H 21-centimeter emission process

    Image 1 Caption: An ABSTRACT diagram of the ground state hyperfine levels of atomic hydrogen (H I): i.e., the parallel spin higher energy hyperfine level (F=1 state) and the antiparallel spin lower energy hyperfine level (F=0 state which is the true absolute ground state).

    The atomic transition from the F=1 state to the F=0 state emits the hydrogen 21-centimeter line (21.1061140542 cm, 1420.4057 5176 67(9) MHz ≅ 1420 MHz). This transition line is in the radio band (fiducial range 3 Hz -- 300 GHz = 0.3 THz, 0.1 cm -- 10**5 km).

    Features:

    1. As aforenoted, the diagram is ABSTRACT. Both proton (the hydrogen nucleus colored in red) and the electron (colored in grey) exist in a spherical cloud-like superposition of positions: the proton in small cloud at the center of the large electron cloud.

    2. Now most elementary particles have spin, an intrinsic and invariable angular momentum. In fact, the only known spin 0 elementary particle (i.e., elementary scalar boson) is the Higgs particle (Wikipedia: Scalar boson: Scalar).

    3. The proton and electron both have spin 1/2 (in units of the Planck constant divided by 2π: symbol ħ).

      Quantum mechanics dictates that only 2 alignments are possible for the proton and electron: the parallel F=1 state and the antiparallel F=0 state.

    4. The downward spin-flip transition from F=1 state to F=0 state emits the emission spectral line the hydrogen 21-centimeter line (21.1061140542 cm, 1420.4057 5176 67(9) MHz ≅ 1420 MHz).

      In space, the downward spin-flip transition is almost always a spontaneous emission (only very rarely a stimulated emission). The putting of the hydrogen atom in the F=1 state may be due to a collisional excitation (and therefore controlled by the local temperature and density of the gas in space) or to the spontaneous emission from a higher energy level that was excited by a photon absorption, a collisional excitation, or following a recombination following at photoionization.

    5. The mean lifetime of the F=1 state for the hydrogen 21-centimeter line is 10.9 Myr (see Wikipedia: Hydrogen line: Cause). The mean lifetime is so huge because the atomic transition is a highly forbidden transition. "Forbidden" in this context means forbidden by the most usual atomic transition mechanism, but allowed by weaker atomic transition mechanisms.

    6. Because mean lifetime of hydrogen 21-centimeter line is so huge, it probably can NEVER be measured in the laboratory. You simply need too huge a sample of low density cold neutral atomic hydrogen gas for the spontaneous emission of the hydrogen 21-centimeter line to be observed. The mean lifetime has to be calculated theoretically from quantum mechanics. Note stimulated emission of the hydrogen 21-centimeter line can be seen in the laboratory using a hydrogen maser (see Wikipedia: Hydrogen line: Cause).

    7. But though it probably can NEVER be measured in the laboratory, the vast volumes of low density cold neutral atomic hydrogen gas in space allow easy detection of the spontaneous emission of the hydrogen 21-centimeter line from that realm. The observation hydrogen 21-centimeter line from space is one of the key tools of radio astronomy (see Wikipedia: Hydrogen line: Uses).

    8. Image 2 Caption: A Grotrian diagram of the fine structure and hyperfine structure of atomic hydrogen (H I). The angular momentum coupling of the different angular momenta causes energy level splitting. The Grotrian diagram is NOT to-scale.

      hydrogen fine and hyperfine structure

      Recall a Grotrian diagram is a diagram illustrating the energy levels of an atom or molecule, either of which could be an ion.

      The 2 lowest energy levels are those illustrated in the ABSTRACT diagram of the ground state shown in Image 1.

    Images:
    1. Credit/Permission: User:Tiltec, 2009 Public domain.
      Image link: Wikimedia Commons: File:Hydrogen-SpinFlip.svg.
    2. Credit/Permission: User:DJIndica, User:Edudobay, 2010 Public domain.
      Image link: Wikimedia Commons: File:Fine hyperfine levels.svg
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