Angular Distribution of the Photoelectron Spectra for Ar, Kr, Xe, H2, N2, and CO

The distribution of photoelectrons as a function of the angle Θ between the direction of the incoming photon and outgoing photoelectron has been measured for Ar, Kr, Xe, H₂, N₂, and CO. The experiments have been carried out with a double focusing electrostatic electron spectrometer to which has been attached a chamber containing a gas discharge lamp that can be freely rotated. The HeI (584 Å) and NeI (736, 744 Å) resonance lines were used as photon sources. For each of the photoionization bands studied the angular distribution could be fitted to the general shape predicted from theory: $\text{1+β/2\hspace{0.17em}(3/2\hspace{0.17em}}{\sin }^2\text{θ −1)}$ . In addition, the values of the parameter β experimentally determined were in good agreement with theoretically determined values for H₂ and for the rare gases but only partially satisfactory agreement for the first ionization potential of N₂. The II states of N₂⁺ and CO⁺ give lower values of β than the Σ state according to expectations. In general the vibrational structure for a given band was independent of Θ, but several exceptions were found for both N₂ and CO, which have been discussed in terms of the presence of autoionization states and possible breakdown in the Born—Oppenheimer approximation.