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The general rule for vibrational Raman Spectroscopy is that the symmetric vibrations give intense Raman lines and non-symmetric vibrations are weak or inactive. There is a rule of mutual exclusion: For a molecule having a center of symmetry, infrared active vibrations are Raman inactive and vice versa. For other molecules, both IR and Raman lines may be observed.
The selection rules for vibrational Raman Spectra are
Δν = 0, ± 1, ± 2 and (15.15 )
ν = νex ± Δεvib (15.16 )
These vibrational Spectra may also show a rotational fine structure with the ΔJ = 0, ± 2 selection rule. |
15.5 |
Time Resolved Spectroscopy |
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Modern laser techniques allow us to generate pulses of the order of femtoseconds. Additional details on lasers and time resolved spectroscopy will be given in the lectures on kinetics. Reaction dynamics and ultrafast spectroscopy are nicely interwoven as the latter provides the techniques to study the former. Activated complexes which are the short lived intermediates between the reactants and products are very short lived. At most, they undergo a few vibrations and a few rotations if they are sufficiently long lived and decay into products. To study the time domain behaviour of these short lived species, we need two pulses of laser light. The first is the pump pulse which initiates the reaction. The second one is the probe pulse which is flashed through the reaction mixture at several delayed time steps to probe the extent of changes occurring in the reaction. |
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Figure 15.9. Sketches of a pump pulse (a) and probe pulses (b) and (c). |
