2. What is Spectroscopy?
The study of the interaction between radiation and matt
er
“Analytical spectroscopy”, as defined in this class, cover
s applications of spectroscopy to chemical analysis
3. History of Analytical Spectroscopy
1666: Isaac Newton (England) shows that white light ca
n be dispersed into constituent colors, and coins the ter
m “spectrum”
– Newton also produced the first “spectroscope” based on lenses,
a prism, and a screen
1800: W. Herschel and J. W. Ritter show that infrared (I
R) and ultraviolet (UV) light are part of the spectrum
1814: Joseph Fraunhofer noticed that the sun’s spectru
m contains a number of dark lines, developed the diffrac
tion grating
1859: G. Kirchoff obtains spectra of the elements, expla
ins the sun’s spectrum
4. The Visible Spectrum of the Sun
(Black lines are absorption by elements in the cooler outer region of the star)
Figure from National Optical Astronomy Observatory/Association of Universities for Research in Astronomy/National Science Foundation, http://www.noao.edu/image_gallery/html/im0600.html
5. History of Analytical Spectroscopy
1870: J. C. Maxwell formalizes and combines the laws
of electricity and magnetism
1900 to present: More than 25 Nobel prizes awarded to
spectroscopists, including:
– 1902: H. A. Lorentz and P. Zeeman
– 1919: J. Stark
– 1933: P. A. M. Dirac and E. Schrodinger
– 1945: W. Pauli
….
– 1999: A. Zewail
6. Introduction to Spectroscopy
Figures from NASA (www.nasa.gov)
The electromagnetic spectr
um
Each color you see is a spe
cific (narrow) range of frequ
encies in this spectrum
8. Properties of Electromagnetic Radiation
Wave/particle duality
Perpendicular E and B c
omponents
– E = electric field
– B = magnetic field
Wave properties:
– Wavelength (frequency)
– Amplitude
– Phase
1 2 3 4 5
-1
-0.5
0.5
1
1 2 3 4 5
-1
-0.5
0.5
1
Long wavelength
(low frequency)
Short wavelength
(high frequency)
c = the speed of light (~3.00 x 108 m/s)
= the frequency in cycles/second (Hz)
= the wavelength in meters/cycle
c
Note – this figure s
hows polarized rad
iation!
9. Interference of Radiation
Monochromatic: radiation containing a single frequency
Polychromatic: radiation containing multiple frequencies
Constructive interference: wh
en two waves reinforce each o
ther
Destructive interference: when
two waves cancel each other
10. The Interaction of Radiation and Matter
Electromagnetic radiation travels fastest in a vacuum
When not travelling in a vacuum, radiation and matter ca
n interact in a number of ways
Some key processes (for spectroscopy):
– Diffraction
– Refraction
– Scattering
– Polarization
– Absorption
11. Transmission of Radiation
The velocity at which radiation travels (or propagates) thr
ough a medium is dependent on the medium itself
When radiation travels through a medium and does not u
ndergo a frequency change, it cannot be undergoing a pe
rmanent energy transfer
However, radiation can still interact with the medium
– Radiation, an EM field, polarizes the electron clouds of
atoms in the medium
– Polarization is a temporary deformation of the electron
clouds
12. Transmission and Refraction
The refractive index (ni) of a medium is given by:
i
i
c
n
c = the speed of light (~3.00 x 108 m/s)
i = the velocity of the radiation in the medium in m/s
ni = the refractive index at the frequency i
Refractive index measures the degree of interaction betw
een the radiation and the medium
– Liquids: ni ~ 1.3 to 1.8
– Solids: ni ~ 1.3 to 2.5
Refractive index can be used to identify pure liquid substa
nces
13. Refraction
When radiation passes through an interface between two
media with different refractive indices, it can abruptly chan
ge direction
Snell’s law:
1
2
2
1
2
1
sin
sin
v
v
n
n
1 = the velocity of the radiation in medium 1 in m/s
n1 = the refractive index in medium 1
Snell’s law is a consequence
of the change in velocity in th
e media
Reflection always occurs at a
n interface. Its extent depen
ds on the refractive indices of
the media
1
2
Medium 1
Medium 2
14. Diffraction
Fraunhofer diffraction:
– Also known as far-field diffraction, parallel beam diffra
ction
– Important in optical microscopy
Fresnel diffraction
– Also known as near-field diffraction
15. Diffraction
Diffraction gratings:
– Widely used in spectroscop
ic instruments to separate f
requencies (can be made p
recisely)
sin
2
d
n
http://www.astro.virginia.edu/research/observatories/40inch/fobos/images/grating.jpg
Bragg diffraction – multiple slit Fraunhofer diffraction:
– Important for instrument design, crystallography
16. Scattering
Rayleigh scattering (an elastic process):
– Scattering of small amounts of radiation by molecules
and atoms (whose size is near to the wavelength of th
e radiation)
Mie scattering: applies to large particles, involves scatteri
ng in different directions.
– Practical use in particle size analysis
4
1
scattering
17. Polarization
Polarization of EM radiation – a simple classical picture:
Figure from Sears, et al., “University Physics”, 7th Ed., 1988
18. Coherent Radiation
Coherent radiation fulfils two
conditions: (1) it has the sa
me frequency or set of frequ
encies, and (2) it has a well-
defined and constant phase
relationship
– Coherent radiation is “cross-c
orelated” in that the properties
of one beam can be used to p
redict those of the other beam
Examples of coherent radiati
on:
– Lasers
– Microwave sources (masers)
Coherent radiation: different frequencie
s (colors) with a defined phase relation
ship interfere to produce a pulse
Diagram from wikipedia.org (public domain)
19. Incoherent Radiation
Produced by “random” emis
sion, e.g. individual atoms in
a large sample emitting phot
ons
Actually is coherent, but just
to a tiny (undetectable) exte
nt
Also known as “continuous”
radiation
Examples of incoherent radi
ation:
– Incandescent light bulbs
– Filament sources
– Deuterium lamps
Incoherent radiation: different frequenci
es (colors) combined to produce contin
uous radiation with varying phase, freq
uency and amplitude
Diagram from wikipedia.org (public domain)
20. More Properties of Electromagnetic Radiation
Wave and particle behavior: photons behave as both wav
es and particles
– Quantum mechanics developed around the concept of
the photon, the elementary unit of radiation
Planck’s law:
E is the energy of the photon in joules
h is Planck's constant (6.624 x 10-34 joule seconds)
is the frequency of the radiation
h
E
21. Absorption and Emission
Absorption is a process accompanied by an energy chang
e
– involves energy transfer of EM radiation to a substanc
e, usually at specific frequencies corresponding to nat
ural atomic or molecular energies
Emission occurs when matter releases energy in the form
of radiation (photons
E = h
Higher energy
Lower energy
Absorption Emission
22. Energy Levels
Several types of quantum-mechanical energy levels occu
r in nature:
– Electronic
– Rotational
– Vibrational (including phonons and heat)
– Nuclear spin (other nuclear energy levels usually need
high energies to access)
For each of these, a discrete quantum state and energy-d
riven transitions between these states can be studied (as
opposed to a continuous range of energies)
23. Selection Rules
Selection rules:
Simple rules that are derived from transition moment i
ntegrals (usually via symmetry arguments) that expres
s which energy level transitions are allowed
Example (for rotational energy levels of a rigid linear r
otor such as a diatomic molecule):
A forbidden transition is usually still possible, but often is
weaker than allowed transitions
1
J
24. The Uncertainty of Measurements
Because the lifetimes of quantum states can persist for sh
ort periods, it can be difficult to measure their energies ac
curately
This is usually stated in the form of an “energy-time uncert
ainty”:
t
E
25. The Uncertainty Principle
The uncertainty principle: it is not possible to know both t
he location and the momentum of a particle exactly – a fu
ndamental limit on all measurements
In Heisenberg’s terms, the act of measuring a particle’s po
sition affects its momentum, and vice versa
In equation form:
– In other words, if you know the position of a particle to within x, t
hen you can specify its momentum along x to px
– As the uncertainty in x increases (x ), that of px decreases (x
), and vice versa
p
x x
2
1
26. Spectra and Spectrometers
Spectra are usually plotted as frequency vs. amplitude
– Instead of frequency, wavelength or energy can also b
e used
– The choice of x- and y-axes is often dependent on the
particular technique, its history, etc…
– In most techniques, a key parameter is the frequency/
energy/wavelength resolution
Spectrometers: instruments that measure the interaction
of radiation with matter, so the properties of such interacti
ons can be studied
27. Spectroscopy in Analytical Chemistry
Widely used approach for characterizing systems ranging
from chemical physics to biology, from individual atoms to
the largest molecules
Some of the most common techniques are:
– UV-Visible spectroscopy
– Fluorescence spectroscopy
– IR spectroscopy
– Raman spectroscopy
– X-ray spectroscopy
– NMR spectroscopy
– EPR spectroscopy
28. Further Reading
P. W. Atkins and R. S. Friedman, Molecular Quantum Mech
anics, 3rd Ed. Oxford University Press, New York (2003).
R. P. Feynman, R. B. Leighton, M. Sands, The Feynman Le
ctures on Physics, Addison-Wesley, Reading, MA (1977).
M. Fox, Optical Properties of Solids, Oxford University Pres
s, New York (2010).
Physics textbooks often contain good discussions of basic s
pectroscopic phenomena.
