Excited state

In the world of quantum mechanics, systems like atoms, molecules, or nuclei exist in different energy levels. The lowest possible energy level is known as the ground state. An excited state is any quantum state that possesses more energy than this absolute minimum. Excitation occurs when a system increases its energy level above a chosen starting point. Usually, this starting point is the ground state, but a system can also be excited from an already excited state. This concept is fundamental to understanding how matter interacts with energy.

Atoms can reach an excited state through several different methods. They can absorb energy from heat, electricity, or light. A common example involves the absorption of a photon, which is a quantum of light energy. In a hydrogen atom, the single electron normally sits in the lowest possible orbital, called the 1s wave function. When the atom absorbs a photon of the correct energy, the electron jumps to a higher orbital. This move results in an excited state with higher quantum numbers. If the photon carries too much energy, the electron may leave the atom entirely. This process turns the atom into an ionized atom. During the rapid shift between states, the electron may exist in a superposition of both states.

Most excited states are temporary because systems naturally seek lower energy. The process of returning to a lower energy level is called de-excitation. This is the inverse of excitation. During de-excitation, the system typically releases a quantum of energy, such as a photon or a phonon. This release of light can create an electromagnetic spectrum. For a hydrogen atom, this spectrum shows specific patterns called emission lines. These include the Lyman, Balmer, Paschen, and Brackett series. However, some states do not decay quickly. These long-lived excited states are called metastable states. Notable examples of metastable states include singlet oxygen and certain nuclear isomers.

Scientists also study atoms that reach very high levels of excitation. An atom in a high excited state is specifically termed a Rydberg atom. When many of these highly excited atoms group together, they can form a condensed state called Rydberg matter. Beyond single atoms, entire collections of molecules in a gas can be excited. This happens when molecules gain enough kinetic energy to change their velocity distribution. This departure from the equilibrium Boltzmann distribution is known as perturbed gas excitation. Researchers study how long it takes for such a gas to relax back to equilibrium.

Another complex process is excited-state absorption, or ESA. This occurs when a system is already in an excited state and absorbs a second photon to reach an even higher energy level. ESA can only happen if the electron has already moved away from the ground state. While ESA is often considered an undesired effect, it is useful in a process called upconversion pumping. Measuring ESA is more difficult than measuring ground-state absorption. Sometimes, scientists must achieve complete bleaching of the ground state to take accurate measurements. They often use pump-probe techniques, such as flash photolysis, to study these interactions.

Excited states also play a major role in chemical changes. When an atom or molecule is in an excited state, it may undergo a chemical reaction. This field of study is known as photochemistry. To understand these complex movements, scientists use advanced mathematical calculations. They employ methods such as coupled cluster theory and Møller–Plesset perturbation theory. They also use multi-configurational self-consistent field, configuration interaction, and time-dependent density functional theory. These tools allow researchers to predict how electrons will behave when they gain energy.

Understanding excitation helps us grasp the behavior of many different physical systems. For instance, the temperature of a group of particles serves as an indicator of their excitation level. This rule applies to almost all systems, though there are notable exceptions like systems exhibiting negative temperature. From the behavior of electrons in a high-Tc superconductor to the way light creates patterns in a gas, excitation is everywhere. It connects the tiny movements of subatomic particles to the observable properties of the physical world.