Breakup dynamics and isotope effects in D 2 H + and H 2 D + dissociative recombination D. Strasser, 1 L. Lammich, 2 H. Kreckel, 2 M. Lange, 2 S. Krohn, 2 D. Schwalm, 2 A. Wolf, 2 and D. Zajfman 1,2 1 Department of Particle Physics, Weizmann Institute of Science, Rehovot, 76100, Israel 2 Max-Planck-Institut für Kernphysik, D-69029 Heidelberg, Germany (Received 17 February 2004; published 15 June 2004) The breakup dynamics of D 2 H + and H 2 D + following dissociative recombination with low energetic electrons has been studied combining two-dimensional imaging and storage ring techniques. The kinematical correlation between the hydrogen and deuterium atoms produced in the three-body channel was measured. We found that the three particles tend to dissociate with a geometry close to linear, and that the deuterium atom has a large probability to be at the center. The data also show that the remaining average internal excitation energy stored in the rotation of the D 2 H + and H 2 D + molecules corresponds to a temperature of less than 70 meV, much less than observed for the H 3 + and D 3 + species previously examined. DOI: 10.1103/PhysRevA.69.064702 PACS number(s): 34.80.Lx The triatomic hydrogen ion H 3 + is the dominant posi- tively charged molecular ion in all hydrogen containing plasma. Its destruction by dissociative recombination (DR) with free electrons, leading to neutral atomic and molecular fragments, H 3 + v, J + e - →  H1s +H1s +H1s or H 2 v', J' +H1s 1 thus plays a key role in understanding the reaction dynamics and the atomic and molecular composition of these ionized systems. In particular, the DR rate coefficient and its influ- ence on the abundance of H 3 + and its isotopomers is a criti- cal ingredient in astrochemical models of the cold interstellar medium [1], as these species play a dominant role in reac- tions which lead to molecule formation [2] and determine the deuteration fraction of interstellar molecules [3]. In view of its astophysical importance and its benchmark character in quantum chemistry, the DR of cold H 3 + ions and its isotopomers with low energetic electrons has been the subject of intensive experimental [4–6] and theoretical [7–9] studies during the last years. But despite these efforts and the apparent simplicity of the system, the DR process turned out to be a challenge to both experiment and theory. Experimen- tally, the control of the initial excitation of H 3 + is the main issue, while theoretically the three dimensionality of the pro- cess constitutes the main difficulty. Complete energy depen- dences for the DR cross section have been measured at sev- eral storage rings [5,10–12] for H 3 + , shown to be vibrationally but not rotationally relaxed prior to recombina- tion [13], and rate coefficients of the order of  =1  10 -7 cm 3 s -1 for 300 K electrons have been deduced. In contrast, recent experiments using the flowing afterglow technique [14,15] have determined an upper limit of only 3  10 -9 cm 3 s -1 for the H 3 + DR rate coefficient. While the difference between the two techniques remains unexplained, a calculation [8,9], which treats the fragmentation reaction in its full dimensionality, is supporting the storage ring results. Using storage rings, DR cross sections were measured also for the isotopomers of H 3 + (Refs. [16–20]) and branching ratios between the two- and three-body dissociation channel [see Eq. (1)] have been determined to be 1:3 for H 3 + ,D 3 + , and H 2 D + [18]; in addition, the HD /2H 2  ratio in the DR of H 2 D + was found to be 1.20±0.05, slightly in favor of HD rather than H 2 production. Recently, we have performed detailed experiments related to the dynamics of the breakup of H 3 + and D 3 + in the DR process [21–23] by measuring the kinematical correlation be- tween the three hydrogen or deuterium atoms [see Eq. (1)]. It was found that the three neutral atoms produced by the DR reaction tend to dissociate along a straight line (while the original structure of the molecular ions is an equilateral tri- angle). An additional important outcome of these experi- ments has been the evidence for high kT rot  230 meV ro- tational excitations of the molecular ions prior to recombination even after storage times of several tens of seconds. These results have led to storage ring experiments [5] and resulted in somewhat lower rate coefficients for H 3 + when using an ion source producing rotationally colder ions. In this work, we present an experimental study of the dissociation dynamics and of isotope effects in the DR of the isotopomers D 2 H + and H 2 D + , investigating the momentum distribution between the three neutral fragments. Our data also indicate that already after a short storage time the re- maining rotational excitation for both D 2 H + and H 2 D + is considerably lower than observed for the symmetric species, an expected result as the asymmetric isotopomers, unlike H 3 + and D 3 + , have a nonvanishing permanent dipole moment (0.60 Debye and 0.48 Debye for H 2 D + and D 2 H + , respec- tively [24]) and can, therefore, radiatively cool both the vi- brational and rotational motion. The experiments were carried out at the Test Storage Ring (TSR) located at the Max-Planck-Institut für Kernphysik, Heidelberg, Germany applying a technique which has been previously described in detail [21]. In the present case, H 2 D + D 2 H +  ions were produced in a “hot” gas discharge ion source, and accelerated using a radio-frequency quadrupole accelerator to a kinetic energy of 1.9 meV for H 2 D + and 1.2 meV for D 2 H + before being injected into the storage ring. After each injection, about 10 6 ions were stored in the ring (55.4 m circumference) in an average vacuum of about PHYSICAL REVIEW A 69, 064702 (2004) 1050-2947/2004/69(6)/064702(4)/$22.50 ©2004 The American Physical Society 69 064702-1