Band system

The most widely employed optical method for the study of chemical reaction dynamics has been laser-induced fluorescence. This detection scheme is schematically illustrated in the left-hand side of figure B2.3.8. A tunable laser is scanned tlnough an electronic band system of the molecule, while the fluorescence emission is detected. This maps out an action spectrum that can be used to detemiine the relative concentrations of the various vibration-rotation levels of the molecule. 

There are several requirements for this to be a suitable deteetion method for a given moleeule. Obviously, tire moleeule must have a transition to a bound, exeited eleetronie state whose wavelength ean be reaehed with tunable laser radiation, and the band system must have been previously speetroseopioally assigned. If the moleeules are fonned with eonsiderable vibrational exeitation, the available speetroseopie data may not extend up to these vibrational levels. Transitions in the visible ean be aeeessed direetly by the output of a tunable dye laser, while transitions in the ultraviolet ean be reaehed by Ifequeney-doubled radiation. The... 

The H + NO2 OH + NO reaetion provides an exeellent example of the use of laser fluoreseenee deteetion for the elueidation of the dynamies of a ehemieal reaetion. This reaetion is a prototype example of a radieal-radieal reaetion in that the reagents and produets are all open-shell free radieal speeies. Both the hydroxyl and nitrie oxide produets ean be eonveniently deteeted by eleetronie exeitation in the UV at wavelengths near 226 and 308 mn, respeetively. Atlases of rotational line positions for the lowest eleetronie band systems of these... 

Engleman R Jr, Rouse P E, Peek H M and Biamonte V D 1970 Beta and gamma band systems of nitric oxide Los Aiamos Scientific Laboratory Report no LA-4364... 

Vibrational transitions accompanying an electronic transition are referred to as vibronic transitions. These vibronic transitions, with their accompanying rotational or, strictly, rovibronic transitions, give rise to bands in the spectrum, and the set of bands associated with a single electronic transition is called an electronic band system. This terminology is usually adhered to in high-resolution electronic spectroscopy but, in low-resolution work, particularly in the liquid phase, vibrational structure may not be resolved and the whole band system is often referred to as an electronic band. 

A group of transitions with the same value of Av is referred to as a sequence. Because of the population requirements long sequences are observed mostly in emission. For example, sequences of five or six members are observed in the band system of N2 in... 

It is clear from Figure 7.18 that progressions and sequences are not mutually exclusive. Each member of a sequence is also a member of two progressions. However, the distinction is usefiil because of the nature of typical patterns of bands found in a band system. Progression members are generally widely spaced with approximate separations of cOg in absorption and co" in emission. In contrast, sequence members are more closely spaced with approximate separations of cOg — co". 

Figure 7.20(b) illustrates the case where r c r". An example of such a transition is the Mulliken band system of C2 (see Table 7.6 and Figure 7.17). The value of is 1.2380 A in the D state and 1.2425 A in the X state. Here the most probable transition is from A to B with no vibrational energy in the upper state. The transition from A to C...

In Figure 7.42 it is seen that the progression is built not on the Og but on the 6g band. The reason for this will become clear when we have seen, in the following section, how non-totally symmetric vibrations may be active in an electronic band system. 

The answer, very often, is that they do not obtain any intensity. Many such vibronic transitions, involving non-totally symmetric vibrations but which are allowed by symmetry, can be devised in many electronic band systems but, in practice, few have sufficient intensity to be observed. For those that do have sufficient intensity the explanation first put forward as to how it is derived was due to Herzberg and Teller. 

Using the Franck-Condon principle in this way we can see that the band system associated with the second lowest ionization energy, showing a long progression, is consistent with the removal of an electron from a bonding n 2p MO. The progressions... 

There is a degeneracy factor of two associated with a n orbital compared with the nondegeneracy of a (7 orbital, so that it might be expected that the integrated intensity of the second band system would be twice that of each of the other two. However, although the second band system is the most intense, other factors affect the relative intensities so that they are only an approximate guide to orbital degeneracies. 

Z states, as indicated on the spectrum (the last two states are almost degenerate). The other two band systems are too weak to be observed. 

The third band system, involving the removal of an electron from the 1 2 orbital, is vibrationally complex, consistent with the orbital being strongly bonding and favouring a linear molecule. Presumably both Vj and V2 are excited but the bands in this system are considerably broadened, making analysis unreliable. 

Figure 9.18 shows a typical energy level diagram of a dye molecule including the lowest electronic states Sq, and S2 in the singlet manifold and and T2 in the triplet manifold. Associated with each of these states are vibrational and rotational sub-levels broadened to such an extent in the liquid that they form a continuum. As a result the absorption spectrum, such as that in Figure 9.17, is typical of a liquid phase spectrum showing almost no structure within the band system. 

Nevertheless, 1,4-difluorobenzene has a rich two-photon fluorescence excitation spectrum, shown in Figure 9.29. The position of the forbidden Og (labelled 0-0) band is shown. All the vibronic transitions observed in the band system are induced by non-totally symmetric vibrations, rather like the one-photon case of benzene discussed in Section 7.3.4.2(b). The two-photon transition moment may become non-zero when certain vibrations are excited. 

Electronic transitions in molecules in supersonic jets may be investigated by intersecting the jet with a tunable dye laser in the region of molecular flow and observing the total fluorescence intensity. As the laser is tuned across the absorption band system a fluorescence excitation spectrum results which strongly resembles the absorption spectrum. The spectrum... 

More commonly, the resonant two-photon process in Figure 9.50(c) is employed. This necessitates the use of two lasers, one at a fixed wavenumber Vj and the other at a wavenumber V2 which is tunable. The first photon takes the molecule, which, again, is usually in a supersonic jet, to the zero-point vibrational level of an excited electronic state M. The wavenumber of the second photon is tuned across the M to band system while, in principle, the photoelectrons with zero kinetic energy are detected. In practice, however, this technique cannot easily distinguish between electrons which have zero kinetic energy (zero velocity) and those having almost zero kinetic energy, say about 0.1 meV... 

Figure 9.5 f shows a ZEKE-PE specfmm of f, 4-difluorobenzene in which fhe firsf laser, of energy /zcvj, was timed to fhe Og band of fhe 5i-5o system. The second laser, of energy hcv2, was fhen timed across fhe M -fo-M ionization band system. 

The ZEKE-PE process shown in Figure 9.50(c) can be modified as shown by changing the wavenumber Vj of the first laser to excite the molecule to an excited vibrational level of M. Then the Franck-Condon factors for the band system are modified. This can allow... 

The UV spectrum of a complex conjugated molecule is usually observed to consist of a few broad band systems, often with fine structure, which may be sharpened up in non-polar solvents. Such a spectrum can often be shown to be more complex than it superficially appears, by investigation of the magnetic circular dichroism (MCD) spectrum, or by introduction of dissymmetry and running the optical rotatory dispersion (ORD) or circular dichroism (CD) spectrum. These techniques will frequently separate and distinguish overlapping bands of different symmetry properties <71PMH(3)397). 

The molecules of Nj so formed are in an excited state (B n ) and give rise to the emission of the first positive band system of the spectrum of molecular N2 in returning to the ground state... 

In general, two low-energy band systems appear, which in benzo-furoxan itself are strongly overlapping. In substituted compounds,... 

The electronic spectrum of S2O has been studied both in absorption and in emission and both in the ultraviolet and the visible regions. The absorption spectrum in the near UV region is extremely intense and well suited to detect S2O in gases even at very low partial pressures. Two band systems are located in the UV region at 340-250 nm and at 230-190 nm [35] while a third system in the visible region at 645-575 nm was discovered only by op-toacoustic detection [36]. The 340-250 nm system has also been observed for matrix-isolated S2O [37]. For more details see [1, 38-47]. 

FIGURE 11.21 Densitograms of PLC of Taxus baccata fraction from preparative column (silica/aqueous methanol) containing unknown taxoid (Tax 1) introduced to the layer with a set of capillaries with simultaneous evaporation of solvent from the starting band. System silica/CH2Cl2 + DX + Me2CO + MeOH (84 10 5 1). Plates double developed (a) 0.3 ml of fraction introduced (b) 0.5 ml of fraction introduced (c) 1 ml of fraction introduced to the layer. 

Hydroxyl radical (OH) is a key reactive intermediate in combustion and atmospheric chemistry, and it also serves as a prototypic open-shell diatomic system for investigating photodissociation involving multiple potential energy curves and nonadiabatic interactions. Previous theoretical and experimental studies have focused on electronic structures and spectroscopy of OH, especially the A2T,+-X2n band system and the predissociation of rovibrational levels of the M2S+ state,84-93 while there was no experimental work on the photodissociation dynamics to characterize the atomic products. The M2S+ state [asymptotically correlating with the excited-state products 0(1 D) + H(2S)] crosses with three repulsive states [4>J, 2E-, and 4n, correlating with the ground-state fragments 0(3Pj) + H(2S)[ in... 

Photoelectron spectroscopy (PES) has been applied to determine the structure of 1-aza- and 1,4,7-triazatricy-clo[5.2.1.04,10]decane 37 and 40 <1997JMT(392)21>. The PES spectrum of ATQ shows four composite bands in the region 7-17 eV. A first band peaked at 7.80 eV is attributed to the NLPO (nitrogen lone-pair orbital). A second prominent broad band system, extending from 10.5 to 13.0 eV is associated with photoionizations from the cr-orbital manifold. The third composite band is produced by two photoemissions. The second band may be attributed to emissions arising from a sequence of seven near-lying MOs. 

In TATCD the uppermost band consists of two components. Due to symmetry restrictions in these NLPOs of TATCD produce only two bands. The second complex band system of ATQ associated with cr-ionizations is displaced toward higher binding energy and is slightly more resolved in the PES spectrum of TATCD. 

Band System Fe(Cp)2 Fe(MeCp)2 Fe(ClCp)2 Assignment (Electron Ionised)... 

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