AC Series photoelectron yield spectrometers in air incorporate an open counter invented by Dr. Masayuki Uda at RIKEN that counts photoelectrons in air. Photoemission yield spectroscopy in air encompasses a wide range of applications, including analysis of the physical quantities such as work function and ionization potential indicating electronic states; the extremely thin nanometer order thickness of films formed on substance surfaces; and the detection of single-molecule films adsorbed on surfaces.