Raman scattering excited state

A small fraction of the molecules are in vibrationally excited states. Raman scattering from vibrationally excited molecules leaves the molecule in the ground state. The scattered photon appears at higher energy, as shown in Figure lb. This anti-Stokes-shifted Raman spectrum is always weaker than the Stokes-shifted spectrum, but at room temperature it is strong enough to be useful for vibrational frequencies less than about 1500 cm 1. The Stokes and anti-Stokes spectra contain the same frequency information. 

Raman Spectroscopy The time-dependent picture of Raman spectroscopy is similar to that of electronic spectroscopy (6). Again the initial wavepacket propagates on the upper excited electronic state potential surface. However, the quantity of interest is the overlap of the time-dependent wavepacket with the final Raman state 4>f, i.e. < f (t)>. Here iff corresponds to the vibrational wavefunction with one quantum of excitation. The Raman scattering amplitude in the frequency domain is the half Fourier transform of the overlap in the frequency domain,... 

A dilute solution of the purine adenine in slightly acidic ultra pure water with a nominal concentration of 1.0x10 " M was prepared and transferred to a clear glass sample vial. The samples were placed 7 m away from the excitation laser source. A 532 mu. Coherent VERDI, sohd state diode was used to excite the Raman scattered radiation at the maximum current standoff distance (based on the limitations of the lab room). Figure 9 shows the spectra acquired when 0.1 mL of colloidal silver suspension was added to 10 mL of adenine solution. 

One of the most interesting features of the Raman spectmm is its dependence on tire incident light frequency, coj. When Wj is on resonance with the excited electronic state, the scattering process closely resembles a process of absorption followed by emission. However, as Uj is detuned from resonance there are no longer... 

Figure Al.6.15. Schematic diagram, showing the time-energy uncertainty principle operative in resonance Raman scattering. If the incident light is detuned from resonance by an amount Aco, the effective lifetime on the excited-state is i 1/Aco (adapted from [15]).

Raman scattering and (b) anti-Stokes Raman scattering. In Stokes scattering, tlie cluomophore is initially in the ground vibrational state, g, and oi > CO2. hr spontaneous anti-Stokes scattering, the cluomophore must be initially m an excited vibrational state,/ Also note that in (b), M2 is (arbitrarily) defined as being greater than... 

As already mentioned, electronically resonant, two-pulse impulsive Raman scattering (RISRS) has recently been perfonned on a number of dyes [124]. The main difference between resonant and nom-esonant ISRS is that the beats occur in the absorption of tlie probe rather than the spectral redistribution of the probe pulse energy [124]. These beats are out of phase with respect to the beats that occur in nonresonant ISRS (cosinelike rather tlian sinelike). RISRS has also been shown to have the phase of oscillation depend on the detuning from electronic resonance and it has been shown to be sensitive to the vibrational dynamics in both the ground and excited electronic states [122. 124]. 

Marzocchi M P, Mantini A R, Casu M and Smulevich G 1997 Intramolecular hydrogen bonding and excited state proton transfer in hydroxyanthraquinones as studied by electronic spectra, resonance Raman scattering, and transform analysis J. Chem. Phys. 108 1-16... 

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). 

Laser Raman Microprobe. A more sophisticated microscope is the Laser Raman Microprobe, sometimes referred to as MOLE (the molecular orbital laser examiner). This instmment is designed around a light microscope to yield a Raman spectmm (45) on selected areas or particles, often <1 ia volume. The data are related, at least distantly, to iafrared absorption, siace the difference between the frequency of the exciting laser and the observed Raman frequency is the frequency of one of the IR absorption peaks. Both, however, result from rotational and vibrational states. Unfortunately, strong IR absorption bands are weak Raman scatterers and vice versa hence there is no exact correspondence between the two. 

The comparison of the vibrational modes energies, determined by i) Raman scattering, ii) fluorescence, iii) ab initio calculation, and iv) absorption, is reported in Table 6-5. We note that, as in the case of T4 [64], the C=C stretching mode of 1460 cm-1 in the ground slate decreases its frequency significantly (1275 cm-1) in the first excited electronic state. 

Figure 3.1. Simple schematic diagram of resonance Raman scattering in a harmonic system. The initial state i> is excited by the incident photon of energy El that is resonant with the set of energy levels v>. The scattering of the photon with energy Eg takes the system from i> to f>.

Laser-based methods of identification are extremely powerful they are able to provide species and structural information, as well as accurate system temperature values. Spontaneous Raman scattering experiments are useful for detection of the major species present in the system. Raman scattering is the result of an inelastic collision process between the photons and the molecule, allowing light to excite the molecule into a virtual state. The scattered light is either weaker (Stokes shifted) or... 

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