A New Technique for Measuring Impact‐broadened Radio Recombination Lines in Hii Regions: Confrontation with Theory at High Principal Quantum Numbers

New, low-noise receivers have allowed detection, in several giant H ii regions, of Rydberg-Rydberg transitions of hydrogen that cover a large range of Δn-values in a single observing window. This, in turn, allows lines covering a large range in principal quantum number n to be detected simultaneously with the same antenna beam. We have employed a new frequency-switching technique which allows a very precise determination of the line widths. We have used this technique with the NRAO 140 foot telescope to observe lines in W51 and Orion A near 6 GHz, with Δn-values that vary by a factor of ~21 (Δn = 1 – ~21) and corresponding n-values that vary by a factor of 2.7 (n = 102–274). By generating Voigt line profiles using Griem's theory of impact broadening by electrons, inserting them into a telescope data file, and processing them in a manner identical to that of the telescope data, we have been able to examine how the observing and reduction techniques affect both the line widths and line areas as n increases. For n ≤ 180,Δn ≤ 6, our restored line widths and areas give densities of Ne = 2500 and 4000 cm–3 in W51 and Orion A, respectively. These densities are higher than reported previously with a 5' beamwidth. For higher n-values we are unable to fit our data using Griem's theory. For n > 180, Δn > 6, our telescope-measured line widths fall rapidly below predicted values, while the line areas simultaneously increase above predicted values. This behavior of the line area as the line widths decrease is inconsistent with Griem's theory or an instrumental effect. Observations of Orion A at 17.6 GHz, with a 17 beamwidth, require a density in excess of Ne = 20,000 cm–3 to fit. Although the detected lines cover a range in n and Δn from 71 to 177 and 1 to 17, respectively, there is no evidence for a line width decrease at the high Δn-values. We conclude from this that the line narrowing seen at 6 GHz is related to the principal quantum number.