EXCIMER-LASER PRODUCED PLASMAS IN COPPER WIRE TARGETS AND WATER DROPLETS

Elastically scattered incident radiation (ESIR) from a copper wire tar get illuminated by a KrF laser pulse at lambda=248 nm shows a distinct two-peak structure which is dependent on the incident energy. The tim e required to reach the critical electron density (n(c) approximate to 1.8 X 10(22) electrons/cm(3)) is estimated at 11 ns based on experime ntal results. Detailed ESIR characteristics for water have been report ed previously by the authors. Initiation of the broadband emission for copper plasma begins at 6.5 +/- 1.45 ns after the arrival of the lase r pulse. However, the broadband emission occurs at 11 +/- 0.36 ns for water. For a diatomic substance such as water, the electron energy rap idly dissipates due to dissociation of water molecules, which is absen ce in a monatomic species such as copper. When the energy falls below the excitation energy of the lowest electron state for water, it becom es a subexcitation electron. Lifetimes of the subexcited electrons to the vibrational states are estimated to be of the order of 10(-9) s. I n addition, the ionization potential of copper (440-530 nm) is approxi mately 6 eV, which is about two times smaller than the 13 eV ionizatio n potential reported for water. The higher ionization potential contri butes to the longer observed delay time for plasma formation in water. After initiation, a longer time is required for copper plasma to reac h its peak value. This time delay in reaching the maximum intensity is attributed to the energy loss during the interband transition in copp er.