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Published online by Cambridge University Press: 12 April 2016
It is well known that molecules have much more complex electronic spectra than atoms due to their rotational and vibrational motions. Most molecular spectra observed in the laboratory and included in model atmospheres involve transitions between two bound electronic states. However, in contrast to atoms, molecules can also have bound-free transitions into states that are repulsive in at least one nuclear coordinate, even below the first ionization potential. These transitions not only lead to dissociation of the molecule, but also provide a source of continuous opacity which is usually not taken into account in model atmospheres. Moreover, photodissociation plays an important role in the non-LTE chemistry of stellar atmospheres and circumstellar envelopes (see Olofsson this volume). In this chapter, a brief overview of our current knowledge of cross sections for continuous absorption and photodissociation is given.