A new technique for measurement of subrotational lifetime of molecular ions

Abstract

Singly and multiply charged molecular ions are found in diverse environments and hold relevance for a wide range of research areas like combustion chemistry, accelerator physics, atmospheric sciences, plasma physics, astrophysics etc. Molecular dications are of special significance as they can be generated and studied comparatively easily in laboratory experiments. And they have enabled exploration of new and exciting phenomenon such as hydrogen migration, inter-atomic Coulombic decay, plasmonic excitations, orbital tomography etc. The lifetime of a molecular dication is one of its fundamental characteristics, whose measurement contributes to strengthening ab initio calculations and in predicting the concentration of its dissociation products. Most of the already reported lifetimes of molecular dications are in the range of nanoseconds to seconds and metastable states with lifetimes of the order of picoseconds have only been theoretical predicted and an experimental verification is pending. We present a method of measuring subrotational lifetimes of molecular dications formed in three-body sequential breakup of polyatomic molecular precursors. Specifically, we have measured the subrotational lifetime of $$\hbox {SO}^{2+}$$ SO 2 + , which is formed as an intermediate in the three-body sequential fragmentation of $$\hbox {SO}_2^{3+}$$ SO 2 3 + . The lifetime against dissociation is determined to be a fraction of the rotational period of $$\hbox {SO}^{2+}$$ SO 2 + and is of the order of few picoseconds. The method proposed is general and is not restricted to triatomic precursors.

Disentangling sequential and concerted fragmentations of molecular polycations with covariant native frame analysis

“…In such cases, under the assumption that rotation occurs within the plane of the diatomic intermediate, the angular distribution takes the form of an exponential decay, with a decay constant determined by the ratio of the lifetime and the rotational period of the intermediate. 28 In the limit of a very long lifetime relative to the rotational timescale, an isotropic angular distribution is observed. 27 In the case of a polyatomic metastable intermediate, however, the situation is more complex, with additional factors that can affect the angular distribution of ion emission within the native frame of the secondary dissociation.…”

“…The intermediate ion can be both rotationally and vibrationally excited by the initial dissociation and, if its lifetime exceeds its rotational period, it will cause the momenta of secondary products to be distributed over a uniform angular range relative to the momentum of the primary product. 27,28,48 The simulation assumes the lifetime of the intermediate greatly exceeds its rotational period, such that the relative angle can take any value. The result is the semicircular distributions seen in the Newton diagram which is a characteristic signature of a sequential three-body breakup.…”

“…In particular, the differences observed between 1-IP and 2-IP indicate the important role of rotation in the propyl dication intermediate, which may occur about multiple axes, in contrast to the simpler case of triatomic fragmentations in which rotation may be assumed to occur in the fragmentation plane. 27,28 Furthermore, evidence suggests that other nuclear motion -namely C-C stretching and bending -has a significant impact on the sequential photofragmentation dynamics. To further investigate these effects in future works, full theoretical trajectory calculations on ab initio potential energy surfaces would be desirable, albeit a significant theoretical challenge given the possible contributions from many electronic states of the polycation.…”

Investigation of the Carbon Monoxide Dication Lifetime Using (CO)$_2$ Dimer Fragmentation

“…The present technique would indeed be particularly well suited for the study of metastable states with lifetimes of the order of 100 fs, where a more precise lifetime estimate could be obtained. With dimer targets produced with low rotational energy and with a well known initial molecular orientation, this technique would also provide access to the measurement of any subrotational lifetime of molecular dications using the native frames representation, as recently performed in [14].…”

“…Another specificity of the molecular system investigated here is the initial quasi-isotropic angular distribution of the CO molecules within the dimer, which prevents the use of information carried through fragment angular correlations. With a well defined initial orientation of the molecules, subrotational lifetimes of the molecular ions could also be accessed more directly using the so-called native frames approach, as recently demonstrated for SO 2+ ions resulting from SO 3+ 2 ions fragmentation [14].…”