• Absorption spectrum of isolated atoms (for instance, low-pressure gases) represents a series of separate narrow lines with a typical thickness of 1/100 to 1/1000 nm. These lines correspond to the frequency of the characteristic oscillations of electrons within the atom.
• Molecular spectra consist of considerably wider areas of absorption, typically of 1/10 to 100 nm. These lines are produced by atomic oscillations in molecules.
• Solids have spectra with continuous areas of 100’s to 1000’s nm wide. Solids consist of atoms and molecules and theoretically should have linear absorption spectrum present . But in reality the substance as a solid is so condensed that strong interactions between the particles lead to collective effects which broaden the spectrum.

Diluted solutions and gases
Consider a situation in which the molecules of a substance under study are well dissolved in a non-absorbing solvent or are distributed in a non-absorbent gas. In such an instance, the extinction coefficient k_l happens to be linearly proportional to the molar concentration C:

k_l = c_l C,

where c_l is a molar extinction coefficient of molar absorptivity. It is also a function of the wavelength l. The molar extinction coefficient c_l is a true light absorption characteristic of a substance independent of its concentration or sample geometry.
The absorption law for diluted solutions and gases, known as Beer-Lambert-Bouguer law, can be defined through the following:

I = I₀ exp(-c_lCL).

This law is correct only for strongly diluted substances and is not always true for real liquids and gases.

Absorbance
In analytical chemistry and other fields, researchers measure the ratio between the intensity of incident light I₀ and the intensity of transmitted light I, known as absorbance A_l:

A_l = - log₁₀ (I/I₀) or sometimes A_l = - log_e (I/I₀)

Absorbance depends on the wavelength l of the incident light and can be expressed as:

A_l = c_lCL,

where c_l is the molar extinction coefficient; L is a sample thickness; C is the molar concentration of species. While Absorption is a physical process of reducing light intensity, Absorbance is a mathematical measure of such reduction.

Absorbance is related to such widely used characteristic as Optical Density (OD), which can be defined as:

OD_l = A_l/L,

where L is the optical path. While Absorbance is non-dimensional, Optical Density is expressed in [1/m] or [1/cm] units. Both are functions of wavelength l.

Limitations of Beer-Lambert-Bouguer Law
Light absorption theory is based on the following three assumptions:

1. Species in the substance do not interact with each other. In other words, atoms/molecules of the substance should be spaced far from each other to prevent collective effects.
2. Intensity of the incident light is not very high. Light absorption causes electrons in atoms to transit to excited higher energy levels. Normally, such transitions are quickly reversible and result in the emission of secondary photons or non-emissive recombinations. High intensity light forces some electrons to stay in the excited high-energy level and subsequently lose their ability to absorb light. Therefore, light absorption becomes non-linear and depends on the intensity of the incident light.
3. The sample does not scatter the light. It implies that the sample should be homogeneous.

Measurements can still be performed outside these conditions, but the theoretical interpretation of experimental results is not going to be correct and reliable. In addition, some precautions should be taken into consideration in order to prevent light losses from reflection.

Recommended Products:

Acton Series Monochromators & Spectrographs
-  Positrak™ grating stabilization offers simple calibration
-  Optimized coatings for higher throughput
-  Interchangeable grating turrets with a wide selection of gratings

PIXIS CCD Cameras
-  Lifetime vacuum guarantee for worry-free operation
-  Deep cooling without the need for liquid circulators
-  Up to 1000 spectra/sec data acquisition

SPEC-10 CCD Cameras
-  Exclusive dual amplifier design offers high SNR or high dynamic range
-  Superior low light sensitivity through deeper cooling

PI-MAX ICCD Cameras
-  5MHz digitization provides up to 15 frames per second
-  500ps gating for ultra fast time resolution
-  Exclusive UNIGEN photocathode for highest sensitivity from deep UV to NIR

OMA V InGaAs Array Cameras
-  Excellent NIR sensitivity for demanding spectroscopy applications
-  Electronic shutter allows intergration times down to 20 µsec
-  1MHz digitization provides up to 1800 spectra/sec