Time-resolved Thermometry in a Condensing Laser-ablated Copper Plasma Plume by Doppler-resolved Laser Induced Fluorescence

Abstract

Doppler-resolved laser-induced fluorescence (LIF) excitation scans of the Cu atom ground state are used for thermometry in laser-ablated plasma plumes. The resulting LIF line shape is analyzed by fitting Voigt profiles to determine the Doppler width of the transition which then yields directly, translational temperature. Temperature is an extremely important parameter in determining the rate and extent of condensation occurring in metal vapor plumes such as the copper plumes which we have been studying. The other seminal controlling parameter, density, has been determined using a combination of hook spectroscopy and planar laser-induced fluorescence (PLIF) as described in several preceding papers1,2 and a newer, more extensive study which is to be published3. In this work, the plume is produced by excimer laser bombardment of an OFC copper target rotating in a vacuum chamber (308 nm, > 20 J/cm2, 1-5 GW/cm2). The copper plasma plume expands rapidly into a helium or argon background gas at pressures of 1 and 10 torr. Scans with 25 torr of background gas yield no useful data as a result of various broadening mechanisms which make fitting unique Voigt profiles difficult. We find that plumes expanding into argon are kinetically hotter and cool more slowly than those in helium. For example, temperatures in 1 torr of helium and delay times between the ablation and probe pulses of 0.5, 1.0 and 3.0 msec are 1800 ± 250 K, 1600 ± 200 K, and 1300 ± 150 K , while temperatures in 1.0 torr of argon for identical delays are 3900 ± 700 K, 3000 ±350 K, and 2600 ± 250 K. In 10 torr of helium, the temperatures are 300 ± 150 K, 300 ± 300 K, and 300 ± 300 K for delays of 0.2, 0.5, and 1.0 msec; whereas temperatures for the identical delay times in argon background gas at 10 torr are 2000 ± 350 K, 1600 ± 200 K, and 1000 ± 100 K. This result helps to explain our earlier observation that the rate of disappearance of Cu atom due to condensation reactions in these plumes is faster in helium than in argon as well as the more general observation that forming clusters and particulate in argon is not as easy as in helium3,4. Physically, this likely results from the higher thermal conductivity of helium relative to argon making helium better suited at removing the excess heat from the plume.