Three-body fragmentation in a new frame

A common problem in several subfields of physics is reconstructing the process by which a body—a molecule, nucleus, or high-energy particle—breaks apart into three or more fragments. Even when all fragments are detected and their velocities measured, it’s not always easy. Across fields, researchers distill and interpret the fragmentation’s essential features with a suite of powerful tools, including the Dalitz plot, a visualization of the kinetic energy carried by each fragment. But ambiguities can still remain.

Consider, for example, the breakup of triply ionized carbonyl sulfide, OCS³⁺, into O⁺, C⁺, and S⁺. Three fragmentation channels are possible: two that are sequential, in which either the O–C or the C–S bond breaks first, and one that is concerted, in which both bonds break at the same time. For each of the sequential channels, conservation of momentum during the first bond-fission step imposes a constraint on the atoms’ final velocities—and thus on the signal’s position on a Dalitz plot. But concerted fragmentation can yield velocities that mimic those of either sequential process, making it difficult to completely disentangle the three channels.

Now Itzik Ben-Itzhak, Brett Esry, and their colleagues at Kansas State University have devised a way to separate sequential from concerted breakup, even when their signals overlap. The key is to translate the system into the center-of-mass frame of one of the putative intermediates—CO²⁺ in this case—and to calculate θ_CO,S, the angle between the C⁺ and S⁺ velocities, as shown in the left panel. If the C–S bond really does break first and the CO²⁺ fragment rotates for a while before dissociating, then all values of θ_CO,S are equally likely. So when θ_CO,S is plotted against some independent quantity, such as the apparent kinetic energy released in the CO²⁺ dissociation, the sequential channel with the CO²⁺ intermediate appears as a vertical stripe, as shown in the right panel.

Apart from experimental artifacts at the top and bottom, the stripe is uniform, so one can easily deduce its width and intensity profile across all values of θ_CO,S to calculate how many events it comprises. The remainder of the data are still a mishmash of the concerted channel and the sequential channel in which the O–C bond breaks first. To separate them, the researchers repeat the process: Translate into the CS²⁺ frame and calculate θ_CS,O. (J. Rajput et al., Phys. Rev. Lett. 120, 103001, 2018, doi:10.1103/PhysRevLett.120.103001 .)