Excited state Raman spectroscopy

The geometry changes which transition metal complexes undergo when excited electronic states are populated are determined by using a combination of electronic emission and absorption spectroscopy, pre-resonance Raman spectroscopy, excited state Raman spectroscopy, and time-dependent theory of molecular spectroscopy. 

Excited state Raman spectroscopy of Fe(CN)5N02-provides direct support for the bending distortion. 

Excited State Raman Spectroscopy A direct method of studying the geometry of an excited state is to measure the vibrational... 

R. Vehring, Red-excitation dispersive Raman spectroscopy is a snitable techniqne for solid-state analysis of respirable pharmaceutical powders, Appl. Spectrosc., 59, 286-292 (2005). 

A variety of spectroscopic methods has been used to determine the nature of the MLCT excited state in the /ac-XRe(CO)3L system. Time-resolved resonance Raman measurements of /ac-XRe(CO)3(bpy) (X = Cl or Br) have provided clear support for the Re -a- n (bpy) assignment of the lowest energy excited state [44], Intense excited-state Raman lines have been observed that are associated with the radical anion of bpy, and the amount of charge transferred from Re to bpy in the lowest energy excited state has been estimated to be 0.84 [45], Fast time-resolved infrared spectroscopy has been used to obtain the vibrational spectrum of the electronically excited states of/ac-ClRe(CO)3(bpy) and the closely related/ac-XRe(CO)3 (4,4 -bpy)2 (X = Cl or Br) complexes. In each... 

The photodissociation of H2S in the 195 nm band has already been discussed in Chapters 9 and 14 in relation to vibrational excitation and Raman spectroscopy. At first glance, one might be tempted to think that it evolves similarly to the dissociation of H2O in the first absorption band. That is not the case, however While the fragmentation of H2O proceeds via a single electronic state, with electronic symmetry B in -configuration, the dissociation of H2S involves two states, 1B and xA<2. (Weide, Staemmler, and Schinke 1990 Theodorakopoulos and Pet-salakis 1991 Heumann, Diiren, and Schinke 1991). 

The use of laser excitation allows Raman spectroscopy to be performed on specimens in almost any state liquid, solution, transparent solid, translucent solid, powder, pellet or gas. Water is a weak scat-terer and therefore an excellent solvent for Raman work. This has important consequences in studies of biochemical interest and in the pharamaceutical industry. Water is a bad solvent in FT Raman because the Raman light is self absorbed. 

As described at the end of section Al.6.1. in nonlinear spectroscopy a polarization is created in the material which depends in a nonlinear way on the strength of the electric field. As we shall now see, the microscopic description of this nonlinear polarization involves multiple interactions of the material with the electric field. The multiple interactions in principle contain infomiation on both the ground electronic state and excited electronic state dynamics, and for a molecule in the presence of solvent, infomiation on the molecule-solvent interactions. Excellent general introductions to nonlinear spectroscopy may be found in [35, 36 and 37]. Raman spectroscopy, described at the end of the previous section, is also a nonlinear spectroscopy, in the sense that it involves more than one interaction of light with the material, but it is a pathological example since the second interaction is tlirough spontaneous emission and therefore not proportional to a driving field... 

Myers A B and Mathies R A 1987 Resonance Raman intensities A probe of excited-state structure and dynamics Biological Applications of Raman Spectroscopy yo 2, ed T G Spiro (New York Wiley-Interscience) pp 1-58... 

Infrared and Raman spectroscopy each probe vibrational motion, but respond to a different manifestation of it. Infrared spectroscopy is sensitive to a change in the dipole moment as a function of the vibrational motion, whereas Raman spectroscopy probes the change in polarizability as the molecule undergoes vibrations. Resonance Raman spectroscopy also couples to excited electronic states, and can yield fiirtlier infomiation regarding the identity of the vibration. Raman and IR spectroscopy are often complementary, both in the type of systems tliat can be studied, as well as the infomiation obtained. 

Resonance Raman Spectroscopy. If the excitation wavelength is chosen to correspond to an absorption maximum of the species being studied, a 10 —10 enhancement of the Raman scatter of the chromophore is observed. This effect is called resonance enhancement or resonance Raman (RR) spectroscopy. There are several mechanisms to explain this phenomenon, the most common of which is Franck-Condon enhancement. In this case, a band intensity is enhanced if some component of the vibrational motion is along one of the directions in which the molecule expands in the electronic excited state. The intensity is roughly proportional to the distortion of the molecule along this axis. RR spectroscopy has been an important biochemical tool, and it may have industrial uses in some areas of pigment chemistry. Two biological appHcations include the deterrnination of helix transitions of deoxyribonucleic acid (DNA) (18), and the elucidation of several peptide stmctures (19). A review of topics in this area has been pubHshed (20). 

Salares VR, Young NM, Carey PR, and Bernstein HJ. 1977. Excited-state (exciton) interactions in polyene aggregates—Resonance Raman and absorption spectroscopic evidence. Journal of Raman Spectroscopy 6(6) 282-288. 

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