Introduction to Spectroscopy

Introduction to Spectroscopy
ER. FARUK BIN POYEN
ASST. PROFESSOR
DEPT. OF AEIE, UIT, BU
faruk.poyen@gmail.com

Contents:
 1. Introduction
 2. Classification of Methods
 3. Atomic Transitions – Theory
 4. Absorption Spectroscopy
 5. Emission Spectroscopy
 6. Scattering Spectroscopy
 7. Atomic Fluorescence Spectroscopy
 8. Few Definitions and Terminologies
 9. Differences between similar terms
 10. What is Electromagnetic Radiation?
 11. Photon as a Signal Source
 12. Basic Components of Spectroscopic Instruments
2

Introduction:
 Spectroscopy is the study of the interaction of electromagnetic radiation in all its forms
with matter. The interaction might give rise to electronic excitations, (e.g. UV), molecular
vibrations (e.g. IR) or nuclear spin orientations (e.g. NMR). Thus Spectroscopy is the
science of the interaction of energy, in the form of electromagnetic radiation (EMR),
acoustic waves, or particle beams, with matter.
 Analytical instruments exploit spectroscopy for both identification (qualitative analysis)
and measurement (quantitative analysis) of atoms, ions and molecules.
 Can be used alone or in combination with chromatography to achieve both separation and
measurement of multicomponent samples.
 The optical system that allows production and viewing of spectrum (VIBGYOR, and
invisible to plain eyesight) is called spectroscope.
 Spectrometry: The measurement of the intensity (I) of radiation, in the form of EMR or
high energy particles (electrons, ions, etc.), with some form of electronic device.
3

 Spectrometer: An instrument that can measure radiation intensity as a function of energy
(E), frequency (ν), wavelength (λ), wavenumber (σ), etc. in order to obtain a spectrum
 Spectrophotometer: A spectrometer that uses a photon detector to measure the ratio of
the radiant power incident on (P0) and emergent power from (P) a sample of matter, as a
function of photon wavelength (λ), frequency (ν), wavenumber (σ) or energy (E photon).
 To see a molecule, the wavelength of light must be smaller than the molecule (1 to 15
angstrom units X – ray region).
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Basic Types:
 Mass Spectrometry: Sample molecules are ionized by high energy electrons. The mass to
charge ratio of these ions is measured very accurately by electrostatic acceleration and
magnetic field perturbation, providing a precise molecular weight. Ion fragmentation
patterns may be related to the structure of the molecular ion.
 Ultraviolet-Visible (UV – Vis) Spectroscopy: Absorption of this relatively high-energy
light causes electronic excitation. The easily accessible part of this region (wavelengths of
200 to 800 nm) shows absorption only if conjugated pi-electron systems are present.
 Infrared (IR) Spectroscopy: Absorption of this lower energy radiation causes vibrational
and rotational excitation of group of atoms within the molecule. As of their characteristic
absorptions, identification of functional groups is easily accomplished.
 Nuclear Magnetic Resonance (NMR) Spectroscopy: Absorption in the low-energy radio-
frequency part of the spectrum causes excitation of nuclear spin states. NMR
spectrometers are tuned to certain nuclei (e.g. 1H, 13C, 19F & 31P). For a given type of
nucleus, high-resolution spectroscopy distinguishes and counts atoms in different
locations in the molecule.
5
Basis: Each chemical element has its own characteristic spectrum.

Classification:
 Spectroscopic measurement is on the basis on either energy absorbed or emitted.
 Methods are differentiated as either atomic (apply on atoms) or molecular (apply on
molecules).
 Absorption Spectroscopy: It uses the range of the EM spectra in which a substance
absorbs. This includes atomic absorption spectroscopy and various molecular techniques,
such as infra-red spectroscopy in that region and Nuclear Magnetic Resonance
spectroscopy in the radio region.
 Emission Spectroscopy: It uses the range of EM spectra in which a substance radiates
(emits). The substance first must absorb energy. This energy can be from a variety of
sources, which determines the name of the subsequent emission, like luminescence.
Molecular luminescence techniques include Spectroflourimetry.
 Scattering Spectroscopy: It measures the amount of light that a substance scatters at
certain wavelengths, incident angles, and polarization angles. The scattering process is
much faster than the absorption/emission process. One of the most useful applications of
light scattering spectroscopy is Raman Spectroscopy.
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Classification:
 Absorption Spectroscopy - Emission Spectroscopy - Scattering Spectroscopy
7

Common Types:
 Fluorescence spectroscopy
 X-ray spectroscopy and crystallography
 Flame spectroscopy
Atomic emission spectroscopy
Atomic absorption spectroscopy
Atomic fluorescence spectroscopy
 Plasma emission spectroscopy
 Spark or arc emission spectroscopy
 UV/VIS spectroscopy IR spectroscopy
 Raman spectroscopy
 NMR spectroscopy
 Photo thermal spectroscopy
 Thermal infra-red spectroscopy
 Mass Spectroscopy
8

Atomic Transitions - Theory
 Atomic electron transition is a change of an electron from one energy level to another
within an atom or artificial atom.
 The probability that an atomic spectroscopic transition will occur is called the transition
probability or transition strength.
 This probability will determine the extent to which an atom will absorb light at a resonant
frequency, and the intensity of the emission lines from an atomic excited state.
 The spectral width of a spectroscopic transition depends on the widths of the initial and
final states.
 The width of the ground state is essentially a delta function and the width of an excited
state depends on its lifetime.
9

Terminologies: Colorimetry
 This is the technique used to determine the concentration of a solution having color.
 It measures the intensity of color and relates the intensity to the concentration of the
sample.
 In colorimetry, the color of the sample is compared with a color of a standard in which the
color is known.
 Colorimeter is the equipment used to measure the colored samples and gives the
appropriate absorptions.
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Terminologies: Spectroscopy
 It is the science of studying the interaction between matter and radiated energy.
 To understand spectroscopy, one must first understand spectrum.
 The visible light is a form of electromagnetic waves. There are other forms of EM waves such
as X-Rays, Microwaves, Radio waves, Infrared and Ultraviolet rays.
 The energy of these waves is dependent on the wavelength or the frequency of the wave.
 High frequency waves have high amounts of energies, and low frequency waves have low
amounts of energies.
 The light waves are made up of small packets of waves or energy known as photons. For a
monochromatic ray, the energy of a photon is fixed.
 The electromagnetic spectrum is the plot of the intensity versus the frequency of the photons.
 When a beam of waves having the whole range of wavelengths is passed through some liquid
or gas, the bonds or electrons in these materials absorb certain photons from the beam. It is due
to the quantum mechanical effect that only photons with certain energies get absorbed. This
can be understood using the energy level diagrams of atoms and molecules. Spectroscopy is
studying the incident spectrums, emitted spectrums and absorbed spectrums of materials.
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Terminologies: Spectrometry
 It is the method used for the study of certain spectrums.
 Ion-mobility spectrometry, mass spectrometry (MS), Rutherford backscattering spectrometry,
and neutron triple axis spectrometry are the main forms of spectrometry.
 In these cases, a spectrum does not necessarily mean a plot of intensity versus frequency.
 For example, the spectrum for MS is the plot between intensity (number of incident particles)
versus the mass of the particle.
 Spectrometers are the instruments used in spectrometry. The operation of each type of
instrument depends on the form of spectrometry used in the instrument.
 Spectrophotometry is the quantitative measurement of the reflection or transmission properties
of a material as a function of wavelength.
 For the visible region, the perfect white light contains all the wavelengths within the region.
Assume, white light is sent through a solution absorbing photons with a wavelength of 570
nm. This means the red photons of the spectrum is now reduced. This will cause a blank or
reduced intensity at the 570 nm mark of the plot of intensity versus wavelength. The intensity
of the light passed, as a proportion to the light projected, can be plotted for some known
concentrations, and the resultant intensity from the unknown sample can be used to determine
the concentration of the solution.
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Terminologies: Photometry
 The term ―photo‖ means light and the term ―metry‖ refers to measurement.
 Photometry is the science of the measurement of light, in terms of its perceived brightness
to the human eye. In photometry, the standard is the human eye.
 The sensitivity of the human eye to different colors is different.
 This has to be considered in photometry.
 Therefore, amplification methods are used so that the effect from each color would be
same as that of the eye.
 Since the human eye is only sensitive to visible light, photometry only falls in that range.
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Terminologies: Spectrophotometry
 Spectrometers have developed into electronically operated complex machines, but they
share the same principle as the initial spectrometers made by Fraunhofer.
 Modern spectrometers use a monochromatic light that passes through a liquid solution of
the material and a photodetector detects the light.
 The changes of the light compared to the source light allow the instrument to output a
graph of the absorbed frequencies. This graph indicates the characteristic transitions in the
sample material. These types of advanced spectrometers are also called
spectrophotometers because it is a spectrometer and photometer combined into a single
device. The process is known as the spectrophotometry.
 Spectrophotometer is the instruments used in this technique. It has two main parts, the
spectrometer, which produces the light with a selected color, and the photometer, which
measures the intensity of light. There is a cuvette where we can place our liquid sample.
The liquid sample will have a color, and it absorbs the complementary color of it when a
light beam is passed through that. The color intensity of the sample is related to the
concentration of the substance in the sample. Therefore, that concentration can be
determined by the extent of absorption of light at the given wavelength.
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Terminologies: Refraction, Reflection
 Refraction: Refraction is an object’s visual proportion being distorted or refined when
passing from one state to another through an angle. Refraction is the bending of light as it
passes from one substance to another. Here, the light ray passes from air to glass and back
to air. The bending is caused by the differences in density between the two substances.
 Reflection: A reflection is a result of light bouncing off an object and hitting another clear
surface, giving out an object’s mirror-like image. Reflection occurs when light changes
direction as a result of "bouncing off" a surface like a mirror
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Terminologies: Diffraction, Scattering
 Diffraction: The process by which a beam of light or other system of waves is spread out
as a result of passing through a narrow aperture or across an edge, typically accompanied
by interference between the wave forms produced.
 Diffraction involves a change in direction of waves as they pass through an opening or
around a barrier in their path.
 Scattering: In scattering, light is intercepted by an object and sent off in many directions;
this movement may appear to be random and not following the law of reflection. Rayleigh
scattering refers to the scattering of light by molecules in air, and is what causes the sky to
have a blue color. As because this type of scattering is proportional to the 4th power of the
frequency, blue light (which has the highest frequency of visible light) scatters the most.
 Rayleigh scattering can be considered an elastic collision of a photon with an atom or
molecule.
 Another type, Raman scattering, is due to an inelastic collision with a molecule, and is
used by chemists and physicists to measure the vibrational quantum state of molecules.
16

Absorption Spectroscopy
 Absorption spectroscopy uses the range of electromagnetic spectra into which a substance
can be absorbed.
 ―Absorption‖ is the phenomenon that occurs when a transition from a lower level to a
higher level takes place with transfer of energy from the radiation field to the atom or
molecule.
 When atoms or molecules absorb light, the incoming energy excites a structure (in energy
quanta) to a higher energy level.
 The type of excitation depends on the light wavelength. Electrons are promoted to higher
orbits by ultraviolet or visible light.
 Vibrations are excited by infrared light and microwaves excite the rotations.
 An absorption spectrum is a way to represent the absorption of light as a function of
wavelength.
 The spectrum of an atom or molecule depends on its energy-level structure, and
absorption spectra are useful for identifying compounds
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 Atomic-absorption (AA) spectroscopy uses the absorption of light to measure the
concentration of gas-phase atoms.
 Since samples are usually liquids or solids, the analyte atoms or ions must be vaporized in
a flame or graphite furnace.
 The atoms absorb ultraviolet or visible light and make transitions to higher electronic
energy levels.
 The analyte concentration is determined from the amount of absorption.
 Applying the Beer-Lambert law directly in AA spectroscopy is difficult due to variations
in the atomization efficiency from the sample matrix, and non-uniformity of concentration
and path length of analyte atoms (in graphite furnace AA).
 Concentration measurements are usually determined from a working curve after
calibrating the instrument with standards of known concentration.
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 Light source
The light source is usually a hollow-cathode lamp of the element that is being measured.
Lasers are also used in research instruments. Since lasers are intense enough to excite atoms
to higher energy levels, they allow AA and atomic fluorescence measurements in a single
instrument. The disadvantage of these narrow-band light sources is that only one element is
measurable at a time.
 Atomizer
AA spectroscopy requires that the analyte atoms be in the gas phase. Ions or atoms in a
sample must undergo desolvation and vaporization in a high-temperature source such as a
flame or graphite furnace. Flame AA can only analyze solutions, while graphite furnace AA
can accept solutions, slurries, or solid samples.
Flame AA uses a slot type burner to increase the path length, and therefore to increase the
total absorbance (see Beer-Lambert law). Sample solutions are usually aspirated with the gas
flow into a nebulizing/mixing chamber to form small droplets before entering the flame.
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The graphite furnace has several advantages over a flame. It is a much more efficient
atomizer than a flame and it can directly accept very small absolute quantities of sample. It
also provides a reducing environment for easily oxidized elements. Samples are placed
directly in the graphite furnace and the furnace is electrically heated in several steps to dry
the sample, ash organic matter, and vaporize the analyte atoms.
 Light separation and detection
AA spectrometers use monochromators and detectors for UV and visible light. The main
purpose of the monochromator is to isolate the absorption line from background light due to
interferences. Simple dedicated AA instruments often replace the monochromator with a
bandpass interference filter. Photomultiplier tubes are the most common detectors for AA
spectroscopy.
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 Change of position of energy bands.
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Emission Spectroscopy
 ―Emission‖ occurs during transition from a higher level to a lower level if energy is
transferred to the radiation field.
 When no radiation is emitted the phenomenon is called ―nonradiative decay.‖ This type
of spectroscopy relies on the range of electromagnetic spectra in which a particular
substance radiates.
 The substance first absorbs energy and then radiates (that is, emits) this energy as light.
The excitement energy that is absorbed first can come from a number of different sources,
including collision (from high temperatures or other means), chemical reactions or light.
 It also should be noted that atoms or molecules once excited to high-energy levels then
can decay to lower levels by emitting radiation.
 This is called emission or luminescence.
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 When atoms are excited by a high-temperature energy source this light emission
commonly is called atomic or optical emission, and for atoms excited with light, it is
called atomic fluorescence.
 Atomic emission spectroscopy (AES or OES [optical emission spectroscopy]) uses
quantitative measurement of the optical emission from excited atoms to determine analyte
concentration.
 Analyte atoms in solution are aspirated into the excitation region where they are
dissolved, vaporized, and atomized by a flame, discharge, or plasma.
 These high-temperature atomization sources provide sufficient energy to promote the
atoms into high energy levels.
 The atoms decay back to lower levels by emitting light. Since the transitions are between
distinct atomic energy levels, the emission lines in the spectra are narrow.
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 The spectra of samples containing many elements can be very congested, and spectral
separation of nearby atomic transitions requires a high-resolution spectrometer.
 Since all atoms in a sample are excited simultaneously, they can be detected
simultaneously using a polychromator with multiple detectors.
 This ability to simultaneously measure multiple elements is a major advantage of AES
compared to atomic-absorption (AA) spectroscopy.
24

Scattering Spectroscopy
 ―Scattering‖ refers to light that is changed in direction (called redirection) from its
interaction with matter.
 It may or may not occur with energy transfer.
 This spectroscopy form measures certain physical properties by determining the amount
of light that a particular substance scatters at different wavelengths, incident angles and
light polarization angles.
 It differs from other spectroscopy types primarily because of speed.
 The scattering process is much faster than absorption or emission.
25

Atomic-Fluorescence Spectroscopy (AFS)
 Atomic fluorescence is the optical emission from gas-phase atoms that have been excited
to higher energy levels by absorption of electromagnetic radiation.
 The main advantage of fluorescence detection compared to absorption measurements is
the greater sensitivity achievable because the fluorescence signal has a very low
background.
 The resonant excitation provides selective excitation of the analyte to avoid interferences.
AFS is useful to study the electronic structure of atoms and to make quantitative
measurements.
 Analytical applications include flames and plasmas diagnostics, and enhanced sensitivity
in atomic analysis.
 As because of the differences in the nature of the energy-level structure between atoms
and molecules, discussion of laser-induced fluorescence (LIF) from molecules is found in
a separate document.
26

Atomic-Fluorescence Spectroscopy (AFS)
 Analysis of solutions or solids requires that the analyte atoms be dissolved, vaporized, and
atomized at a relatively low temperature in a heat pipe, flame, or graphite furnace.
 A hollow-cathode lamp or laser provides the resonant excitation to promote the atoms to
higher energy levels.
 The atomic fluorescence is dispersed and detected by monochromators and
photomultiplier tubes, similar to atomic-emission spectroscopy instrumentation.
27

Difference: Colorimetry and Spectrophotometry
 A colorimeter quantifies color by measuring three primary color components of light (red,
green, blue), whereas spectrophotometer measures the precise color in the human-visible
light wavelengths. .
 Colorimetry uses fixed wavelengths, which are in the visible range only, but
spectrophotometry can use wavelengths in a wider range (UV and IR also).
 Colorimeter measures the absorbance of light, whereas the spectrophotometer measures
the amount of light that passes through the sample.
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Difference: Photometry and Spectrophotometry
 Spectrophotometry is applied to the whole electromagnetic spectrum, but photometry is
only applicable to the visible light.
 Photometry measures the total brightness as seen by the human eye, but
spectrophotometry measures the intensity at each wavelength on the whole range of the
electromagnetic spectrum for which the measurements are necessary.
29

Difference: Spectrometry and Spectroscopy
 Spectroscopy is the science of studying the interaction between matter and radiated
energy while spectrometry is the method used to acquire a quantitative measurement of
the spectrum.
 Spectroscopy does not generate any results. It is the theoretical approach of science.
Spectrometry is the practical application where the results are generated.
30

Difference: Diffraction and Refraction
 Diffraction is bending or spreading of waves around an obstacle, while refraction is
bending of waves due to change of speed.
 Both diffraction and refraction are wavelength dependent. Hence, both can split white
light in to its component wavelengths.
 Diffraction of light produces a fringe pattern, whereas refraction creates visual illusions
but not fringe patterns.
 Refraction can make objects appear closer than they really are, but diffraction can not do
that.
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Difference: Absorption and Emission Spectra
 When an atom or molecule excites, it absorbs a certain energy in the electromagnetic
radiation; therefore, that wavelength will be absent in the recorded absorption spectrum.
 When the species come back to the ground state from the excited state, the absorbed
radiation is emitted, and it is recorded. This type of spectrum is called an emission
spectrum.
 In simple terms, absorption spectra records the wavelengths absorbed by the material,
whereas emission spectra records wavelengths emitted by materials, which have been
stimulated by energy before.
 Compared to the continuous visible spectrum, both emission and absorption spectra are
line spectra because they only contain certain wavelengths.
 In an emission spectrum there’ll be only few colored bands in a dark back ground. But in
an absorption spectrum there’ll be few dark bands within the continuous spectrum. The
dark bands in the absorption spectrum and the colored bands in the emitted spectrum of
the same element are similar.
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Difference: Spectrometer and Spectrophotometer
 Spectroscopy is the study of methods of producing and analyzing spectra using spectrometers,
spectroscopes, and spectrophotometers.
 The basic spectrometer developed by Joseph von Fraunhofer is an optical device that can be
used to measure the properties of light. It has a graduated scale that allows the wavelengths of
the specific emission/absorption lines to be determined by measuring the angles.
 Spectrophotometer is a development from the Spectrometer, where a spectrometer is combined
with a photometer to read relative intensities in the spectrum, rather than the wavelengths of
emission/absorption.
 Spectrometers were only used in the visible region of the EM spectrum, but spectrophotometer
can detect IR, visible, and UV ranges.
 A spectrometer is an optical instrument used to measure properties of light over a specific
portion of the electromagnetic spectrum. The variable measured is most often the light's
intensity but could also, for instance, be the polarization state.
 A spectrophotometer is a photometer (a device for measuring light intensity) that can measure
intensity as a function of the color, or more specifically, the wavelength of light.
33

Electromagnetic (EM) Radiation
 EM radiation—light—is a form of energy whose behavior is described by the properties
of both waves and particles. Some properties of EM radiation, such as its refraction when
it passes from one medium to another, are explained best by describing light as a wave.
Other properties, such as absorption and emission, are better described by treating light as
a particle.
Particle Properties of EM Radiation
When matter absorbs EM radiation it undergoes a change in energy. The interaction between
matter and EM radiation is easiest to understand if we assume that radiation consists of a
beam of energetic particles called photons. When a photon is absorbed by a sample it is
―destroyed,‖ and its energy acquired by the sample.
The energy of a photon, in joules, is related to its frequency, wavelength, and wavenumber by
the following equalities
𝐸 = hν = hc / λ =hcν
where h is Planck’s constant, which has a value of 6.626 × 10–34 J⋅s
34

Photon as a Signal Source
 There are several characteristic properties of EM radiation, including its energy, velocity,
amplitude, frequency, phase angle, polarization, and direction of propagation.
 A spectroscopic measurement is possible only if the photon’s interaction with the sample
leads to a change in one or more of these characteristic properties.
 We can divide spectroscopy into two broad classes of techniques.
 In one class of techniques there is a transfer of energy between the photon and the sample.
 In the second broad class of spectroscopic techniques, the electromagnetic radiation
undergoes a change in amplitude, phase angle, polarization, or direction of propagation as
a result of its refraction, reflection, scattering, diffraction, or dispersion by the sample.
35

Photon as a Signal Source
Table1 Examples of Spectroscopic Techniques Involving an Exchange of
Energy Between a Photon and the Sample
Type of Energy
Transfer
Region of
Electromagnetic
Spectrum
Spectroscopic Techniquea
Absorption γ-ray Mossbauer spectroscopy
X-ray X-ray absorption spectroscopy
UV/Vis UV/Vis spectroscopy
atomic absorption spectroscopy
IR infrared spectroscopy
raman spectroscopy
Microwave microwave spectroscopy
Radio wave electron spin resonance spectroscopy
nuclear magnetic resonance
spectroscopy
Emission (thermal
excitation)
UV/Vis atomic emission spectroscopy
Photoluminescence X-ray X-ray fluorescence
UV/Vis fluorescence spectroscopy
phosphorescence spectroscopy
atomic fluorescence spectroscopy
Chemiluminescence UV/Vis chemiluminescence spectroscopy
36
Table 2 Examples of Spectroscopic Techniques That Do Not
Involve an Exchange of Energy Between a Photon and the
Sample
Region of
Electromagnetic
Spectrum
Type of
Interaction
Spectroscopic Technique
X-ray diffraction X-ray diffraction
UV/Vis refraction refractometry
scattering nephelometry
turbidimetry
dispersion optical rotary dispersion

Basic Components of Spectroscopic Instruments
 Sources of Energy
 Wavelength Selection
 Detectors
 Signal Processors
37

Basic Components – Sources of Energy
 All forms of spectroscopy require a source of energy.
 In absorption and scattering spectroscopy this energy is supplied by photons.
 Emission and photoluminescence spectroscopy use thermal, radiant (photon), or chemical
energy to promote the analyte to a suitable excited state.
 Chemical Sources of Energy: Exothermic reactions also may serve as a source of energy.
In chemiluminescence the analyte is raised to a higher-energy state by means of a
chemical reaction, emitting characteristic radiation when it returns to a lower-energy state.
When the chemical reaction results from a biological or enzymatic reaction, the emission
of radiation is called bioluminescence. Commercially available ―light sticks‖ and the flash
of light from a firefly are examples of chemiluminescence and bioluminescence.
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 Sources of Electromagnetic Radiation. A source of electromagnetic radiation must provide
an output that is both intense and stable. Sources of electromagnetic radiation are
classified as either continuum or line sources. A continuum source emits radiation over a
broad range of wavelengths, with a relatively smooth variation in intensity. A line source,
on the other hand, emits radiation at selected wavelengths.
 Sources of Thermal Energy. The most common sources of thermal energy are flames and
plasmas. Flames sources use the combustion of a fuel and an oxidant to achieve
temperatures of 2000–3400 K. Plasmas, which are hot, ionized gases, provide
temperatures of 6000–10 000 K.
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 Common Sources of Electromagnetic Radiation
40
Source Wavelength Region Useful for...
H2 and D2 lamp continuum source from 160–380 nm molecular absorption
Tungsten lamp continuum source from 320–2400 nm molecular absorption
Xe arc lamp continuum source from 200–1000 nm molecular fluorescence
Nernst glower continuum source from 0.4–20 μm molecular absorption
Globar continuum source from 1–40 μm molecular absorption
Nichrome wire continuum source from 0.75–20 μm molecular absorption
Hollow cathode lamp line source in UV/Visible atomic absorption
Hg vapor lamp line source in UV/Visible molecular fluorescence
Laser line source in UV/Visible/IR atomic and molecular absorption,
fluorescence, and scattering

Basic Components – Wavelength Selection
 The ideal wavelength selector has a high throughput of radiation and a narrow effective
bandwidth.
 A high throughput is desirable because more photons pass through the wavelength
selector, giving a stronger signal with less background noise.
 A narrow effective bandwidth provides a higher resolution, with spectral features
separated by more than twice the effective bandwidth being resolved.
 These two features of a wavelength selector generally are in opposition.
 Conditions favoring a higher throughput of radiation usually provide less resolution.
 Decreasing the effective bandwidth improves resolution, but at the cost of a noisier signal.
 For a qualitative analysis, resolution is usually more important than noise, and a smaller
effective bandwidth is desirable. In a quantitative analysis less noise is usually desirable.
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 Wavelength Selection Using Filters:
 The simplest method for isolating a narrow band of radiation is to use an absorption or
interference filter.
 Absorption filters work by selectively absorbing radiation from a narrow region of the
electromagnetic spectrum.
 Interference filters use constructive and destructive interference to isolate a narrow range
of wavelengths.
 A simple example of an absorption filter is a piece of colored glass. A purple filter, for
example, removes the complementary color green from 500–560 nm.
 Commercially available absorption filters provide effective bandwidths of 30–250 nm,
although the throughput may be only 10% of the source’s emission intensity at the low
end of this range. Interference filters are more expensive than absorption filters, but have
narrower effective bandwidths, typically 10–20 nm, with maximum throughputs of at
least 40%.
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 Wavelength Selection Using Monochromators:
 A filter has one significant limitation—because a filter has a fixed nominal wavelength, if
you need to make measurements at two different wavelengths, then you need to use two
different filters.
 A monochromator is an alternative method for selecting a narrow band of radiation that
also allows us to continuously adjust the band’s nominal wavelength.
 Radiation from the source enters the monochromator through an entrance slit.
 The radiation is collected by a collimating mirror, which reflects a parallel beam of
radiation to a diffraction grating.
 The diffraction grating is an optically reflecting surface with a large number of parallel
grooves.
 The diffraction grating disperses the radiation and a second mirror focuses the radiation
onto a planar surface containing an exit slit. In some monochromators a prism is used in
place of the diffraction grating.
43

 Schematic diagram of a monochromator that uses a diffraction grating to disperse the
radiation.
44

 Radiation exits the monochromator and passes to the detector.
 A monochromator converts a polychromaticsource of radiation at the entrance slit to a
monochromatic source of finite effective bandwidth at the exit slit.
 The choice of which wavelength exits the monochromator is determined by rotating the
diffraction grating. A narrower exit slit provides a smaller effective bandwidth and better
resolution, but allows a smaller throughput of radiation.
 Monochromators are classified as either fixed-wavelength or scanning.
 In a fixed-wavelength monochromator we select the wavelength by manually rotating the
grating. Normally a fixed-wavelength monochromator is used for a quantitative analysis where
measurements are made at one or two wavelengths.
 A scanning monochromator includes a drive mechanism that continuously rotates the grating,
allowing successive wavelengths to exit from the monochromator. Scanning monochromators
are used to acquire spectra, and, when operated in a fixed-wavelength mode, for a quantitative
analysis
45

Basic Components – Detectors
 In initial days, human eyes served as detectors but they are only limited to visible spectra
and vary from person to person.
 In modern days, sensitive transducers convert the signal consisting of photons into an
easily measurable electrical signal.
 Ideally the detector’s signal, S, is a linear function of the electromagnetic radiation’s
power, P,
𝑺 = 𝒌𝑷 + 𝑫
where k is the detector’s sensitivity, and D is the detector’s dark current, or the background
current when we prevent the source’s radiation from reaching the detector.
 There are two broad classes of spectroscopic transducers: thermal transducers and photon
transducers.
46

Basic Components – Detectors
Transducer Class Wavelength Range Output Signal
Phototube photon 200–1000 nm current
Photomultiplier photon 110–1000 nm current
Si photodiode photon 250–1100 nm current
Photoconductor photon 750–6000 nm change in resistance
Photovoltaic cell photon 400–5000 nm current or voltage
Thermocouple thermal 0.8–40 μm voltage
Thermistor thermal 0.8–40 μm change in resistance
Pneumatic thermal 0.8–1000 μm membrane
displacement
Pyroelectric thermal 0.3–1000 μm current
47
Transducers for Spectroscopy

Basic Components – Detectors 48
 Photon Transducers:
 Phototubes and photomultipliers contain a photosensitive surface that absorbs radiation in
the ultraviolet, visible, or near IR, producing an electrical current proportional to the
number of photons reaching the transducer
 Other photon detectors use a semiconductor as the photosensitive surface. When the
semiconductor absorbs photons, valence electrons move to the semiconductor’s
conduction band, producing a measurable current.
 One advantage of the Si photodiode is that it is easy to miniaturize.
 Groups of photodiodes may be gathered together in a linear array containing from 64–
4096 individual photodiodes.
 With a width of 25 μm per diode, for example, a linear array of 2048 photodiodes requires
only 51.2 mm of linear space.
 By placing a photodiode array along the monochromator’s focal plane, it is possible to
monitor simultaneously an entire range of wavelengths.

Basic Components – Detectors 49
 Thermal Transducers:
 Infrared photons do not have enough energy to produce a measurable current with a
photon transducer.
 A thermal transducer, therefore, is used for infrared spectroscopy. The absorption of
infrared photons by a thermal transducer increases its temperature, changing one or more
of its characteristic properties.
 A pneumatic transducer, for example, is a small tube of xenon gas with an IR transparent
window at one end and a flexible membrane at the other end.
 Photons enter the tube and are absorbed by a blackened surface, increasing the
temperature of the gas.
 As the temperature inside the tube fluctuates, the gas expands and contracts and the
flexible membrane moves in and out.
 Monitoring the membrane’s displacement produces an electrical signal.

Basic Components – Signal Processors
 A transducer’s electrical signal is sent to a signal processor where it is displayed in a form
that is more convenient for the analyst.
 Examples of signal processors include analog or digital meters, recorders, and computers
equipped with digital acquisition boards.
 A signal processor is also used to calibrate the detector’s response, to amplify the
transducer’s signal, to remove noise by filtering, or to mathematically transform the
signal.
50

References:
 http://www.webassign.net/question_assets/ucscgencheml1/lab_6/manual.html
 https://xpssimplified.com/ion_scattering_spectroscopy.php
 http://www.renishaw.com/en/a-basic-overview-of-raman-spectroscopy--25805
51

Introduction to Spectroscopy

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