Excimer laser produced plasmas in copper wire targets and water droplets

Abstract

Elastically scattered incident radiation (ESIR) from a copper wire target illuminated by a KrF laser pulse at λ=248 nm shows a distinct two-peak structure which is dependent on the incident energy. The time required to reach the critical electron density (nc ≊ 1.8× 1022 electrons/cm3) is estimated at 11 ns based on experimental results. Detailed ESIR characteristics for water have been reported previously by the authors. Initiation of the broadband emission for copper plasma begins at 6.5 ±1.45 ns after the arrival of the laser pulse. However, the broadband emission occurs at 11 ± 0.36 ns for water. For a diatomic substance such as water, the electron energy rapidly dissipates due to dissociation of water molecules, which is absence in a monatomic species such as copper. When the energy falls below the excitation energy of the lowest electron state for water, it becomes a subexcitation electron. Lifetimes of the subexcited electrons to the vibrational states are estimated to be of the order of 10−9 s. In addition, the ionization potential of copper (440–530 nm) is approximately 6 eV, which is about two times smaller than the 13 eV ionization potential reported for water. The higher ionization potential contributes to the longer observed delay time for plasma formation in water. After initiation, a longer time is required for copper plasma to reach its peak value. This time delay in reaching the maximum intensity is attributed to the energy loss during the interband transition in copper.