12. Optical Spectroscopy in the VUV Region

Abstract

Publisher Summary This chapter focuses on optical spectroscopy in the vacuum ultraviolet (VUV) region. Atomic and molecular spectra in the VUV region are major contributors to the experimental characterizations of the structures of excited electronic states. Such spectra also are important to the identification of species participating in physical and chemical processes in environments ranging from terrestrial to astrophysical. In the chapter, VUV implies wavelengths from ∼2000 A (200 nm) range into the edge of the soft x-ray region (∼150 A). In some VUV spectroscopic observations, resolving power (RP) is limited by line broadening (lifetime and/or Doppler) rather than by instrumental effects. Usually, lifetime broadening is of greater importance because the upper state of a VUV spectroscopic transition often lies energetically above ionization and/or dissociation limits. The chapter presents an example of the way resolution affects both the separability of different spectral features lying close together and the line shape and apparent intensities of such features. Because wavelength is inversely proportional to energy, relative and absolute uncertainties in wavelength determinations translate directly to analogous uncertainties in transition energies and empirical energy level structures. Traditional high RP photographic plate detection has been especially important in the area of precise VUV wavelengths because hundreds of Angstroms of a spectrum can be measured against standard reference lines on a single photographic plate—a process that ensures high relative precision and simplifies absolute wavelength determinations. The chapter discusses absolute photoabsorption cross sections and their application to atmospheric science, interstellar molecules, and so on.