Photoelectron Spectroscopy

The photoelectron spectroscopy (PES) method is widely used for investigation of the basic and "rough" structure of molecules, particularly position of the potential energy levels corresponding to different molecular orbitals. In this method the wide spectral range of laser sources is usually more important than their narrow radiation bandwidth. Within the PES method tunable laser radiation is used for ionization molecular species $M$ in the reaction

$$M + h\nu \rightarrow M^+ + e^-$$ (73)

and the kinetic energy distribution of the produced photoelectrons $e^-$ is analyzed.

According to the energy conservation low the photoelectron kinetic energy $E_k$ can be obtained from the expression

$$E_{k} = h\nu + U_{orb} - E_i,$$ (74)

where $h \nu$ is a photon energy, $U_{orb}$ is the energy of the corresponding molecular orbital, and $E_i$ is the ionization potential of the molecule.

Usually, high energy UV photons are needed for ionizing molecules. Tuning the photon energy and determining the kinetic energy of the corresponding photoelectrons, one can calculate from eq. (74) $U_{orb}$ and thus obtain energy spectrum of molecular orbitals. The resolution of the PES method is usually not very high, because the electron kinetic energy cannot be analyzed with great accuracy. Therefore, one can usually determine electronic and vibrational structure of the molecular energy levels, but not rotational structure and other smaller details. In the far UV and X-ray spectral range, where tunable lasers are not available, the synchrotron radiation is found to be the best source for PES. As an example we consider the PES spectrum of $CO$ molecule in the UV spectral range. The fastest photoelectrons at 7 eV correspond to photoionization of the electrons from the higher occupied molecular orbital (HOMO) $3\sigma$. The lower energy photoelectron group at 3-5 eV corresponds to photoionization of the electrons from the first inner orbital $1\pi$ and the lowest energy electrons at 1-2 eV corresponds to to photoionization from the second inner orbital $2\sigma$. The vibrational structure of each orbital is also clearly seen.

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