INTRODUCTION :
In spectroscopy, transitions are induced in a chemical species by its interaction with the photons of electromagnetic radiation. The most important consequence of electromagnetic interaction is that energy is absorbed or emitted by matter in discrete amounts called quanta. Spectroscopic techniques are generally employed to measure the energy difference between different molecular energy levels and to determine the atomic and molecular structures. The instruments used in such studies, called spectrophotometers, are devised to measure the relative energy that is emitted, transmitted or reflected in the infrared, ultraviolet or visible regions, as a function of wavelength or wave number Special devices are incorporated in spectrophotometers for the automatic recording of spectra.
The spectrum of the molecule is a continuous graph, obtained by plotting either absorption or transmittance of electromagnetic radiation as a function of wavelength or wave number over a particular range. Spectrometry, which is quite different from usual spectroscopic techniques, depends upon the absorption or emission of electromagnetic radiation. For instance, in mass spectrometry the photographic plate is calibrated to measure various values of m/z. In many branches of spectroscopy the system interacts with the electric field. However, in case of magnetic resonance spectroscopy it interacts with the magnetic field.
ELECTROMAGNETIC RADIATION :
Electromagnetic radiation ranges from the electric waves of low frequency through UV rays, visible spectrum and IR rays to the high frequency (low wavelength) X-rays and ɣ-rays. It is said to have a dual nature (wave and particle characteristics).
A. Wave Theory of Electromagnetic Radiation :
Maxwell (1864) found that the electromagnetic radiation is made up of two mutually perpendicular sinusoidally oscillating electric and magnetic fields in planes at right angles to each other as a sine wave as shown in Fig. 1. E and B represent the electric field vectors and magnetic field vectors respectively.
In study of spectroscopy, the effects associated with the electrical component of the electromagnetic wave are important. The electromagnetic radiation travels in the third plane OYZ (Fig. 1) . The ratio of E/B associated with an electromagnetic wave is equal to the velocity of light provided E is taken in e.s. units and B in e.m. units. As with this structure of wave no medium is involved, electromagnetic radiation can travel in vacuum. This velocity of light in vacuum has a value of 3 x 108 ms-1 An electromagnetic wave is characterised by their wavelengths or frequencies or wave numbers. These parameters are:
1. Wavelength ( λ) :
The distance between two peaks or two troughs is called the wavelength. The wavelength is expressed in either Angstrom (1Å=10-8 cm =0.1 nm =10-10 m), metres (m), millimetres (1mm=10-3m), micrometres (1μm =10-6 m) or nanometers (1 nm = 10-9 m = 10-7cm).
2. Frequency (ν) :
The number of complete wavelength units passing through a given point per second is called frequency: It is measured as cycles per second (cps) called Hertz (Hz after H. R. Hertz) or as Fresnel.
1 Hz=1 cps. 1 Fresnel=1012 Hz
Bigger units are kilocycles per second (kcps or kHz) and mega cycles per second (Mcps or MHz).
1kHz =103 Hz and 1 MHz =106 Hz
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Note that :
(i) The frequency of a visible light determines its colour.
(ii) Frequency of radiation is a direct measure of its energy. High frequency radiations have high energy and vice versa.
(iii) Wavelength and frequency are inversely proportional.
(iv) The wavelength measures the periodicity in space and frequency the periodicity in time.
It is generally wavelength which is measured and always frequency is more significant for the interpretation of spectra, v and λ are the characteristic quantities of the radiation and are related by the relation
ν = c/λ
where c is the velocity (3x 108 ms-1) of the electromagnetic wave.
3. Wave Number (v̅)
It is the number of waves in a length of 1 cm. v =1/λ. It is expressed in the units of cm-1 called Kaysers (K). This representation is particularly useful in the optical region of the spectrum (ultraviolet, visible and infrared). Sometimes kilo Kayser (Kk) is also used.
1 kK =1000 K =1000 cm-1
4. Velocity (v)
The product of frequency and wavelength is equal to the velocity of the wave in the medium. i.e.,
v=λ×ν
B. Quantum Theory of Electromagnetic Radiation
The phenomenon of reflection, retraction, reinforcement and destructive interference constitute wave properties of radiation. But the wave nature of electromagnetic radiation fails to explain several phenomenon like the photoelectric effect. Hence quantum theory describes electromagnetic radiation as consisting of a stream of energy packets, called photons or quanta, which travel in the direction of propagation of the beam with the velocity of light. Thus during absorption or emission of light by chemical species, the change in energy becomes only discreetly (integral multiples of small units of energy, called photons). The energy E of the photon is proportional to the frequency of radiation and is
E=hν or hc/λ
=hc v̅
where Planck’s constant h=6.626 x 10-34 Js, ν is in s–¹ and λ in nm. Energy of photons is called quantum of energy and it depends only on the frequency not on the intensity of radiation.
Note that higher the frequency of radiation the greater will be its energy (X-rays are more energetic than rays of visible light). A photon of ultraviolet light has more energy than a photon of visible light).
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THE ELECTROMAGNETIC SPECTRUM :
The arrangement of all types of electromagnetic radiations in order of their increasing wavelengths or decreasing frequencies is called electromagnetic spectrum (Fig. 2).
An electromagnetic spectrum is considered to be consisting of the regions of radiant energy ranging from wavelengths of 10 m to 1×10-14 m. If all types of electromagnetic radiation are arranged in order of their increasing wavelengths then the radio waves have the highest wavelengths lowest frequencies) while cosmic rays have the shortest wavelengths (highest frequencies).
Cosmic rays < ɣ- rays < X-rays < UV rays < Visible light < IR rays < Microwaves < Radio Waves.
All types of electromagnetic radiation travel with the same speed (the velocity of light). but they differ in wavelength from each other. Thus,
E = hν = hc/λ
The increasing order of energies of the electromagnetic radiation is:
Radio Waves < Microwaves < IR < Visible < UV < X-rays <ɣ- rays < Cosmic rays.
When a molecule absorbs electromagnetic radiation (e.. ΔE= hv), it can undergo various types of excitations such as electronic excitation, rotational excitation, excitation leading to a change in nuclear spin, excitation resulting in bond formation and so on. In addition, the vibrational and rotational energies of the molecules as a whole are quantized. Thus, any wavelength of light that a particular molecule will absorb will be due to changes in the electronic, vibrational and rotational energy levels permissible for its atoms. Since each mode of excitation requires a specific quantity of energy, the different absorptions occur in different regions of the electromagnetic spectrum (Table 1).
Note that :
1. Visible and ultraviolet radiations cover the wavelength range from 200 nm to 780 nm. The absorption of radiation in this region causes the excitation of π electrons in a conjugated or an unconjugated system.
2. Infrared rays are outside the upper limit of the visible spectrum and produce high thermal effects. The IR radiations cover the wavelength range from 0.75 to 2.5 μm (near IR region) and from 50-1000 µm (far IR region). The most useful region lies in the range of 2-5 to 50 µm. The absorptions in the fingerprint region are most characteristic of the compound.
3. NMR technique utilises radiations of long wavelength range, i.e. radiowaves.
Thus the interaction of radiation with matter in different ways by various processes gives rise to following branches of spectroscopy (Table 2).
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INTERACTION OF ELECTROMAGNETIC RADIATION WITH MATTER :
When a beam of light interacts with matter, numerous changes occur in both light and matter. These changes provide the basis for several research tools such as spectrographs, colorimeters, polarimeter and refractometer etc. Electromagnetic radiation is characterized by wavelength λ. frequency V0 , intensity l0 and direction. Important changes that may occur are summarised below.
1. The direction of the incident beam of light can be changed by reflection and refraction.
2. The beam of light can be transformed into other beams by diffraction, double refraction and scattering
3. If scattering occurs, the scattered light may exhibit the same frequency as incident light. This type of scattering is referred to as Rayleigh scattering.
4. If the scattered beam exhibits either higher or lower frequency, it is called Raman scattering or the Raman effect.
5. If the incident beam is plane polarised, the plane of polarisation may be rotated by passing through the compound. It is known as optical rotation and is measured by polarimeter
6. The intensity of the incident beam gets reduced or even disappear when passed through the substance. It is absorption of light.
(a) If there is an exchange of energy between the light beam and the molecules, a pattern of wavelength of light absorbed with an indication of the energy absorbed at each wavelength constitute the absorption spectra.
Absorption occurs only when the radiation supplying the right packet of energy impinges on the matter. This forms the basis of molecular spectra. The absorption of radiation depends on the molecular structure of the compound.
(b) The extent of absorption may depend on the orientation of the plane polarisation in the incident beam of radiation. This is called dichroism.
(c) The absorption of radiation causes the atom or molecule to be in an excited state.
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Since excited states are short lived (10-8 s), the electron may return to its ground state with the emission of certain amount of energy. When this emission of light is instantaneous, the phenomenon is known as fluorescence, if delayed it is called phosphorescence.
(d) If the light absorption produces chemically reactive substances, the process is called photo-activation and photochemical reaction
7. Matter can be made to emit light if it is properly excited. The resulting radiation may contain several discrete and reproducible wavelengths in ultraviolet and visible
regions. Thus a pattern of wavelength of radiation emitted constitute emission spectra.
The absorption and emission spectra provide the same information about the energy level separation in the molecule. Interaction of radiation with matter provide significant informations for the determination of the molecular structure. The phenomenon associated to frequency and intensity of radiation include (i) Absorption, (ii) Scattering (Rayleigh). (iii) Raman scattering. (Iv) Fluorescence and phosphorescence (Fig.3).
Thus employing sophisticated techniques for handling electromagnetic radiation and data processing constitute the major field of spectroscopy.
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SOME PHENOMENON RELATED TO RADIATION :
Diffraction of Radiation :
Electromagnetic radiation normally travels in straight paths. But if a beam of radiation is passed through a narrow opening, a part of it appears to be in the shadow of the object in its path of travel. This bending of radiation is termed as diffraction. It is a direct consequence of interference.
Refraction of Radiation :
When a beam of radiation is allowed to pass from one medium to another having different density, an abrupt change in the direction of beam is seen. This phenomenon due to the differences in the velocity of radiation in two media is called refraction. The extent of refraction is given by
sin θ1 /sin θ2 = n2/n1 = v1/v2
Reflection of Radiation :
Reflection occurs when a beam of radiation is allowed to cross an interface between media of different refractive indices. When a beam travel normal to the interface, the fraction of reflected beam increases with increasing difference in refractive index and is given by
Ir / I0 = (n2 – n1)2/ (n2 + n1)2
where l0 and Ir , are the intensities of incident and reflected radiation, n1 and n2 are the refractive indices of two media.
Scattering of Radiation :
If the incoming energy radiation strikes upon particles which are suspended in a medium having a refractive index different from that of the suspended particles, the light which is transmitted at angles other than 180ο from the incident light is said to be scattered as the radiation passes through the sample. The size, shape and concentration of colloidal particles and suspensions may be determined from this. Nephelometry and turbidimetry are based upon this ability of particles to scatter light. Rayleigh scattering is scattering by molecules or aggregates of molecules with dimensions, which are smaller than the wavelength of the radiation . [For detail see Raman spectroscopy].
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Dispersion of Radiation :
The variation of refractive index of a substance with wavelength of the transmitted light is known as dispersion. In the regions of high transparency if wavelength increases then the refractive index decreases, but not linearly. In the regions of high absorbance, the refractive index increases sharply with wavelength. Dispersion is of two types.
1. Normal dispersion :
It occurs in that region in which the refractive index increases gradually with increasing frequency or decreasing wavelength.
2. Anomalous dispersion :
It takes place in that region in which a short change in refractive index is observed. Typical dispersion curves are shown in Fig. 4.
Transmission of Radiation :
Transmission in a medium may be considered as a stepwise process involving atoms, ions or molecules as intermediates. The interaction which occurs during transmission may be due to alternating electrical field of radiation that causes oscillation of bound electrons of the particles with respect to their fixed nuclei. Hence periodic polarisation of the particles occurs. Each polarised particle emit radiation in all possible directions. For small particles, destructive interference prevents the propagation of radiation in any direction other than that of the original path of beam. However, if the medium contains large particles, the destructive effect is not complete. Thus a part of the beam will be scattered as a consequence of the interaction step.
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