Stark Broadening of ${\mathrm{H}}_{\beta }$, ${\mathrm{H}}_{\gamma }$, and ${\mathrm{H}}_{\delta }$: An Experimental Study

Stark-broadened ${\mathrm{H}}_{\beta }$, ${\mathrm{H}}_{\gamma }$, and ${\mathrm{H}}_{\delta }$ line profiles from a spatially uniform plasma column which was formed in a critically damped discharge tube were recorded with a rapid-scan spectrometer. A plasma temperature of ∼2 eV was inferred from both ${\mathrm{H}}_{\beta }$- and ${\mathrm{H}}_{\gamma }$-to-continuum ratios. Values of the electron density were determined to an estimated accuracy of better than ±2% by means of two multiple-pass laser interferometers operating at infrared and visible wavelengths. Systematic errors in recording and analyzing the line profiles are estimated to be less than ±1.5%. For interferometric values of the electron density in the range (1.3-8.5)×${10}^{16}$ ${\mathrm{cm}}^{-3\mathrm{}}$, the measured half-intensity widths of all the recorded ${\mathrm{H}}_{\beta }$ profiles agreed to within ±2% of the widths predicted by Stark-broadening theory. In this comparison, the ±2% represents the maximum scatter of the measured half-intensity widths where the theoretical widths were determined using the interferometric values of electron density which were taken as the standard. The total systematic error in this comparison is estimated to be less than ±3.5%. For ${\mathrm{H}}_{\gamma }$ the measured half-intensity widths ranged from 11 to 25% larger than that predicted by theory for electron densities in the range (1.3-7.1)×${10}^{16}$ ${\mathrm{cm}}^{-3\mathrm{}}$. For ${\mathrm{H}}_{\delta }$ the measured half-intensity widths are 7% narrower than that predicted by theory for electron densities in the range (1.3-2.8)×${10}^{16}$ ${\mathrm{cm}}^{-3\mathrm{}}$. Because the ${\mathrm{H}}_{\beta }$, ${\mathrm{H}}_{\gamma }$, and ${\mathrm{H}}_{\delta }$ profiles were recorded from the same plasma, systematic errors in measuring the electron density will affect the comparison of theory and experiment for each line in the same sense. Thus the relative systematic errors between the comparison of all three lines with the theory are estimated to be less than ±1.5%.