The spectral lines are usuallyquantum system (Usuallyatom, but sometimesmoleculeorNucleus）And singlephotonIt is generated by interaction.When the energy of the photon is indeed consistent with a change in the energy level of the system (in the case of atoms, usuallyElectronicsChange the orbit), photons are absorbed.Then, it will spontaneouslylaunchIt may be the same frequency as the original or phased, but the total energy emitted by the photon will be the same as the energy absorbed at the beginning, and the direction of the new photon will not have any correlation with the original photon direction.
According to the geometric relationship among gas, light source and observer, the spectrum seen will beAbsorption lineorEmission line。If the gas is located between the light source and the observer, the intensity of light will be weakened at this frequency, and most of the photons emitted will be different from the direction of the original photon, so the observer will see the absorption spectrum line.If the observer looks at the gas but is not in the direction of the light source, then the observer will only see the re emitted photons at a narrow frequency, so what he sees is the emission spectrum line.
The absorption and emission spectral lines have a specific relationship with atoms, so it is easy to distinguish the chemical composition of light passing through the medium (usually gas).somewhatelement, likehelium、thallium、ceriumWait, they are all found through spectral lines.Spectral lineIt also depends on the physical state of the gas, so they are widely used infixed starAnd otherscelestial bodiesIt is impossible to use other methods to complete this work.
Homonuclear shiftBecause the atomic nucleus that absorbs photons is different from the atomic nucleus that emits photonselectron density。
In addition to the atom photon interaction, other mechanisms can also generate spectral lines.Photons generated according to actual physical interactions (molecules, individual particles, etc.) are widely distributed in frequency and can span fromradio wavesreachGamma ray, all observableElectromagnetic wave spectrum。
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In the radiation spectrum of various celestial bodies, there are often many spectral lines, some are emission lines, and some are absorption lines.Spectral lines are formed by transitions between discrete energy levels of a system.
If E1 and E2 are two discrete energy levels of a system, and E2>E1, when the system transitions from E2 to E1, the emission frequency is V=(E2 – E1)/h radiation;On the contrary, when the system transitions from E1 to E2, the absorption frequency is v radiation.
If the emission process is superior to the absorption process, the emission line will be generated;Conversely, an absorption line is generated.
In stellar spectra, spectral lines are caused by transitions between discrete energy levels of atoms, ions and molecules.For example, D1, D2 and H, K lines in the solar spectrum are caused by the transitions of sodium atom and calcium ion between discrete energy levels, respectively.
In the radio band, there are also spectral lines.For example, the 21cm spectral line of neutral hydrogen is composed of hydrogen atomsHyperfine structureCaused by transitions between energy levels.The hyperfine structure energy level is generated by the coupling between the spin quantity of the atomic nucleus and the total angular momentum of the electron (see the hyperfine structure of the atom).Many millimeter wave spectral lines are found in interstellar clouds, and most of the radio spectral lines are formed by the transitions of various rotational energy levels of various interstellar molecules.
Spectral lines also began to be found in the high-energy bands of X-rays and gamma rays.For example, in the X-ray spectrum of X-1 in Hercules, a 58 kiloelectron volt spectrum line was found, which may be formed by the transition between the Landau energy levels of electrons moving in a strong magnetic field.
Any spectral line is not infinitely narrow, but always has a certain width.This width is partly due to the limited resolving power of observation instruments, and partly due to the celestial radiation itself.There are many reasons for this broadening, but it can be roughly divided into two categories: one is because the energy level of the microscopic system forming the spectral line is not infinitely narrow, but has a certain width.The spectral line generated by the energy level with a certain width must also have a certain width, which is called the natural width of the spectral line.This effect is called radiation damping.The other is caused by superposition, because the radiation we observe is the superposition of radiation from various emission or absorption systems.
Generally speaking, the motion state of each emission or absorption system and the interaction state with the surrounding substances are different, and the frequency of their emission or absorption is also different, which causes the broadening of spectral lines.Thermal Doppler effect, collision damping, statistical broadening, rotation, expansion and turbulence can all broaden the spectral line through superposition effect.
