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:
- 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.
- 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).
- 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.
- 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.
- 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.
- 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).
- 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).
- 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.
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 shown in Image 1.
-
Images:
- Credit/Permission: User:Tiltec,
2009
Public domain.
Image link: Wikimedia Commons: File:Hydrogen-SpinFlip.svg.
- Credit/Permission:
User:DJIndica,
User:Edudobay,
2010
Public domain.
Image link: Wikimedia Commons: File:Fine hyperfine levels.svg
File: Atomic file: atom_001_h_001_21_cm_line.html.