Third excimer continuum of argon excited by a heavy-ion beam

The emission of the third excimer continuum of argon in the wavelength range between 175 and 250 nm has been studied by time-resolved optical spectroscopy. Argon gas at pressures between 5 and 100 kPa was excited with a pulsed beam of 100-MeV ³²S⁹⁺ ions from the Munich tandem van de Graaff accelerator. Wavelength spectra recorded in different time windows after the 2-ns beam pulses show that two different components contribute to the third continuum of argon. A radiative lifetime of 5.71±{}0.08 ns for the Ar₂²⁺ molecule and a rate coefficient k₃=(1.46±{}0.12)×{}10^-30 cm^-6/s for the reaction Ar²⁺+2Ar{→}Ar₂²⁺+Ar were determined from the pressure dependence of time spectra at a wavelength of 190 nm. Additional time spectra were measured at 210 and 230 nm.