ELECTRON-EXCITATION ENERGY-TRANSFER FROM HIGHLY EXCITED CS ATOMS FORMING HIGH RYDBERG STATE ATOMS AND MOLECULES

The reactions of excited Cs atoms with various gases have been studied in a quadrupole mass spectrometer ion source. The excited atoms are formed in a thermionic converter (TIC) plasma and are sampled into the ion source as a molecular beam. Direct field ionization of the excited states from the plasma at field strengths up to 600 V/cm in the ion source gives ions Cs+, Cs2+, and H-2+. The intensity of the doubly ionized Cs is often larger than that of singly ionized Cs, and it is believed to be formed from Cs2*. The doubly excited Cs is found to transfer its excitation energy to gas molecules M in the ion source, as Cs2* + M --> Cs* (or Cs) + M*. The excited molecules are field ionized to M+ or dissociate to smaller ions. Charge-exchange reactions are excluded, especially due to the low electron affinity of Cs. The variation of the positive current of M+ is studied as a function of gas pressure between 10(-6) and 10(-4) mbar. During inlet of some gases, e.g. CO2, almost all mass peaks vary linearly with pressure, which is interpreted as excitation transfer from the Rydberg states of CO2 to other atoms and molecules. The initial excitation-transfer cross sections are estimated to be 4000-10 000 angstrom-2, corresponding approximately to n = 10 for Cs2*.