| excited states | |
|---|---|
| Name | Excited state |
| Caption | Energy level diagram illustrating an excited state above a ground state |
| Type | Quantum state |
| Field | Quantum physics |
excited states
Excited states are quantum states of a physical system that have higher energy than the lowest‑energy or ground state. They occur in atoms, molecules, nuclei, and solid‑state systems and determine phenomena from spectroscopy and chemical reactivity to emission of photons. Understanding excited states is central to atomic physics, molecular physics, and condensed matter physics and bears on social outcomes through technologies for energy, health, and information.
In quantum theory an excited state is any eigenstate or quasistationary state with energy greater than the ground state. For an atom such as hydrogen atom the first excited state corresponds to principal quantum number n=2; for molecules excited states include electronic, vibrational, and rotational excitations. Excited states underpin radiative processes (absorption and emission), nonradiative decay, and transient phenomena exploited in lasers, LEDs, and photovoltaic devices. Recognition of excited‑state lifetimes and transition probabilities is important for equitable deployment of technologies in energy and public health applications.
Mathematically, excited states are solutions to the time‑independent Schrödinger equation with discrete or quasi‑continuous eigenvalues above the ground energy. In many‑body systems they can be characterized by occupation numbers, quasiparticles (e.g., exciton, polariton), or collective modes such as phonons. Approaches include Hartree–Fock theory, configuration interaction (CI), coupled cluster (CC) methods, and time‑dependent formulations like time-dependent density functional theory (TDDFT). Model systems and solvable models—particle in a box, harmonic oscillator, and the hydrogenic atom—illustrate selection rules derived from symmetries and operators such as angular momentum and parity.
Excited states are probed experimentally by absorption spectroscopy, emission spectroscopy, fluorescence spectroscopy, photoelectron spectroscopy, and pump–probe spectroscopy. Landmark experimental platforms and institutions—Bell Labs, Lawrence Berkeley National Laboratory, and Max Planck Institute for Quantum Optics—have advanced spectroscopic techniques and ultrafast science. Observables include transition energies, oscillator strengths, lineshapes affected by Doppler broadening and homogeneous broadening, and lifetimes measured with time-correlated single-photon counting or ultrafast lasers (e.g., Ti:sapphire systems). Spectroscopy connects to environmental justice through monitoring pollutants and enabling affordable sensors.
In chemistry, excited electronic states determine photochemistry, photoinduced electron transfer, and processes in photosynthesis and photovoltaic cells. Molecules such as rhodopsin and chromophores exhibit excited states crucial for vision and organic electronics. In condensed matter, excitations include excitons in semiconductors (e.g., silicon, gallium arsenide), magnons in magnetic materials, and collective excitations in superconductivity and topological insulators. Excited‑state properties influence catalysis, atmospheric chemistry, and materials design for equitable energy solutions.
Dynamics after excitation follow pathways including radiative decay (spontaneous emission), nonradiative relaxation (internal conversion, intersystem crossing), energy transfer (Förster and Dexter mechanisms), and ionization. Timescales range from femtoseconds for electronic relaxation to milliseconds for phosphorescence. Competing processes are affected by coupling to phonon baths, solvent dynamics studied in chemical dynamics labs, and coherence phenomena relevant to quantum coherence studies. Understanding these dynamics supports more efficient solar energy conversion and phototherapy.
Controlled excited states are exploited in lasers, LED lighting, OLED displays, and single‑photon sources for quantum cryptography. Solid‑state qubits in NV centers in diamond, quantum dot excitons, and superconducting circuits use excited levels for encoding and manipulating quantum information. Research at institutions like IBM Research, Google Quantum AI, and MIT targets scalable quantum processors where excited‑state control, leakage, and decoherence present both opportunity and equity concerns in technology access and governance.
Accurate prediction of excited states remains challenging due to electron correlation, open quantum system effects, and large basis requirements. Methods balancing accuracy and cost include multireference CI, equation‑of‑motion CC (EOM‑CC), and many‑body perturbation techniques such as the GW approximation and the Bethe–Salpeter equation (BSE). Advances in algorithms, high‑performance computing at facilities like Argonne National Laboratory and software such as Gaussian and VASP enable better modeling. There is growing attention to reproducibility, open data, and inclusive training to democratize computational resources and address disparities in scientific capacity.
Category:Quantum states Category:Spectroscopy Category:Quantum chemistry