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The third line shape that has a theoretical basis is the Voigt function, a convolution of a Gaussian and a Lorentzian, Lifetime broadening. According to the uncertainty principle the uncertainty in energy, Δ E and the lifetime, Δ t, of the excited state are related by An atomic transition is associated with a specific amount of energy, E. However, when this energy is measured by means of some spectroscopic technique, the line is not infinitely sharp, but has a particular shape. Numerous factors can contribute to the broadening of spectral lines. Broadening can only be mitigated by the use of specialized techniques, such as Lamb dip spectroscopy. The principal sources of broadening are: E Δ t ⪆ ℏ {\displaystyle \Delta E\Delta t\gtrapprox \hbar } This determines the minimum possible line width. As the excited state decays exponentially in time this effect produces a line with Lorentzian shape in terms of frequency (or wavenumber).

Observed spectral line shape and line width are also affected by instrumental factors. The observed line shape is a convolution of the intrinsic line shape with the instrument transfer function. [3] where p 0 {\displaystyle p_{0}} is the position of the maximum (corresponding to the transition energy E), p is a position, and w is the full width at half maximum (FWHM), the width of the curve when the intensity is half the maximum intensity (this occurs at the points p = p 0 ± w 2 {\displaystyle p=p_{0}\pm {\frac {w}{2}}} ). The unit of p 0 {\displaystyle p_{0}} , p {\displaystyle p} and w {\displaystyle w} is typically wavenumber or frequency. The variable x is dimensionless and is zero at p = p 0 {\displaystyle p=p_{0}} . Doppler broadening. This is caused by the fact that the velocity of atoms or molecules relative to the observer follows a Maxwell distribution, so the effect is dependent on temperature. If this were the only effect the line shape would be Gaussian. [1] Proximity broadening. The presence of other molecules close to the molecule involved affects both line width and line position. It is the dominant process for liquids and solids. An extreme example of this effect is the influence of hydrogen bonding on the spectra of protic liquids.Each of these mechanisms, and others, can act in isolation or in combination. If each effect is independent of the other, the observed line profile is a convolution of the line profiles of each mechanism. Thus, a combination of Doppler and pressure broadening effects yields a Voigt profile. Main article: Spectral line §Line broadening and shift Absorption spectrum of an aqueous solution of potassium permanganate. The spectrum consists of a series of overlapping lines belonging to a vibronic progression

Pressure broadening (Collision broadening). Collisions between atoms or molecules reduce the lifetime of the upper state, Δ t, increasing the uncertainty Δ E. This effect depends on both the density (that is, pressure for a gas) and the temperature, which affects the rate of collisions. The broadening effect is described by a Lorentzian profile in most cases. [2] The subsidiary variable, x, is defined in the same way as for a Lorentzian shape. Both this function and the Lorentzian have a maximum value of 1 at x = 0 and a value of 1/2 at x=±1. V ( x ; σ , γ ) = ∫ − ∞ ∞ G ( x ′ ; σ ) L ( x − x ′ ; γ ) d x ′ , {\displaystyle V(x;\sigma ,\gamma )=\int _{-\infty } where L signifies a Lorentzian function standardized, for spectroscopic purposes, to a maximum value of 1; [note 1] x {\displaystyle x} is a subsidiary variable defined as