1C566 Master Lecture Notes Lecture 24 Which vibrational modes are “active” in electronic spectra(active mode = progressions for that vibrational mode are observed in the spectrum) Bottom Line For totally symmetric vibrational modes, in an electronic transition, = 0, 1, 2, … For non-totally symmetric modes, = 0, 2, 4, … But, the fact that something is allowed does not mean you’ll see it! Overlap integrals for non-totally symmetric modes are generally very small (there Q for non-totally symmetric modes is ZERO unless the molecule in the different electronic states belongs to different point groups!, so the = 0 FCF is close to 1, and the = 2 will be very small), Analysis of Vibrational Structure in S1S0transitions Assign observed vibronic lines to quantum changes in particular vibrational modes. Learn about vibrational frequencies in S1state- back out information about bonding, relate to electronic structure change in S1S0transition Set up studies of excited state dynamics (how does energy flow in the excited state?) Tools available: Helpful to know vibrational frequencies on the ground state (S0) oRaman spectroscopy oIR Single vibrational level fluorescence Supersonic jets- low temperatures simplify vibrational structure by depopulating excited vibrational levels in . High-resolution band contours- distinguish between x, y, z, get vibrational symmetry, structural parameters.
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