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A hydrogen atom with proton and electron spins aligned (top) undergoes a flip of the electron spin, resulting in emission of a photon with a 21 cm wavelength (bottom) The hydrogen line, 21 centimeter line, or H I line [a] is a spectral line that is created by a change in the energy state of solitary, electrically neutral hydrogen atoms.
Lines are named sequentially starting from the longest wavelength/lowest frequency of the series, using Greek letters within each series. For example, the 2 → 1 line is called "Lyman-alpha" (Ly-α), while the 7 → 3 line is called "Paschen-delta" (Pa-δ). Energy level diagram of electrons in hydrogen atom
It is a popular observing frequency used by radio telescopes in radio astronomy. [1] The strongest hydroxyl radical spectral line radiates at 18 centimeters, and atomic hydrogen at 21 centimeters (the hydrogen line).
Lyman-alpha, typically denoted by Ly-α, is a spectral line of hydrogen (or, more generally, of any one-electron atom) in the Lyman series.It is emitted when the atomic electron transitions from an n = 2 orbital to the ground state (n = 1), where n is the principal quantum number.
Electromagnetic – common cordless telephone frequency in the US 10 9: 1 gigahertz (GHz) 1.42 GHz: Electromagnetic – the hyperfine transition of hydrogen, also known as the hydrogen line or 21 cm line 2.4 GHz: Electromagnetic – microwave ovens, wireless LANs and cordless phones (starting in 1998) 2.6–3.8 GHz
The "visible" hydrogen emission spectrum lines in the Balmer series. H-alpha is the red line at the right. Four lines (counting from the right) are formally in the visible range. Lines five and six can be seen with the naked eye, but are considered to be ultraviolet as they have wavelengths less than 400 nm.
In principle, any emission line can be used to make intensity maps if it can be detected. Other emission lines that have been proposed as cosmological tracers include: Rotational transitions in molecules, such as carbon monoxide [13] Fine structure transitions from species such as ionized carbon [14] Lyman-alpha emission from hydrogen [15]
The spectrum appears in a series of lines called the line spectrum. This line spectrum is called an atomic spectrum when it originates from an atom in elemental form. Each element has a different atomic spectrum. The production of line spectra by the atoms of an element indicate that an atom can radiate only a certain amount of energy.