Dissociation dynamics of the water dication following one-photon double ionization. I. Theory

Abstract

The measurement of the triple differential cross section in the body frame for double photoionization of a molecule can be made in principle by detecting the ionic fragments and the two photoelectrons in coincidence-but only if the dynamics and geometry of dissociation of the doubly charged molecular ion are known. A classical trajectory study of the nine lowest states of the water dication is presented using high quality ab initio potential energy surfaces. Sampling from a semiclassical initial distribution of positions and momenta is used to approximate ionization from the Frank-Condon region of the ground vibrational state of neutral H2O. Excellent agreement in comparison with preliminary experimental momentum imaging measurements of double photoionization of water show that eight dication states can be unambiguously identified in the experiment with the aid of theory. The theoretical trajectory results allow body frame measurements of double photoionization to yield all eight states even though the usual assumption of direct dissociation, the "axial recoil" approximation, breaks down for three of the dication electronic states seen in the experiment.

Dissociation dynamics of the water dication following one-photon double ionization. II. Experiment

“…These features correspond to the lowest three energy terms of the neutral oxygen fragment: 3 P , 1 D, and 1 S, the asymptotic limits of the dissociating dication potential energy surfaces leading to H + + H + + O and discussed in great detail by Gervais et al [37] and in Ref. [1]. These features are labeled in Fig.…”

“…These new diagrams are introduced to identify and isolate, with the help of theory, all eight dication states relevant in this PDI experiment. We mention the unambiguous failure of the axialrecoil approximation, discovered and discussed in detail in the theoretical companion paper by Streeter [1]. In Section V we will conclude the investigation with a summary and outlook towards possible kinematically complete experiments on water molecules in the future.…”

“…Gervais et al [37], and more recently Ref. [1], inform us that the PDI data is likely a combination of primarily the 3 A 2 , 1 1 A 1 , 1 A 2 , and 2 1 A 1 dication states. However, these states cannot be separated in the current data set.…”

“…Knowledge of the water dication potential surfaces and their asymptotes from Ref. [1] leads us to conclude that the features in Fig. 10 must correspond to the various dication states populated by the PDI.…”

“…Of these five chan-nels only two have been observed in our PDI experiment, namely one two-body (2) and one threebody breakup (4). Channel (1) has been observed in proton-water collisions [29,30]. Experiments of PDI below the vertical ionization threshold of 39 eV have observed channel (3) [31], while electron ionization experiments have recorded fragments from all five channels [32].…”