in: Soft matter at aqueous interfaces, PR Lang,Y Liu, Editor, Springer, London/Berlin , Amsterdam, pp.459-490, 2015
This chapter summarises the physical principles of optical absorption spectroscopy and its use for the characterisation of surfaces and interfaces. After a brief discussion of the fundamentals of absorption spectroscopy and its relation to quantum mechanics, the chapter discusses the basics of optics at interfaces, focusing on the absorption of light by molecules in the interfacial region. Because of fundamental similarities, the chapter will touch on spectroscopy of both electronic and vibrational transitions, with a strong focus on infrared absorption experiments. There is a brief discussion, with reference to examples, of experiments in internal and external reflection geometry, including a brief discussion of the measurement of spectra on different classes of substrates (metallic vs. transparent).