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Fragmentation of Small Molecules after Core Excitation and Core Ionization Studied by Negative-Ion/Positive-Ion Coincidence Experiments


We have studied the fragmentation of the methanol molecule after core excitation and core ionization by observing coincidences between negative and positive ions.
Kivimäki et al.; J. Phys. Chem. A  122 (2018) 224







Small molecules usually break into parts after the absorption of an X-ray photon. More than 99% of the states with a hole in a C 1s, N 1s or O 1s shell have been calculated to decay through electron emission. Subsequent dissociation mainly produces positive and neutral fragments owing to the positive charge of the molecular ion after normal and resonant Auger decay. Negative ions (or anions) have also been observed at the core edges, but there is very little information about their formation. We have studied the fragmentation of the methanol molecule after core excitation and core ionization by observing coincidences between negative and positive ions. 
Our experimental setup consists of two time-of-flight (TOF) spectrometers facing each other; one of the TOF spectrometers is used to detect positive ions, the other for negative ions. Electrons, which often interfere with negative ion detection, are deflected by a weak magnetic field created by permanent magnets, placed outside the vacuum chamber. Coincidences between negative particles (anions + residual electrons) and positive ions were recorded using a constant extraction field in the interaction region. The arrival times are analyzed afterwards, searching for arrival-time-differences (ATD) between the particles. This procedure allows for the possibility that a negative ion can arrive before or after the positive ions, depending on their masses. The analysis can thus yield also multiple negative-ion/positive-ion coincidences.
For methanol five different negative ions ― H-, C-, CH-, O-, and OH- - were observed both at the C 1s and O 1s edges. As negative ion formation occurs after resonant and normal Auger decay of core-hole states, it is necessarily linked with the release of positively charged fragments. Our data show that such fragmentation can happen in many different ways: We found approximately 30 negative-ion/positive-ion/positive-ion coincidence (NIPIPICO) channels. All involve only singly charged positive ions. Fragmentation channels leading to atomic ions are the most probable, but positive molecular ions are also frequently found in the context of anion formation. We could also verify the occurrence of four-ion coincidences, which involved one negative ion (H- or O-) and three positive ions.
The coincidence detection of negative and positive ions not only helps us to identify negative ions released by a given sample molecule, but also gives information on dissociation channels that produce these anions. The most intense NIPIPICO channels belong to the series O/H+/CHn+ (n =0−3), H/H+/CHn+ (n = 0−2), and H/H+/COHn+ (n = 0,1), where the intensities typically decrease when n increases. As an exception, the O/H+/CH3+ channel gains much intensity at the excitations of O 1s electrons to high-Rydberg orbitals.
 

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      Fragmentation of Methanol Molecules after Core Excitation and Core Ionization Studied by Negative-Ion/Positive-Ion Coincidence Experiments
Antti Kivimäki, Christian Stråhlman, Robert Richter, and Rami Sankari
J. Phys. Chem. A  122 (2018) 224
 
Last Updated on Thursday, 03 December 2020 15:24