Journal of Physical Chemistry & Biophysics
Open Access
ISSN: 2161-0398
ISSN: 2161-0398
Yosr EE-D Gamal
Posters-Accepted Abstracts: J Phys Chem Biophys
A numerical study is carried out on the phenomenon of water breakdown induced by ultra-short pulses of laser radiation to investigate electron dynamic and mechanical processes associated with the plasma generation. The investigation is based on comparative study of the breakdown process applying the following models; the classical breakdown model (Raizer, 1966), the moving breakdown model (Docchio et al, 1988) and the femtosecond breakdown model (Fan and Longtin, 2001). The first model takes into account only the temporal variation of the laser intensity in the focal volume, while the second model considers the influence of its temporal and spatial variations. The third model accounts for the effect of laser pulse propagation in the focal volume. These models are based on a numerical solution of the rate equation that proposed to investigate the transient progression of plasma formation in water. The equation describes the temporal evolution of the electron density in the focal volume under the combined effect of both multi-photon ionization and cascade ionization processes as well as the effect of electron removal processes namely electron diffusion out of the focal volume and electron recombination (Gaabour et al, 2012). In this study, adopting the same assumption made in Kennedy (1995), the water is treated as amorphous semiconductor of an energy gap of 6.5 eV. On this basis the rate equation is solved numerically using the Rang-Kutta fourth order technique with adaptive time step control. This equation is incorporated into a computer program together with the cross-sections and rate constants corresponding to each of the physical processes considered in this analysis. Computations are performed under the experimental conditions given by Noack and Vogel (1999) to determine the threshold intensity of water breakdown and plasma formation under the effect of different laser wavelengths (532 nm, 800 nm, 1064 nm) with a wide range of pulse duration (10 fs up to 10 ns). The result of calculations showed a reasonable agreement between the calculated threshold intensities and the experimentally measured ones. Moreover, a comparison is under taken for the obtained results using these models to study the effect of laser parameters on the shape of the formed plasma, the role played by loss processes and the temporal and spatial variation of the electron density during laser irradiation in the focal volume. In addition, the results of computations also clarified the effect of pulse length and in turn its propagation length on the temporal variation of the electron density as well as the spatial distribution of both laser intensity and shape of the formed plasma in the focal volume. For completeness, computations are carried out to determine the dynamics of cavity bubble and shock wave generation in water plasma by 40 fs and 170 ps laser pulses of wavelength 800 nm under the experimental conditions given by El kamhwy (2006).In addition, a study is performed on the effect of pulse length on the plasma shielding since it is one of the important parameters which plays a principal role for laser medical application (i.e.) microsurgery in ophthalmology.