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Spectroscopic Analysis
Part 2 – Electromagnetic Radiation
Chulalongkorn University, Bangkok, Thailand January 2012
Dr Ron Beckett
Water Studies Centre
School of Chemistry
Monash University, Melbourne, Australia
Email: [email protected]
Water
Studies
Centre
1
Spectroscopy
• Spectroscopy deals with the interaction of electromagnetic
radiation (EMR) with matter.
• For example the absorption of visible light by a solution
• The amount of light absorbed depends on the colour or
wavelength of the light and the concentration of the solution
• Spectroscopy can be used to identify compounds by
studying the absorbance versus wavelength plots (spectrum)
• Spectroscopy can be used to analyse the concentration of a
solution by measuring the amount of light absorbed
• There are other types of interactions of EMR with matter
e.g. fluorescence, scattering, photo-electron emission
2
Interactions of EMR with Matter
• Absorption – no re-emission energy dissipated as heat
• Absorption/Re-emission – if the same wavelength is
emitted then it is called light scattering
• Absorption/Re-emission – if different wavelength
then it is called fluorescence or phosphorescence
• Reflection from Surfaces – mirror, special geometry
• Photoemission of Electrons – EMR detectors
3
Types of Electromagnetic Radiation (EMR)
1. Visible light – colours (VIBGYOR)
2. Infrared – radiated heat
3. Ultraviolet – sunburn
4. X-rays - medicine
5. g-rays – cancer therapy
4
Dual Nature of EMR
Electromagnetic radiation has properties that
can be described in terms either:
1. A wave train consisting of oscillating
electric and magnetic fields travelling
through space
e.g. interference patterns, diffraction
2. A stream of particles (photons)
e.g. absorption and emission spectroscopy
photoemission of electrons from metals5
EMR as a Wave
(a)
(b)
6
EMR as a Wave
(a)
7
EMR as a Wave
(b)
8
Wavelength and Wavenumber
Wavelength (l in m) is the distance travelled during a
complete oscillation of the wave
Wavenumber (n- in m-1) is the reciprocal of wavelength
n =1/l
9
EMR can be Described by a Sine Wave
10
Period and Frequency
Period (T in s) is the time for a complete oscillation
Frequency (n in s-1 or Hertz i.e. Hz) is the number
of oscillations per second
n = 1/T
11
Velocity of EMR
The velocity of EMR (c in m s-1) in a given medium is
constant irrespective of the wavelength or frequency
In vacuum
c = 3.00 x 108 m s-1
And
c = l/T = ln
12
Effect of Medium on EMR
Air
Water
Air
• Frequency remains constant
• Velocity decreases with refractive index
• Wavelength decreases with RI
since during one oscillation the wave travels
a shorter distance in water than in air
13
Einstein’s Interpretation of the
Photoelectric Effect
EMR can eject electrons
from certain materials but
only if the frequency is
above a given threshold
e
Einstein argued that this must mean EMR consists of a
stream of particles (now called photons) and the energy
of a photon depends on the frequency of the EMR
For electrons to be emitted the photon energy must
exceed the work required for the electron to escape
from the material
14
EMR as a Particle
EMR consists of a stream of particles called photons
The energy of a photon (E in J photon-1 or J mol-1) is
E = hn
Or
E = hn N 0
(J photon-1 )
(J mol-1 )
Plank’s constant h = 6.63x10-34 J s
Avagadro’s number N0 = 6.02x1023 mol-1
15
Intensity of EMR
• Intensity is the energy transmitted through unit
area at right angles to the beam per second
• It is the energy per photon (Ephoton ) times the number
of photons passing unit area per second (N)
I = Ephoton x N
Units – J m-2 s-1
=hnN
Unit Area
16
The type of EMR depends on
wavelength, frequency, energy,etc
Infrared -
n
3x1012 – 3x1014 Hz
100 mm – 800 nm
Visible light -
n
3x1014 – 6x1014 Hz
800 nm – 400 nm
Ultraviolet -
n
6x1014 – 3x1016 Hz
800 nm – 10 nm
X-rays -
n
3x1016 – 3x1018 Hz
10 nm – 100 pm
l
l
l
l
17
Absorption or emission of specific types of EMR
cause changes in different kinds of
atomic or molecular energy
1. Microwave - molecular rotation
2. Infrared – molecular vibration
3. Visible light – outer shell electrons
4. Ultraviolet – outer valence electrons
5. X-rays – inner shell electrons
18
The type of EMR depends on wavelength, frequency, energy,etc
Absorption or emission of specific types of EMR cause changes
in different kinds of atomic or molecular energy
19
The EMR Spectrum
Electron ic
excitation
1020
1018
g-rays
X-rays
1016
UV
Visible
Cosmic
rays
n
Vibration
1014
Rotation
1012
In frared
108
Radio
Bon d b reaking
and ion ization
Microwave
Visib le Spectrum
400
500
600
Wavelength (nm)
700
20
Energy is Quantized
The energy of atoms and molecules is quantized.
They can only exist in allowed energy states or levels
Electronic energy levels
in a H atom
1s 2s 2p 3s 3p 3d
The lowest energy state
has the single electron in
the 1s orbital
1s1
21
Absorption and Emission of EMR
When EMR is absorbed or emitted by matter it
does so in whole photons only (NOT fractions)
Absorption involves
promotion from a
lower energy state to
a higher one
Emission results in a
jump from a higher
energy level to a
lower energy level
E2
DE = hn
E1
E2
DE = hn
22
E1
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