Double photoionization of propylene oxide: A coincidence study of the ejection of a pair of valence-shell electrons

Abstract

Propylene oxide, a favorite target of experimental and theoretical studies of circular dichroism, was recently discovered in interstellar space, further amplifying the attention to its role in the current debate on protobiological homochirality. In the present work, a photoelectron-photoion-photoion coincidence technique, using an ion-imaging detector and tunable synchrotron radiation in the 18.0-37.0 eV energy range, permits us (i) to observe six double ionization fragmentation channels, their relative yields being accounted for about two-thirds by the couple (CH, CHO) and one-fifth by (CH, CHO); (ii) to measure thresholds for their openings as a function of photon energy; and (iii) to unravel a pronounced bimodality for a kinetic-energy-released distribution, fingerprint of competitive non-adiabatic mechanisms.

Production and Characterization of Molecular Dications: Experimental and Theoretical Efforts

“…By this technique, we were able to produce and characterize several molecular dications such a CO 2 2+ [ 77 , 78 ], N 2 O 2+ [ 79 , 80 ], C 6 H 6 2+ [ 81 , 82 ], and C 2 H 2 2+ [ 83 , 84 ], and more recently C 3 H 6 O 2+ [ 85 , 86 ], C 2 H 5 NO 2+ [ 87 ] (the dications of methyloxirane and N-methylformamide, respectively), pointing out their important role in the upper atmosphere of planets of the Solar system. In all cases, we performed double photoionization experiments using the tunable linearly polarized synchrotron radiation (with photon energy resolution of about 1.5 meV) as it is available at the GasPhase and CiPo beamlines of Elettra measuring: (i) the threshold energy for the different open fragmentation channels; (ii) the relative cross sections; (iii) the KER distribution of fragment ions at different photon energies; (iv) the angular distributions of fragment final ions with the relative anisotropy parameter β [ 75 , 76 ].…”

Angular Distribution of Ion Products in the Double Photoionization of Propylene Oxide

“…Nowadays, and already for several decades, a wide number of experiments are carried out world-wide using circularly polarized tunable light sources of high intensity as synchrotron radiation and, in order to detect chirality in molecules, imaging photoelectron circular dichroism (Turchini et al, 2004; Stranges et al, 2005; Piancastelli et al, 2007; Alberti et al, 2008; Janssen and Powis, 2014). Moreover, techniques of molecular alignment with the pioneering work done by Zare et al (Sinha et al, 1974; Caldwell and Zare, 1977) and by B. Friedrich and D. R. Herschbach (Friedrich and Herschbach, 1991; Friedrich et al, 1991) have been found of great relevance to control the stereodynamics of elementary chemical processes occurring in the gas phase and at surfaces, and are arguably crucial in chirality issues (Falcinelli et al, 2018a). In particular, previous studies (Caldwell and Zare, 1977; Karny et al, 1978; Sanders and Anderson, 1984; Pullman et al, 1990; Aquilanti et al, 1994) indicate that molecular directionality and alignment should strongly amplify dichroism and provide stereodynamical mechanisms for discrimination of enantiomers, other than via circularly polarized light (Herwig et al, 2013; Pitzer et al, 2013; Tia et al, 2017).…”

“…In particular, previous studies (Caldwell and Zare, 1977; Karny et al, 1978; Sanders and Anderson, 1984; Pullman et al, 1990; Aquilanti et al, 1994) indicate that molecular directionality and alignment should strongly amplify dichroism and provide stereodynamical mechanisms for discrimination of enantiomers, other than via circularly polarized light (Herwig et al, 2013; Pitzer et al, 2013; Tia et al, 2017). Very recently, in our laboratory we are working, to couple a mechanical molecular velocity selector, specifically constructed ad hoc (able to control the velocity dependence of the molecular alignment) with a PEPIPICO (photo-electron-photo-ion-photo-ion coincidence) imaging device, in order to be used with a synchrotron radiation (Falcinelli et al, 2018a), delivering tunable intense photon sources with high degree of both linearly and circularly polarized light of both helicities. Double photoionization studies on propylene oxide enantiomeric molecules have been done by exploiting Auger spectroscopy (Piancastelli et al, 2007; Alberti et al, 2008).…”

“…In these experiments, using high photon energy synchrotron radiation (in the 289–380 eV energy range), the authors indirectly estimated the first double ionization threshold, allowing the observation of a weak circular dichroism effect (Alberti et al, 2008). In our recent experimental work performed at the Synchrotron Radiation Facility of Elettra (Trieste, Italy) we were able to identify six two body fragmentation channels produced by double photoionization of propylene oxide in the 18.0–37.0 eV photon energy range, including their relative threshold energies (Falcinelli et al, 2018a). Such two body fragmentation channels involve the direct ejection of two valence electrons, and allowed us to determine with more accuracy the first double photoionization energy of propylene oxide, obtaining the values of 28.3 ± 0.1 which is 0.9 eV below the previous indirect estimation of 29.2 eV by Auger spectroscopy (Piancastelli et al, 2007; Alberti et al, 2008).…”

“…This allows the formation of an intermediate molecular dication able to dissociate by Coulomb explosion, as we have discussed in our previous investigations (Alagia et al, 2004a, 2006a, 2007, 2010; Falcinelli et al, 2014, 2016a,b), even if a debate is still open on whether the multiply charged molecule obeys pure Coulomb explosion model during its breakup or not (Sheehy and DiMauro, 1996; Mathur, 2004). Preliminary data already published on the double photoionization of propylene oxide concern: (i) the threshold energy for the recorded six two-body dissociation channels producing C 2 H4+/CH 2 O + , C 2 H3+/CH 3 O + , CH2+/C 2 H 4 O + , CH3+/C 2 H 3 O + , O + /C 3 H6+, and OH + /C 3 H5+ ion pairs; (ii) their relative cross sections as a function of the investigated photon energy; and (iii) the kinetic energy released (KER) distributions of each ion pair products as a function of the photon energy (Falcinelli et al, 2018a). In the present paper we report on the angular distributions of ion pair products formed in the two-body fragmentation reactions mentioned above.…”