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 u nderlying simplified mathematical model describes the source as well a s the absorber as Doppler-broadened line profiles with characteristic temperatures. The experimental apparatus consists of a Lyman-alpha sou rce (a DC glow discharge of hydrogen diluted in helium), the system un der investigation (absorber) and a simple VUV spectrometer, which is n ot able to resolve the line profile. The three parts are separated by MgF2 windows or lenses, which are fairly transparent to Lyman-a radiat ion. By knowing the gas kinetic temperature of the lamp as well as tha t of the absorber, atomic hydrogen densities can be determined from th e absorption measurements. In order to check the reliability of the me thod, a special furnace was used to ensure a well-defined absorber. In vestigations clearly demonstrated that absolute hydrogen atom densitie s can be determined with an accuracy better than +/-50%, which is suff iciently 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 ra nge.