At a microscopic level, the world is in constant, jittery motion. Vibrational excitation is a process where molecules absorb energy and increase the intensity of this natural, internal jiggling. The chemical bonds that hold atoms together in a molecule are not rigid rods; they behave more like flexible springs. This phenomenon is a temporary change in the molecule’s energy state.
Imagine a tuning fork, which is designed to vibrate at a specific pitch when struck. A molecule is similar, but it can vibrate in multiple, distinct ways, each corresponding to a different “note” or energy level. When a molecule absorbs a precise amount of energy, it jumps from a lower energy vibration to a higher one.
The Physics of Molecular Vibration
The atoms within any molecule are in a state of perpetual motion, performing a continuous dance of stretching, bending, and twisting. A principle governing this behavior is quantization. This means a molecule cannot possess just any amount of vibrational energy; it is restricted to specific, discrete energy levels. It’s much like climbing a staircase, where you can stand on the first step or the second step, but never in the empty space between them.
A molecule can be elevated from a lower vibrational “step” to a higher one through a couple of primary mechanisms. The most common way is by absorbing a photon, which is a particle of light. For this to happen, the energy of the photon must exactly match the energy difference between the molecule’s current vibrational state and a higher one. This energy corresponds to the infrared portion of the electromagnetic spectrum.
Another method of excitation is through inelastic collisions. When one molecule collides with another, kinetic energy—the energy of motion—can be transferred. If this transfer is inelastic, some of that kinetic energy is converted into vibrational energy within one or both of the colliding molecules. This process effectively uses the force of the impact to make the molecular “springs” oscillate more intensely.
Detecting Vibrational Excitation
Scientists observe and measure vibrational excitation using a technique called infrared (IR) spectroscopy. This method provides a window into the vibrational world of molecules by monitoring how they interact with infrared light. The process involves passing a beam of infrared light, which contains a range of frequencies, through a sample of a substance. As the light passes through, the molecules within the sample will absorb photons of specific frequencies.
By detecting which frequencies of light are absorbed by the sample, a unique spectrum is generated. This spectrum acts as a “molecular fingerprint” because the pattern of absorbed light is distinct for every different type of molecule. The specific masses of the atoms and the strengths of the bonds between them determine the exact frequencies at which a molecule will vibrate and absorb light.
This resulting IR spectrum is a direct map of the molecule’s allowed vibrational transitions. Each peak in the spectrum signifies a particular type of vibration, such as the stretching of a carbon-hydrogen bond or the bending of a water molecule. Researchers use these fingerprints to identify unknown substances, verify the purity of a compound, and study the structure of molecules.
Impact on Chemistry and the Environment
Vibrational excitation has consequences that extend from the laboratory to the planet’s climate system. In chemistry, exciting molecular vibrations is a way to influence chemical reactions. For a reaction to occur, molecules need to overcome an “activation energy” barrier, the minimum energy required to initiate the breaking and forming of chemical bonds. Pumping enough vibrational energy into a molecule can stretch its bonds to the breaking point, helping it surmount this barrier and transform into new products.
This mechanism of energy absorption is central to the Earth’s greenhouse effect. Gases in the atmosphere, such as carbon dioxide (CO2) and methane (CH4), are good at absorbing infrared radiation. As the Earth’s surface radiates heat back towards space in the form of infrared light, these greenhouse gas molecules absorb it, causing them to enter a state of vibrational excitation. They then re-radiate this energy in all directions, including back towards the surface, effectively trapping heat and warming the planet.
On a domestic scale, vibrational excitation is the principle behind how microwave ovens heat food. Microwaves are a form of electromagnetic radiation that causes water molecules to become excited. This excitation is primarily rotational, but it is coupled with vibrational motion. The rapidly oscillating electric field of the microwaves causes the water molecules in food to vibrate and rotate vigorously, generating thermal energy, which then spreads throughout the food and cooks it.