Ablation of CdTe with 100 μs pulses from Nd:YAG laser: Velocity distribution of emitted particles

Investigations of the velocity and the angular distributions of particles (Cd, Te, Te2) emitted into vacuum during the ablation of CdTe with pulsed Nd:YAG laser are presented. The laser pulse duration was 100 μs and the energy ranged from 0.16 to 0.28 J. Three kinds of targets were used in the experiments: target made of bulk crystal, target made of CdTe pressed powder, and target made of loose powder. Quadrupole mass spectrometer time-of-flight (TOF) measurements of the particle velocities revealed that their distributions are substantially narrowed. These effects are ascribed to strong collisional effects in the gas phase. The particle velocity distributions are well described by the shifted Maxwell-Boltzmann distribution. From fitting this distribution to the TOF experimental data we found that the centre-of-mass-velocity, u, is in the range of (220-600) m/s and the most probable velocity in the centre-of-mass system, v0 is in the range (140-240) m/s. The average velocity in the laboratory system depends both on the laser pulse energy and on the target preparation method. At lower pulse energies, the values of u decrease with the particle mass, but with an increase in energy, the values of u level. This levelling can be explained by a model of entrainment of heavy particles (Te2) by the stream of lighter particles (Cd, Te). The model is assumed to be applicable for a gas phase with a large number of particle collisions. The found angular dependence of the particle velocities is smaller than expected. It is apparently associated with the roughness of the target surface arising in the ablation process.