  We observed radio recombination lines (RRLs) toward the W51 molecular cloud
complex, one of the most active star forming regions in our Galaxy. The UV
radiation from young massive stars ionizes gas surrounding them to produce HII
regions. Observations of the W51 IRS1 HII region were made with the Arecibo 305
m telescope. Of the full 1-10 GHz database, we have analyzed the observations
between 4.5 and 5 GHz here. The steps involved in the analysis were: a)
bandpass calibration using on-source/off-source observations; b) flux density
calibration; c) removing spectral baselines due to errors in bandpass
calibration and d) Gaussian fitting of the detected lines. We detected alpha,
beta and gamma transitions of hydrogen and alpha transitions of helium. We used
the observed line parameters to 1) measure the source velocity (56.6 $\pm$ 0.3
km s$^{-1}$) with respect to the Local Standard of Rest (LSR); 2) estimate the
electron temperature (8500 $\pm$ 1800 K) of the HII region and 3) derive the
emission measure (5.4 $\pm$ 2.7 $\times$ 10$^{6}$ pc cm$^{-6}$) of the ionized
gas.
