Abstract
The angular distribution of O 1s photoelectrons emitted from uniaxially oriented methanol is studied experimentally and theoretically. We employed circularly polarized photons of an energy of hν = 550 eV for our investigations. We measured the three-dimensional photoelectron angular distributions of methanol, with the CH3–OH axis oriented in the polarization plane, by means of cold target recoil ion momentum spectroscopy. The experimental results are interpreted by single active electron calculations performed with the single center method. A comparative theoretical study of the respective molecular-frame angular distributions of O 1s photoelectrons of CO, performed for the same photoelectron kinetic energy and for a set of different internuclear distances, allows for disentangling the role of internuclear distance and the hydrogen atoms of methanol as compared to carbon monoxide.
Highlights
We investigate by calculations, which we benchmark against our experiment on methanol, how variations in the molecular structure—such as bond length changes and addition of hydrogen atoms—influence the MFPADs
We investigated angular distributions of O 1s photoelectrons emitted from uniaxially oriented methanol molecules
The molecular CH3 –OH axis has been fixed in the polarization plane of the circularly polarized ionizing radiation
Summary
In the emission of photoelectrons from molecules, the photoelectron’s angular emission distribution in the bodyfixed frame of the molecule is an exquisitely sensitive probe of molecular shape resonances [1,2,3,4,5,6], molecular structure [2, 7,8,9,10,11], localization of core holes [12,13,14,15,16], electron correlation [3, 17,18,19,20,21,22], multi-electron processes [17, 23,24,25,26], initial electronic state [27,28,29], nuclear dynamics [30, 31], and allows to probe Auger decay [32,33,34]. The second chosen route, which is routinely employed in synchrotronrelated studies, consists of an a posteriori measurement of the molecular orientation, and does not consist of an active spatial alignment of the molecule In order for this approach to be applicable, the molecule needs to dissociate during the ionization process. If the dissociation occurs rapidly, the emission direction of the ionic fragments corresponds (e.g. for a diatomic molecule) to the direction of the molecular bond at the instant of the ionization (an assumption which is known as the so-called ‘axial recoil approximation’ [42]) This approach is typically employed in cases, where several charges are created after the primary ionization by Auger decay and is the basis for Coulomb explosion imaging [43, 44]
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