We were discussing about the electromagnetic
radiation.
Now I am going to show you a pictorial representation
of that.
This is the pictorial representation.
Here I have in this slide the red one.
You can imagine a waves along this black line,
a wave going like this and then coming down
cutting it again going up like this, that
is 1 wave.
Imagine another wave perpendicular to that
going like this and then again coming here
and going out like this etc., this is sort
of how an electromagnetic radiation will move
along this direction.
It has got a direction, it has the electric
field is aligned perpendicular to that and
it is aligned magnetic field is also aligned
perpendicular to that in the x, y and z direction.
The distance between the two peaks is known
as wavelength.
I think most of you are familiar with the
wave forms.
We will simply say that all the electromagnetic
radiations are characterized by the wavelength
denoted by λ or frequency(ν).
The term wavelength is defined as the distance
between successive maxima or minima.
Every wave reaches a peak point and then it
reaches 0 and then it reaches another peak
point in the opposite direction that is negative.
I have 1 maxima, 1 minima (maxima and minima
are the plural terms).
1 maxima, 1 minima and again 1 maxima again
1 minima like that the waves will move on.
The frequency is the number of cycles occurring
per second.
If I start from this end the red one I am
marking it here, if I start from here reach
0 and then reach -1 and then again reach 0
here(watch the lecture for better understanding)
this is known as 1 frequency.
The number of frequencies per second how much
it moves, is the frequency per second.
The number of cycles occurring per second
is the frequency.
How these two are related to each other?
A very simple relation lambda is equal to
v/mu and where, v is the velocity of the propagation.
All electromagnetic radiations travel in the
space with or without matter at the same rate
i.e same velocity 2.9979 × 1010 cm/sec.
For simplicity we call it 3 × 1010 cm/sec.
That is the speed of light visible to us,
sun’s rays etc., that is also the same velocity
that is 3×1010cm/sec or 3×108 km/sec.
You can measure the distance between the sun
and the earth, by the time the sun’s radiation
reaches the earth and dividing it by the speed.
It is possible for us to determine the exact
distance of all planets or all astronomical
bodies by the time the light from the astronomical
body reaches the earth.
It is a very separate science by itself.
We also known that λ is nothing but wavelength,
speed of light divided by frequency.
Now frequency is expressed as number of cycles
per second or as wave numbers.
This wave number is important as far as IR
is also concerned.
Wave number is given by mu bar =1/lambda.
This also is the parameter we are suppose
to remember.
The distribution of spectral intensity in
black body radiation was best explained by
Max Plank in 1900 by uniting the corpuscular
theory that is assuming that, the electromagnetic
radiations are also the radiations containing
particles that is a corpuscular theory.
Just because it moves in the wave form it
is also called as wave theory.
That the relation between the energy of a
quantum of radiation to the frequency that
is given by delta e=h mew mu or hc/lambda.
That means, the energy of the radiation can
be determined by wavelength.
If I know the wavelength then, hc/lamda where,
c is the velocity of light i.e 3×1010 , h
is plank’s constant.
If I know the lambda I can find out what is
the corresponding energy of the electromagnetic
radiation, that is very simple.
Here in this equation delta E is the energy
of the quantum of radiation and v mu is the
frequency of radiation, h is in ergs seconds
that is 6.624×10-27 ergs second.
The 
interaction of matter with radiation thus
involves the exact quantized energy of the
substance, why?
Because, all the energy of the wave length,
electromagnetic radiation is also quantized.
Every elementary system whether nucleus, atom
or a molecule thus has a number of quantized
energy levels which we have seen earlier that
in the form of n, l, s and m.
Absorption or emission of energy takes place
only if the energy of the matter is equivalent
to the difference in energy states.
This is another quantum mechanical theory
that, changes in the absorption or emission
of energy takes place if the energy of the
matter is also equivalent to the difference
between the energy states.
Otherwise the radiation is transmitted without
any change through the matter.
This is an important concept we should remember.
The energy of the frequency has to match the
energy of the electromagnetic radiation and
of the particles.
I can write an equation like this, h mu that
is hc/lambda, that is the energy of the final
state-energy of the initial state.
Delta E, this is the energy that is to be
matched by the electromagnetic radiation as
well as the particulates.
That are interacting with the electromagnetic
radiation.
This equation is known as Bohr’s equation.
In this equation Ef and Ei are the energies
of the final and initial states of the substance.
In case of the energy of the radiation is
given by h mu==hc/lambda that is Ei-Ef.
Here I am defining two aspects, one is emission
another is absorption.
Absorption is, E energy of the final state-E
energy of the initial state and emission is
the energy difference between the energy of
the initial state and energy of the final
state.
But the quantum always says that, in emission
Ei and Ef refer to the energy states of the
particulates.
In actual practice what happens is, whenever
a radiation of multiple frequencies interacts
with the matter (multiple frequencies means
I cannot choose a radiation with only one
single wavelength.
That is a little difficult not that, it cannot
be done but I think those of you who are familiar
with scientific advances and who are familiar
with the science know that we do have laser
rays etc with single wavelength) But in general
if I am dealing with an electromagnetic radiation
coming from a particular body i.e black body
or any other body I get a set of waves having
different energies and set of having different
wavelengths, different frequencies, it is
a bundle of energy.
Whenever I am getting bundle of energy I can
call it as multiple frequency radiation.
If I am getting only one single wavelength
then it is a single wavelength radiation.
When I get multiple energy radiations passing
through a given space containing empty space,
electrons, neutrons, air, molecules etc.,
lot of things can happen!
What can happen?
Part of the energy maybe absorbs by the matter
or it may be reflected by hitting on the particulate
and then going in some other direction.
And another possibility is, it may enter a
glass like material go through the glass bend
a little bit and come out on the other side.
If it is a black color glass or something,
all of it will be absorbed.
It can be emitted, it can be reflected as
it hitting on some hard surface coming here
and going of, it is reflection like a mirror
and then refraction, it passes through the
matter but with a change in the direction
and then it can get diffracted.
It can get scattered in all directions, so
many other possibilities.
If we consider the interaction of electromagnetic
radiation, we should consider all these things
i.e absorption, emission, reflection, refraction,
diffraction, scattering and all these things.
The sum total of the energy when it is interacting
is given by, look at the slide now.
E=E absorbed, energy emitted, energy reflected,
energy refracted, energy diffracted, energy
scattered and so many other forms of energy
dispersion.
But the sum total of the energy should be
same, because that is the law of conservation
of energy.
I think most of you are familiar with the
law of conversation of energy that is, energy
can be neither created nor destroyed.
The total energy available on the surface
of the earth or anywhere else in the whole
universe is essentially same.
It can get converted from one form and another
but you cannot create energy.
Its a sort of human restriction, may be God
can do it but we do not know!
But as far as humans are concerned the law
stands very firm.
You cannot create energy nor you can destroy
energy you can only alternate to some extent.
Various types of quantized energy changes
occur in each region of the spectrum.
The magnitude of energies involved have been
traditionally used for a variety of spectrochemical
techniques.
These energy changes are generally classified
into different waves, these energy changes
also had different wavelengths and different
frequencies.
So, what are the different kinds of energies
or electromagnetic radiation we talk about?
Yesterday I had mentioned during my introduction
that, electromagnetic radiations are composed
gamma rays, X-rays, vacuum ultraviolet, ultraviolet,
visible rays, infrared, microwaves, near infrared,
far infrared, microwaves and radio waves.
The absorption of radiation occurring at different
wavelengths and the associated spectroscopic
techniques are listed in table 1.
This is the electromagnetic radiation.
It is a Google picture.
The whole electromagnetic radiation contains
gamma rays, X-rays, ultraviolet rays, visible
rays, infrared, microwaves, FM radio waves,
AM radio waves, long radio waves.
On the top I have listed the frequencies.
High energetic electromagnetic radiations
are the gamma rays.
There are slightly more than that also called
as cosmic rays but I am not showing you that.
Most of the gamma rays frequency ranges from
1024 to 1019 and X-rays having 1018 frequency.
Ultraviolet rays are around 1016.
Visible range are around 1015 and frequencies
for IR are 1012, 1014 etc., the frequency
keeps on decreasing as we go through the electromagnetic
radiation from left to right.
If the frequency is decreasing wavelength
keeps on increasing.
I think you are familiar with that relation
now.
I have put the wavelength also at the bottom
of this strip.
The wavelength of gamma rays if it is 1024
, per second 24 waves should pass through
a given space and the wavelength should be
approximately 10-16, it keeps on decreasing
again.
That means wavelength keeps on increasing
and then it reaches somewhere in all Amstrong’s
up to 10-8 centimeter.
And then it comes to IR etc.
They keep on increasing.
They are slightly longer than visible rays
and still longer than UV rays and it keeps
on increasing.
Radio waves and long radio waves around 104,
106, 108 centimeters.
As the frequency decreases wavelength increases
that is the inverse relationship we keep on
talking.
You should also see that, the radiation I
am showing you (in slide) a small portion
with blue, red and green that I have expanded
below.
When I expand that below, I get colors like
this that is one side it is dark blue and
another side it is somewhat red and violet.
The visible range as we experience them covers
from 400 to approximately 700 nm.
The wavelength range corresponding to this
is, I am describing in nanometers (10-6 centimeter)
not in Amstrong units or something like that.
This is where the visible range is there as
part of the electromagnetic radiation.
Several types of molecular transitions occur
when the electromagnetic radiations interact.
Whenever you take a gamma rays and make them
bombard on the element or on a metal, changes
in the nuclear structure take place.
That’s what I have written above gamma rays
nuclear.
If you take X-rays and make them impinge on
the metals, I get core level electron changes
that is, X-rays, soft X-rays etc., so the
energy of the matter that comes out would
be in the form of about 10-10 centimeters.
Then comes ultraviolet region.
In ultraviolet region it is the valance electron
i.e electrons moving around their nucleus.
Those electrons get affected and they may
get excited to next higher energy level or
lower energy level.
I have listed those interactions as valance
electron interaction with electromagnetic
radiation.
These are all atomic and molecular transitions
happening in s-orbital, p-orbital, d-orbital
and f-orbitals.
Microwave also that is the region where molecular
rotations takes place and electron spin changes.
If you take electromagnetic radiation in microwave,
impinge them on the metal elements then, the
electrons will change their spin from positive
half to negative half.
Still lower energy radio waves they result
in nucleus itself may be moving like this.
That kind of changes also will be taking place
in the electromagnetic radiation.
This is how electromagnetic radiation you
should visualize.
In table 3.1 I am talking about the visible
radiation.
If you remember 400 to 700 nm is the visible
range.
That visible range I am cutting into different
ranges again and that is less than 380.
Most of it is ultraviolet range and 380 to
435 nanometers is, what color of the radiation
is transmitted?
That means, if I pass the violet radiation
it will pass through the matter, it is just
like taking a violet glass and what comes
out is violet glass whatever is the light
on the other side.
What comes out on the other side is known
as transmitted light, what is held in the
glass is the complementary hue.
That is how the glass will look like.
Whenever electromagnetic radiation passes
through the matter there are two components.
One part of it is absorbed and part of it
is transmitted.
What is transmitted is not absorbed and whatever
is not transmitted is all absorbed.
Suppose, I take violet color, violet is absorbed
and the remaining six colors from the VIBGOR
all constitute another color which is known
as complementary hue.
That is how it looks to us.
The radiation coming out will look violet
but the actual material will look the combination
of all other colors that are retained in the
material, that is known as complementary hue.
Now look at the slide, from 380 to 435 I have
violet color and the complementary hue is
the material will look like yellowish green.
So, 435 that means if a material looks yellowish
green to you as an observer you should say
that, it transmits violet color.
If the material is yellow then what is transmitting
is blue color.
If it is orange it is greenish blue or bluish
green and green.
If it is transmitting green color it will
look purple to you and similarly other colors
like yellowish green, violet, yellow, blue,
orange, red.
So, you can see that most of these transmitted
color and complementary hues are approximately
inversely related.
Yellow is at the top of the complementary
hue and orange is transmitted between 590
to 625 but it looks like greenish blue.
Greenish blue corresponds to 480 to 590.
For any visible range the transmitted color
and complementary hues are different.
So, the same thing is true in other radiations
also.
But we may not be able to see them unless
they are in the visible range.
Infrared and many other radiations they predominantly
look red only.
Microwaves you may not be able to see them
at all.
Near infrared you can see sometimes color
or not.
Near ultraviolet you will see them light yellow
or colorless.
That is all human eye is capable of seeing
the colors.
It must be noted here that there are no sharp
differences in color or wavelength.
For example in the previous slide, I was showing
you 480 to 435 and 435 to 480.
Now what about 440?
380 to 435 if it is violet, how would 440
look like?
I have no way of telling exactly, because
it all depends upon the observer, your eye,
your capability of how good your eyes are,
how sharp your eyes are etc.,
The merging of the colors is sort of general.
To a lot of people it may look the same but
actually some people have colorblind also.
It looks different to them many times.
The remaining colors merge into one another
in a diffused manner like a rainbow.
The absorption of radiation involves transfer
of energy to the medium and this process is
a specific phenomena related to the characteristic
molecular structure.
For a given excitation process a molecule
absorbs a discrete quantity of an absorption
line.
However since a group of molecules exists
in a number of vibrational and rotational
energy states each differing by a very small
amount of energy.
A series of absorption lines appear over a
small range and thus it gives rise to an absorption
peak or an absorption band.
You should look at this slide a little more
carefully and try to understand what is written
here.
The absorption of radiation involves transfer
of energy and this process is a specific phenomenon.
That means if I have a specific phenomenon
then the absorption line should correspond
to only one wavelength.
But actually when you see a spectrum you see
that, it is a peak.
Now, why do I get a peak?
The answer is, for a given excitation process
molecule definitely absorbs discrete amount
of energy corresponding to an absorption line.
But, a group of molecules exists in a slightly
differing energy levels.
Therefore even though the exact quantity of
energy is absorbed by the molecule with higher
energy it absorbs the same amount of energy.
It appears a slightly higher.
The molecule at lower energy will appear a
little lower.
So, with the result that we have a figure
like a peak and that is the reason why we
see instead of a single absorption line a
peak appears over a small range that gives
rise to an absorption band.
Here I am showing you, the energy level diagram
of different orbitals.
Here it is 1s, 2s, 2p, 3s, 3p, 3d and 4s,
4p, 4d etc., and I am showing you here by
arrow marks and you can see that these energies
are quantized.
That means if I supply energy somewhere here
the transition does not take place, when I
supply the energy corresponding to 2s to 1s
then transition ΔE3 will take place.
Similarly between 2s and 2p ΔE2 will take
place only when it matches the energy difference
.
For monoatomic substances, that means atoms
with single elements with single element that
is hydrogen, nitrogen etc.
Such substances normally exist in gaseous
state.
Very low atomic weight and atomic number etc.
They absorb radiation only through an increase
in their electronic energy, that is the energy
level between 1s, 2s, 3s etc.
Transition between 2s and 2p, 3s and 3d like
that it does not happen.
For polyatomic transitions, that is not the
case because they involve molecular orbitals
requiring energy in the ultraviolet region
and these are for more complex involving vibrational
and rotational energy levels.
The total energy maybe considered as the sum
of the contributions from all states of energy.
For each electronic energy state of the molecule
there exist several possible vibrational states.
For each of these vibrational states there
are numerous rotational energy states.
This is what I was trying to explain to you.
Many molecules maybe in different vibrational
energy levels and different rotational energy
levels.
You can imagine them to be like steps in a
multistory building.
In a multistory building if you want to climb
the stairs you will have to go from ground
floor to first floor, second floor, third
floor like that and then there will be landing.
If you stand at the bottom of a ground floor,
you kick a ball to the first floor it may
land on the first floor or it may land 1 or
2 steps before that or 1 or 2 steps above
that also.
Imagine a molecule is absorbing energy corresponding
to 1 step below, 1 step exactly same step
and exactly 1 step above that.
All these three molecules if they represent,
I get a peak plotting in terms of energy.
These energy states are represented as vibrational
states, for each of these in terms of rotational
energy states also.
Electronic transitions in organic molecules
are characterized by the promotion of electrons
from bonding or non bonding orbitals to the
excited state anti bonding orbitals.
The bonding orbitals are designated as sigma(σ)
orbital and non bonding orbitals are designated
as sigma star(σ*).
Now you should have a figure like this.
I will show you in the next class but try
to understand what I am writing here.
The electronic transitions are all characterized
by the promotion of electrons from ground
state to higher energy state.
That means all bonded in a single bond, double
bond etc., they are all bonding electrons.
Those electrons are getting excited by interaction
with the electromagnetic radiation.
Those electrons go to a state defined as non
bonding orbital or anti bonding orbital.
The bonding orbitals are designated as sigma
(σ) orbitals and non bonding excited orbitals
are called as sigma star(σ*).
These are virtual energy levels 
not exactly defined and you cannot visualize
them in actual practice.
But pictorially we can represent that.
In addition from sigma to sigma star transitions
I have many molecules which may contain many
electrons that are not directly involved in
bonding.
These are mainly located in atomic orbitals
of elements like oxygen, sulphur, nitrogen
and halogens.
For those of you who are familiar with the
chemistry, we know that there are two lone
pair of electrons in oxygen, which are usually
present in water i.e 2 hydrogen, 1 oxygen.
But on the oxygen there are 2 lone pair of
electrons.
Similarly there are unshared pair of electrons
on elements like sulphur, nitrogen and halogens
and when they are combined with other elements,
we have the generalized shapes of n orbitals
can be shown in lines, solid line, dash line
etc.,
We will study them in detail in our next class.
Thank you very much.
