Download Absorption of Radiation

Atomic and
Molecular
Spectroscopy
Chap 1 T2
Chap 5 T1
Chap 6 R1
•Spectroscopy, is the measurement and
interpretation of electromagnetic radiation absorbed,
scattered or emitted by species (atoms, molecules,
ions etc)
• interaction of radiation with species can cause
redirection of the radiation and/or transitions
between the energy levels of the atoms or molecules
• these interactions are associated with changes in
the energy states of the species, thus spectroscopy
can be used to identify the interacting species.
Electromagnetic Radiation:
• consists of discrete packets of
energy called ‘photons’
• a photon consists of an
oscillating electric field (E)and
magnetic field component,
(M).
• electric and magnetic fields are orthogonal
(perpendicular) to each other, and they are orthogonal
to the direction of propagation of the photon.
Characteristics of EM radiations:
•electric and magnetic fields
flip direction as the photon
travels.
• the number of flips, or
oscillations, that occur in one
second is c/a the frequency, υ.
•Frequency has the units of
oscillations per second, or
simply s-1 (this unit is given
the name Hertz).
•distance over which the electric and magnetic fields
of a photon make one complete oscillation is called the
wavelength, λ, of the electromagnetic radiation
• or wavelength of the radiation, λ, is the distance
between maxima of either electrical or magnetic
component, i.e.. from crest to crest.
In spectroscopy, wavelengths are expressed in a
variety of units
•microwave region - centimeters or millimeters,
•infrared region – micrometers
• visible and ultra-violet region – Angstrom
(1A=10-10m) or nanometer (10-9m)
• Wave number, ϋ: it is the number of waves per
centimeter or defined as the reciprocal of the
wavelength in cm (cm-1) ,(used in infrared
spectroscopy)
Monochromatic light: radiation of one discrete
wavelength
Polychromatic light: radiation of wide distribution of
wavelength
• when a radiation passes through a medium
containing species its propagation is slowed due its
interaction with bounded electrons in the species
•since the frequency is invariant and fixed by the
source, the wavelength must decrease
•Velocity = frequency X wavelength
Regions of EM Spectrum:
• is a broad range of radiation extending from cosmic
rays (λ~10-9nm) to radio waves (λ~1000km)
• photons in all of these regions have the same
electromagnetic nature, but because of their very
different energies (E = hυ = hc/λ)they interact with
matter very differently
•Effects of various types of radiation are very different.
•in radio wave region the energy of photon is very
low and is concerned with the reorientation of
nuclear spin state of the species in a magnetic field.
(NMR)
•in micro wave region the energy of photon is
concerned with reorientation of electron spin state
of the species with unpaired electrons in a magnetic
field. (ESR)
•in infrared region, absorption of radiation causes
changes in rotational and vibrational energy state
of molecules (IR)
•in visible and ultraviolet regions absorption of
radiation changes the energy of the valence
electrons (+ in case of molecules, rotationalvibrational changes)
•X rays causes the ejection of inner electrons from
matter, and,
• gamma rays can cause changes or rearrangement
in the nucleus.
Frequency
Type of
Radiation Range (Hz)
Waveleng
Type of Transition
th Range
gammarays
1020-1024
<1 pm
nuclear
X-rays
1017-1020
1 nm-1
pm
inner electron
ultraviolet 1015-1017
400 nm-1
nm
750 nm400 nm
outer electron
visible
4-7.5x1014
nearinfrared
1x10144x1014
2.5 µm750 nm
outer electron
molecular vibrations
infrared
1013-1014
25 µm-2.5
µm
molecular vibrations
microwav
es
3x1011-1013
1 mm-25
µm
molecular rotations,
electron spin flips*
radio
waves
<3x1011
>1 mm
nuclear spin flips*
outer electron
Interaction of radiation with matter:
Diffraction, Transmission, Refraction, Reflection,
Scattering and Polarization
In a beam of ordinary light (ex from bulb) the
oscillation of electric field are occurring in all
possible planes perpendicular to the direction of
propagation, c/a Unpolarized light
Transmission
• Transmission:The rate of radiation
propagated through medium is less than its
velocity in vacuum
• Rate depends on type and conc. Of
molecules in media
• No frequency change ( Wave length
changes)
Transmission continued
• Refractive index (n) is a measure of interaction of
radiation with matter = c / v
c=velocity of radiation in vacuum
v= velocity of radiation in media
• Interaction
– Interaction of electric field of radiation with
electrons of medium
– Brief polarization of ions,atoms, etc.
– Path of beam unaltered
Dispersion
• Dispersion : variation of refractive index with
wavelength or frequency
– Normal dispersion: Gradual increase in
dispersion with frequency
– Anomalous dispersion : Sudden change in
dispersion with frequency ( when frequencies
correspond to natural frequency of
components) simultaneous absorption observed
– Important for choosing optical components eg
normal dispersion for lens ( gives minimum
chromatic aberration) whereas anomalous
Refraction of Radiation
• Refraction : Change in direction of beam
when radiation passes from one media to
another differing in physical densities
-Shift toward normal when passing from
rarer
medium to denser media
- Shift away from normal when passing
from denser to rarer media
• Refraction=sinθ1/ sinθ2=n1/n2=v1/v2
Diffraction
• Diffraction : A process by which parallel
beam of light is bend on passing through
sharp barrier or narrow opening
– Wave property
– More pronounced when wavelength and slit of
same order of magnitude
– Diffraction is a consequence of interference
Reflection
• Reflection : When radiation crosses an interface
between media with differeing refractive indexes
reflection results
• Larger the difference more the refraction
• Ir/I0=n2-n1/n2+n1
• I0::Intensity of Incident beam
• I1:Intensity of reflected beam
• n1 & n2 refractive index of the 2 media
• Reflective loss increases till 600 then reflection
increases to 100% ot 900
Scattering
• Scattering: Occurs when when
media has large particle size as
compared to wavelength
• A very small fraction of radiation
transmitted in all directions
• Colloidal size particles
• Nephlometry
Diffraction
• Diffraction : A process by which parallel
beam of light is bend on passing through
sharp barrier or narrow opening
– Wave property
– More pronounced when wavelength and slit of
same order of magnitude
– Diffraction is a consequence of interference
Interaction of radiation with matter:
Diffraction, Transmission, Refraction, Reflection,
Scattering and Polarization
In a beam of ordinary light (ex from bulb) the
oscillation of electric field are occurring in all
possible planes perpendicular to the direction of
propagation, c/a Unpolarized light
•when an unpolarized light passes through a
polarizer, it interacts with the electrical field
• resultant light which emerge from the polarizer has
their electric field vector oscillating in only one
direction. Such light is c/a plane-polarized light
• can be polarized in different directions
•Rotaion angle depends on
• Rotation angle : Depends on temperature of solvent,
wavelength of plane polarized light, concentration &
number of symmetric molecules
•circularly polarized light: the electric field vector is
rotating around the axis of light propagation.
• electric field vector can rotate in either the right or left
direction, and the light is called right (Clockwise)
circularly polarized or left (anticlockwise) circularly
polarized, respectively.
Energy States of Chemical Species:
According to the quantum theory:
a) Atoms,ions and molecules can exist only in certain
discrete states, characterized by definite amounts of
energy
b) When a species changes its states, it absorbs or emits
an amount of energy (radiation) exactly equal to the
energy difference between the states
c) The frequency or the wavelength of the radiation is
related to the energy difference between the states
• Energy of atomic state arises from the motion of
electrons around the nucleus, and the energy states
are c/a “electronic states”
• Molecules, in addition to having electronic states,
also haves quantized “vibrational states” and
“rotational states”
• vibrational energies are associated with the energy of
interatomic vibration
• rotational energies arise from rotation of molecules
around their centers of gravity
• Generally at room temperature, chemical species are
in their ground state.
Absorption of Radiation:
• absorption is a process in which matter (species)
can capture electromagnetic radiation and convert
the energy into internal energy
• due to this the species gets transformed from
ground state to one or more higher-energy state
• for absorption to occur, the energy of the
exciting photon must exactly match the energy
differences between the ground state and one of
the excited state of the absorbing species
• since the energy levels of matter are quantized,
only light of energy that can cause transitions
from one level to another will be absorbed.
• type of excitation (or transition) depends on the
wavelength of the light.
uv or visible light –promote electrons to higher
orbitals
infrared light –
promotes vibrations
radio wave- promotes reorientation of nucleus spin;
X-ray-promotes ejection of electrons
absorption spectrum- the
absorption of light as a
function of wavelength.
spectrum of an atom or
molecule depends on its
energy-level structure, thus
useful for identifying
compounds.
absorption spectra vary widely
in appearance
a) Atomic Absorption:
• When a polychromatic radiation (UV or Vis) pass
through a sample containing monoatomic particles
(viz.gaseous Hg or Na) it results in the absorption
of a well-defined frequencies (wavelengths) (Fig a)
•
relative simplicity of spectra is due to small
number of possible energy states (only electronic
state) for the absorbing species (atoms)
ex. Na vapors exhibits two sharp absorption peaks
(589.0 and 589.6nm) due to excitation of 3s
electron to two 3p orbital
b) Molecular Absorption:
• Absorption spectra of molecules are more complex
than atomic spectra because the number of energy
states of molecules is enormous (fig b,c,d)
• Energy E associated with the bands of a molecule is
made up of 3 components:
E = Eelectronic + Evibrational + Erotational
Eelectronic-energy associated with bonding electrons
Evibrational –energy associated with interatomic vibrations in
molecule
Erotational –energy associated with various rotational motions
within a molecule
• for each electronic energy
state of a molecule, several
possible vibrational states
exists,
•and for each to these
vibrational states, numerous
rotational states are possible
• thus molecular absorption
spectra are characterized by
regions of substantial
wavelength range (b and c) and
is complex
c) Absorption induced by a magnetic field:
• when a molecule is placed in a strong magnetic field
additional quantized energy levels are observed, due
to the magnetic properties of their particles
• differences in energy between the induced states are
small and transitions between the states are brought
about only by absorption of long wavelength radiation
with nuclei, radio wave (λ=103-60cm) are involved
(NMR)
For electron, microwaves (λ=3 cm) are involved (EPR)
Emission of Radiation:
• radiation is emitted when excited species (atoms,
ions or molecules) relax to lower energy levels by
giving up their excess energy as photons
Emission spectra: a plot of intensity of emitted
radiation as a function of wavelength
3 types of emission spectra
a) Line spectra: produced
when the radiating species
are individual atomic
particles that are well
separated, in a gas phase
b) Band spectra: are
encountered in sources
when gaseous radicals or
small molecules are
present
c) Continuum spectra: are encountered when large
molecules are present in the source
Absorption methods:
• Quantitative absorption methods require two intensity
measurements: one before a radiation beam passed
through the sample (P0) and other after (P)
Fig shows a beam of radiation before and after it has
passed through a medium that has a thickness of b cm
and a concentration c of absorbing species
• due interaction between the photons and absorbing
species, the intensity of the beam is attenuated from
P0 to P.
• Transmittance (T) of the medium is the fraction of
incident radiation transmitted by the medium
T= P/P0
• Absorbance (A) of a medium is defined as
A = -log10 T = log P0/P
Absorbance of a medium increases as attenuation of the
beam become greater
Components of optical Instruments: Typical
instruments contains 5 components:
a) A stable source of radiant energy
b) A transparent container for holding the sample
c) A device that isolate a region of the spectrum
for measurement (ie. wavelength selector)
d) A radiation detector which convert radiant
energy to a usable signal(electric) and
e) A signal processor and readout which displays
the transduced signal
1. Sources of Radiation: Lamps convert electrical
energy into radiation.
Continuum sources:
For UV region- deuterium lamp
Visible region- tungsten filament lamp
For IR region- heated inert solids
Line sources:
For all atomic spectroscopy- hollow-cathode lamps
Radiation sources for spectroscopic instruments
2. Wavelength Selectors: for most spectroscopic
analyses radiation of a limited, narrow continuous
group of wavelengths (c/a band) is required
• A narrow bandwidth not only enhances the sensitivity
of absorbance measurement but also is a requirement
to obey LB laws
Two types of wavelength selectors:
a) Filters: relay on optical interference (interference
filter) or absorbing certain portion of the
spectrum(absorption filter), to provide narrow bands
of radiation
b) Monochromators:For many spectroscopic methods it
is necessary to vary the wavelength continuously
over a considerable range.
ex prism and diffraction grating
3. Sample containers:must be of material that is
transparent to radiation of interest
Visible region-plastic container/UV region-quartz or
fused silica
Sample
holder
Wavelengt
h selector
4. Radiation Transducer (detector): 2 types
a) Photoelectric transducer- absorbed radiation
(photon), causes emission of electrons and
development of a photocurrent
ex photovoltic cells, phototubes, and
photomultiplier tube
Used largely for measurement of UV, Vis, near-IR
radiation
b) Heat transducer- responds to the average power
power of the incident radiation,
Used largely for measurement of IR radiation,
because photons in this region lack the energy to
cause photoemission of electron
Detectors
5. Signal Processors and Readouts:
• Signal processor is an electronic device that not only
amplifies the electrical signal form the transducer but
also alter the signal (dc to ac,change the phase of
signal etc)
• Several types of readout devices are found in modern
instruments ( digital meter, recorder,cathode-ray
tubes etc.)