Fast excited state formation and decay in the pulse radiolysis of gaseous argon--iodine systems

Processes responsible for the formation and decay of excited states of I2 emitting in the 320--345 nm region in the pulse radiolysis of gaseous argon--iodine systems are investigated using an electron pulse of half-width less than 40 psec and an analyzing system with a time resolution slightly better than 1 nsec. The observed emission behavior is interpreted on the basis of a competition between light emission from vibrationally excited levels of an excited electronic state and vibrational deactivation of these levels by collision with argon. Vibrational deactivation rate constants in the range of 2 x 10-11 cm3 sec-1 are observed. The lifetime tau (=1/k) for the emission at the longer wavelength end of the emission region is 6.7 nsec; the pressure independence of this value indicates that it is the radiative lifetime. Possible processes responsible for the formation of the excited I2 are discussed. We conclude that an excited state of I2 is produced on a subnanosecond time scale via a direct interaction of energetic electrons with iodine molecules in their ground electronic state. Collisions of these excited I2 molecules with argon are necessary to reach the excited states of I2 which emit in the 320--345 nm region. The processes investigated in this study occur too rapidly to be connected with ion-recombination processes or collisional transfer of energy to I2 from excited argon atoms or Ar2 excited dimers