General issues of spectroscopies II c So Hirata Department of Chemistry University of Illinois at UrbanaChampaign This material has been developed and made available online by work supported jointly by University of Illinois the National Science Foundation under Grant CHE1118616 CARE ID: 265735
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Lecture 32General issues of spectroscopies. II
(c) So Hirata, Department of Chemistry, University of Illinois at Urbana-Champaign.
This material has
been developed and made available online by work supported jointly by University of Illinois, the National Science Foundation under Grant CHE-1118616 (CAREER), and the Camille & Henry Dreyfus Foundation, Inc. through the Camille Dreyfus Teacher-Scholar program. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the sponsoring agencies
.Slide2
General issues of spectroscopiesWe will learn two types of spectroscopies: absorption/emission and scattering.
We will learn their relationship to dipole moment and
polarizability
as well as perturbation
theories.Slide3
Absorption/emission versus scattering
spectroscopy
Absorption/emission spectroscopy
: microwave, IR, and UV/
vis
absorption as well as fluorescence
Scattering spectroscopy
: Raman
IR
RamanSlide4
Absorption spectroscopy
One-photon process
1
st
order perturbation theorySlide5
Scattering (Raman) spectroscopy
Two-photon process
2
nd
order perturbation theory
C. V. Raman
Public domain image from WikipediaSlide6
Scattering (Raman) spectroscopy
Rayleigh
Anti-Stokes
Raman
(a hot band)
Stokes
RamanSlide7
Scattering (Raman) spectroscopy
Resonant
Rayleigh
Resonant
Raman
RamanSlide8
Quantum in nature
Why is sky blue?
Resonance Rayleigh scatteringSlide9
Quantum in nature
Why is the Sun yellow?
Resonance Rayleigh scatteringSlide10
Dipole moment and polarizability
Absorption spectroscopies (such as IR spectroscopy)
are related to the
dipole moment
and its changes.
Scattering spectroscopies (such as Raman spectroscopy) are related to the polarizability
and its changes.Polarizability – softness of wave function; the larger the polarizability, the more easily the wave function is distorted by external electric field to create an induced dipole.Slide11
Dipole moment
E
0
E
0
+
µE
+ + + + + + + + + + + +
–
– – – – – – – –
First-order perturbation theory
Electric field
Perturbation
Think of
molecular
length!Slide12
Polarizability
E
0
E
0
+
µE +
α
E
2
=
E
0 + (µ
+ αE)E
+ + + + + + + + + + + +
–
– – – – – – – –
Second-order perturbation theory
Induced dipole
Think of
molecular
volume!Slide13
Polarizability
+ + + + + + + + + + + +
–
– – – – – – – – Slide14
Classical (
Smekal
) theory of Rayleigh and Raman scattering
An oscillating electric field (incident photon) causes the molecule to have an
induced
dipole:
Polarizability varies with molecular vibration; so does induced dipole: Slide15
Absorption/emission versus Raman spectroscopies
Absorption/emission occurs when molecular vibration, rotation, etc.
alter dipole moment
. The transition tends to transform as
x
, y, z.
Raman occurs when molecular vibration, rotation, etc. alter polarizability. The transition tends to transform as xx, yy, zz, xy, yz, zx
.Slide16
SummaryWe have the general theories of absorption/emission spectroscopy and scattering (Raman) spectroscopy.
We have learned the relationship between absorption/emission spectroscopies to dipole moment and that between scattering (Raman) spectroscopy and
polarizability
.
We have made references to first- and second-order perturbation theories.