Target charging during laser ablation of polyethylene

It is known that exposure of a target to a focused laser beam results in the occurrence of a time-varying current between the target and the grounded vacuum chamber. This current is composed by three distinct phases, namely, the ignition phase, in which the laser pulse drives the electron emission, while electrons coming from the ground through the target holder balance the positive charge generated on the target. The active phase appears at post-pulse times and it is characterized by the presence of peaked structures in the time-resolved current, representing characteristics of the target composition. Finally, the afterglow phase is determined by a current of electrons flowing from the target to the ground. In the active phase of target current resulting from polymers ablation with an UV KrF laser, negative target current peaks have been observed, whose origin is still unknown. We investigate the dependence of these current structures on the dimensions of the target, using ultra-high molecular weight polyethylene disks of different thickness. We provide evidence to explain the origin of such negative peaks. We found, indeed, that target initially charges positively under the action of the laser pulse, leading to a first negative target current peak. Then, a net charge unbalance is produced that further attracts free electrons on target surface. This behavior is enhanced if an opportune static electric field is imposed between the target and an opposite electrode.