A new type of a ZEKE (zero kinetic energy electrons) spectrometer was developed for improved detection efficiency and energy resolution. A typical resolution of 2 - 3 cm-1 (approximately 0.3 meV) was easily achieved at signal levels of 10(3) electrons/pulse. This spectrometer was used to interrogate the dynamics of excited Rydberg state population in mercury. The dynamics of the Rydberg state population was found to be governed by collisions with the background gas atoms.