A previously developed electron cascade model is extended and used to examine the laser wavelength-dependence on the threshold intensity in laser-induced electrical breakdown of molecular gases. The analysis is based on numerical solution of the time-dependent Boltzmann equation and a set of rate equations describing the population of the involved excited states. The calculations are restricted to molecular hydrogen at atmospheric pressure irradiated with tunable laser radiation ( lambda =720-800 nm) with a pulse duration (FWHM) of 18 ns. The computed threshold intensities are compared with those experimentally measured. The calculated electron energy distribution function and its parameters provide an important correlation between the laser wavelength and the relevant physical processes involved during the interaction.

Studies of laser-induced breakdown in molecular nitrogen were carried out to investigate the dependence of the threshold irradiance on the wavelength at various pressures. The analysis was based on the numerical solution of the time-dependent Boltzmann equation for the electron energy distribution function (EEDF) and a set of rate equations describing the rate of change of the excited states population. The rate coefficients and cross-sections as functions of the electron energy were introduced into this analysis in order to probe the exact contribution of each physical process to the breakdown phenomenon. The calculations were performed under the experimental conditions of Davis et al. In this experiment the breakdown of nitrogen was measured at wavelengths of 1064, 532, 355 and 266 nm, over gas pressures in the range 25-760 Torr, with laser irradiances in the range 2 × 10 10 to 3 × 10 11 W cm −2 . The computed thresholds were found to be in good agreement with the measured ones at all wavelengths. The calculated EEDF and its parameters showed that, at λ = 355 nm, vibrational losses are dominant. Collisional ionization of ground and excited state molecules was found to make a minor contribution to the breakdown phenomenon at 532, 355 and 266 nm. However, the contribution to this process at 1064 nm was more effective. Therefore, the breakdown phenomenon proceeds via an electron-cascade process that converts the molecules only into the excited states, whence multiphoton ionization plays its role.

Our aim was (1) to detect the presence of fibrosis by Cardiac magnetic resonance imaging (CMR) in the pediatric age group. (2) Correlate CMR findings with demographic data, LV function, and other echocardiographic parameters. We studied 40 pediatric patients diagnosed as HCM by echocardiography. All patients were subjected to clinical examination (in which the NYHA classification was determined for each patient), echocardiography, and CMR. CMR was done on a 1.5T Philips Achieva scanner in SSFP with delayed myocardial enhancement (DE-MRI). All demographic and functional parameters as well as pressure gradient across left ventricular outflow tract (LVOT) were correlated with the percentage of myocardial enhancement. We studied 13 female and 27 male patients from 45 days up to 18 years. The mean percentage of DE-MRI was 9.7 ± 9%. We found significant correlation between the NYHA classification and the pressure gradient across the LVOT (P = < 0.001) as well as the percentage of DE-MRI (P = 0.004). The percentage of DE-MRI showed positive correlation with LV myocardial mass index (P = 0.042). It didn't correlate with any other demographic or LV functional cardiac parameters. A good positive correlation was detected between the percentage of DE-MRI and the severity of pressure gradient across LVOT measured by echocardiography (r = 0.69 and P = <0.001). We found a significant correlation between the percentage of DE-MRI in children with HCM and the pressure gradient across LVOT, NYHA classification, and LV myocardial mass. This may help in the further management of those patients, planning for follow-up, and prognosis of the disease.

A theoretical investigation of the physical processes associated with the interaction of ultraviolet pulses of laser radiation with argon gas is presented. Analyses are based on an extended electron cascade model which is applied to two sets of experimental conditions, in which argon, in the pressure range of 200-10000 Torr, is illuminated by laser pulses of durations 15 and 500 ns at a wavelength of 0.35 mu m. The predicted threshold intensities as a function of gas pressure showed reasonable agreement with the two sets of experimental data. The strong dependence of the threshold intensity on the gas pressure indicated that ionization growth proceeds mainly via collisional processes. The calculated energy distribution function and its parameters (namely electron density and electron mean energy) revealed information about the importance of loss processes in the breakdown phenomenon. The results of the calculations also provided a satisfactory explanation for the observed reduction of threshold intensity when ultraviolet laser radiation induced breakdown in argon gas.

This article reports on the laser action of pyrromethene 597 and pyrromethene 567 doped in the novel solid polymeric matrix glycidyl methacrylate. Mirrorless lasing was observed when samples with diameters of 10 mm and lengths of 20 mm, with different dyes concentrations, were transversely pumped at 532 nm. The influence of the dye concentration on the laser measurements included the peak wavelengths, output energies, conversion efficiencies, and laser gains. Relatively high lasing efficiencies (up to $ 60%) were obtained from these new polymeric materials with very good photostability, where a decrease of only 65% in the initial laser output energy was observed after pumping by 60,000 shots of 15 mJ at 10 Hz.