Photon Induced Fluorescence Spectroscopy of atoms and molecules by Synchrotron Radiation

Tuneable synchrotron radiation is continuously increasing the opportunity to study the energy dependence of different decay pathways from highly excited states. Among the various techniques available, Photon Induced Fluorescence Spectroscopy (PIFS) is particularly useful when high resolution is required for the study of the electronic excitation. The PIFS experimental set-up available at the Gas Phase beamline of Elettra will be described, as well as examples of synchrotron radiation studies of electronic transitions in atoms and molecules will be presented, in which new information on the primary photo excitation and its decay is obtained through the analysis of secondary fluorescent decay in the UV-visible range. Helium is the simplest atom in which electron correlation can be studied, and its doubly excited states below the N=2 threshold are a benchmark system for testing our understanding [1]. These states can decay by autoionisation or fluorescence; close study of the latter channel only became possible to study with third generation synchrotron light [2]. The fluorescence decay is usually a cascade of a VUV photon, a UV/visible photon and another VUV photon. By measuring the second step we have been able to determine quantitatively the branching ratios of the first step, and compare the results with calculated values. Good agreement was found. Molecules can also be studied by PIFS [3], with the advantage over electron and ion spectroscopies that neutral particles can be characterised. For example the decay of O 1s core excited water results in the creation of excited hydrogen atoms, detected by their Balmer emission in PIFS. We show that the O 1s Rydberg states create excited hydrogen atoms whose quantum numbers follow those of the primary excitation, that is, after Auger decay and fragmentation there is a "memory" of the primary excitation. [1] R. P. Madden and K. Codling, Phys. Rev. Lett. 10, 516 (1963); M. Domke, K. Schulz, G. Remmers, G. Kaindl, and D. Wintgen, Phys. Rev. A 52, 1424 (1996). [2] M. K. Odling-Smee, E. Sokell, P. Hammond, and M. A. MacDonald, Phys. Rev. Lett. 84, 2598 (2000); J.-E. Rubensson, C. Såthe, S. Cramm, B. Kessler, S. Stranges, R. Richter, M. Alagia, and M. Coreno, Phys. Rev. Lett. 83, 947 (1999). [3] J. Álvarez Ruiz, M. Coreno et al, Chem. Phys. Lett. 372 (2003) 139.