The aim of the present study is to discuss the possibility of “counting atoms” by laser ionization technique, which has been applied to post-ionization in sputtered neutral mass spectrometry. The detection of one H2 molecule was tried under an extremely high vacuum (XHV) condition, since the molecule has a higher ionization energy (15.4 eV) than most of elements and the existence probability of it within laser ionization region is less than one under the condition.

Fig.1 shows schematically the experimental setup. The second harmonics (SH) of a picosecond YAG laser was focused with a spherical lens (f=250 mm) into the center of a home-made XHV chamber (Fig. 1a). The focused laser power density (W) is more than 1013W/cm2 for input pulse energy of 30 mJ, and the saturation of H2 ionization can occur under the condition. The repetition rate of the laser was 10 Hz. The XHV chamber can be evacuated to a minimum pressure of 5×10−11n Pa. To the chamber, either an ion counting detector or an ion imaging detector was attached. The ion counting detector (Fig.1b) is composed of ion collecting electrodes, an electron multiplier (EM), and a pulse counter.