General issues of spectroscopies I 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 CAREE ID: 268559
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Lecture 31General issues of spectroscopies. I
(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
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General issues of spectroscopiesIn 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
linewidthsSlide3
Important physical quantitiesλ
(wave length) (typically in nm)
v
(frequency) (typically in Hz = s
–1
) = c / λ (wave number) (in cm–1) = 1 / λ = v / cVisible light : 400 – 700 nm1 eV = 8065 cm–1 298 K = 207 cm–1 10000000 / 400 nm = 25000 cm–1 = 3.1 eVSlide4
Photon energies and spectroscopies
Radio-wave
Micro-wave
IR
Visible
UVX-rayγ-ray>30 cm30 cm – 3 mm33–13000 cm–1 700–400 nm3.1–124 eV100 eV –100 keV
>100 keVNuclear spinRotation
VibrationElectronicElectronicCore
electronicNuclearSlide5
Electronic, vibration, and rotation
Born-Oppenheimer approximation
Exact separation
Rigid rotor approximationSlide6
Electronic, vibration, and rotation
kT
Vibrational spectroscopy
IR/Raman spectroscopies
Electronic spectroscopy
UV/
vis spectroscopyRotational spectroscopy
Microwave spectroscopySlide7
Three elements of
spectroscopy
1. Source
Sample
Reference
2. Dispersing element3. DetectorSlide8
Sources of radiation
The sun and stars
Various conventional lamps
Newer radiation
sources:
LasersSynchrotron radiationPublic domain image created by U.S. Department of EnergyAdvanced Light Source at Argonne National LaboratorySlide9
The dispersing elements: prism
air
glassSlide10
The dispersing
elements: diffraction gratingSlide11
The dispersing
elements: Fourier transform technique
M
ovable mirror
Mirror
Laser Interferometer Gravitational Observatory (LIGO) at Hanford, WA Copyrighted image in courtesy of LIGO LaboratoryHalf mirrorSlide12
Detectors
CCD
CCD
Digital camera
Photodiode
Photodiode
Pyroelectric
Pyroelectric
Remote control
Optical mouse
Barcode reader
Heat sensing missile
Night vision goggle Slide13
Stimulated
absorption
Stimulated
emission
Spontaneous
emissionEinstein’s theory of three modes of optical transitionsAbsorption always needs the help of photon – stimulated absorption.Emission occurs in two ways – stimulated or spontaneous emission.Slide14
Three modes of optical transitions
Stimulated
absorption
Stimulated
emission
Spontaneous
emission
A
B
B
'
ρ
ρ
N
N
'Slide15
Three modes of optical transitions
Equilibrium: no net excitation or deexcitation
Blackbody radiationSlide16
Three modes of optical transitions
Same effects on both states. If it were not for
A
,
N
= N'
Einstein
A coeff
Stimulated
absorptionStimulated emission
Spontaneousemission
A
B
B
'
ρ
ρ
N
N
'
Einstein
B
coeff
The
greater the
frequency, the
the greater the rate of the spontaneous
emission, causing Boltzmann distribution Slide17
Lasers
High power
Monochromatic and polarized
Coherent
Low divergence and long
path lengthsSlide18
Population inversion
Thermal equilibrium
Pumping
Laser actionSlide19
Applications of laserHigh power
Nonlinear/
multiphoton
spectroscopy (including Raman)
High sensitivity
MonochromaticState-to-state reaction dynamics; Laser isotope separationHigh resolutionSlide20
Linewidths
: lifetime broadening
Collisional deactivation
Natural
linewidthSlide21
Linewidths: Doppler broadeningSlide22
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.