Two-Photon Ionization Spectrometry of Alkali Atoms in Flames

Abstract

Nonlinear optical spectroscopy (1-3) and laser ionization mass spectrometry (4-6 ) are important, well-established techniques that generally rely on intense, short-pulse lasers. Fortunately, such lasers are now inexpensive, user friendly, and widely available. This summary describes a new experiment that introduces students to pulsed laser-based instrumentation with an optogalvanic technique we call two-photon ionization spectrometry (TPIS).Educational experiments based on multiphoton spectroscopy are rare (6-10). Most standard vibrational or electronic spectroscopy lab procedures involve linear techniques such as absorption, fluorescence, or Raman scattering, in which molecules are excited by single photons from a continuous light source. In contrast, multiphoton excitation requires simultaneous absorption of two or more photons, which is practical only with short-pulse laser sources. Such excitation is generally advantageous, however, because it provides spectroscopic access to quantum states having a wider range of energy and angular momentum. Since stand-alone instruments for multiphoton ionization spectrometry are not yet commercially available, students and instructors in advanced undergraduate or beginning graduate-level courses will still find it necessary to go beyond the typical black-box approach to using an instrument. ApparatusAlkali atom samples for TPIS are prepared by aspirating aqueous salt solutions into an acetylene-air flame, as shown in Figure 1. A commercial 5-cm slot burner provides good path length and the stability needed for quantitative measurements, although a natural gas burner with homemade aspirator also provides sufficient sample. A stainless-steel