Introduction of spectroscopy :
Spectroscopy is, therefore, the study of the molecule's response when it is exposed to certain kinds of radiation.
One of the best ways of knowing the structure of the molecule is to 'see' the molecule directly - however, molecule dimensions are so small - of the order of 10-7 cm to 10-8 cm even the most powerful microscope available today, cannot ' show 'us the structure of the molecule directly. Therefore, all our method involves subjecting the molecule for various experiments and interpreting the results obtained in terms of structure.
In spectroscopy, the molecule is exposed to some kind of radiation, and the response given by the molecule is recorded generally in the form of a graph 📉 called ' Spectrum.'(By studying the spectrum, it is possible to throw light on the structure of molecules.
When exposed to radiation, a given molecule absorbs part of it, gets excited, and goes to higher energy. The amount and the wavelength of radiation absorbed by the molecule to reach the excited state depend upon the structural features of the molecule. Therefore, by studying the type of radiation absorbed, predicting what kind of structural features are present in the molecule. The absorption of different types of kind of structural features is present in the molecule. The absorption of different types of radiation such as UV, visible, infrared, microwave, radio waves produce different kinds of excitations of the molecule, and each excitation provide some important information about the structure. Since the nature of the radiation absorbed is studied in all these methods, the spectroscopic methods are designed as 'absorption spectroscopy.'
Once the molecule absorbs the radiation, it goes to the higher energy state or the excited state. However, the lifetime of this state is tiny, and molecules would lose absorbed energy in the form of heat or other kinds of radiation and come back to the ground state. The radiation emitted by the molecule can also be studied to reveal the structure of molecules. This is called 'emission spectroscopy.
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Phosphorescence and emission of certain colors when inorganic salts are directly heated in flame 🔥 are examples of emission spectroscopy. However, emission spectroscopy has not so far provided as much information about the molecule's structure as 'absorption spectroscopy' and hence not yet very popular.
In absorption spectroscopy, the molecule is subjected to different kinds of radiation.
Therefore, we begin our study of absorption spectroscopy with the study of certain properties of radiation.
Nature of electromagnetic radiation:
. Radiation exhibits dual nature - particle and waveform. In studying spectroscopy, our emphasis is more on the waveform of radiation.
When propagating in the waveform, all radiation produces an electric ⚡ field and a magnetic 🧲 field. The direction of propagation and these two fields are mutually perpendicular to each other.
Suppose the 'X' axis represents the direction of the propagation of radiation. 'Z' and 'Y' axis represent the direction of electric and magnetic fields, respectively. Let us examine some parameters of the waves.
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