Electron Density and Temperature

Two vitally important parameters of the plasma are the electron temperature and electron density. In ISP laboratories we have studied the electron density and electron temperature of laser induced plasma from different target materials and also studied how the ambient gases affect these parameters. Measurements of line widths of stark broadened emission lines gives information about electron density while the line intensity ratios directly give information on electron temperature. The experimental set-up used for these measurements is similar to that used for temporal and spatial analysis of the plasma. But here we have extra provision to insert Langmuir probes into the plasma chamber. For studies involving Langmuir probes, the target is kept at 45⁰ to the laser beam propagation direction. The probe is positively biased through an external power supply and the transient probe current is monitored across a load resistance using a digital storage oscilloscope. Such a setup gives true pulse shapes of the plasma electron current. The oscilloscope is interfaced with a personal computer for data acquisition and analysis. 

We have shown that the electron density varies as z-1 and t-2 while the electron temperature shows a z-0.1 and t-2 type dependence where z is the distance from the target surface and t, the time after the termination of the laser pulse . Studies have also been carried out to determine the electron density and temperature in superconducting YBa2Cu3O7 plasma, graphite plasma and different metal plasmas. Laser interferometric techniques are also employed to measure electron densities and such measurements demonstrate plasma shielding at high laser intensities. Our studies revealed the effect of different ambient gases on the dynamics of laser ablated carbon plasma. The electron temperature and density show an abrupt change with the addition of ambient gases and these parameters also depend on the nature and composition of the gas used. It is noted that hotter and denser plasmas are formed in Ar atmosphere compared to He and air as a result of the difference in the efficiency of cascade-like growth of the electron number density and plasma absorption coefficient. The electron density exhibits a similar behavior irrespective of the background gas atmosphere. The inclusion of ambient atmosphere cools the hot electrons by collisions, leading to a more efficient electron impact excitation and plasma recombination, which lead to plasma confinement and in turn decrease of the electron density.

We have observed a twin peak distribution of the electron pulses occured during the interaction of infrared radiation from a pulsed Nd:YAG laser with a silver target. The first peak corresponds to laser heated electrons escaping from the interaction volume before the absorbed energy is transferred to the lattice. The second peak comprising of low energy electrons has a comparatively broader temporal distribution and corresponds to those in the silver plasma. These prompt electron pulses can effectively be used as an excellent short duration excitation source in atomic and molecular spectroscopy. Also by using even shorter duration pulsed lasers for irradiating metal surfaces one can get ultrashort electron pulses and this may act as a very effective electron source required for electron-vibration energy relaxation experiments.