Altermatt Lecture:   The Solar Spectrum


4.3:  Atmospheric absorption – an overview*

This page looks at deeper causes for absorption to understand, how different ranges of the solar spectrum are shaped by different absorption processes.

In the ultraviolet (UV) range – at very short wavelengths λ – the photons have a very large energy Eph, which gets absorbed by shell-transitions in molecules that are simple and therefore are strongly bound (ie. their atomic shells are energetically wide apart). Molecules such as H2, O2, and N2 are very abundant and, hence, the atmosphere is completely opaque below 300 nm.

In the near UV, we need to search for heavier and less strongly bound molecules. The first molecule that absorbs a considerable amount of sunlight is ozone (O3). Because it is a rather complicated molecule, there is a rather broad range of shell-transitions possible, so ozone is the only absorber in a broad wavelength range (from 230 to 290 nm) and plays a key role in the amount of UV light that reaches the earth surface.

In the infrared (IR) range – at long λ – the photons have a small Eph and are unable to excite electrons to higher shells. However, they can cause changes to rotations and vibrations in molecules that are rather large or asymmetric such as water and carbon dioxide. There are many different possibilities for rotations and vibrations, causing broad absorption bands. Therefore, the atmosphere is completely opaque beyond the near-IR end of the visible spectrum (down to radio frequencies).

In the visible range (λ ≈ 380–780 nm), the photons have neither enough energy to excite electrons in most simple molecules, nor is their Eph small enough to cause vibrations and rotations in most larger molecules. Hence, only a small amount of light gets absorbed (mainly by water vapor), allowing for the atmospheric window in the visible range.

Sources of absorption in AM1-5d

Figure: Sources of absorption in the AM1-5d spectrum.


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