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Investigation of the 1s Decay Channels of Sodium by Photoelectron-photoion Coincidence Measurements B. Kanngießer, S. Brünken, K. Godehusen, W. Malzer, W. Benten, A. Tutay, and P. Zimmermann Institut für Atomare und Analytische Physik, Technische Universität Berlin, 10623 Berlin, Germany The determination of transition probabilities and branching ratios is a very important part of the characterization of ionization processes after innershell excitation of free atoms. Here, the simplest systems, i.e. atoms with closed shells, are experimentally as well as theoretically investigated to the greatest extent. Experimental data of metal vapors are more rare and also theoretical calculations are more complicated due to the multiplet splitting of the open shell. A detailed picture of the complex decay routes can be gained by modern coincidence methods. With these methods a quantitative determination of the ratios of non-radiative to radiative, and of single to multiple Auger decays for the decay of a specific initial hole state is possible. In this work we investigate the decay of the sodium 1s hole state after photionization by photoelectronphotoion coincidence measurements. The experiments were carried out at the BW3 beamline of HASYLAB. The principle of the method is the correlation of a certain initial electronic hole state with the final ionic states after deexcitation. The initial hole state is chosen by measuring the respective kinetic energy of the photoelectrons emitted in the photoionization process. In coincidence with these photoelectrons the photoions are measured indicating the respective final ionic state. With this technique corruption by satellite lines, by the continuos energy distribution of the double Auger electrons and by the different angular distribution of photons and electrons can be avoided. Generally, the emission of a fluorescence photon can be distinguished from the emission of an Auger electron in the photoionization process by comparing the charge state of the initial state to the one of the final states. If the resulting charge state remains the same as the initial state, only a fluorescence photon could have been emitted. Each increase of the charge state indicates the emission of an Auger electron. If the final ionic state is reached directly from the initial state, the charge distribution of the photoions gives immediately the fluorescence yield and the Auger decay probabilities [1]. In the case of the 1s photoionization of sodium the fluorescence yield is the direct ratio of the 1+ final ionic state to the sum over all final ionic states. For the higher final ionic states single and direct double and triple Auger decays are responsible. The free sodium atoms were produced by an atom oven, in which solid samples are evaporated. The atoms are then excited by an intensive beam of monochromatized synchrotron radiation. The electrons generated in the interaction zone are analyzed according to their kinetic energy by a cylindrical mirror analyzer (CMA) under the “magic” angle. The corresponding ions are analyzed by their mass to charge ratio in a time-of-flight spectrometer (TOF). A time correlation between electrons and ions is established by using the signals of the electron spectrometer to trigger a high voltage pulse at the ion-spectrometer that pulls the photoions into the TOF. With the same signals the ion time-of-flight measurement is started. If the analyzer energy of the CMA is fixed during the measurement, a TOF coincidence spectrum of photoions correlated with electrons of a specific kinetic energy is obtained. To account for false coincidences in the spectrum measured, the distribution of false coincidences is determined by a separate reference measurement where the TOF is triggered randomly by a pulse generator with the same rate as the electron signal rate from the CMA. The distribution of false coincidences is then subtracted from the coincidence spectrum taking dead time effects into account [2]. Figure 1 shows the time-of-flight spectra of the final ionic states obtained in coincidence with the 1s photoelectrons (spectrum at the top) and with a random trigger (spectrum at the middle). The spectrum at the bottom depicts the distribution of true coincidences which has been evaluated out of the first two spectra. The spectra were taken at an excitation energy of 1135.5 eV, which is high enough to omit near threshold effects. The resulting mean values of the decay probabilities measured can be found in table 1.
500
3+
400
coincidence mode
2+
300 200 100
1+
4+
Intensity [counts]
0 500 400
reference mode
300 200
Na hν = 1135.5 eV
100
Ekin = 56.5 eV (1s )
-1
0 300
true coincidences 200
x5 100
0 50
100
150
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250
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TOF Channel
Fig. 1: Sodium 1s coincidence measurements. Na 1s-1
1+ [%]
2+ [%]
3+ [%]
4+ [%]
Decay to
1.39(16)
63.6(3)
33.0(3)
1.93(12)
Table 1: Decay probabilities of the Na 1s hole state into the different final ionic states. Earlier work by other groups and by ourselves have directly demonstrated that the photoelectron-photoion coincidence method is very useful to investigate complex decay channels of inner shell hole states. Now we could show that this coincidence method can also be successfully employed to determine fluorescence yields of free atoms, especially for light elements, with a precision of about 10 %. The K-shell fluorescence yield of sodium has been determined for the first time. For neon [3] as well as for sodium the K-shell fluorescence yield has been overestimated in theoretical calculations by around 30 %. This shows the need for further investigations on other atoms and for refined theoretical calculations. For elements with an atomic number greater than eleven, i.e. for complex cascade processes, additional information can be gained by the detection of the corresponding fluorescence photon or Auger electron. Furthermore, a comparison between the investigations of decay processes of free atoms and molecules and solids should be done. The authors gratefully acknowledge the support of the staff of HASYLAB.
References [1] B. Kanngießer, S. Brünken, K. Godehusen, Ch. Gerth, W. Malzer, M. Richter, and P. Zimmermann, Nucl. Instr. and Meth. A (accepted). [2] T. Luhmann, Ch. Gerth, M. Groen, M. Martins, B. Obst, M. Richter, and P. Zimmermann, Phys. Rev. A 57, 282 (1998). [3] B. Kanngießer, M. Jainz, S. Brünken, W. Benten, Ch. Gerth, K. Godehusen, K. Tiedtke, P. von Kampen, A. Tutay, and P. Zimmermann, K. F. Demekhin, and A.G. Kochur, Phys. Rev. A 62, 014702 (2000).
