Temporal and Spatial Studies

The plasma for the temporal and spatial analysis is generated by focusing 1.06 micrometer laser pulses from a Q-switched Nd:YAG laser. The repetition rate is 10 Hz and pulse width is ~10 ns. The target material is placed in a partially evacuated plasma chamber provided with quartz window. The laser beam is focused to the target material which is rotated to avoid non-uniform pitting of the surface. The bright plasma emission can be observed through a side window at 90^o with respect to the laser beam axis. The spectra from different regions of the plasma can be recorded by focusing the optics to required region of the plasma plume. 

In time resolved analysis, a part of the plasma plume is focused onto the slit of a monochromator coupled to a photomultiplier tube and a digital storage oscilloscope / box-car averager. In order to study the time evolution of a particular species produced by laser abalation, the characteristic lines are selected using the monochromator and the PMT output is fed to the digital storage oscilloscope. The amplitudes of the pulse shapes were monitored by a gated integrator and box-car averager. 

We have studied a variety of target materials like metals, insulators, semiconductors, polymers and high temperature superconductors. We have observed double peak structure in the temporal history of emission from electronically excited C2 molecules in laser induced plasma produced from a graphite target. Our work points to the existance of various mechanisms of the formation of C2 in the plasma. We have studied the influence of ambient gas on spatial and temporal features of emission from C2. Such studies have been carried out in an extensive manner by varying the laser energy under different ambient gas environments. The length of the plume in the laser induced plasma is maximum under a low pressure Helium ambient while it is considerably shortened in an Argon atmosphere. 

We have also studied the CN species in graphite plasma. Our investigations demonstrate that the emission intensities from CN species are sensitive to laser power density, pressure of the background gas, time after the elapse of laser pulse, and spatial separation from target. At low laser irradiance the emission bands due to C2 and CN predominate while at higher irradiance the multiple ionised species up to C IV have been observed along with CN and C2 species.

When YBCO superconducting material was used as target , the temporal profile of emission from Yttrium atoms and YO molecules yielded a twin peak structure while this was absent in the case of Y+ . This can be interpreted as due to the different formation mechanisms of the particular species. For example, excited YO molecules could be ablated directly form the surface of the target giving characteristic diatomic molecular emission spectrum from a certain section of the plasma plume. It is also possible that collision of Yttrium and Oxygen atoms can lead to formation of excited YO molecules in the region in front of the target and this could yield another YO emission peak. .