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Lecture 31General issues of spectroscopies. I
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General issues of spectroscopies
In this lecture, we have an overview of spectroscopies: Photon energies and dynamical degrees of
freedom and spectroscopies Three elements of spectroscopy Three modes of optical transitions Lasers Spectral line widths
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Important physical quantities
λ (wave length) (typically in nm) v (frequency) (typically in Hz = s–1) = c / λ (wave number) (in cm–1) = 1 / λ = v / c
Visible light : 400 – 700 nm 1 eV = 8065 cm–1 298 K = 207 cm–1 10000000 / 400 nm = 25000 cm–1 = 3.1 eV
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Photon energies and spectroscopies
Radio-wave
Micro-wave
IR Visible UV X-ray γ-ray
>30 cm 30 cm – 3 mm
33–13000 cm–1
700–400 nm
3.1–124 eV
100 eV –100 keV
>100 keV
Nuclear spin
Rotation Vibration Electronic Electronic Core electronic
Nuclear
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Electronic, vibration, and rotation
all
3n+3N
electronic
3n
nuclear
3N
translational
3
relative
3N−3
rotational
3 or 2
vibrational
3N−6 or 3N−5
Born-Oppenheimer approximation
Exact separation
Rigid rotor approximation
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Electronic, vibration, and rotation
kT Vibrational spectroscopyIR/Raman spectroscopiesElectronic spectroscopyUV/vis spectroscopyRotational spectroscopyMicrowave spectroscopy
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Three elements of spectroscopy
1. Source
Sample
Reference
2. Dispersing element
3. Detector
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Sources of radiation The sun and stars Various conventional lamps Newer radiation sources:
Lasers Synchrotron radiation
Public domain image created by U.S. Department of Energy
Advanced Light Source at Argonne National Laboratory
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The dispersing elements: prism
air glass
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The dispersing elements: diffraction grating
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The dispersing elements: Fourier transform technique
Movable mirror
Mirror
Laser Interferometer Gravitational Observatory (LIGO) at Hanford, WA Copyrighted image in courtesy of LIGO
Laboratory
Half mirror
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Detectors
CCD
CCD
Digital camera
PhotodiodePhotodiode
Pyroelectric Pyroelectric
Remote controlOptical mouseBarcode reader
Heat sensing missileNight vision goggle
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Stim
ulat
ed
abso
rptio
n
Stim
ulat
ed
emis
sion
Spo
ntan
eous
emis
sion
Einstein’s theory of three modes of optical transitions
Absorption always needs the help of photon – stimulated absorption.
Emission occurs in two ways – stimulated or spontaneous emission.
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Three modes of optical transitions
W NBS
timul
ated
ab
sorp
tion
Stim
ulat
ed
emis
sion
Spo
ntan
eous
emis
sion
A
B
B'
ρ
ρ
N
N'
W N A B
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Three modes of optical transitions
NB W W N A B
/
/ /
/ / /hv kT
N A A B A B
NB N B N N B B e B B
3 3
/
8 /
1h kT
h c
e
Equilibrium: no net excitation or deexcitation
Blackbody radiation
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Three modes of optical transitions
B BSame effects
on both states. If it
were not for A, N = N'
Einstein A coeffS
timul
ated
ab
sorp
tion
Stim
ulat
ed
emis
sion
Spo
ntan
eous
emis
sion
A
B
B'
ρ
ρ
N
N'
Einstein B coeff 3
3
8 hA B
c
The greater the
frequency, the the greater the rate of the spontaneous
emission, causing Boltzmann distribution
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Lasers
High power Monochromatic and
polarized Coherent Low divergence and
long path lengths
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Population inversion
Thermal equilibrium
Pumping
Laser action
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Applications of laser
High power Nonlinear/multiphoton spectroscopy (including
Raman) High sensitivity
Monochromatic State-to-state reaction dynamics; Laser isotope
separation High resolution
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Line widths: lifetime broadening
Collisional deactivation Natural line width
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Line widths: Doppler broadening
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Summary We have discussed photon energies,
molecular dynamical degrees of freedom, and spectroscopies.
We have surveyed three elements (light source, dispersing element, and detector) of spectroscopy.
We have characterized three modes of optical transitions (stimulated absorption and emission as well as spontaneous emission).
We have learned the origins of line widths.