Available at: http://publications.ictp.it IC/2008/079 United Nations Educational, Scientific and Cultural Organization and International Atomic Energy Agency STUDYING THE ROLE OF THRESHOLD INTENSITY DEPENDENCE ON VIBRATIONAL LOSSES IN THE BREAKDOWN OF MOLECULAR NITROGEN BY WIDE RANGE OF LASER WAVELENGTHS Yosr EE-D Gamal 1 National Institute of Laser Enhanced Science, Cairo University, El-Giza, Egypt and The Abdus Salam International Centre for Theoretical Physics, Trieste, Italy. Abstract As an extension of our previous paper (Gamal et al. 1999), in this study a numerical investigation of the actual contribution of vibrational losses on the threshold irradiance during the breakdown of molecular nitrogen is presented. The calculations are carried out adopting the previously developed electron cascade model of Evans and Gamal (1980) under the experimental conditions of Davies et al. (1991). In this experiment measurements are undertaken to determine the threshold irradiance when molecular nitrogen over a pressure range (50- 760 torr) is irradiated by a Nd:YAG laser source operating at the first four harmonics of wavelengths 1064, 532, 355 and 266 nm with pulse duration 8.5, 7.5, 6.5 and 5.5 ns, respectively. To get an idea about the effect of gas pressure, in the present study computations are carried out at two values of the gas pressure namely: 400 torr and 760 torr. The results of calculations revealed that at a gas pressure of 400 torr vibrational losses act noticeably on the threshold irradiance for the laser wavelengths 355 and 532nm. While at atmospheric pressure its effect is pronounced only at 355 nm. On the contrary, no observable effect on the threshold irradiance due to this loss process is noticed at 1064 nm. For the laser wavelength 266 nm the calculations of the breakdown irradiance showed an unexpected behavior of the vibration excitation where this process affect the threshold irradiance as a gain process rather than loss process for the two gas pressure values considered in this analysis. MIRAMARE – TRIESTE December 2008 1 Senior Associate of ICTP. gamaly@yahoo.com