第4讲：叙述分子光谱学的原则Lecture 4 Principles for Describing Molecular Spectroscopy.pdf

Lecture Notes: Principles of Molecular Spectroscopy
What variables do we need to characterize a molecule?
Nuclear and electronic configurations: What is the structure of the molecule? What are
the bond lengths? How strong or stiff are the bonds? What is the symmetry? Where is
the electron density?
Molecular Behavior: How much do the nuclei move (vibration/rotation)? How do the
structural variables change with time?
EXAMPLE: H−F
What is your picture of the structure of this molecule? 2 nuclei + 10 e−? Molecular orbitals?
ψab
b H F
ψ=c1 ψ1s + c2 ψ2p
1s z
ψab = c ψH − c ψF
2pz 2 1s 1 2pz
ψb
We quantitatively characterize the structure through the electronic energy as a function of
nuclear configuration:
Eelec
ψab
H + F
0
ψb
-Do
repulsive attractive
forces forces
r
r e
Eelec = electronic energy relative to isolated atoms
Lecture Notes: Principles of Molecular Spectroscopy Page 1
If we characterize this energy curve, we have learned much about our molecule:
1) re: What is the equilibrium separation between atoms?
2) D0: What is the strength of the bond? How stable is molecule?
b ab
3) Shape of ψ or ψ: Is re fixed? How stiff is bond? How does re change if you put energy
into bond?
4) Splitting between surfaces: What is the nature and energy of the e− orbitals involved in
bonding?
How can you probe these energy surfaces with light?
• The light field will exert a force on the charged particles. Different frequencies of light will
interact with the particles depending on: (1) if they are nuclei or electrons, and (2) the shape
of the potential for the particle.
• The potential energy reflects the coulombic interactions between the charged nuclei and
electrons.
Different frequencies of light will interact with different nuclear and electronic
motions. To simplify problem, treat nuclear and electronic motion separately.
• : The basis fo