Photoelectron spectroscopy (PES) is a technique based on the photoelectric effect. In PES, monochromatic light is directed onto a sample, causing electrons to be emitted. The difference between the energy of the photons and the kinetic energy of the electrons corresponds to the ionization energy. Suppose that a PES experiment is performed, in which certain gas X is irradiated with ultraviolet light of photon energy 12.1 eV. The kinetic energy of the emitted electrons is measured to be 4.8 eV. What is the first ionization energy of X?

Photoelectron spectroscopy (PES) is a technique based on the photoelectric effect. In PES, monochromatic light is directed onto a sample, causing electrons to be emitted. The difference between the energy of the photons and the kinetic energy of the electrons corresponds to the ionization energy. Suppose that a PES experiment is performed, in which certain gas X is irradiated with ultraviolet light of photon energy 12.1 eV. The kinetic energy of the emitted electrons is measured to be 4.8 eV. What is the first ionization energy of X?