atomic absorption, emission, spectral lines,, energy level diagram

1. Lecture No. 01
Course title:
Atomic Spectroscopy
Topic: Introduction to Atomic Spectroscopy
Course instructor: Dr. Salma Amir
GFCW Peshawar

2. Atomic spectroscopy
 Atomic spectroscopy refers to the absorption and emission of
ultraviolet–visible (UV-VIS) light by atoms and monoatomic ions
and is conceptually similar to the absorption and emission of UV-VIS
light by molecules
Broadly classified as
 Flame Emission Spectroscopy (FES)
 Atomic Absorption Spectroscopy (AAS)
 Atomic Florescence Spectroscopy (AFS)

3. Emission, absorption, and fluorescence by atoms in a flame. In atomic
absorption, atoms absorb part of the light from the source and the
remainder of the light reaches the detector. Atomic emission comes from
atoms that are in an excited state because of the high thermal energy of the
flame. To observe atomic fluorescence, atoms are excited by an external
lamp or laser. An excited atom can fall to a lower state and emit radiation.

4. Production of gaseous-phase atoms
The procedure by which gaseous metal atoms are produced in the flame
may be summarized as follows.
When a solution containing a suitable compound of the metal to be
investigated is aspirated into a flame, the following events occur in rapid
succession:
 1. Evaporation of solvent leaving a solid residue;
 2. Vaporization of the solid with dissociation into its constituent
atoms, which initially, will be in the ground state; and
 3. Some atoms may be excited by the thermal energy of the flame to
higher energy levels, and attain a condition in which they radiate
energy.

5.  The resulting emission spectrum thus consists of lines originating
from excited atoms or ions. These processes are conveniently
represented diagrammatically

6. Absorption by atoms
 Atoms in which no electrons are in the higher levels are said to be in
the ground state. This state is designated in energy level diagrams as E0
Atoms in which there is an electron in the higher level are said to be in an
excited state. Excited states are designated in energy level diagrams as E1,
E2, E3, etc.
An energy level diagram consists of short horizontal lines representing the
levels or states, with each line labeled as E0, E1, etc. Often, an energy level
diagram shows the movement of electrons between levels with longer
vertical arrows. The movement of an electron between electron energy
levels is called an electronic energy transition.

7. Where can an electron obtain the required energy to be
promoted to a higher level?
One way is for it to come into contact with light of the same energy. When the light of
this energy “strikes” an atom and causes an electron to be promoted to a higher
energy level, the energy of the light becomes part of the electron’s energy and
therefore “disappears.” It is absorbed. It is important to keep in mind that the light
coming in must be exactly the same energy as the energy difference between the two
electronic levels; otherwise, it will not be absorbed at all. If it is not absorbed and is
in the visible region, we see it. It becomes part of the light that is reflected and
therefore detected by our eyes. The absorption of light by atoms consists of the
absorption of only a few very specific wavelengths because the energy difference
between two levels is very specific.

8. Absorption spectra
(origin of spectral lines)
 Absorption spectra for atoms are due to atoms in the gas phase
absorbing UV-VIS light from a light source and are characterized by
very narrow wavelength absorption bands called spectral lines.
 These very narrow bands result from energy transitions between
electronic energy levels in which there are no vibrational levels
superimposed.
 An absorption spectrum is a plot of the amount of light absorbed by a
sample vs. the wavelength of the light. The amount of light absorbed
is called the absorbance. It is symbolized as A . An absorption
spectrum is obtained by using a spectrometer to scan a particular
wavelength region and to observe the amount of light absorbed by the
sample along the way.

9. Emission by atoms
 Under certain conditions, analytes present in samples of matter will emit light,
and this light can be useful for qualitative and quantitative analysis.
 For example, most processes used to obtain free ground state atoms in the gas
phase from liquid phase solutions of their ions result in a small percentage of the
atoms being elevated to the excited state, even if no light beam is used. Whether
a light beam is used or not, excited atoms return to the ground state because the
ground state is the lowest energy state.
 The energy loss that occurs when the atoms return to the ground state may be
dissipated as heat, but it may also involve the emission of light because the
difference in energy between the ground state and the excited state is equivalent
to light energy.
 Energy level diagrams can be used to depict such a process, and an emission
spectrum, the plot of emission intensity vs. wavelength, may be plotted.

10. Emission spectra
 Emission spectra for atoms and monoatomic ions are due to gas
phase atoms or monoatomic ions in excited states emitting UV-
VIS light because they are returning to the ground state.
 Emission spectra are also characterized by very narrow
wavelength bands (also called spectral lines) because they too
involve electronic energy levels in which there are no
vibrational levels superimposed.
 In fact, for a given kind of atom, the emission lines occur at the
same wavelengths as the absorption lines because they involve
the same energy levels.

11.  Consider the simplified energy-level diagram, where Eo represents
the ground state in which the electrons of a given atom are at their
lowest energy level and E1,E2 , E3 , etc., represent higher or excited
energy levels

12. Absorption vs Emission
 The process of emission is the reverse of that of light absorption by atoms, so
downward-pointing arrows are used to indicate the return to the ground
state; the wavelengths emitted are the same as those that are absorbed. Thus
the lines in an emission spectrum of a metal are often at the same
wavelengths as the lines in the absorption spectrum for the same metal.