Experimental study of the triatomic hydrogen molecule through the collisional sequence ${\mathrm{H}}_3^{+}$ → ${\mathrm{H}}_3$ → ${\mathrm{H}}_3^{+}$ undergone by fast beams in argon

The neutralization-ionization method using two gas targets was employed to study the sequence ${\mathrm{H}}_3^{+}$ → ${\mathrm{H}}_3$ → ${\mathrm{H}}_3^{+}$. The existence of long-lived states of the neutral molecule ${\mathrm{H}}_3$ ($\tau \gtrsim 3\times {10}^{-7\mathrm{}}$ s) is demonstrated, any ambiguity due to the presence of HD molecules being definitively removed. Dissociation cross sections of the molecular ion in argon have been measured and the dissociation cross section of the neutral molecule, the electron-capture, and the electron-loss cross sections have been estimated. The capture cross section for ${\mathrm{H}}_3^{+}$ is extremely small (∼1 b at 1 MeV) and decreases very rapidly with the velocity of the incident ion beam. It is suggested that these measured cross sections concern the formation of ${\mathrm{H}}_3$ Rydberg states in a narrow band of $n$ and $l$ values.