Coulomb explosion in femtosecond laser ablation of Si(111)

The ablation of ionized silicon is studied using near-infrared femtosecond laser pulses in the fluence regime from 1 to 9 J/cm2. Two major peaks are observed in the mass spectrum corresponding to Si+ and Si2+. In the time-of-flight transients of Si+, a bimodal structure is observed. The fast Si+ peak corresponds to a velocity half of that observed for Si2+. This momentum scaling is clearly indicative of a Coulomb explosion.