Ionizing Collisions of the Metastable and Long-lived Highly Excited States of Helium Atoms with Water Vapor

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Long-lived excited states of He^*(2¹S and 2³S)and He^**(in highly excited states), produced by impact of 20-200eV electrons were allowed to collide with H₂O and D₂O molecules. It was confirmed that ions H⁺, HeH⁺, OH⁺ and H₂O⁺ were produced by impact of He^* with H₂O. An ion produced by, a rearrangement chemi-ionization, HeH⁺, was identified for the first time in the He^*+H₂O system. Another process, where He^** is ionized into He⁺ by collision with H₂O, was also observed. These processes were discriminated by the difference in the threshold energies of the electrons used for exciting the helium atoms. The corresponding deuterated ions were observed when D₂O was used for the target.