DETERMINATION OF ABSOLUTE HYDROGEN-ATOM DENSITIES BY LYMAN-ALPHA ABSORPTION

We developed a method for the determination of absolute hydrogen atom densities based on the absorption of the hydrogen Lyman-a! line. The underlying simplified mathematical model describes the source as well as the absorber as Doppler-broadened line profiles with characteristic temperatures. The experimental apparatus consists of a Lyman-alpha source (a DC glow discharge of hydrogen diluted in helium), the system under investigation (absorber) and a simple VUV spectrometer, which is not able to resolve the line profile. The three parts are separated by MgF2 windows or lenses, which are fairly transparent to Lyman-a radiation. By knowing the gas kinetic temperature of the lamp as well as that of the absorber, atomic hydrogen densities can be determined from the absorption measurements. In order to check the reliability of the method, a special furnace was used to ensure a well-defined absorber. Investigations clearly demonstrated that absolute hydrogen atom densities can be determined with an accuracy better than +/-50%, which is sufficiently accurate for model calculations of the gas phase chemistry in our plasma-based diamond deposition process. However, at present such high accuracies can be obtained only within a restricted parameter range.