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In quantum mechanics, a singlet state usually refers to a system in which all electrons are paired. The term 'singlet' originally meant a linked set of particles whose net angular momentum is zero, that is, whose overall spin quantum number =. As a result, there is only one spectral line of a singlet state.
A spectral line is a weaker or stronger region in an otherwise uniform and continuous spectrum. It may result from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to identify atoms and molecules.
Molecular orbital diagram of two singlet excited states and triplet ground state of O 2. From left to right, the diagrams are for: 1 Δ g singlet oxygen (first excited state), 1 Σ + g singlet oxygen (second excited state), and 3 Σ − g triplet oxygen (ground state).
Spectral line shape or spectral line profile describes the form of an electromagnetic spectrum in the vicinity of a spectral line – a region of stronger or weaker intensity in the spectrum. Ideal line shapes include Lorentzian , Gaussian and Voigt functions, whose parameters are the line position, maximum height and half-width. [ 1 ]
The two singlet-triplet transitions are forbidden both because of the spin selection rule ΔS = 0 and because of the parity rule that g-g transitions are forbidden. [11] The singlet-singlet transition between the two excited states is spin-allowed but parity-forbidden. The lower, O 2 (1 Δ g) state is commonly referred to as singlet oxygen.
In spectroscopy, a forbidden mechanism (forbidden transition or forbidden line) is a spectral line associated with absorption or emission of photons by atomic nuclei, atoms, or molecules which undergo a transition that is not allowed by a particular selection rule but is allowed if the approximation associated with that rule is not made. [1]
The numeral I is used for spectral lines associated with the neutral element, II for those from the first ionization state, III for those from the second ionization state, and so on. [1] For example, "He I" denotes lines of neutral helium , and "C IV" denotes lines arising from the third ionization state, C 3+ , of carbon .
The analysis of line intensity ratios is an important tool to obtain information about laboratory and space plasmas. In emission spectroscopy, the intensity of spectral lines can provide various information about the plasma (or gas) condition. It might be used to determine the temperature or density of the plasma. Since the measurement of an ...