Understanding Spectroscopy for Chemical Identification and Analysis

Spectroscopy Quiz

When electromagnetic radiation passes through a sample containing chemical compounds, they interact in one of two ways. This interaction gives rise to a spectrum that can be used to identify the compound.

Infrared radiation has less energy than visible or UV light but enough to cause the atoms in molecules to vibrate. This creates a fingerprint region on a spectrum that is unique to each compound.

Emission Spectra

Emission Spectra are series of bright lines in a spectrum that correspond to emission of light at a particular frequency. The emission occurs when electrons in atoms or molecules jump between different energy levels, and emit radiation that contains electromagnetic waves with wavelengths in the visible light spectrum.

When an element is heated, its electrons move to higher energy levels or orbitals. When they are cooled back to their ground state, they release photons with frequencies or wavelengths that are distinct from the background radiation. This results in a spectrum that contains a number of well-defined, color-coded “bright” lines whose wavelength characteristics are unique to that element.

Each element has a unique emission and absorption spectrum, due to the differences in their atomic structures and the number and arrangements of their electrons. For example, the emission spectra of hydrogen and helium are very different, since their electrons can occupy different energy levels in their atomic orbitals. These differences allow astronomers to tell the composition of stars from their appearance in the electromagnetic spectrum. The emission spectra of chemical compounds are more complicated than the spectra of individual elements, but they still have sharp bright regions that are separated by dark regions. These lines are the result of the individual vibrations or rotations of the atoms in the compound that give it its unique spectrum.

Absorption Spectra

Absorption spectroscopy is the technique used to directly measure the amount of light that is absorbed by a sample. This type of spectroscopy provides important information about the electronic structure of a molecule, including its band theory.

It also provides key information about a material’s properties, such as its density and molecular vibrations. It can be used to detect and quantify gases in an atmosphere.

When a electromagnetic radiation wave passes through a sample, it interacts with the molecules in the sample to produce an electric field that is the opposite of the direction of the light wave. This interaction causes some of the molecules in the sample to absorb some of the energy from the radiation, causing them to vibrate and emit photons. The wavelengths of these photons are recorded on an absorption spectrum, which is the inverse of a transmission spectrum.

The width of an absorption line depends on several factors, including the atomic energy levels and physical environment of the absorbing molecule. A liquid tends to have broader lines than a gas. The line width can also depend on the spectrometer that is used to record the spectrum. The instrument has an inherent limit to how narrow a line it can resolve and the wider a line is, the closer it is to this resolution limit.

Linear Spectra

Spectroscopy is the study of how atoms emit and absorb electromagnetic radiation. Spectroscopy allows us to tell things about objects in space like their age, if they are moving (redshift), temperature, and what they are made of.

Emission spectra are used to identify elements in matter with unknown composition. They are made up of the frequencies of electromagnetic radiation emitted when an atom or molecule makes an electron transition from one energy state to another. Each of these transitions has a different energy difference and therefore a different emitted wavelength. The collection of all the possible transitions, with their corresponding emitted wavelengths, is called an emission spectrum.

Line spectra are the bright lines in a spectrum that are emitted by atoms that have specific frequencies of electromagnetic radiation absorbed. This occurs when an atom, or group of atoms, receives enough energy from a photon to change their quantum state and jump from a low energy orbital to a high energy orbital. This atom then spontaneously re-emits the energy as a photon with a particular frequency.

The energy of the absorbed photon is related to its frequency by the Planck constant h and the width of the spectral line is given by the equation Dn = DE/h 1/Dt where Dn is the spectral line width, h is the wavelength of the absorbing photon and t is the lifetime of the excited atomic or molecular state. The higher the conjugation of the molecule, the greater the maximum absorbing wavelength will be.

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