Absorbance Spectroscopy

In absorbance spectroscopy (also known as absorption spectroscopy), an optical spectrometer measures the amount of light absorbed by a sample as a function of wavelength. This can provide important information about a samples molecular structure, concentration or optoelectronic properties. Depending on the sample, absorbance measurements can also give you key insights into dynamic material properties, such as phase or composition changes.

To measure absorbance, calculate the logarithmic of the ratio of the initial light intensity and light intensity transmitted through the sample. Absorbance values are unitless and always wavelength dependent. However, for solution and solid samples other values can be used, such as the molar attenuation coefficient and extinction coefficient respectively.

Absorbance Theory

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Within an atom, electrons exist in regions around the nucleus known as orbitals. Depending on the electronic structure of an atom, these orbitals (or energy levels) are either completely filled, partially filled, or empty. When you bring multiple atoms together in a molecule, solid or solution, this will change its orbitals, electron distribution and therefore the samples electronic and optical properties.

Absorbance simply tells you the amount of light that is absorbed by a sample. However, by measuring the wavelengths of light that are absorbed by a material, you can probe these different energy levels and study how electrons move between them - which can give you some useful information about your sample.

For many materials, absorption occurs when the the energy of an incoming photon (E_photon) is equal or larger than energy difference (ΔE) between the highest occupied molecular orbit (HOMO) and the lowest unoccupied molecular orbit (LUMO).

If its energy is high enough, an incoming photon can be absorbed by an electron in the HOMO level. It is then be excited into a higher energy state. This light will not be transmitted through the sample, hence it had been absorbed.

Some materials however can only absorb light at very specific energies. This is often described in terms of the photons wavelength (λ). Wavelengths of the absorbed light correspond to energy through the following equation.

The types of electronic transitions that are allowed often vary with different material properties. Measuring the wavelengths at which absorbance occurs will therefore tell you a lot about the electronic properties of a material, molecule or thin film.

Measuring absorbance requires a broadband light source and an optical spectrometer with appropriate spectral range. Absorbance values are closely related to transmission measurements, as in both cases you actually measure the transmitted light.

As light moves through a sample, it is more likely to be absorbed. Therefore, the intensity of transmitted light will fall exponentially with distance. You can calculate absorbance by the comparing the reference light spectrum, I_0, to the detected light that has passed through the sample, I, using this equation.

Defining Absorbance

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Absorbance is a relative measurement so is therefore unitless. Having said this, absorption properties of different samples can be defined in a number of different ways.

Molar Attenuation Coefficient

When you measure the absorbance spectrum of a solution, one important factor is the wavelengths where maximum absorbance occurs. Identifying these wavelengths can can help you determine certain molecular properties of your sample. You can also note the strength of absorbance at these wavelengths.

By looking at these peaks, you can establish the molar attenuation coefficient, ε. This has also been referred to historically as the molar extinction coefficient and is also known as the molar absorptivity coefficient. This will tell you about the kinds of electronic transitions that are occurring within your sample associated. For allowed transitions, ε > 1000 whereas for forbidden transitions, ε < 100.

The molar attenuation coefficient, ε, is featured in Beer Lambert's Law:

Where A is absorbance, c is the molar concentration of the molecule in solution, and l is the path length through the sample (often the width of the cuvette, or the total film). You can use this calculation to measure the concentration of a molecule in a thin film. The graphs below show the variation in absorbance intensity with concentration.

Molar attenuation coefficient has property has dimensions of

Therefore, the SI units of the molar absorptivity coefficient is m²M^-1 or cm²M^-1.

Absorption Coefficient

For bulk solids, you can define absorbance in terms of an absorption coefficient, α(λ). This is a property of a solid material and is wavelength dependent. The absorption coefficient, α(λ) relates to absorbance measurements through the following equation:

Here, d is the distance light has travelled into (or through) the bulk solid.

The SI units of the absorption coefficient of a bulk solid are m^-1 or cm^-1.

Absorbance Units

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Property	Units	Sample Type
Absorbance	Unitless	General
Absorption Coefficient	m-1	Bulk Solid
Molar Absorptivity Coefficient	m2M-1	Molecule in solution

Absorbance Uses

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You can use absorbance spectroscopy to investigate the suitability of a material for specific purposes. For example:

• Organic dyes require high absorbance over a small wavelength range so that only light of a matching colour is emitted from the dye, e.g. for a yellow dye, blue light (435 - 480 nm) must be absorbed.
• Materials that convert visible light into different forms of energy (such as solar cells, photosensors) will require high absorption, ideally across the visible region. These materials frequently have desirable band gaps, which can be determined using absorbance spectroscopy.
• Transparent materials need to absorb as little visible light as possible. You can confirm this using UV-Vis spectroscopy.
• In organic compounds, UV-vis spectroscopy can help illuminate the amount of conjugated pi bonds in a molecule.

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