What’s actually slowing this PC down?
Pick the symptom - the matching free tool is one click away.
An electron state is a quantum-mechanical description of an electron’s allowed condition. Scientists do not photograph or inspect a state directly: they measure how a system responds to light, emitted or transmitted electrons, or a scanning probe, then infer its electronic structure from the signal. The right method depends on whether the sample is an atom, molecule, surface, or solid—and whether the question concerns transitions, occupied or unoccupied states, local structure, or excitations.
What an electron state means
A quantum state describes the allowed properties of an electron in a particular physical system. For an electron in an atom, the state can be labeled with quantum numbers such as n, l, ml, and ms, or with n, l, j, and mj. These labels encode quantities such as energy, angular momentum, and their projections; they do not describe a tiny planet-like orbit. NIST’s Atomic Spectroscopy: An Introduction explains these atomic labels.
As an Amazon Associate I earn from qualifying purchases.
The useful description changes with the system. Molecules are described using electronic and vibrationally structured energy levels, while crystalline solids are often described through energy bands and a density of states. These concepts are related, but they are not interchangeable: a set of atomic quantum numbers is not a complete description of every molecule or solid.
What scientists actually measure
Experiments record observables: for example, light intensity versus wavelength, the kinetic energy of emitted electrons, the energy lost by transmitted electrons, or tunneling current versus applied voltage and probe position. Researchers use calibration and physical models to interpret those signals as transition energies, levels, bands, or densities of states. A measured spectrum is therefore evidence about electronic structure, not an assumption-free picture of it.
#1 Best Overall
Different methods answer different questions. Optical spectroscopy measures transitions between states; photoelectron methods reveal information about electrons removed from a sample; inverse photoemission probes unoccupied states; scanning tunneling spectroscopy maps energy-dependent surface response; and electron energy-loss spectroscopy detects excitations and electronic information in thin specimens. More than one method may be needed to build a fuller picture.
Which measurement method fits the question?
| Method | What is measured | What it can reveal | Key qualification |
|---|---|---|---|
| Absorption and emission spectroscopy | Light absorbed or emitted at particular wavelengths or energies | Transition energies between states | A transition energy is not automatically the energy of a single isolated state; interpretation depends on the system and transitions involved. |
| Photoelectron spectroscopy, including XPS and UPS | Kinetic energies of electrons ejected after the sample absorbs photons | Ionization energies and information about occupied electronic structure | Energy balance and spectral interpretation matter. Photoelectric cross sections affect signal intensities, and final-state effects can influence features. |
| Inverse photoemission | Photons emitted when incoming electrons enter available states | Unoccupied states | It complements photoemission, which primarily accesses occupied states; the methods do not provide identical information. |
| Scanning tunneling spectroscopy (STS) | Tunneling response as the probe position and voltage change | Energy-resolved local electronic structure at a surface | The signal depends on the surface and tunneling conditions; it is not a universal bulk-state measurement. |
| Electron energy-loss spectroscopy (EELS) | Energy and angular distributions of electrons transmitted through a thin specimen | Excitations such as plasmons and excitons, and information on densities of states, interband transitions, and bandgaps | Interpretation depends on the material and specimen, including thickness and measurement resolution. |
NIST’s overview of electronic and chemical properties of systems for solid-state devices describes methods that span occupied and unoccupied density of states, as well as photoemission electron microscopy for spatial imaging. For EELS and imaging, see NIST’s 2019 publication.
How the main methods work
Light reveals transitions
When matter absorbs or emits light, the photon energy corresponds to a transition between allowed states. Measuring the wavelengths or energies in absorption or emission spectra lets scientists identify transition energies and compare them with models of the system.
Quick wins for a faster PC:
Repair Windows errors before they cause bigger problemsFix Now →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Photoelectrons reveal occupied states and ionization
In photoelectron spectroscopy, photons of known energy eject electrons from a sample. The instrument measures the electrons’ kinetic energies; applying energy conservation helps determine ionization energies and infer the electronic structure left behind. For molecules, assigning spectral features can depend on ionization-potential references. A vertical ionization quantity describes removal before the molecular geometry relaxes, whereas an adiabatic quantity accounts for the change in geometry; the distinction can matter when comparing values. NIST’s Chemistry WebBook provides molecular vibrational and electronic energy-level information.
Photoelectron signal intensity is not a simple count of how many electrons occupy each state. Photoelectric cross sections can weight features differently, and final-state effects can shape spectra. An analysis of electronic densities of states from X-ray photoelectron spectroscopy by C. S. Fadley and D. A. Shirley discusses these interpretive issues in the NIST Journal of Research.
Inverse photoemission probes unoccupied states
Conventional photoemission starts with electrons in occupied states and ejects them. Inverse photoemission reverses the direction of the energy exchange: incoming electrons enter available states and the emitted photons are measured. This makes it useful for examining unoccupied states, which ordinary photoemission does not directly access.
Scanning tunneling spectroscopy resolves surface response by position and energy
A scanning tunneling microscope brings a sharp conducting tip close to a surface. Electrons tunnel across the gap, and the response depends on the availability of electron states. By varying the bias and recording the response at different positions, STS measures energy-resolved local electronic structure. NIST describes the STM as “inherently sensitive to surface electronic properties due to the dependence of the tunneling process on the availability of electron states” on its scanning-probe instruments project page.
A specialized NIST scanning-probe program reports atomic-scale wavefunction mapping and energy-level resolution at the microelectronvolt scale. Its Scanning Probe Metrology page also describes a particular instrument environment of 10 mK (0.01 K) and 15 T. Those are facility-specific capabilities, not general requirements for STS or universal resolution limits.
Best Value
EELS tracks energy transferred inside thin specimens
In EELS, electrons pass through a thin sample and lose energy through interactions with it. Measuring their energy and angular distributions can reveal excitations such as plasmons and excitons, along with information about electronic states and transitions. In scanning transmission electron microscopy (STEM), spectra can be collected at image pixels, linking energy-loss data to location. The specimen’s material and thickness, instrument resolution, and the model used to interpret the losses all affect what can be concluded.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to choose a method
Start with the question, not with a favorite instrument. A useful method choice depends on the sample, which part of the electronic structure is accessible, whether spatial detail matters, and how the signal must be interpreted.
- For transition energies: use absorption or emission spectroscopy.
- For ionization energies or occupied electronic structure: consider photoelectron spectroscopy.
- For unoccupied states: consider inverse photoemission.
- For local, energy-resolved structure at a surface: consider STS.
- For excitations and electronic information in a thin specimen: consider EELS.
- When location matters: check whether the technique provides an averaged signal, an image, or a position-resolved measurement.
Methods are often complementary. For example, photoemission and inverse photoemission address occupied and unoccupied states, while optical measurements focus on transitions; each offers a different view of the system.
Crashes, No Sound, or Screen Glitches?
Random freezes, missing sound and display glitches usually trace back to one bad driver. Find and replace yours safely.Free scan · under a minutePC Slower Than It Used to Be?
A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11Quick Recap
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
