The photodissociation cross sections of a beam of Ar2+formed by associative ionization in a conventional electron impact ion source have been measured over the wavelength range from 265 to 715 nm. The measured cross sections are for transitions from the2Sgr;u+ground state to the repulsive2Sgr;g+excited state and are dependent on the ion source conditions. At low pressures (0.01 Torr) in the ion source, the cross section is 1times;10minus;17thinsp;cm2and varies little over the entire wavelength range. When pressures are increased to 0.1 Torr, a peak in the cross section at about 300 nm is obtained with a value of 1.7times;10minus;17thinsp;cm2. A simplified two state mechanism involving high and low lying vibrational levels of Ar2+in its ground state is proposed to explain the qualitative effects of pressure on the cross sections. Measurement of the Ar2+and Ar+beam currents as a function of repeller voltage showed that collisional dissociation is occurring in the ion source under conditions of relatively high pressure and positive repeller voltage at rates comparable to the Langevin rate. With zero repeller voltage an average collisional stabilization rate coefficient of 8times;10minus;11thinsp;cm3thinsp;secminus;1thinsp;moleculeminus;1is obtained for vibrational relaxation of Ar2+from rsquo;rsquo;highrsquo;rsquo; to rsquo;rsquo;lowrsquo;rsquo; lying levels. Using the calculated cross sections of Michels and Wright lsqb;J. Chem. Phys. 69, 5151 (1978)rsqb; for each vibrational level, a vibrational population is derived from the observed cross section versus wavelength plot. A comparison of the vibrational population derived from the low and high pressure results confirms that collisional relaxation is occurring, but that at 0.1 Torr the distribution in the ion source is still rsquo;rsquo;hotterrsquo;rsquo; than a 300deg;thinsp;K Boltzmann distribution.
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