Valence photoelectron spectroscopy of N2 and CO: Recoil-induced rotational excitation, relative intensities, and atomic orbital composition of molecular orbitals

Abstract

Recoil-induced rotational excitation accompanying photoionization has been measured for the X, A, and B states of N(2)(+) and CO(+) over a range of photon energies from 60 to 900 eV. The mean recoil excitation increases linearly with the kinetic energy of the photoelectron, with slopes ranging from 0.73×10(-5) to 1.40×10(-5). These slopes are generally (but not completely) in accord with a simple model that treats the electrons as if they were emitted from isolated atoms. This treatment takes into account the atom from which the electron is emitted, the molecular-frame angular distribution of the electron, and the dependence of the photoelectron cross section on photon energy, on atomic identity, and on the type of atomic orbital from which the electron is ejected. These measurements thus provide a tool for investigating the atomic orbital composition of the molecular orbitals. Additional insight into this composition is obtained from the relative intensities of the various photolines in the spectrum and their variation with photon energy. Although there are some discrepancies between the predictions of the model and the observations, many of these can be understood qualitatively from a comparison of atomic and molecular wavefunctions. A quantum-mechanical treatment of recoil-induced excitation predicts an oscillatory variation with photon energy of the excitation. However, the predicted oscillations are small compared with the uncertainties in the data, and, as a result, the currently available results cannot provide confirmation of the quantum-mechanical theory.

Energy Transfer into Molecular Vibrations and Rotations by Recoil in Inner-Shell Photoemission

“…The accurate determination of both the vibrational profiles and the ionization energies is essential for extracting correct chemical information. However, these quantities are also affected by other phenomena such as photoelectron recoil [1][2][3][4][5] and diffraction [6][7][8], which have often not been properly accounted for. Furthermore, photoelectron diffraction and recoil effects can themselves be a valuable source of information about the molecular environment or be a source of novel features observed in photoelectron and even atomic Auger electron spectra [9].…”

“…Photoelectron recoil can lead to vibrational excitation, as has been observed in the carbon 1s ionization of CH 4 [1], graphite [10], and CF 4 [2], and rotational excitation, as seen in the valence ionization of N 2 [3,4]. However, no measurements have been reported in which both vibrational and rotational excitation have been observed and quantified simultaneously, and recoil-induced rotational excitation during core ionization has not been observed at all.…”

Vibrational branching ratios in the photoelectron spectra of N2 and CO: interference and diffraction effects

“…More recent theoretical investigations have addressed the features appearing in the photoelectron spectra at high energy, mostly in core photoionization. In particular, results for N 2 and CO obtained from these methods have led to the observation of interesting interference phenomena that are often difficult to identify in total photoelectron spectra, especially at high photon energies 4,11,61,64,69,[74][75][76][77][78] . Two of these phenomena, which lie at the heart of quantum mechanics, have been recently observed in vibrationally resolved photoelectron spectra from valence-shell orbitals 78 : (i) two-center coherent electron emission leading to Young's type double-slit interferences 79 and (ii) electron diffraction by the molecule's atomic centers.…”

Vibrational scattering anisotropy in O2—dynamics beyond the Born–Oppenheimer approximation

“…The combined experimental resolution, obtained from a least-squares curve fit to the typical measured electron spectra, was 96 ± 0.1 meV. The contributions to the combined broadening can be decomposed as a series of Gaussian broadenings from: the monochromator bandwidth about 30 meV at the photon energyhω = 539 eV; about 40 meV from the electron energy analyzer, and about 51 meV accounting for the well-established translational [56], and recently discussed rotational Doppler broadenings [50,57], as well as from other causes including unresolved rotational excitation or recoil-induced rotational broadening [58].…”