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 regi ons, of Rydberg-Rydberg transitions of hydrogen that cover a large range of Delta n-values in a single observing window. This, in turn, allows lines c overing a large range in principal quantum number n to be detected simultan eously with the same antenna beam. We have employed a new frequency-switchi ng technique which allows a very precise determination of the line widths. We have used this technique with the NRAO 140 foot telescope to observe lin es in W51 and Orion A near 6 GHz, with Delta n-values that vary by a factor of similar to 21 (Delta n = 1-similar to 21) and corresponding Delta n-val ues that vary by a factor of 2.7 (n = 102-274). By generating Voigt line pr ofiles using Griem's theory of impact broadening by electrons, inserting th em 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 i ncreases. For n less than or equal to 180, Delta n less than or equal to 6, our restored line widths and areas give densities of N-e = 2500 and 4000 c m(-3) in W51 and Orion A, respectively. These densities are higher than rep orted previously with a 5' beamwidth. For higher n-values we are unable to fit our data using Griem's theory. For n > 180, Delta n > 6, our telescope- measured line widths fall rapidly below predicted values, while the line ar eas simultaneously increase above predicted values. This behavior of the li ne 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 1'.7 be amwidth, require a density in excess of N-e = 20,000 cm(-3) to fit. Althoug h the detected lines cover a range in n and Delta n from 71 to 177 and 1 to 17, respectively, there is no evidence for a line width decrease at the hi gh Delta n-values. We conclude from this that the line narrowing seen at 6 GHz is related to the principal quantum number.