quantum mechanics Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, q ...

, a repulsive state is an

electronic state A quantum mechanical system or particle that is bound—that is, confined spatially—can only take on certain discrete values of energy, called energy levels. This contrasts with classical particles, which can have any amount of energy. The t ...

of a

molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bio ...

for which there is no minimum in the

potential energy In physics, potential energy is the energy held by an object because of its position relative to other objects, stresses within itself, its electric charge, or other factors. Common types of potential energy include the gravitational potentia ...

. This means that the state is unstable and unbound since the potential energy smoothly decreases with the interatomic distance and the atoms repel one another. In such a state there are no discrete vibrational energy levels; instead, these levels form a continuum. This should not be confused with an

excited state In quantum mechanics, an excited state of a system (such as an atom, molecule or nucleus) is any quantum state of the system that has a higher energy than the ground state (that is, more energy than the absolute minimum). Excitation refers t ...

, which is a

metastable In chemistry and physics, metastability denotes an intermediate energetic state within a dynamical system other than the system's state of least energy. A ball resting in a hollow on a slope is a simple example of metastability. If the ball i ...

electronic state containing a minimum in the potential energy, and may be short or long-lived. When a molecule is excited by means such as UV/VIS spectroscopy it can undergo a

molecular electronic transition Molecular electronic transitions take place when electrons in a molecule are excited from one energy level to a higher energy level. The energy change associated with this transition provides information on the structure of a molecule and determin ...

: if such a transition brings the molecule into a repulsive state, it will spontaneously

dissociate Dissociation in chemistry is a general process in which molecules (or ionic compounds such as salts, or complexes) separate or split into other things such as atoms, ions, or radicals, usually in a reversible manner. For instance, when an aci ...

. This condition is also known as predissociation since the

chemical bond A chemical bond is a lasting attraction between atoms or ions that enables the formation of molecules and crystals. The bond may result from the electrostatic force between oppositely charged ions as in ionic bonds, or through the sharing o ...

is broken at an energy which is lower than what might be expected. In electronic spectroscopy, this often appears as a strong, continuous feature in the

absorption Absorption may refer to: Chemistry and biology *Absorption (biology), digestion **Absorption (small intestine) *Absorption (chemistry), diffusion of particles of gas or liquid into liquid or solid materials * Absorption (skin), a route by which ...

emission spectrum The emission spectrum of a chemical element or chemical compound is the spectrum of frequencies of electromagnetic radiation emitted due to an electron making a transition from a high energy state to a lower energy state. The photon energy of t ...

, making repulsive states easy to detect. For example,

triatomic hydrogen Triatomic hydrogen or H3 is an unstable triatomic molecule containing only hydrogen. Since this molecule contains only three atoms of hydrogen it is the simplest triatomic molecule and it is relatively simple to numerically solve the quantum mech ...

has a repulsive ground state, which means it can only exist in an excited state: if it drops down to the ground state, it will immediately break up into one of the several possible dissociation products.

References

* Spectroscopy {{spectroscopy-stub