spectral line
- Key People:
- Joseph von Fraunhofer
What causes spectral lines?
What is the difference between emission and absorption spectral lines?
How are atomic energy levels measured?
What is the Balmer series?
What happens during the Hα line transition in hydrogen?
spectral line, a bright or dark feature in a spectrum caused when a photon of a specific energy changes the state of an ion, atom, or molecule.
Spectral lines come in two types: emission and absorption. In emission, the ion, atom, or molecule moves from a high-energy state to a low-energy state and emits the energy difference as a photon. In absorption, it is the reverse, with the atom, ion, or molecule absorbing a photon and moving from a low-energy state to a high-energy state.
An isolated atom can be described in terms of certain discrete states called quantum states. Each quantum state has a definite energy associated with it, but several quantum states can have the same energy. These quantum states and their energy levels are calculated from the basic principles of quantum mechanics.
Atomic energy levels are typically measured by observing transitions between two levels. For example, an atom in its lowest possible energy state (the ground state) can be excited to a higher state only if energy is added by an amount that is equal to the difference between the two levels. Thus, by measuring the energy of the radiation that has been absorbed by the atom, the difference in its energy levels can be determined. The energy levels are identical for atoms of the same type; allowed energies of a particular atom of silver are equal to those for any other atom of the same isotope of silver.
Other isolated systems, including molecules, ions (charged atoms or molecules), and atomic nuclei, have discrete allowed energies. The analysis of these simple systems is carried out with techniques that are analogous to those that were first applied to simple atomic spectra.
If an atom in its ground state is given some amount of energy so that it is promoted to an excited state, the atom will release that extra energy spontaneously as it moves back into lower states, eventually returning to the ground state. For an isolated atom, the energy is emitted as electromagnetic radiation. The emitted energy E equals the upper-state energy minus the lower-state energy; this energy is usually carried by a single quantum of light (a photon) having a frequency ν in which photon energy (E) is equal to a constant times the frequency, E = hν, where h, Planck’s constant, equals 6.626 × 10−34 joule second. This relationship determines the frequencies (and wavelengths, because λ = c/ν) of light emitted by atoms if the energies of the states are known. The light emitted or absorbed at these wavelengths are spectral lines.
Spectral lines that are related are called spectral line series. Hydrogen displays five of these series in various parts of the spectrum, the best-known being the Balmer series in the visible region. Johann Balmer, a Swiss mathematician, discovered in 1885 that the wavelengths of the visible hydrogen lines can be expressed by a simple formula: the reciprocal wavelength (1/λ) is equal to a constant (R) times the difference between two terms, 1/4 (written as 1/22) and the reciprocal of the square of a variable integer (1/n2), which takes on successive values 3, 4, 5, etc.; i.e., 1/λ = R(1/22 − 1/n2). The constant R is known as the Rydberg constant. When n = 3, Balmer’s formula gives λ = 656.21 nanometers (1 nanometer = 10−9 meter), the wavelength of the line designated Hα, the first member of the series (in the red region of the spectrum), and when n = ∞, λ = 4/R, the series limit (in the ultraviolet). In the Balmer series, the electron in the hydrogen atom loses energy from a state with a principal quantum number n greater than 3, emits a photon, and ends up in the n = 2 state. The Hα or Balmer α line happens when the electron goes from the n = 3 state to the n = 2 state.
