lecture20_2011

lecture20_2011 - 18Feb2011 Chemistry 21b Spectroscopy...

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18Feb2011 Chemistry 21b – Spectroscopy Lecture #20 – Electronic Spectroscopy of Simple Polyatomic Molecules The electronic spectra of polyatomic molecules can become hopelessly congested at high resolution because of the very high density of eigenstates. Furthermore, a sometimes bewildering array of radiative and non-radiative processes become important. Before turning to the spectroscopy of formaldehyde as an example of of some of the more important “rules” of polyatomic electronic spectroscopy, we first present a brief tabulation of terms: I. A Brief Glossary of Terms in Electronic Spectroscopy A. Absorption and Emission Bound State-Bound State Transitions: Fluorescence – Spin allowed, τ < 10 7 sec Phosphorescence – Spin forbidden, τ > 10 6 sec Raman effects – Spontaneous, resonant, stimulated Franck-Condon factors Bound-Free Transitions: Photoioinization – resonant, non-resonant, pulsed field Photodissociation – direct, predissociation, coupled states, excimers . . The Reflection approximation and the Mulliken difference potential B. Non-radiative Transitions Predissociation (curve crossing) Yablonski diagrams – internal conversion (IC), intersystem crossing (ISC), intramolecular vibrational redistribution (IVR), inverse electronic relaxation (IER), ... C. Popular Techniques Laser Induced Fluorescence: Excitation spectra (tune laser, measure total yield) Dispersed fluorescence (fix laser, disperse emission w/gratings, prisms,...) Stimulated emission pumping (SEP, including four-wave mixing) Photoelectron spectroscopy (lasers & mass spectrometers) Multi-photon dissociation (MPD) & ionization (REMPI) Picosecond/femtosecond pulses (pump-probe, CARS, ionization...) We’ll now look at some of these processes using H 2 CO as a test case. Formaldehyde: A Quick Rovibrational Review As the figure below reminds you, formaldehyde is a four atom non-linear molecule. The point group to which it belongs is C 2 v , and it is a nearly prolate top with an asymmetry parameter near -1. The permanent dipole moment is directed along the a -axis. 164
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Since it is non-linear, there are 3(4)-6 = 6 non-degenerate normal modes of vibration. In the order derived by G. Herzberg, these six modes are: H H O C H H O C H H O C H H O C H H O C H H O C H H O C - + + + ν ν ν ν ν ν 1 2 5 6 3 4 z , b y , b x , c C 2 v σ v σ A = 9.405 cm B = 1.295 cm C = 1.134 cm -1 -1 -1 κ = -0.96 Figure 20.1 – The structure and vibrational normal modes of formaldehyde. What are the symmetries of the normal modes? Clearly, the C 2 v symmetry of the molecule is maintained in the ν 1 , ν 2 , and ν 3 modes, and so they are all A 1 . The ν 6 , or out-of-plane bending, mode, is B 1 , and the remaining ν 4 and ν 5 modes are B 2 . The ν 1 , ν 2 , and ν 3 modes have dipole derivatives that change along the a -axis, and so these vibrational transitions are A -type bands with ∆ K p = 0 and ∆ K
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