FOUNDATIONS OF SPECTROSCOPY Incande

FOUNDATIONS OF SPECTROSCOPY
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Diffraction Grating
Spectroscope
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Real Spectrum RReeaall SSppeeccttrruumm
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by DR. STEPHEN THOMPSON MR. JOE STALEY
The contents of this module were developed under grant award # P116B-001338 from the Fund for the Improvement of Postsecondary Education (FIPSE), United States Department of Education. However, those contents do not necessarily represent the policy of FIPSE and the Department of Education, and you should not assume endorsement by the Federal government.
FOUNDATIONS OF SPECTROSCOPY
CONTENTS
2 Introduction 3 Electromagnetic Radiation Ruler: The ER Ruler 4 Numbering Electromagnetic Radiation 5 Waves 6 Frequency And Wavelength 7 Questions For The ER Ruler 8 Fundamental Processes 9 Spectroscope 10 Interference: The Principle By Which Light Is Broken Up 11 Diffraction 12 An Atomic Line Emission Spectrum From A Discharge Tube 13 Fluorescent Lights 14 Continuous Emission 15 The Origins Of Band Spectra 16 Origins Of band Spectra 17 The Frank-Condon Principle 18 Band Spectra 19 Lasers 20 Simple Spectra 21 Complex Spectra 22 Spectral Questions
FOUNDATIONS OF SPECTROSCOPY
INTRODUCTION
Spectroscopy is the study of the interaction of electromagnetic radiation with matter. When matter is energized (excited) by the application of thermal, electrical, nuclear or radiant energy, electromagnetic radiation is often emitted as the matter relaxes back to its original (ground) state. The spectrum of radiation emitted by a substance that has absorbed energy is called an emission spectrum and the science is appropriately called emission spectroscopy. Another approach often used to study the interaction of electromagnetic radiation with matter is one whereby a continuous range of radiation (e.g., white light) is allowed to fall on a substance; then the frequencies absorbed by the substance are examined. The resulting spectrum from the substance contains the original range of radiation with dark spaces that correspond to missing, or absorbed, frequencies. This type of spectrum is called an absorption spectrum. In spectroscopy the emitted or absorbed radiation is usually analyzed, i.e., separated into the various frequency components, and the intensity is measured by means of an instrument called a spectrometer. The resultant spectrum is mainly a graph of intensity of emitted or absorbed radiation versus wavelength or frequency. There are in general three types of spectra: continuous, line, and band. The sun and heated solids produce continuous spectra in which the emitted radiation contains all frequencies within a region of the electromagnetic spectrum. A rainbow and light from a light bulb are examples of continuous spectra. Line spectra are produced by excited atoms in the gas phase and contain only certain frequencies, all other frequencies being absent. Each chemical element of the periodic chart has a unique and, therefore, characteristic line spectrum. Band spectra are produced by excited molecules emitting radiation in groups of closely spaced lines that merge to form bands. These categories of emission and absorption spectra contain tremendous amounts of useful information about the structure and composition of matter. Spectroscopy is a powerful and sensitive form of chemical analysis, as well as a method of probing electronic and nuclear structure and chemical bonding. The key to interpreting this spectral information is the knowledge that certain atomic and molecular processes involve only certain energy ranges. Page 3 shows the regions of the electromagnetic spectrum and the associated energy transitions that occur in atomic and molecular processes.
Much of the scientific knowledge of the structure of the universe, from stars to atoms, is derived from interpretations of the interaction of radiation with matter. One example of the power of these techniques is the determination of the composition, the velocities, and the evolutionary dynamics of stars. The source of the incredible amount of energy produced by the sun is nuclear fusion reactions going on within the hot interior (temperature 40 x 106K). Two fusion cycles, the carbon cycle and the proton cycle, convert hydrogen nuclei into helium nuclei via heavier nuclei, such as carbon 12 and nitrogen 14. The enormous radiation of energy from the hot core seethes outwards by convection. This radiation consists of the entire electromagnetic spectrum as a continuous spectrum. Towards the surface of the sun (the photosphere), the different elements all absorb at their characteristic frequencies. The radiation that shoots into space toward earth is a continuous emission spectrum with about 22,000 dark absorption lines present in it (Fraunhofer lines), of which about 70% have been identified. These absorption lines - i.e., missing frequencies - prove that more than 60 terrestrial elements are certainly present in the sun.
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FOUNDATIONS OF SPECTROSCOPY
ELECTROMAGNETIC RADIATION RULER: THE ER RULER
Energy Level Transition Energy Wavelength
Joules 10-27
Plank’s constant, h = 6.63 x 10-34J s. Nuclear and electron spin
The speed of light, c = 3.0 x 108 m s-1.
10-26
10-25
109
nm 104 106 108 1010 10-2 10-4 10-6 1 RF μμW UV X γγIR 2 4 6 2 4 6 8 2 4 6 4 110022 6 8 2 10-1 10
W
avelength aavveelleennggtthh
10-24
10-23
700 nm
Molecular rotations
107
10-22
650 nm
10-21
105
Molecular vibrations
10-20
600 nm
10-19
1011003 VIS
550 nm
Valence electrons
10-18
Middle-shell electrons
10-17
500 nm
10-16 450 nm
Inner-shell electrons
10-15
10-14
400 nm
10-13
10-3
RF = Radio frequency radiation
Nuclear
10-12
μW = Microwave radiation IR = Infrared radiation VIS = Visible light radiation
10-11
UV = Ultraviolet radiation
10-5
X = X-ray radiation 10-10
γ = gamma ray radiation
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FOUNDATIONS OF SPECTROSCOPY
NUMBERING ELECTROMAGNETIC RADIATION
Radiation may be described in one of two ways: either as a stream of energy pulses (photons) or as energy waves sent out from a source at the speed of light. Scientists use whichever interpretation works best to explain an experiment involving radiation. The photon and wave theories are linked by Plank’s law:
E = hν
where E is the photon energy in Joules (J), ν is the frequency of the radiation (Hz or s-1) and h is Plank’s constant (6.63 x 10-34 J s).
Wavelength and frequency are related by c = λν
where c is the speed of light (3 x 108 m s-1), λ is the wavelength of the radiation (often reported in nm), and ν is the frequency.
PREFIXES AND SCIENTIFIC NOTATION Prefix Symbol Value Yotta Y 1,000,000,000,000,000,000,000,000 Zetta Z 1,000,000,000,000,000,000,000 Exa E 1,000,000,000,000,000,000 Peta P 1,000,000,000,000,000 tera T 1,000,000,000,000 giga G 1,000,000,000 mega M 1,000,000 kilo k 1,000 hecto h 100 deka da 10
1.0
deci d 0.1 centi c 0.01 milli m 0.001 micro µ 0.000001 nano n 0.000000001 pico p 0.000000000001 femto f 0.000000000000001 atto a 0.000000000000000001 zepto z 0.000000000000000000001 yocto y 0.000000000000000000000001
How many meters are in a nanometer?
How many megabytes in a gigabyte? How many kilobytes in a terabyte? What is the frequency of a one nanometer photon? What is its energy? A rough calculation suggests that there might be 1025 stars in the observable universe. How many Yotta
stars is that?
Could you see a 500 nm speck of dust? Why? You can calculate the kinetic energy, in Joules, of a moving object by multiplying its mass, in kilograms,
times the square of its velocity, in meters per second,
and dividing the result by two. What is the kinetic energy of a 1.0 Mgram elephant flying at 10 meters per second? Of a 10 gram meteoroid moving at Mmeters per second?
Notation 1024 1021 1018 1015 1012 109 106 103 102
101 100 10-1 10-2 10-3 10-6 10-9
10-12 10-15 10-18 10-21 10-24
Some Greek letters which we will use: α = “alpha”; as in α particle. β = “beta”; as in β particle. γ = “gamma”; as in γ ray. δ = “delta”; used with a plus or minus sign
to represent partial charges in
polarized molecules. Δ = “delta”; a capital delta is used for a
change in a quantity. λ = “lambda”; used for wavelength. ν = “nu”; used for frequency. μ = “mu”; used for the prefix micro. π = “pi” is the ratio of the circumference of
a circle to its diameter. Ω = “omega”; a capital omega is used for
electrical resistance.
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FOUNDATIONS OF SPECTROSCOPY
WAVES
The fisherman rises and falls with the passing waves. The (vertical) distance the fisherman moves through is called the amplitude of the wave. The distance between crests of the wave is called the wavelength. The time it takes for the fisherman to go from one wave top (crest), down to the bottom (trough) and back up to the top is called the period; its inverse is called the frequency. If the sea is choppy we could say that the number of waves going by the fisherman in a second is the frequency.
There is a general relationship for all waves that the velocity of the wave is the product of the wavelength times the frequency.
v = λν
Measure the wavelength and amplitude, in centimeters, of the fisherman’s wave.
If the frequency of the wave is 2.5 Hz, calculate its velocity.
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FOUNDATIONS OF SPECTROSCOPY
FREQUENCY AND WAVELENGTH
TIME
0.000
s
0.125
s
0.250
s
0.375
s
0.500s
0.625
s
0.750
s
0.875
s
1.000
s
0.000 s 0.125 s
0.250
s
0.375
s
0.500s
0.625
s
0.750
s
0.875
s
1.000 s
Each of the vertical sequences of pictures above shows a wave moving to the right. Since electromagnetic radia-Does the wave on the left have the same tion always travels at the same velocity (in vacuum and nearly so in air) and that velocity is equal to the product of wavelength and frequency, then increasing the frequency must decrease the wavelength.
velocity as the wave on the right?
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FOUNDATIONS OF SPECTROSCOPY
QUESTIONS FOR THE ER RULER
Plank’s constant, h = 6.63 x 10-34 J s