyperflne structure, Doppler broadening and electromagnet-ically induced absorption, Eur. Phys. J. D1 (1998) 57. 

Nevertheless, even for polyacetylene, the electronic structure is not that of a simple metal in which the bond-alternation and the tc-tc gap have gone to zero there are infrared active vibrational modes (IRAV) and a pseudo-gap. This is indicated by the spectra in Figure 2 which demonstrate the remarkable similarity between the doping-induced absorption found with heavily doped trans-(CH)x, and the photoinduced absorption spectrum observed in the pristine semiconductor containing a very few photoexcitations. Not only are the same IRAV mode spectral features observed, they have almost identical frequencies. 

---