Intensity of Photoelectron Bands

In addition to energy analysis, the relative intensities of the bands themselves in a photoelectron spectrum also offer important information of the molecular orbital, which is needed in order to be able to analyse the photoelectron spectra of molecules whose ionic states have not already been identified. The area under a photoelectron band in a spectrum is approximately proportional to the relative probability of ionisation to the different ionic states. For a single-electron model using polarised incident light the intensity of ejection will be

where s_T(E_kin) is the total cross section for ionisation of an electron with photoelectron kinetic energy E_kin and J is the angle between the direction of polarisation and direction of the outgoing photoelectron. If the radiation is unpolarized, as is normally the situation for the photon sources employed in ESCA, then the expression becomes

where J = Y + q is the angle between the incident photon beam and the direction of the ejected photoelectron. The parameter b depends on the nature of the given molecular orbital and it provides information of a preferred forward or backward direction of the photoejected electrons. In addition, there are experimental factors that must also be considered such as electron energy analyser design and ionisation cross sections.