14. High-Resolution Spectroscopy at Short Using Pulsed Dye Lasers

The invention of the dye laser [14.1, 2] 25 years ago has brought about an enormous development of optical spectroscopy (for reviews see, e.g., [14.3, 4]). The tunability and high spectral intensity of these sources in combination with the possibility to achieve extremely narrow bandwidths for cw lasers, or pulses of extremely short duration, have greatly facilitated many spectroscopic investigations and made entirely new types of experiments possible. In the present chapter we will discuss high-resolution laser spectroscopy of free atoms and especially focus on the UV and VUV wavelength region. (For a more detailed discussion of this topic we refer to 1-14.5]). Dye lasers do not operate at such wavelengths, but their radiation can be frequency converted to this spectral region using nonlinear optics techniques. Such conversion can most conveniently be performed with pulsed lasers, that, however, necessarily have a much larger linewidth than the single-mode cw systems available at longer wavelengths. The topic of this chapter is primarily to discuss how a resolution, limited only by the Heisenberg uncertainty relation, can still be obtained by combining intense, broadband excitation with classical high-resolution techniques such as optical double-resonance (ODR), level-crossing (LC) and quantum-beat (QB) spectroscopy. Such techniques proved very useful at an earlier stage of development of dye lasers. At the beginning of the 1970s only multi-mode cw lasers were available and in a very limited wavelength range. Such lasers, used for stepwise excitations and combined with ODR and LC spectroscopy, allowed a substantial extension of high-resolution alkali-atom spectroscopy (see e.g. [ 14.6, 7]). At the same time, QB spectroscopy was shown to effectively extend the wavelength range of high-resolution laser spectroscopy [14.8]. While awaiting the development of broadly tunable, narrow-band cw laser sources allowing the full utilization of Doppler-free laser spectroscopy methods using collimated atomic or ionic beams, spectral holeburning, two-photon absorption and cooled atoms or ions in traps, the ODR, LC and QB methods can provide precision spectroscopic data on atoms and ions. However, it must be remembered that these resonance and coherence methods are only useful for the measurement of level splittings due to, e.g., fine and hyperfine interactions, Zeeman and Stark effects. Measurements of isotopic shifts, scalar Stark interaction, and of course investigations of the absolute wavelength/frequency of the optical transition, still require the narrow-band laser sources. For selected wavelengths such radiation can be achieved, e.g., for Lamb-shift/Rydberg