Absorption, emission, fluorescence
Schematic representation of absorption, emission, and
Atomic Absorption Spectrometry
In this method, the atomic vapours
containing free atoms of an element in
the ground state are subjected to a UVVIS radiation source emitting a
characteristic frequency of the element
present in atomic vapours. The
radiation is absorbed and the intensity
of the radiation is attenuated. The
absorbed radiation causes excitation of
electrons from the ground state to an
excited level. The extent of absorption
is a quantitative measure of the concn.
of the atomic vapours of the element in
the ground state. It is an electronic
excitation and the energy of transitions
lies in the to UV-VIS region of the
Atomic Emission Spectrometry (AES)
In this method, a sample is normally
excited by the thermal energy of a flame,
argon plasma or an electrical discharge. The
atoms in the sample absorb thermal energy,
causing the excitation of the outer orbital
electrons. As the excited state is short
lived, the excited atoms return back to the
ground state after a very short lifetime
(typically10-6 to 10-9 s). This is accompanied
by the emission of EMR, normally in the
form of light in the UV-VIS region. The
wavelength of the emitted radiation and
its intensity provide the qualitative and
quantitative information about the
analyte. The atomic emission spectroscopy
employing flame as a means of excitation
is called flame photometry or flame
emission spectroscopy (FES).
A blackbody emits a continuous spectrum
If you look directly at a blackbody, you will see this continuous spectrum
Clouds of gas absorb certain wavelengths (colors) of light
A continuous spectrum that hits a cloud of cool gas will be partially absorbed
The transmitted spectrum is called an absorption line spectrum (because
certain lines are absorbed), and is continuous except for the colors that were
absorbed by the gas.
•Anything that absorbs also emits.
•A cloud of cool gas that absorbs certain colors from a blackbody will emit
exactly those colors as the gas atoms de-excite
•If we look at the cloud without the blackbody in our line of sight, we will see an
emission line spectrum.
•The lines of emission have the same color as the absorption lines in the
absorption line spectrum
•If you added an emission line spectrum and an absorption line spectrum, you
would get a continuous spectrum.
• Atomic spectra: single external electron
Basic components of an atomic spectrophotometer
Molecular spectra are generally broad. These consist of a number of closely spaced
lines constituting what is called a band spectrum. The band nature of the spectrum
is due to a number of factors like, the quantisation of the rotational and vibrational
motions of the molecules along with the quantisation of electronic energy levels. In
addition, the width of the spectrum is also dependent on some of the instrumental
In contrast to the above, the atomic spectra consist of a number
of very sharp lines, characteristic of the atomic species. It implies that
these give rise to line spectra. The atomic spectra are generally much
sharper because in atomic systems, the rotational and the vibrational
motion are not quantised and the transitions are observed amongst the
electronic energy levels of the absorbing species. A typical atomic
spectrum is shown in Fig. 7.2.
Similar to the molecular spectrum,
the signals in an atomic spectrum
are also characterised in terms of
These are given below.
Position of the Signal
The position of the spectral signals is determined by the difference in the
energies of different energy levels. If E2 and E1 represent the energy of the
higher and lower energy levels concerned, the energy change during the
transition from E2 to E1 level
may be defined as follows.
where, h is the Planck’s constant, c is the velocity of light, v the frequency and
λ the wavelength of the emitted radiation. The wavelength of the radiation is
characteristic of the atoms of the particular element from which it is emitted.
When flame photometry is employed as an analytical tool, the
wavelength of the radiation emerging from a flame tells us about the elements
that are present in that flame
Intensity of the Signal
The intensity of a signal depends on the population of the energy level
from which the transition is originating and the probability of such a
transition. According to the Boltzmann statistical distribution, the
population of the ground state i.e., the number of species in the ground
state is highest and it keeps on decreasing as we go to higher energy
levels. In case of the atoms the population of any excited state relative
to that of the ground state is given by the following formula.
N* and N0 = Number of atoms in excited state
and ground state, respectively,
E = Energy difference between ground and
excited state (in J),
g* and g0 = Statistical factors that depend on
the degeneracies of the levels involved,
k = Boltzmann constant (= 1.28 × 10-23 J k-1),
T = Absolute temperature.
According to Eq. after the ground state, the lowest energy excited
state will be most populated and the population of the higher excited
states would decrease progressively. Eq. can be used to determine the
population of an excited state with respect to the ground state.
Let us illustrate this with the help of an example.
Q: The characteristic yellow emission of sodium vapours consists
of a pair of lines at 589 nm and 589.6 nm. These arise from the
emission of radiation by the gaseous sodium atoms in
excited state to 3s ground state.
Compute the ratio of the sodium atoms in the excited state to the
Therefore, to evaluate the ratio of the atoms in the excited state to that in the
ground state we need to know the statistical factors g* and g0 and ΔE.
As the 3s and 3p levels have two and six quantum states i.e., the statistical
factors are 2 and 6 respectively, the ratio of g* and g0 comes out to be 6/2 = 3
The change in energy, ΔE, can be calculated by using the formula,
The wavelength is taken
as an average of the two,
We have learnt above that the intensity
of a signal depends on the population
of the level from which the transition
Substituting the value of ΔE in the above
originates and the probability of such a
eqn, we get the following.
These are the intrinsic parameters of
the analyte being determined.
In addition to these, the intensity of a
signal does depend on an imp. external
parameter, viz., the concn. of the
Intensity Concentration Relationship
The intensity of the emitted radiation (P) is proportional to the no. of
excited atoms N*.
we see that the number of atoms in the excited state, N*, at a given
temperature are proportional to N0. Therefore, we can write the
P = K. C
(as, N0 is directly proportional to metal concn)
Thus, the intensity of the emitted light will be directly
proportional to the concentration of the element introduced
into the flame.
To a first approximation, absorption by free atoms is similar to absorption
by molecules and there is a linear relationship between absorbance and the
“concn” of the sample. This relationship is given by the Beer-Lambert Law
it is found that B-L relationship cannot
be sustained because flame atomizers
are generally used as the “sample cell”
and the population of free atoms in a
flame is far from homogeneous.
Homogeneity of the sample is a basic
requirement for the application of
e = charge of the electron,
m = mass of the electron
c = the speed of light,
N = the no. of free absorbing atoms in the light path
f = is the oscillator strength of the absorption line.
It can be seen that there are a number of constants in this equation.
The only variables are N, the total number of atoms in light path,
and f the oscillator strength.
The relationship between them and the total amount of light
absorbed is a basis for quantitative analysis. The degree of
absorption for each element and each absorption line depends on the
oscillator strength which is a direct measure of how strongly each
atom will absorb at that wavelength.
The oscillator strength in emission spectroscopy is a measure of
how closely an atom resembles a classical oscillator in its ability
to emit radiation.
The greater the oscillator strength the greater the emission
intensity for a given set of conditions.
A , = the transition probability between energy levels i and j ,
f, = the oscillator strength of the associated emission line;
h = wavelength of the emission line.
The relationship between the absorption oscillator strength and the
emission oscillator strength is given by the equation
Widths of atomic lines are quite
important in atomic spectroscopy.
Narrow lines in atomic and emission
spectra reduce the possibility of
interference due to overlapping lines.
Atomic absorption and emission lines
consists of a symmetric distribution of
wavelengths that centers on a mean
wavelength (λ 0) which is the wavelength
of maximum absorption or maximum
intensity for emitted radiation.
The energy associated with λ 0 is equal to
the exact energy difference between two
absorption or emission.
A transition between two discrete, singlevalued energy states should be a line with
line-width equal to zero.
However, several phenomena cause line
broadening in such a way that all atomic
lines have finite widths.
Line width or effective line width (∆λ 1/2) of
an atomic absorption or emission line is
defined as its width in wavelength units
when measured at one half the maximum
Atomic Line Widths
Sources of broadening:
(1) Uncertainty effect
(2) Doppler effect
(3) Pressure effects due to collisions
(4) Electric and magnetic field effects
It results from the uncertainty principle postulated in 1927
by Werner Heisenberg.
One of several ways of formulating the Heisenberg
uncertainty principle is shown in the following equation:
Δt x ΔE = h/2π
The meaning in words of this equation is as follows: if the
energy E of a particle or system of particles – photons,
electrons, neutrons or protons – is measured for an exactly
known period of time Δt, then this energy is uncertain by at
least h/ 2πΔt.
Therefore, the energy of a particle can be known with zero
uncertainty only if it is observed for an infinite period of
For finite periods, the energy measurement can never be
more precise then h/ 2πΔt.
• The lifetime of a ground state is typically long, but the lifetimes of
excited states are generally short, typically 10-7 to 10-8 seconds.
• Line widths due to uncertainty broadening are called natural
line widths and are generally 10-5nm or 10-4Å.
The "natural line width" indicates the lower limit of the absorption lines width. It
can be calculated from the uncertainty principle which states that
where Δτ is the lifetime of the excited state and ΔE
the range of energy over which the line emits, i.e. the line
width in terms of energy.
Note: ∆λ = ∆λ1/2
In a collection of atoms in a hot
environment, such as an atomizer,
atomic motions occur in every
The magnitude of the Doppler shift
increases with the velocity at which the
approaches or recedes the detector.
For relatively low velocities, the
relationship between the Doppler shift
(Δλ) and the velocity (v) of an
approaching or receding atom is given
Δ λ / λ 0= v / c
Where λ 0 is the wavelength of an un-shifted line
of a sample of an element at rest relative to the
transducer, and c is the speed of light.
Emitting atom moving: (a) towards a
photon detector, the detector sees wave
crests more often and detect radiation of
higher frequency; (b) away from the
detector, the detector sees wave crests
less frequently and detects radiation at
The result is an statistical distribution of
frequencies and thus a broadening of
Pressure Effects Due to
• Pressure or collisional broadening
is caused by collisions of the
emitting or absorbing species with
other atoms or ions in the heated
• These collisions produce small
changes in energy levels and hence
a range of absorbed or emitted
broadening that is two to three
orders of magnitude grater than
the natural line widths.
• Eg. :Hollow-cathode lamps (HCL):
• Pressure in these lamps is kept
really low to minimize collisional
• Glass tube is filled with neon or
argon at a pressure of 1 to 5 torr.
Light Source: Hollow Cathode Lamp
Cup made of
metal of interest
• The electric potential ionizes rare gas atoms and
accelerates them into the cathode where they sputter metal
atoms into the gas phase
• Collisions with gas atoms or electrons excite the metal
• On decay the metal atoms emit light
An illustration of the hollow cathode is given in Figure 2. In this
system the metal of interest is used as the material from which the
cathode is made.
The light source is filled with an inert gas, such as neon, which
is ionized by the anode. The positively charged neon ions are then
attracted by the negative charged cathode and accelerated towards it.
On arrival at the cathode the neon strikes the surface of the cathode.
If it has sufficient energy it causes atoms of the cathode to be ejected.
This process is called “sputtering”.
The sputtered atoms are invariably excited
and emit radiation characteristic
of the cathode metal(s). The emitted lines
are generally very narrow in band width .
The sample atoms absorb only at their own
characteristic wavelengths. It is therefore
essential that the light source emit at
Exactly the same wavelength. This can be
accomplished by using a hollow cathode
made of the same element as the element
The exact mechanism of the excitation process in the hollow cathode lamp
(hcl) is of interest. Figure 11 is a close-up view of a typical lamp and of the
mechanism. The lamp itself is a sealed glass envelope filled with argon or
neon) gas. When the lamp is on, neon atoms are ionized, as shown, with
the electrons drawn to the anode (+ charged electrode), while the neon
ions, Ne+, "bombard" the surface of the cathode (- charged electrode). The
metal atoms, M, in the cathode are elevated to the excited state and are
ejected from the surface as a result of this bombardment. When the atoms
return to the ground state, the characteristic line spectrum of that atom
is emitted. It is this light, which is directed at the flame, where unexcited
atoms of the same element absorb the radiation and are themselves raised
to the excited state. As indicated previously, the absorbance is measured and
related to concn.
Fig11. The hcl and the process of metal atom excitation and light emission
Illustration of how the light emitted by the hollow cathode lamp is the
exact wavelength needed to excite the atoms in the flame.
Hollow Cathode Lamp
As stated before, the light from this lamp is exactly the light required
for the analysis, even though no monochromator is used. The reason
for this is that atoms of the metal to be tested are present within the
lamp, and when the lamp is on, these atoms are supplied with energy,
which causes them to elevate to the excited states. Upon returning to
the ground state, exactly the same wavelengths that are useful in the
analysis are emitted, since it is the analyzed metal with exactly the
same energy levels that undergoes excitation.
The light emitted by such a lamp consists of the line spectra of all the
kinds of atoms present. No interference will usually occur as long as
the sufficiently intense line for a given metal can be found which can
be cleanly separated from all other lines with the monochromator.
PRINCIPLE OF FLAME PHOTOMETRY
This method is based upon the measurement of intensity of radiation emitted,
in the visible region, when a metal atom is introduced into a flame. The
wavelength of the radiation (or the colour), emitted tells us what the element
is, and the intensity of the radiation tells us how much of the element is
IUPAC Committee on Spectroscopic Nomenclature has recommended
abbreviation FAES (flame atomic emission spectrometry) for this technique.
In a typical flame photometric experiment, a solution containing the relevant
substance to be analysed is aspirated into the burner and dispersed into the
flame as a fine spray. This process is called nebulisation.
The five processes occurring in the flame can be summarised as follows.
v) Emission of radiation:
Electrons in the excited state are very
unstable and move back down to the ground
state or a lower energy state quite quickly. As
they do so, they emit the energy in the form of
radiation of characteristic wavelength, which
is measured by a detector.
Inductively Coupled Plasma Source
A plasma is a hot, partially ionized
gas. It contains relatively high
concentrations of ions and electrons.
Argon ions, once formed in a plasma, are
capable of absorbing sufficient power
from an external source to maintain the
temperature at a level at which further
ionization sustains the plasma
indefinitely. The plasma temperature is
about 10 000 K.
After Manning T.J. and Grow
A plasma is an ionized gas that is macroscopically neutral (i.e.
with the same number of positive particles (ions) and
negative particles (electrons)). If a monoatomic gas, X, is
used, a plasma can be described by the following eqlm.:
where Xn+ is an ion with n charges and e is the electron.
In contrast to a flame, it is necessary to supply an external energy in the
form of an electrical field in order to ionize the gas and to sustain the
plasma, which, in turn, will transmit part of this energy to the sample to
atomize, ionize and excite it.
● direct current plasma (DCP) is obtained when a direct current field
is established across electrodes,
● ICP is obtained when a high-frequency (hf) field is applied through
● microwave-induced plasma (MIP) is obtained when a microwave
field is applied to a cavity
The gas that is used to generate the plasma (plasma gas) is argon. Like any
noble gas, argon is a monoatomic element with a high ionization
energy (15.76 eV), and is chemically inert.
(i) a simple spectrum is emitted by argon in contrast to a flame
where primarily molecular spectra are observed;
(ii) argon has the capability to excite and ionize most of the
elements of the Periodic Table;
(iii) no stable compounds are formed between argon and the analytes.
In atomic emission spectrometry (AES), a source will have actually two
1st step consists of the atomization of the sample to be analyzed
so as to obtain free analyte atoms, usually in the ground state,
2nd step consists of a partial ionization of the analyte atoms, and
of the excitation of the atoms and the ions to higher-energy states.
The plasma acts as reservoir of energy provided by the rf field, and
transfers this energy to an analyte, M. It should be noted that the
atomization of a sample is a relatively long process (of the order of a
few ms), while ionization and excitation are very fast processes.
The major species are not only the argon ions, Ar+, and the
electrons, e, but also the excited argon atoms, Ar*,with the
special case of the metastable levels, Arm .
The main ionization processes are:
while the main excitation processes for the analyte atom are:
electron impact excitation
Quantitative analysis will be possible if the intensity of the line can
be related to the concentration of the emitting species. The intensity
of a line is proportional to:
(i) the difference in energy between the upper level, Em, and the lower
level, Ek, of the transition,
(ii) the population of electrons, nm, in the upper level, Em,
(iii) the number of possible transitions between Em and Ek per unit time.
This value is expressed by the transition probability A, and has
been defined by Einstein. Therefore, the intensity I is proportional to:
As seen above, it is possible to relate the population nm to the total
population N through the Boltzmann equation. The intensity of a line can
be, therefore, written as:
where Φ is a coefficient to account for the emission being isotropic over a solid
angle of 4 steradian.