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From Ideal to Reality: Why Are Real Spectral Line Shapes Always So Broad?
The shape of the spectral line, that is, the spectral line profile, refers to the intensity change near a certain spectral line in the electromagnetic spectrum. Ideally, the shapes of these spectral lines can be described by Lorentz type, Gaussian type, and Voigt type, but in fact, real spectral lines are often broadened due to a variety of factors. These factors include lifetime width, Doppler effect, collision broadening, and proximity effects.
Real spectral lines are not infinitely sharp, but have a specific shape, which is the result of various factors.
The origin of spectral line shapes
Spectral lines are generated mainly from electronic transitions of atoms, molecules or ions, and these transitions are accompanied by specific energy changes. It is important to note, however, that the measurement of these energies is not always precise and is subject to various effects, resulting in broad lines. The root of this phenomenon lies in.
The fundamental reason can be traced back to the uncertainty principle, which makes the uncertainty of energy have a certain relationship with the lifetime of the excited state.
The source of the breadth of spectral lines
There are several main sources of spectral line width: The first is the width due to the lifetime of the excited state, which is due to the uncertainty principle. Second, the Doppler width is affected by the velocity distribution of atoms or molecules relative to the observer, which usually results in a Gaussian-shaped line shape. Furthermore, the collision width is caused by collisions between atoms or molecules. This collision shortens the lifetime of the upper-level state, thereby increasing the energy uncertainty. Finally, the near effect mainly affects spectral line shapes in liquids and solids.
Spectral line shape function
Spectral line shapes can be described by several functions: Lorentz type, Gaussian type and Voigt type. The commonly used form of the Lorentz type linear function is defined as a curve with long tail characteristics, while the Gaussian type is a symmetrical bell-shaped curve. The Voigt type is the convolution of the two, making it more complex in many cases.
Application examples
For atoms in the gas phase, the main factors are Doppler and pressure span, so in applications such as atomic absorption spectroscopy, it can provide clear elemental analysis. When discussing molecular spectroscopy and nuclear magnetic resonance (NMR), changes in line shapes show different characteristics and widths, especially in different phase states, and their impact is particularly obvious.
These line shape changes are of great significance for various spectroscopic applications, especially in the field of analysis and diagnosis.
Application of curve deconvolution
Numerical deconvolution of spectral curves can be applied to improve spectral resolution. Especially in NMR spectroscopy, deconvolution becomes relatively simple due to the properties of the Lorentzian, which can reduce the half-width of the line in the frequency domain. In addition, this technique is also applicable to other types of spectra, although the processing process is more cumbersome.
Conclusion
The shape and breadth of spectral lines are the result of the interaction of many factors. Understanding these factors is not only important for academic research, but also crucial for accuracy in practical applications. So when we are faced with such a broad spectrum of spectral lines, should we rethink our understanding of the spectrum?
