Super Hot Hydrogen Atoms in Microwave Plasmas

``Super hot'' (with kinetic energy in the range 4 - 8 eV) and ``hot'' (kinetic energy $\sim $ 0.3 eV) hydrogen atoms were detected throughout the volume of a surface wave (500 MHz) generated H$_{2}$ plasma column, at pressure p = 0.01 mbar, from the analysis of the H$_{\beta }$, H$_{\gamma }$, H$_{\delta }$and H$_{\varepsilon }$ emission line profiles. The profiles were found to evolve from single Gaussian to bi-Gaussian towards the column end. Population inversion between the levels 5$\to $4 and 6$\to $4 was detected from the measured relative intensities of transitions within the Balmer series. The Doppler temperatures corresponding to the H$_{\beta }$, H$_{\gamma }$, H$_{\delta }$, H$_{\varepsilon }$ line broadening are much higher than the rotational temperature, as measured from the hydrogen molecular Fulcher-$\alpha $ band (350 -- 500 K), and than the wall temperature (300 -- 450 K). At pressure p = 0.2 mbar, ``super hot'' atoms were not detected while ``hot'' atoms are present. It has also been found that the kinetic temperature of excited H ($n$ = 4 -7) atoms, as determined from the fitting of the spectral lines with a single Gaussian profile, increases with the upper level principal quantum number. These experimental results are analyzed in the framework of a kinetic model, which accounts for the generation of three groups of atoms.