The concept of spectral classes is discussed in more detail on the next page. The study of many thousands of stellar spectra in the late Nineteenth Century led to the development of our modern classification system for stars. Stars of different temperatures, size and metallicities will have different spectra but most exhibit absorption lines even if they do not all show strong Balmer lines as in this star. This can be used to determine the effective temperature of the star. The overall shape of the spectrum approximates a black body curve with a peak wavelength. Note the characteristic absorption line features including strong lines for Hα, Hβ, Hγ and Hδ - the Balmer Series. The spectrum below is an intensity plot of a star. Let us know use these basic principles to account for and compare spectra produced by different types of astronomical objects. The spectrum formed is an emission or bright line spectrum, as shown by the middle spectrum in Figure 1. As these photons can re emitted in any direction an external observer will detect light at these wavelengths. When they de-excite they emit photons of specific frequency and wavelength. If this cloud can be excited by a nearby source of energy such as hot, young stars or an active galactic nucleus then the electrons in atoms of the gas cloud can get excited. Stellar spectra typically look like this.Įmission spectrum: A third possibility occurs if an observer is not looking directly at a hot black body source but instead at a diffuse cloud of gas that is not a black body. This means that the resultant spectrum will show dark absorption lines or a decrease in intensity as shown in the dips in the absorption spectrum top right in the diagram above. The net effect of this is that the intensity of light at the wavelength of that photon will be less in the direction of an observer. The direction of this re-emission however is random so the chances of it travelling in the same path as the original incident photon is very small. Eventually the electron will de-excite and jump down to a lower energy level, emitting a new photon of specific frequency. Photons of specific frequency can be absorbed by electrons in the diffuse outer layer of gas, causing the electron to change energy levels. The photons emitted from the core cover all frequencies (and energies). If we were able to view the light from this source directly without any intervening matter then the resultant spectrum would appear to be a continuum as shown bottom left in the Figure 1 above.Ībsorption spectrum: Most stars are surrounded by outer layers of gas that are less dense than the core. In the gas phase collisions are rare compared to those of solvent in the condensed phase and so vibrational relaxation can no longer compete with fluorescence and so the fluorescence spectrum can arise from a mixture of vibrational levels not just v'=0.Figure 1: How continuous, emission and absorption spectra can be produced from same source.Ĭontinuum spectrum: In this diagram, a dense hot object such as the core of a star acts like a black body radiator. In the question the molecule also shows a mirror image but the picture is in wavelength not wavenumber (or energy) so is a bit distorted. Assuming that the ground and excited state have a similar shape the fluoresce and absorption spectra are mirror images of one another as shown in the figure. Thus all the excited molecules are now in v'=0 in the excited state. The intensity of absorption/emission is proportional to the transition moment squared, $d_$. Swenberg, Electronic Processes in Organic Crystals and Polymers, 1999)Īssuming harmonic oscillations, the overlap between each two vibrational wavefunctions (different energy level for different frequencies/number of nodes) is defined as the nuclear vibrational overlap, which in turn contributes to the total transition moment. When an electron is excited from a ground state $S_0$ to the first excited state $S_1$, the bond is stretched and the internuclear separation increases: As mentioned, the reason for this difference is the contribution of nuclear vibrational overlap to the transition moment.
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