Download lec20320spectroscopy2020kk20sept2009

Spectroscopy
• “Spectroscopy” : Webster’s definition
• “Spectro”
• Radiant energy consisting of component
waves which can be dispersed and
focused into individual wavelengths
• “ Scopy “
• Viewing, observation.
•
The observation of the interaction of light
( energy ) with matter
Example of a spectral observation:
A beam of sunlight is refracted and dispersed by a
prism to produce a display of colours.
What spectrum would this be ?
What wavelengths does it involve ?
Spectroscopic methods apply the physics of light
energy to the analysis of a sample (analyte)
In General
The intensity of this light energy is either imparted
to (re-emitted) or changed (reduced) by the
analyte upon interaction with it, depending on
the characteristics of the analyte itself
This change in the nature of the light energy is
then detected and converted to an electrical
signal which is proportional (ideally) to the
concentration of the analyte
INCREASING λ
Wavelength (l) = distance between peaks
Frequency (u) = waves/sec, # of wave crests passing a point per unit time
Speed of light (C) = 3 x 1010 cm s-1 = 186,000 miles s-1
C = lu
As a particle, light behaves as discrete packet of energy:
Quantum of light, or a photon, equals a unit of electromagnetic radiation.
Quantum Energy (E)
= hu
h= 6.626 x 10-34 J·s (Plank's constant)
.
If E = hu and C = lu
then u = C/l
and E = h (C/l)
According to this last equation, shorter
wavelengths of light have higher quantum energy
PROPERTIES SHARED BY ALL
FORMS OF ELECTROMAGNETIC
RADIATION
• Can travel through empty space
• Speed of light is constant in space
• Wavelength of light described similar to
a wave of water (crests/troughs)
Kirchoff's Laws
After looking at and recording many different spectra, Gustav
Kirchoff listed three laws consistent with his observations in
1859
• A hot, dense object (solid, liquid or gas) emits light of all
wavelengths and therefore produces a continuous spectrum
• A low density gas will produce an emission spectrum of
distinct bright lines
• If a low density, cool gas is placed in front of a continuous
source, there will be dark lines superimposed on the
continuous spectrum, producing an absorption spectrum
Kirchoff's Laws
Spectral Fingerprints
• Kirchoff and other scientists noticed that by burning chemicals,
they could reproduce the same lines over and over
• For example, heating sodium will produce the two lines seen
below, whether in emission or absorption
• Each element had a unique pattern of lines that never
changed
Sodium in emission
Sodium in absorption
Spectral Fingerprints
The Solar Spectrum
• Astronomers also took a closer
look at the spectrum of the Sun
• The Solar spectrum is an
absorption spectrum that is filled
with many dark lines
• Around 1810, a German scientist
named Fraunhofer recorded and
named over 600 of these lines
• By comparing these lines to the
elemental lines known on Earth,
astronomers realized that
elements like hydrogen, calcium,
iron, and many others must be
present in the Sun
The Formation of Spectral Lines
• Scientists now knew that spectral lines were
somehow tied to the elements that produced them
• Each spectral line and pattern of lines
corresponded uniquely to a given element
• In order to fully understand the processes behind
these lines, science needed to explain the
structure of these elements on an atomic scale
CONTINUOUS SPECTRA
EMISSION SPECTRA
ABSORPTION SPECTRA
The Atom
• During the early 1900's, physicist
began to realize that at a very
small level, all matter is made up
of atoms
• The center of an atom is the
nucleus, made up of protons (+
charge) and neutrons (no charge)
• Electrons (- charge) orbit the
nucleus at a set distance
• The distance that an electron
orbits the nucleus is set at very
specific levels (quantized)
The Hydrogen Atom
The Photon
• Physicists realized that something
must be adding energy to an atom
in order to change the orbit of an
electron
Outgoing photon
Grounded electron
Excited electron
• This led to the idea of light as a
particle or a photon
• A photon can be thought of a little
packet of energy that strikes the
atom and gives it extra energy
• After a very short time, the atom
releases that energy back in the
form of another photon
Grounded electron
Incoming photon
The Energy of a Photon
• If photons could be thought of as little light packets
of energy, what determined the energy of a
photon?
• Einstein proposed that the energy of a photon was
directly proportional to the frequency of the light
– The higher the frequency, the more energetic the photon
• German physicist Max Planck formulated the exact
relation: Energy = h x frequency
– h is a universal constant called Planck's constant
– h is equal to 6.63 x 10-34 Joules/Hz
Photon Energy Example
• Gamma rays are one of the highest energy forms of
light we encounter, with frequencies around 1022Hz
• What is the energy of a gamma ray?
E = h u
E = (6.63 1034 J
Hz
)  (1022 Hz )
E = 6.63 10
12
J
• One joule is about the energy it takes to lift an
apple one meter off the ground
– The energy of a single gamma ray photon is small, but is
enough to damage cells in the human body
Kirchoff's Laws Explained
• Suppose we have a cloud of
cool hydrogen gas in front of a
cluster of very hot stars
• A continuous spectra is
produced by the stars
• The hydrogen gas can absorb a
select few of these photons,
creating an absorption
spectrum
• Depending on how the electron
returns to the ground state, the
cloud may also emit light of its
own
Complexity of Spectra
• A simple atom like
hydrogen (one electron)
produces a simple
spectrum of just a few lines
Helium atom
• As the complexity of the
atom increases, so does
the complexity of the
spectra it produces
Carbon atom
• The more electrons an
atom has, the more options
it has to produce lines at
different wavelengths
Molecular Spectra
• Molecules, or groups of atoms, can
also emit spectra
• Sometime atoms will share electrons
in a molecule
– This changes the energy of the
electron and therefore the energy of
the photons the molecule can
produce
• Molecules can change vibration
speed, absorbing or releasing a
photon
• Molecules can also speed up or slow
down their rotation by absorbing or
releasing a photon
Radial Velocity
• Remember that the Doppler effect can shift light towards the
red or blue depending on whether it is moving toward or
away from us
• The same concept applies to spectral lines
• By measuring how far a line has shifted, astronomers can
precisely measure the velocity of the object
Spectroscopy
• When light is broken into its component colors
or spectra, sometimes distinct lines can be
seen at certain wavelengths
• These spectral lines can be dark or bright and
these lines are used to reveal information
about a sample
• Text Chapter 22
The Spectroscope
• An instrument used to
look at spectra is called
a spectroscope
• The main components
include:
– A slit to create a beam of
light
– A prism or grating to split
the light into a spectrum
– A screen or detector to
record the spectrum
• Simple or complex, all
spectroscopes follow
this design
SINGLE BEAM SPECTROPHOTOMER
DOUBLE BEAM SPECTROPHOTOMETER
GRATING MONOCHROMATOR
PRISM MONOCHROMATOR
Optical Materials
• visible region- silicate glass
• UV region- quartz
• infrared region- halide salts or
polymers
Continuous Spectra
• The spectra you see is
directly related the object's
blackbody curve
• A simple object, like a light
bulb, emits a continuous
spectra, with a smooth
transition from color to color
intensity
• When you use a
spectroscope to look at
different objects, you will
see each displays a different
spectra
wavelength
Emission Spectra
intensity
• For some objects, only
isolated bright lines are
seen against a black
background
• These lines are known as
an emission spectrum
• Notice that the peaks of
each of these lines still
follows a blackbody curve
wavelength
Absorption Spectra
• These lines are known as an
absorption spectra
• The blackbody curve is still
present, but it appears that
slices have been removed
intensity
• Some spectra appear
continuous except for the
presence of distinct dark
lines
wavelength