The following terminology has evolved to describe atomic and/or molecular spectra:
According to classical thinking, the electron moving around the nucleus has a magnetic dipole moment, because it is charged. The interaction of this magnetic dipole moment with the magnetic moment of the nucleus (due to its spin) leads to hyperfine splitting.
However, due to the electron's spin, there is also hyperfine splitting for s-shell electrons, which have zero orbital angular momentum. In this case, the magnetic dipole interaction is even stronger, as the electron probability density does not vanish inside the nucleus ().
This interaction obeys the Lande interval rule: The energy level is split into energy levels, where denotes the total electron angular momentum and denotes the nuclear spin.
Usually, is of order of GHz; the hyperfine splitting is orders of magnitude smaller perturbation than the fine structure.
In a more advanced treatment, one also has to take the nuclear magnetic quadrupole moment into account. This is sometimes (?) referred to as "hyperfine structure anomaly".
In 1935, M. Schiiler and T. Schmidt proposed the existence of a nuclear quadrupole moment in order to explain anomalies in the hyperfine structure.
Carl Sagan and Frank Drake considered the hyperfine transition of hydrogen to be a sufficiently universal phenomenon so as to be used as a base unit of time and length on the Pioneer plaque and later Voyager Golden Record.
In radio astronomy, heterodyne receivers are widely used in detection of the electromagnetic signals from celestial objects. The separations among various components of a hyperfine structure are usually small enough to fit into the receiver's IF band. Because optical depth varies with frequency, strength ratios among the hyperfine components differ from that of their intrinsic intensities. From this we can derive the object's physical parameters.
Due to the accuracy of hyperfine structure transition-based atomic clocks, they are now used as the basis for the definition of the second. One second is now defined to be exactly 9,192,631,770 cycles of the hyperfine structure transition frequency of caesium-133 atoms.
Since 1983, the meter is defined by declaring the speed of light in a vacuum to be exactly 299,792,458 metres per second. Thus:
The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.
The frequency associated with the states' energy separation is in the microwave region, making it possible to drive hyperfine transitions using microwave radiation. However, at present no emitter is available that can be focused to address a particular ion from a sequence. Instead, a pair of laser pulses can be used to drive the transition, by having their frequency difference (detuning) equal to the required transition's frequency. This is essentially a stimulated Raman transition.
US Patent Issued to K.K. Toshiba on Dec. 18 for "Light-Transmitting Metal Electrode Having Hyperfine Structure and Process for Preparation Thereof" (Japanese Inventors)
Dec 20, 2012; ALEXANDRIA, Va., Dec. 20 -- United States Patent no. 8,334,547, issued on Dec. 18, was assigned to K.K. Toshiba...
Laser spectroscopy of hyperfine structure in highly charged ions: a test of QED at high fields (1).(quantum electrodynamics)(Technical report)
May 01, 2007; Abstract: An overview is presented of laser spectroscopy experiments with cold, trapped, highly charged ions, which will be...
Theoretical progress in the [H.sup.+.sub.2] molecular ion: towards electron to proton mass ratio determination.(Report)
Jan 01, 2011; 1. Introduction The electron to proton mass ratio [m.sub.em.sub.p] is known with a relative accuracy of 4.3 x [10.sup.-10] ....