Introduction to spectroscopy

Introduction to Spectroscopy: 

Introduction to Spectroscopy Anuj Chawla M Pharm

Contents: 

Contents Radiant Energy Electromagnetic Spectrum Region of Electromagnetic Spectrum Properties of Electromagnetic radiation Spectroscopy Spectrometry Types of Spectroscopy Classification of Methods

Radiant Energy: 

Radiant Energy Energy of electromagnetic waves, which is transmitted away from one body to another in the form of radiations. The quantity may be calculated by integrating radiant flux (or power) with respect to time and, like all forms of energy, its SI unit is the joule.

Electromagnetic spectrum: 

Electromagnetic spectrum When a narrow beam of light is allowed to pass through a prism or grating it is dispersed into seven colors from red to violet and the set of colors or band produced is called spectrum . The arrangement obtained by arranging various types of electromagnetic waves or radiations inorder of their increasing wavelengths or decreasing frequencies is called electromagnetic spectrum.

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Arrangement of different types of electromagnetic radiation with increasing order of their wavelength is : Cosmic rays <Gamma rays < X-rays < Ultra violet rays < Infra red rays < Micro waves < Radio waves. v = c/ λ Increasing order of wavelength means decreasing order of frequency.

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All types of electromagnetic radiations travel with the same speed, the velocity of light, but they differ in wavelength or frequency from each other. Radiowaves are least energetics and cosmic waves are highly energetic. E = hv = hc / λ Equation indicate that greater the wavelength, lower will be the energy and vice versa.

Regions of Electromagnetic Spectrum: 

Regions of Electromagnetic Spectrum Region Limits X-rays 0.1-100 Å Far Ultra violet 10-200 nm Near Ultraviolet 200-400 nm Visible 400-750 nm Near Infrared 0.75-2.2 µm Mid Infrared 2.5-15 µm Far Infrared 15-200 µm Microwaves 0.1-100 cm Radiowaves 1-1000 m

Properties of Electromagnetic Radiations: 

Properties of Electromagnetic Radiations Characterised easily by ascribing a wave nature to its propagation and to portray these waves by such parameters as velocity, frequency, wavelength and amplitude. It comprises of oscillating electric and magnetic fields. Both are propagated as waves with the two fields perpendicular to one another.

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Relation Between Wavelength Frequency and Amplitude

Wavelength: 

Wavelength Distance between two successive peaks of waves, that is, length of one wave Usually expressed in cm, the units of wave length are: Angstrom Å (1Å = 10 -8 cm = 10 -10 m) Nanometer nm (1nm = 10 -7 cm = 10 -9 m ) Micron µ ( 1µ = 10 -4 cm = 10 -6 m )

Frequency (v): 

Frequency (v) The number of waves per second is called frequency of an electromagnetic radiation Frequency = speed of light in cm/sec wavelength in cms Velocity of light c = 3.0 × 10 10 cm/sec The unit of frequency is cycles per second (Hz)

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Velocity The distance travelled by the wave in one second is called velocity of the wave. It is denoted by c Wave number(v) Number of waves spread in a length of one centimeter. It is equal to the reciprocal of wavelength in centimeter. Thus, v = 1/ λ The wave number is a direct measure of the energy of radiation. Radiation with large wave number has high energy and vice versa.

Spectroscopy: 

Spectroscopy Study of the interaction between radiation and matter as a function of wavelength( λ ). Historically, spectroscopy referred to the use of visible light dispersed according to its wavelength, e.g. by a prism. Later the concept was expanded greatly to comprise any measurement of a quantity as a function of either wavelength or frequency . Thus, it also can refer to a response to an alternating field or varying frequency ( ν)

Spectrometry: 

Spectrometry Spectroscopic technique used to assess the concentration or amount of a given chemical (atomic, molecular, or ionic) species. The instrument that performs such measurements is a spectrometer , spectrophotometer, or spectrograph.

Uses of Spectroscopy/Spectrometry: 

Uses of Spectroscopy/Spectrometry Used in physical and analytical chemistry for the identification of substances through the spectrum emitted from or absorbed by them. Also used in astronomy and remote sensing.

Types of Spectroscopy: 

Types of Spectroscopy Atomic Spectroscopy Molecular Spectroscopy

Atomic Spectroscopy: 

Atomic Spectroscopy Interaction of electromagnetic radiations with atoms which are most commonly in their lowest energy state called the ground state.

Molecular Spectroscopy : 

Molecular Spectroscopy Interaction of electromagnetic radiation with molecules. Results in transition between rotational and vibrational energy levels in addition to electronic transition. Spectra of molecules are much more complicated than those of atoms

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Molecular spectra extend from the visible through infrared into the microwaver region. Current interest in molecular spectroscopy is very great because the number of known molecules are extremely large as compared with free atoms.

Different Types of Molecular Energies: 

Different Types of Molecular Energies In addition to the ordinary energy of transitional motion which is not of concern in molecular spectroscopy, a molecule may posses internal energy which can be subdivided into three classes : Rotational energy Vibrational energy Electronic energy

Translational Energy: 

Translational Energy Energy is associated with the uniform motion of a molecule as a whole. This motion is generally described with respect to the centre of mass of the molecule. The energy due to translational motion is given by E = ½mv 2

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Due to translational motion the molecule is free to move in the three perpendicular directions. It means that it has three degrees of freedom.

Rotational Energy: 

Rotational Energy Energy is associated with the overall rotation of the molecule with the atoms considered as fixed point masses. The value of a rotational energy is given by E = ½Iw 2 Where I is the moment of inertia And w is angular velocity of rotating molecule

Vibrational Energy: 

Vibrational Energy Energy is associated with the oscillation of atoms which is considered as point masses about equilibrium positions. It can be treated on a quantum mechanical basis. The vibrational energy of a molecule may be given as E = h v (v+½) Where v is vibrational frequency And v is vibrational quantum number.

Electronic Energy: 

Electronic Energy Energy is associated with the motion of electrons while considering the nuclei of atoms as fixed points. The increase in the electronic energy of a molecule occurs due to an increase in the kinetic energy and potential energy of the electrons of the molecule. The energy required to move the electron from the ground state to an excited state varies with the type of molecular bond in which the electron exists.

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Type of transition Region of electromagnetic spectrum Electronic Ultraviolet and visible Vibrational Infrared Rotational Far infrared and microwave

Classification of methods: 

Classification of methods Nature of excitation measured Measurement process

Nature of excitation measured : 

Nature of excitation measured The type of spectroscopy depends on the physical quantity measured. Electromagnetic spectroscopy - interactions of matter with electromagnetic radiation, such as light. Electron spectroscopy - interactions with electron beams. Dielectric spectroscopy - the frequency of an external electrical field Mechanical spectroscopy - the frequency of an external mechanical stress, e.g. a torsion applied to a piece of material

Measurement process: 

Measurement process Absorption spectroscopy uses the range of the electromagnetic spectra in which a substance absorbs. This includes atomic absorption spectroscopy and various molecular techniques, such as infrared, ultraviolet-visible and microwave spectroscopy. Emission spectroscopy uses the range of electromagnetic spectra in which a substance radiates (emits). The substance first must absorb energy. This energy can be from a variety of sources, which determines the name of the subsequent emission, like luminescence.

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