Interface kinetics during pulsed laser ablation

This work investigates evaporation kinetics — the relation between the surface temperature and pressure during excimer laser ablation. Nickel targets are ablated by excimer laser pulses in a laser fluence range between 1 and 6 J/cm2, with the upper limit exceeding the threshold of phase explosion (5 J/cm2). The surface pressure is determined with a polyvinylidene fluoride (PVDF) piezoelectric transducer. When phase explosion occurs, the surface temperature is known to be near the thermodynamic critical temperature, therefore, by measuring the surface pressure, the surface temperature-pressure relation is determined at the threshold fluence of phase explosion. The surface temperature and the threshold fluence of phase explosion are also estimated from the measured velocity of the vapor plume and gas dynamics calculations. It is shown that, during excimer laser ablation, the temperature and pressure relation deviates significantly from the equilibrium kinetic relation. PACS: 79.20.Ds; 68.10.Jy; 68.35.Rh This work investigates the kinetics at the evaporating surface during excimer laser ablation. The relation between the transient surface temperature and the transient surface pressure, which determines the evaporation mechanism and the evaporation rate, is of prime interest. It is known that during laser ablation superheating in the liquid could occur. When the liquid is superheated to the limit of thermodynamic stability, the spinodal point, an explosive type of phase change termed “phase explosion” or “explosive phase change” occurs [1]. The explosive phase change has been observed experimentally during nanosecond pulsed excimer laser ablation of nickel at laser fluences higher than 5 J/cm2 [2]. Using a numerical procedure, it is also shown that at laser fluences higher than 5 J/cm2 the surface temperature of nickel reaches the spinodal point [3]. It has also been suggested that during the evaporation process induced by a pulsed excimer laser, the surface ∗Corresponding author. COLA’99 – 5th International Conference on Laser Ablation, July 19–23, 1999 in Göttingen, Germany temperature-pressure relation does not follow the binode or the Clausius–Clapeyron relation. However, to date there has been no report in the literature of experimental studies of the actual surface temperature-pressure relation during laser ablation. It has been common practice to use the equilibrium Clausius–Clapeyron relation to compute the surface pressure and the evaporation rate. The present work is intended to determine the deviation of the surface temperature-pressure relation from the Clausius–Clapeyron relation experimentally. The surface pressure is measured with the use of a thin polyvinylidene fluoride (PVDF) transducer (K-tech Co., Albuquerque, NM). The surface temperature is known to be near the spinodal temperature (∼ 0.9 Tc, where Tc is the thermodynamic critical temperature) when phase explosion occurs. However, measurements of the surface temperature are not attempted since the plasma plume produced by laser ablation disrupts any information coming out from the surface. Instead, the surface temperature is estimated from a gas dynamics calculation using the measured velocity of laser-evaporated vapor as the input parameter. Using these approaches, the surface temperature-pressure relation is reconciled. The experiments are performed in a laser fluence range between 1 and 6 J/cm2, with the lower end near the evaporation threshold and the upper end exceeding the threshold of phase explosion. 1 Pressure measurement with the PVDF transducer The PVDF transducer is a well-established technique to study pressure in solids. There are also a number of studies on using the PVDF transducer to measure stress waves in solids produced by laser irradiation [4-6]. The PVDF transducer offers many advantages in stress measurements, including fast time resolution (∼ 1 ns), large measurement range (0.1 bar to 10 Mbar), and large signal output. In addition, PVDF transducer signals are highly reproducible under recurrent shock loading, which is often lacking in conventional piezoelectric shock sensors. The properties and responses of PVDF transducers to pressure have been well characterized [7]. Under shock loading, the PVDF transducer delivers a voltage, V(t), proportional to the stress difference between the two surfaces