Question

How would the absorption and emission spectra of a powdered 1:1 GaP and GaAs physical mixture differ from the spectra that would be seen for a GaP_0.5As_0.5 solid solution?

Answer 1

First of all, many compounds do not emit light after absorption of radiation. so these molecules do not have an excitation spectrum. The reason is that the absorbed energy can dissipate in the molecule without emitting light/photons.

Secondly, molecules absorb light to different excited states. Emission comes from the lowest of these states, called the S1 state. When a molecule is excited to a higher state it often ends up in this lowest excited state S1 and then emits radiation. In this case the excitation spectrum is the same as the absorption spectrum. However from a higher excited state a molecule does not have to go to the lowest excited state. It can directly go to the ground state for instance. Then it does not emit radiation.

In this case the excitation spectrum only has the lowest energy absoption features. In spectroscopy terms: the absorption spectrum shows the So-S1, the So-S2, So-S3 etc bands, but the excitation spectrum only shows the So-S1 band.

An absorption spectrum measures wavelengths at which a molecule absorbs light , while an excitation spectrum determines the wavelengths of light necessary to produce emission or fluorescence from the molecule, as monitored at a particular wavelength.

It is intresting to know tthat the intensity of peaks seen in the excitation spectrum is directly proportional to the number of photons absorbed. By contrast, in absorption spectroscopy, one usually measures the absorbance, which is log (base10) of incident light intensity over transmitted light intensity. Thus peaks in the excitation spectrum will not have the same relative intensities as peaks in the absorption spectrum, although they should appear at the same wavelengths.

The two peaks in the given mixture of compounds absorption spectrum with absorbances willbe in ratio 2 and 1, respectively. Of course the first peak will appear to be twice as "high" as the second, on an absorbance scale. But in terms of absorbed light, for the first peak, 99% (1% transmitted) of the light is absorbed, while for the second peak 90% is absorbed (10% transmitted). So in the excitation spectrum, the first peak would have an intensity only about 1.1 times that of the second peak. As one goes to a very dilute sample, however, the relative intensities in the two types of spectrum approach the same value