Femtosecond photoelectron spectroscopy of I 2 2 „ Ar ... n clusters „ n 5 6 , 9 , 12 , 16 , 20 ...

The photodissociation of I2 2 embedded in mass-selected I2 (Ar)n clusters (n56 – 20) was studied using femtosecond photoelectron spectroscopy. The Ã8←X̃ transition in the I22 chromophore was excited using a femtosecond pump pulse, and the subsequent dynamics were followed by photodetachment with a femtosecond probe pulse and measurement of the resulting photoelectron spectrum. In all clusters, dissociation of the I2 2 is complete by 300 fs. From 300 fs to 1 ps, the spectra yield the number of Ar atoms interacting with the I fragment. At later times, recombination of I2 2 occurs in I2 (Ar)n>12 on both the X̃ and à states. Analysis of the spectra yields the time scale for X̃ state vibrational relaxation and solvent evaporation. In I2 (Ar)20 , energy transfer from I2 2 to Ar atoms through vibrational relaxation is slightly faster than energy loss from the cluster through Ar evaporation, indicating the temporary storage of energy within Ar cluster modes. © 1999 American Institute of Physics. @S0021-9606~99!01245-3#