The inner-shell ionization and fragmentation of selenophene at 120 eV

Abstract

The inner-shell ionization of selenophene at 120 eV produces a rich array of fragmentation dynamics, including many originating from Auger-Meitner processes. In this report, three-dimensional velocity-map imaging and covariance analysis were used to identify and characterize over 50 distinct selenophene fragmentation channels. The majority resulted in two or three 'heavy' products containing selenium or carbon, many of which had identical mass-to-charge ratios but different chemical compositions due to the degree of hydrogenation and the selenium isotope involved. Covariance analysis was used to isolate these reaction channels and to provide estimates of their relative yields. In combination with prior similar studies on thiophene and furan, the current results indicate that the nature of the heteroatom significantly influences the charge redistribution and bond cleavage dynamics induced by the Auger-Meitner process, and demonstrate the sensitivity of inner-shell ionization dynamics to the molecular and electronic structures of heterocyclic systems.

Fig. 1

(a) The mass-to-charge (m/z) spectrum of selenophene following ionization at 120 eV. The grey regions highlight the ranges of ions that contain the same numbers of carbon and selenium atoms. (b) Covariance mapping was used to identify pairs of correlated ion species. The covariance map was calculated using ten contingent covariance subsets, as described in the Methods section. The geometry of the neutral parent molecule is given in the top right. Note the nonlinear m/z axes, which are plotted linearly in time-of-flight, and that the minimum m/z shown is five, as no meaningful covariance was seen below this limit due to scattered light signals. Features corresponding to the major groups of two- and many-body fragmentation channels are indicated by white and cyan boxes, respectively. The regions enclosed by the white boxes are given in greater detail in panels (c-f).

Fig. 2

(a) The recoil-frame covariance map of the fragment momentum distribution of (m/z = 93 u) with respect to . The momenta (p) are given in atomic units (a.u.). (b) The summed relative intensities of fragmentation channels producing and all co-fragments incorporating the different selenium isotopes, as described in Table 1. The error bars correspond to uncertainty values obtained from an adapted bootstrapping method outlined in the Methods section. Red dots indicate the natural selenium isotope abundances.

Fig. 3

Selenophene illustrations showing the C-Se and/or C-C bonds required to break to form the products of two-body fragmentation channels (iii), (v), and (vi) in Table 2.

Fig. 4

(a) and (b) are the Newton-frame covariance maps of the momenta of and relative to (with 0 or 2), calculated without momentum constraints. In each case, the momentum corresponding to the remainder of the molecule (‘/’ or ‘/CH’ assuming a three-body fragmentation process) is plotted in the bottom half of the image. Momentum constraints, as described in the main text, were applied to extract the components of the overall distributions that correspond to different primary fragmentation mechanisms; (c) and (d) are the distributions obtained for mechanisms where is a primary product (), while (e) and (f) are the distributions obtained when it is a secondary product (). The Newton-frame covariance maps are all normalized to their own maxima, and a Gaussian blur has been added to mitigate the low signal-to-noise ratios of these channels.

Fig. 5

Experimental and simulated Newton-frame covariance maps of correlated and ( 0 or 2) momenta. The recoil distributions of secondary relative to primary are given in (a) and (b), and vice versa in (c) and (d). The recoil distributions of primary relative to secondary are given in (e) and (f), and vice versa in (g) and (h). Each Newton-frame covariance map is normalized to its own maximum, and a Gaussian blur has been added to mitigate the low signal-to-noise ratios of these channels.