09_580ln_fa08

09_580ln_fa08 - MIT OpenCourseWare http:/ocw.mit.edu 5.80...

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MIT OpenCourseWare http://ocw.mit.edu 5.80 Small-Molecule Spectroscopy and Dynamics Fall 2008 For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms .
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5.80 Lecture #9 Fall, 2008 Page 1 Lecture #9: The Born-Oppenheimer Approach to Transitions -Selection Rules -Relative Intensities First of 3 lectures illustrating simplest patterns in 3 main types of transitions - mostly for diatomi c molecules permanent µ pure rotation (microwave) v 0 change in µ rotation-vibration (IR) v ±1 electronic symmetries rotation-vibration-electronic (UV – VIS) v = any How does the Born-Oppenheimer Approximation help us to predict what to expect in the spectrum? Begin reading Hougen monograph http://physics.nist.gov/Pubs/Mono115/contents.html Chapter 6 of Bernath KEY TOPICS  * Electric dipole transitions: e r α M if (R) α vector electrons in body integrate over r * DIRECTION COSINES LAB XYZ body xyz integrate over θ , φ , χ * Selection Rules symmetry and propensity * Hönl-London rotational linestrength factors sum over M J Last time I was concerned with how to go H H ° + H E evJ exact simplified missing exact stuff This was mostly formal. As spectroscopists we care much less about how to compute spectra ab initio than how to extract information from real spectra. The reason the Born-Oppenheimer approximation is so important is that it enables us to simplify our interpretation of spectra.
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5.80 Lecture #9 Fall, 2008 Page 2 It is very helpful to think of E evr = T i + G i (v) + F i,v (J) Φ i (r;R) | JM χ i,v (R) V i,J (R) and that all electronic properties vary slowly with R, and all observable quantities normally vary smoothly with v,J.
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09_580ln_fa08 - MIT OpenCourseWare http:/ocw.mit.edu 5.80...

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