OBT751 - ANALYTICAL METHODS AND

INSTRUMENTATION

LECTURE - 4

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1.2 Components of optical instruments
 Optical spectroscopic methods are most often based on six phenomena:
(1) absorption - absorption of  electromagnetic radiation  is how
matter (typically  electrons  bound in atoms) takes up a photon’s
energy  — and so transforms  electromagnetic energy  into 
internal energy of the absorber (for example,  thermal energy).
(2) fluorescence
(3) phosphorescence, - Fluorescence and phosphorescence result
from absorption of electromagnetic radiation and then
dissipation of the energy emission of radiation. The major
distinction between fluorescence and phosphorescence is the
time scale of emission, with fluorescence being prompt and
phosphorescence being delayed.
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1.2 Components of optical instruments
(4) Scattering - Scattering of electromagnetic radiation is caused
by the interaction of radiation with matter resulting in the
reradiation of part of the energy to other directions not along
the path of the incident radiation.  Scattering effectively removes
  energy from the incident beam.
(5) Emission - The emission spectrum of a chemical element or  is the
spectrum of frequencies  of electromagnetic radiation emitted due to
an atom  or molecule making a transition from a high energy state to a
lower energy state.
(6) Chemiluminescence - Chemiluminescence (CL) is defined as the
production of electromagnetic radiation (ultraviolet, visible or
infrared) observed when a chemical reaction yields an electronically
excited intermediate or product, which either luminesces (direct CL)
or donates its energy to another molecule responsible for the emission3
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1.2 Components of optical instruments
 Typical spectroscopic instruments contain five components:
(1) a source of radiant energy;
(2) a container for holding the sample;
(3) a device that isolates a restricted region of the
spectrum for measurement;
(4) a radiation detector, which converts radiant
energy to a usable electrical signal; and
(5) a signal processor and readout, which displays
the transduced signal on a digital display, a
computer screen, or another recording device.

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1.2 Components of optical instruments
 Figure illustrates the three ways these components are configured to
carry out the six types of spectroscopic measurements

In (a), the arrangement for absorption measurements is shown. Note that source radiation of the selected wavelength is
sent through the sample, and the transmitted radiation is measured by the detector–
signal processing– readout unit. With some instruments, the position of the sample and wavelength selector is reversed.

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1.2 Components of optical instruments

In (b), the configuration for fluorescence measurements is shown. Here,

two wavelength selectors are needed to select the excitation and
emission wavelengths. The selected source radiation is incident on the
sample and the radiation emitted is measured, usually at right angles to
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avoid scattering.
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1.2 Components of optical instruments

In (c), the configuration for emission spectroscopy is shown. Here, a

source of thermal energy, such as a flame or plasma, produces an analyte
vapor that emits radiation isolated by the wavelength selector and
converted to an electrical signal by the detector.
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1.2 Components of optical instruments
 The first two instrumental configurations, which are used for
the measurement of absorption, fluorescence, and
phosphorescence, require an external source of radiant
energy.
 For absorption, the beam from the source passes into the
wavelength selector and then through the sample, although
in some instruments the positions of the selector and sample
are reversed.
 For fluorescence and phosphorescence, the source induces
the sample, held in a container, to emit characteristic
radiation, which is usually measured at an angle of 90° with
respect to the source.
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1.2 Components of optical instruments
 Emission spectroscopy and chemiluminescence
spectroscopy differ from the other types in that no external
radiation source is required; the sample itself is the emitter
(c).
 In emission spectroscopy, the sample container is a plasma, a
spark, or a flame that both contains the sample and causes it
to emit characteristic radiation.
 In chemiluminescence spectroscopy, the radiation source is a
solution of the analyte plus reagents held in a transparent
sample holder. Emission is brought about by energy released
in a chemical reaction in which the analyte takes part
directly or indirectly.
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1.2 Components of optical instruments
 The optical characteristics of all the components shown in Figure below:

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1.2 Components of optical instruments

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1.3 Sources of Radiation
 For spectroscopic studies, a source must generate a beam with
sufficient radiant power for easy detection and measurement.
 In addition, its output power should be stable for reasonable periods.

 Typically, the radiant power of a source varies exponentially with the

voltage of its power supply. Thus, a regulated power source is almost
always needed to provide the required stability.
 Alternatively, the problem of source stability can sometimes be
circumvented by double-beam designs in which the ratio of the signal
from the sample to that of the source in the absence of sample serves
as the analytical variable.

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1.3 Sources of Radiation

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1.3 Sources of Radiation

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1.3 Sources of radiation
 There are two types of sources:

a) Continuum sources

b) Line sources

 Continuum sources - emits radiation that changes in

intensity smoothly as a function of wavelength.
 Line sources - emit a limited number of lines, or
bands of radiation, each of which spans a limited
range of wavelengths.
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1.3 Sources of radiation
Continuum Sources:
 Continuum sources are useful in several types of
spectroscopy including absorption and fluorescence.
1) deuterium lamp is used for UV region
2) High-pressure, gas-filled arc lamps that contain argon,
xenon, or mercury are used when a particularly intense
source is required.
3) tungsten filament lamp is used for many applications in
visible region of the spectrum
4) The common IR sources are inert solids heated to 1500
to 2000 K, a temperature at which the maximum radiant
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output occurs at wavelengths of 1.5 to 1.9 μm

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1.3 Sources of radiation
Line Sources:
 Sources that emit a few discrete lines are widely used in
atomic absorption spectroscopy, atomic and molecular
fluorescence spectroscopy, and Raman spectroscopy
 The familiar mercury and sodium vapor lamps provide a
relatively few sharp lines in the UV and visible regions and
are used in several spectroscopic instruments.
 Hollow-cathode lamps and electrodeless discharge lamps are
the most important line sources for atomic absorption and
fluorescence methods.
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1.3 Sources of radiation
Laser Sources:
Lasers are highly useful sources in analytical instrumentation because
of their
 high intensities,
 narrow bandwidths, and
 the coherent nature of their outputs.

Lasers sources have become important in several routine

analytical methods, including
 Raman spectroscopy,
 Molecular absorption spectroscopy,
 fluorescence spectroscopy,
 Emission spectroscopy, and
 as part of instruments for Fourier transform IR spectroscopy. 18
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1.3 Sources of radiation
 Lasers
 The term laser is an acronym for light amplification by stimulated
emission of radiation.
 Because of their light-amplifying characteristics, lasers produce
spatially narrow (a few hundredths of a micrometer), extremely
intense beams of radiation.
 The process of stimulated emission, produces a beam of highly
monochromatic (bandwidths of 0.01 nm or less) and remarkably
coherent radiation.
 Now, however, tunable lasers are available that provide fairly narrow
bands of radiation in several different spectral regions.
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1.3 Sources of radiation
Components of Lasers:
The schematic representation that shows the components
of a typical laser source is shown below:

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1.3 Sources of radiation
 The heart of the device is the lasing medium. It may be a solid
crystal such as ruby, a semiconductor such as gallium arsenide, a
solution of an organic dye, or a gas such as argon or krypton.
 The lasing material is often activated, or pumped, by radiation
from an external source so that a few photons of proper energy
will trigger the formation of a cascade of photons of the same
energy.
 Pumping can also be accomplished by an electrical current or by
an electrical discharge. Thus, gas lasers usually do not have the
external radiation source.
 Instead, the power supply is connected to a pair of electrodes
contained in a cell filled with the gas.
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1.3 Sources of radiation
 A laser normally functions as an oscillator, or a resonator,
in the sense that the radiation produced by the lasing action
is caused to pass back and forth through the medium numerous times
by means of a pair of mirrors.
 Additional photons are generated with each passage, leading to
enormous amplification. The repeated passage also produces a beam
that is highly parallel, because nonparallel radiation escapes from the
sides of the medium after being reflected a few times.
 One of the easiest ways to obtain a usable laser beam is to coat one of
the mirrors with a sufficiently thin layer of reflecting material so that a
fraction of the beam is transmitted rather than reflected.

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Mechanism of Laser Action:
The four processes of Laser action are :
(a) pumping,
(b) spontaneous emission (fluorescence),
(c) stimulated emission, and
(d) absorption.

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 Two of the several electronic energy levels of each are shown as

having energies Ey and Ex.
 Slightly different vibrational energy levels depicted as Ey,
E”y , E’ y .
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1.3 Sources of radiation
 Pumping : Pumping, is a process by which the active species of a
laser is excited by means of an electrical discharge, passage of an
electrical current, or exposure to an intense radiant source.
 During pumping in a molecular system, several of the higher
electronic and vibrational energy levels of the active species are
populated.
 One electron is shown as being promoted to an energy state E”y.

 The second is excited to the slightly higher vibrational level

E’’’y.
 The lifetime of an excited vibrational state is brief, so after 10-13 to 10-
15 s, the electron relaxes to the lowest excited vibrational level and an

undetectable quantity of heat is produced.

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1.3 Sources of radiation
 Spontaneous Emission: In an excited electronic state may lose all or
part of its excess energy by spontaneous emission of radiation.
 The wavelength of the fluorescence radiation is given by the
relationship

where h is Planck’s constant and c is the speed of light.

o It is to be noted that the instant at which emission occurs and the
path of the resulting photon vary from excited molecule to excited
molecule because spontaneous emission is a random process;
o Thus, the fluorescence radiation produced by one of the species
differs in direction and phase from that produced by the second
species. Spontaneous emission, therefore, yields incoherent 27
monochromatic radiation.
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1.3 Sources of radiation
 Stimulated Emission: Stimulated emission, is the basis
of laser behavior.
 Here, the excited laser species are struck by photons that have
precisely the same energies (Ey – Ex) as the photons produced by
spontaneous emission.
 Collisions of this type cause the excited species to relax immediately
to the lower energy state.
 It simultaneously emits a photon of exactly the same energy as the
photon that stimulated the process.
 The emitted photon travels in exactly the same direction and is
precisely in phase with the photon that caused the emission.
 Therefore, the stimulated emission is totally coherent with the
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incoming radiation.
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1.3 Sources of radiation
 Absorption: The absorption process, competes with
stimulated emission.
 Here, two photons with energies exactly equal to (Ey – Ex) are
absorbed to produce the metastable excited state.
 This state is identical to the state attained in by pumping.

Population Inversion and Light Amplification:

 To have light amplification in a laser, the number of photons
produced by stimulated emission must exceed the number lost by
absorption.
 This condition prevails only when the number of particles in the
higher energy state exceeds the number in the lower; in other
words, there must be a population inversion from the normal
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distribution of energy states.
 Population inversions are created by pumping.
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 Figure below shows the effect of incoming radiation on a
noninverted population with that of an inverted one.

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 In each case, nine molecules of the laser medium are in the
two states Ex and Ey.
 In the non-inverted system, three molecules are in the
excited state and six are in the lower energy level.
 The medium absorbs three of the incoming photons to
produce three additional excited molecules, which
subsequently relax very rapidly to the ground state without
achieving a steady-state population inversion.
 The radiation may also stimulate emission of two photons
from excited molecules resulting in a net attenuation of the
beam by one photon. 31
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1.3 Sources of radiation
 As shown in Figure -b, pumping two molecules into
virtual states En followed by relaxation to Ey creates
a population inversion between Ey and Ex.
 Thus, the diagram shows six electrons in state Ey
and only three electrons in Ex.
 In the inverted system, stimulated emission prevails
over absorption to produce a net gain in emitted
photons.
 Light amplification, or lasing, then occurs.
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 Three- and Four-Level Laser Systems
Figure shows simplified energy diagrams for the two common
types of laser systems.

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 In the three-level system, the transition responsible for laser radiation
is between an excited state Eγ and the ground state E0.
 in a four-level system, on the other hand, radiation is generated by a
transition from Eγ to a state Ex that has a greater energy than the
ground state.
 But the transitions between Ex and the ground state is rapid.

 The advantage of the four-level system is that the population

inversions essential for laser action are achieved more easily

than in three-level systems.
 In a four-level system, it is only necessary to pump sufficiently to
make the number of particles in the E γ energy level exceed the number
in Ex. 34
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 The lifetime of a particle in the Ex state is brief,
however, because the transition to E0 is fast; thus,
the number in the Ex state is generally negligible
relative to the number that has energy E0 with
respect to the number in the Eγ state.
 Therefore, the four level laser usually achieves a
population inversion with a small expenditure of
pumping energy.

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