ABSTRACT
Due to their unprecedented degree of monochromaticity and directionality, lasers have revolutionized the field of interferometry and spectroscopy. In the former branch, just think of the practical development of holography as well as of the ambitious experiments that are in progress to detect gravitational waves [375]. In the latter field, through the introduction of novel high-resolution and high-sensitivity interrogation techniques, lasers have allowed the study of atomic/molecular spectra with an unprecedented precision. In a nutshell, the resolution limit is now determined by the width of the spectral lines of the substance under investigation rather than by the instrumental width of the spectral apparatus. In turn, the implementation of more and more sophisticated spectroscopy-based frequency measurement schemes have improved the laser performance, eventually leading to the realization of optical standards. In this chapter we review the most advanced spectroscopic techniques for gaseous samples and the realization of standards using either absorption cells or effusive beams. Essentially, the same spectroscopic interrogation methods are also at the basis of frequency standards using samples of cold/trapped atoms and ions. These will be dealt with in Chapter 7.
