When an atom X absorbs radiation with a photon energy than the ionization energy of the atom, the atom is ionized to generate an ion X^(+) and the electron (called a photoelectron) is ejected at the same time. In this event, the energy is conserved as shown in Figure 1, that is, Photon energy (h)=ionization energy (IE) of X+ kinetic energy of photoelectron. When a molecule, for example, H_(2), absorbs short-wavelength light, the photoelectron is ejected and an H_(2), ion with a variety of vibrational states is produced. A photoelectron spectrum is a plot of the number of photoelectrons as a function of the kinetic energy of the photoelectrons. Figure-2 shows a typical photoelectron spectrum when H_(2) in the lowest vibrational level is irradiated by monochromatic light of 21.2 eV. No photoelectrons are detected above 6.0 eV. (eV is a unit of energy and 1.0 eV is equal to 1.6xx10^(-19) J) Figure 1 Schematic diagram of photoelectron spectroscopy. Figure 2 Photoelectron spectrum of H_(2). The energy of the incident light is 21.2 eV ul("Calculate") the threshold energy E_(E) (eV) of the following dissociative reaction to the first decimal place: H_(2) rarr H^(**) (n=2) +H^(+)+e^(-) If you were unable to determine the values for E_(B) and E_(C), then use 15.0 eV and 5.0 eV for E_(B) and E_(C) respectively. When H_(2) absorbs monochromatic light of 21.2 eV, the following dissociation process occurs at the same time. H_(2) overset(21.2 eV)(rarr)H(n=1)+H(n=1) Two hydrogen atoms move in opposite direction with the same speed.