Temperature and population measurements of n = 2 hydrogen atoms in H2RF discharges from Hα (Balmer) spectral profiles obtained by intracavity laser spectroscopy Original Research Article

Spatially resolved Hα (Balmer) absorption profiles are obtained using intracavity laser spectroscopy for 13.56 MHz RF discharges of hydrogen gas at pressures from 25 mTorr to 1.0 Torr. At the high spectral resolution employed, fine structure is observed in the profiles which originate from the three closely spaced levels of n = 2 atomic hydrogen (H∗). The profiles indicate that the H∗ temperature changes significantly over the pressure range. In the model developed to explain the profiles, two groups of H∗ are indicated. The sub-states for one H∗ group are at thermodynamic equilibrium with the walls of the container (≈ 320 K), whereas the sub-states for the other H∗ group are at higher temperatures (500 to 2500 K) and are not statistically distributed. For the latter H∗ atoms, the computed temperatures decrease with increasing H2 pressure. For the pressures and electron densities (≈109–1o10 cm−3) employed, only Doppler broadening needs to be considered in explaining the observed profiles.