AN UPDATE ON FUNDAMENTALS OF UV- VISIBLE SPECTROSCOPY

ABOUT AUTHORS:
Prashanta Kr. Deb1*, Kaushik Nath Bhoumik2,
1Dept. of Pharmacy, Tripura University; Suryamaninagar –799022, Agartala, Tripura (W), India.
2Regional Institute of Pharmaceutical Science & Technology, Agartala–799005, Tripura (W), India.
*shaandeb2010@gmail.com

INTRODUCTION
Spectroscopy is the measurement and interpretation of Electromagnetic radiation (EMR) absorbed or emitted when the molecules or atoms or ions of a sample move from one energy state to another energy state. This change may be ground state to exited state or vice-versa. EMR is made up of discrete particles called photons. EMR has got both wave characteristic as well as particle characteristics. This means that it can travel in vacuum also.

REFERENCE ID: PHARMATUTOR-ART-1724

The energy of an EMR can be given by the following equation…

E=hν

Where,
E= Energy of radiation
h= Plank’s constant (6.624 x 10-34 jSec)
ν= frequency of radiation

Frequency = c/λ or velocity of light in vacuum/ wavelength

Hence, E = hc/ λ = hcv

Where, v = wave number

Therefore the energy of a radiation depends upon frequency and wavelength of the radiation. Frequency is defined as the number of complete wavelength units passing through a given point in unit time. Frequency is measured in Hz or cps.

Wavelength is the distance between two successive maxima or minima or distance between two successive through or peas. It can be measured in meters, cm, mm, nm or Å.

Electromagnetic Spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object. The electromagnetic spectrum extends from low frequencies used for modern radio communication to gamma radiation at the short-wavelength (high-frequency) end, thereby covering wavelengths from thousands of kilometers down to a fraction of the size of an atom. It is for this reason that the electromagnetic spectrum is highly studied for spectroscopic purposes to characterize matter.  The limit for long wavelength is the size of the universe itself, while it is thought that the short wavelength limit is in the vicinity of the Planck length, although in principle the spectrum is infinite and continuous.

Regions of the spectrum
The types of electromagnetic radiation are broadly classified into the following classes.

  1. Gamma radiation
  2. X-ray radiation
  3. Ultraviolet radiation
  4. Visible radiation
  5. Infrared radiation
  6. Microwave radiation
  7. Radio waves

Spectrum of Electromagnetic Radiation

Region

Wavelength
(Angstroms)

Wavelength
(centimeters)

Frequency
(Hz)

Energy
(eV)

Radio

> 109

> 10

< 3 x 109

< 10-5

Microwave

109 - 106

10 - 0.01

3 x 109 - 3 x 1012

10-5 - 0.01

Infrared

106 - 7000

0.01 - 7 x 10-5

3 x 1012 - 4.3 x 1014

0.01 - 2

Visible

7000 - 4000

7 x 10-5 - 4 x 10-5

4.3 x 1014 - 7.5 x 1014

2 - 3

Ultraviolet

4000 - 10

4 x 10-5 - 10-7

7.5 x 1014 - 3 x 1017

3 - 103

X-Rays

10 - 0.1

10-7 - 10-9

3 x 1017 - 3 x 1019

103 - 105

Gamma Rays

< 0.1

< 10-9

> 3 x 1019

> 105

This classification goes in the increasing order of wavelength, which is characteristic of the type of radiation. While, in general, the classification scheme is accurate, in reality there is often some overlap between neighboring types of electromagnetic energy. The distinction between X-rays and gamma rays is partly based on sources: the photons generated from nuclear decay or other nuclear and sub nuclear/particle process, are always termed gamma rays, whereas X-rays are generated by electronic transitions involving highly energetic inner atomic electrons. In general, nuclear transitions are much more energetic than electronic transitions, so gamma-rays are more energetic than X-rays, but exceptions exist. The convention that EM radiation that is known to come from the nucleus, is always called "gamma ray" radiation is the only convention that is universally respected, however. Many astronomical gamma ray sources (such as gamma ray bursts are known to be too energetic (in both intensity and wavelength) to be of nuclear origin. Quite often, in high energy physics and in medical radiotherapy, very high energy EMR (in the >10 MeV region) which is of higher energy than any nuclear gamma ray, is not referred to as either X-ray or gamma-ray, but instead by the generic term of "high energy photons."

The region of the spectrum in which a particular observed electromagnetic radiation falls, is reference frame-dependent (due to the Doppler shift for light), so EM radiation that one observer would say is in one region of the spectrum could appear to an observer moving at a substantial fraction of the speed of light with respect to the first to be in another part of the spectrum.

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