Biography
Biography: A A Zaidi
Abstract
Solid methane at 77K was irradiated with laser pulses of wavelength 800 nm and energy of 275 μJ. Laser pulse duration was 100 fs. This irradiation experiment shows formation of higher molecular weight pure carbon clusters Cn where n = 1, 2, 3, ---, and the formation of alkane molecules of higher molecular mass than the original target molecule. These molecules have the composition (CH4)n where n = 1, 2, 3-- The time of flight spectrum also shows the formation of polyynes and cumulenes as ablation products. The laser ablation of graphite has been ex-tensively studied because of graphite’s unique properties. It is well known that the laser ablation of graphite with nanosecond pulses produces pure carbon clusters and fullerenes such as C50, C60 and C70 as well as more complex species including nanotubes, depending on ablation conditions. It is then important to investigate the ablation of molecular systems containing C-H sigma bonds using femtosecond laser pulses in order to see the effect of hydrogenation of the target molecule. The use of femto-second, rather than nanosecond laser pulses eliminates heating of the target. Alkanes are the simplest fully hydrogenated C-H molecular systems that occur naturally and methane (CH4), containing only C-H sigma bonds, is the simplest member of this family. A study of the ablation of solid methane is then important in understanding ablation in more complicated alkane systems. Experiments were carried out using a 5×10-7 torr vacuum chamber attached to a time of flight (TOF) spectrometer. By flowing methane gas onto a cold finger made of copper plate in the vacuum chamber, a 0.5 mm-thick film of solid methane (a van der Waals solid with a density of 0.522 gm cm-3) was obtained on the cold finger. Five to eight successive laser pulses chopped from a 100-fs 1-KHz 800nm 300-μJ mode-locked Ti:sapphire laser system were focused on the solid methane by a 10-cm focusing lens. Each focused laser pulse has a peak intensity 8×1014 W/cm2. A laser plume was generated which was extended from the cold finger surface up to a few millimeters above it. Positively charged particles generated in the laser plume were accelerated by a 4kV linear accelerator toward a micro-channel plate (MCP) detector. Signals from the MCP detector were recorded by a scope and sent to the PC.