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 ±rst 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,
Phosphorescence – Spin forbidden,
Raman e²ects – Spontaneous, resonant, stimulated
Photoioinization – resonant, non-resonant, pulsed ±eld
Photodissociation – direct, predissociation, coupled states, excimers .
The Re³ection approximation and the Mulliken di²erence 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 ³uorescence (±x laser, disperse emission w/gratings, prisms,.
Stimulated emission pumping (SEP, including four-wave mixing)
Picosecond/femtosecond pulses (pump-probe, CARS, ionization.
We’ll now look at some of these processes using H
CO as a test case.
Formaldehyde: A Quick Rovibrational Review
As the ±gure below reminds you, formaldehyde is a four atom non-linear molecule.
The point group to which it belongs is
, and it is a nearly prolate top with an asymmetry
parameter near -1. The permanent dipole moment is directed along the