The results of the experimental determination and quantitative description of the energy and spectral-brightness characteristics of near surface laser plasma of ablating polymer targets in vacuum for near IR - VUV spectral range (hn~0.1-70 eV) are given. Photoelectric and bolometric methods are employed to record the radiation and convective fluxes, and the described experimental technology of double open and closed ionization chambers is applied (as radiation detectors which do not require any absolute energy calibration). In a wide range of control parameters of the effect of the radiation at standard laser frequencies (and II-V harmonics) on spatially restricted targets, new data are obtained on multifactor radiation-gasdynamic processes, which are necessary to determine the emission and mass-flow-rate characteristics of ablating targets.
The obtained frequency dependences of the spectral brightness and brightness temperatures of the plasma-surface zone of (CH2O)n, (CH2)n, and (C2F4)n targets of different shapes (flat, conical, hemispherical) are indicative of the significant effect of the geometry of the ablating target on the emission spectrum of laser plasma and on the short-wave radiation diagram. These data, along with the results of direct calorimetric measurements of heat fluxes performed using a metal bolometer with a bismuth thermal resistor arranged on the rear side of the target irradiation zone, enable one to estimate the contribution of the convective and radiation components to the balance of energy expenditure for ablation in view of reemission and to determine the dependence of the mass flow rate and of the dynamics of spectral brightness characteristics on the control parameters of the laser impact pulse and geometry of the irradiation zone.
These results provide the basis of the electronic bank of experimental data on the thermodynamic, optical, and transport properties of laser plasma, and are used to construct the spectral-energy and dimensional scaling analysis of the mass-flow-rate characteristics of ablating polymers in a wide range of control parameters for optical erosion discharges in vacuum conditions as VUV powerful (~10^6 - 10^7 W/cm^2) non-synchrotrone-type light source for EUV lithography. |