Introduction to Molecular Spectroscopy

Electromagnetic radiation has both wave and particle features. It includes electric and magnetic field components that oscillate in planes perpendicular to each other. These fields are also perpendicular to the direction the radiation moves through space.

As a wave, electromagnetic radiation is described by wavelength and frequency. Wavelength is the distance between two consecutive peaks or troughs. Frequency is the number of wave cycles that pass a point in a fixed time. The electromagnetic...

Electromagnetic radiation and matter interact in ways that reveal details about atoms and molecules. In spectroscopy, scientists study how EM radiation is absorbed, emitted, transmitted, reflected, or scattered by a substance. This helps identify its chemical composition and properties.

EM radiation behaves as both a wave and a particle. As a wave, it has intensity and amplitude. Intensity, also called irradiance, is the number of photons that strike a given area in a certain time. Amplitude...

Molecular spectroscopy describes how molecules absorb and emit light by moving between energy levels. These levels are called quantum states. Unlike atoms, molecules also have rotational and vibrational energy levels, so their energy structure is more complex.

Each level is separated by an energy gap. The gaps between electronic, vibrational, and rotational levels are very different in size. A diatomic molecule can be used to show these three kinds of energy levels.

A molecule can absorb a...

Spectrophotometry measures how a material absorbs, reflects, diffracts, or transmits electromagnetic radiation. It compares the intensity of radiation before and after it interacts with the material. The result shows how the material responds to different wavelengths.

A spectrophotometer is the device used for this measurement. It includes a source of electromagnetic radiation, a slot for the sample, and a detector. The detector measures the radiation that leaves the material, including...

UV-Vis spectroscopy measures how materials absorb ultraviolet and visible light. It is a useful analytical method because it is versatile, simple to use, and gives results quickly. Scientists use it for both qualitative and quantitative analysis.

When UV-Vis radiation passes through a sample, molecules absorb specific wavelengths based on the energy needed for electronic transitions. This absorption moves atoms or molecules from a ground state to an excited state. If a molecule absorbs UV...

UV-Vis spectroscopy shows how molecules absorb ultraviolet and visible light. It is used to study electronic structure by tracking how electrons move between molecular orbitals. The absorbed light must match the energy gap between the ground state and an excited state.

In this process, an electron moves from the highest occupied molecular orbital, or HOMO, to the lowest unoccupied molecular orbital, or LUMO. This jump raises the electron to a higher energy level. The energy difference between...

A UV-visible spectrophotometer measures how much UV and visible light a sample absorbs. It uses a light source, a wavelength selector, a sample holder, and a detector to build the spectrum. This makes it useful for checking liquid samples in the lab.

The light source depends on the region being measured. A deuterium lamp provides UV radiation, and a tungsten lamp provides visible radiation. A monochromator or prism acts as a diffraction grating and splits the incoming light into different...

UV-Vis spectroscopy shows how a molecule absorbs light in the ultraviolet and visible ranges. When light passes through a substance, some of it is absorbed and the rest is reflected or transmitted. Molecules that absorb between 180 and 400 nm give a UV spectrum, while absorption in the 400 to 780 nm range gives a visible spectrum.

A UV-Vis spectrum is a plot of absorbance versus wavelength. The y-axis shows molar absorptivity, ε, or log ε, and the x-axis shows wavelength. Absorbance describes...

The Beer-Lambert law links absorbance, concentration, and path length in a solution. It is based on ideas from Johann Heinrich Lambert and August Beer. Their work helps explain how light changes as it passes through a medium.

Lambert's law says that when light passes through a medium, the loss in intensity is directly proportional to the original intensity and the path length. Beer’s law adds that the transmittance of a solution stays constant if the product of concentration and path length...

Photoluminescence is light emission from molecules after they absorb light energy. A substance takes in photons, which raises electrons to excited energy levels. The electrons then relax back to the ground state and release light.

This process appears in many materials, including semiconductors, organic compounds, and inorganic compounds. The main role of electron spin helps separate photoluminescence into fluorescence and phosphorescence. In a singlet spin state, two electrons share the same...

Fluorescence and phosphorescence depend on several factors that change how molecules emit light. These effects matter in analytical chemistry, biological imaging, and materials science because they help scientists detect molecular properties and see cellular structures.

Molecular structure is one of the main influences. Aromatic compounds with low-energy pi-bonding to pi-non-bonding transitions often give strong fluorescence. Aliphatic and alicyclic carbonyl compounds, as well as highly...

Jablonski diagrams show how excited molecules lose energy after absorbing light. They are used to track both light-emitting and non-light-emitting steps in luminescence. The diagram gives a simple picture of the energy states a molecule can move through.

The lowest line in a Jablonski diagram is the ground-state energy of the molecule. This state is usually a singlet state, and it represents the condition of most molecules in solution at room temperature. The lines above it show vibrational...

Photoluminescence is useful in chemical analysis because it is sensitive and selective. It can support both direct and indirect measurements of an analyte, which is the substance being tested. When an analyte has a strong quantum yield, it can be measured directly by fluorescence or phosphorescence.

Indirect analysis is used when the analyte is not naturally fluorescent or phosphorescent, or when its quantum yield is poor. One approach is to react the analyte with a reagent to make a...

Fluorometers and spectrofluorometers are instruments used to measure molecular fluorescence. They do this by choosing specific excitation and emission wavelengths. The two instruments use different light sources and wavelength-selection methods.

A fluorometer uses absorption interference filters to select the excitation and emission wavelengths. Its excitation source is usually a low-pressure mercury vapor lamp. This lamp gives intense lines across the ultraviolet and visible regions.