Transcript Slide 1
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 Frequency
( l ) = distance between peaks ( u ) = waves/sec, # of wave crests passing a point per unit time
Speed of light
(C) = 3 x 10 10 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) = h
u .
h= 6.626 x 10 -34 J·s (Plank's constant)
If E = h
u
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 • 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 Outgoing photon Excited electron Grounded electron
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 10 22 Hz • What is the energy of a gamma ray?
E
=
h
u
E
= ( 6 .
63 10 34
J Hz
) ( 10 22
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 • The more electrons an atom has, the more options it has to produce lines at different wavelengths Carbon atom
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
• When you use a spectroscope to look at different objects, you will see each displays a different 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 wavelength
Emission Spectra
• 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
• Some spectra appear continuous except for the presence of distinct dark lines • These lines are known as an absorption spectra • The blackbody curve is still present, but it appears that slices have been removed wavelength