Energy-sensitive imaging detector applied to the dissociative recombination of ${\mathrm{D}}_2$H${}^{+}$

We report on an energy-sensitive imaging detector for studying the fragmentation of polyatomic molecules in the dissociative recombination of fast molecular ions with electrons. The system is based on a large area ($10\times 10$ cm${}^2$) position-sensitive, double-sided Si-strip detector with $128$ horizontal and $128$ vertical strips, whose pulse height information is read out individually. The setup allows us to uniquely identify fragment masses and is thus capable of measuring branching ratios between different fragmentation channels, kinetic energy releases, and breakup geometries as a function of the relative ion-electron energy. The properties of the detection system, which has been installed at the Test Storage Ring (TSR) facility of the Max-Planck Institute for Nuclear Physics in Heidelberg, is illustrated by an investigation of the dissociative recombination of the deuterated triatomic hydrogen cation ${\mathrm{D}}_2$H${}^{+}$. A huge isotope effect is observed when comparing the relative branching ratio between the ${\mathrm{D}}_2$ $+$ H and the HD $+$ D channel; the ratio $2B$(D${}_2$ $+$ H)/$B$(HD $+$ D), which is measured to be $1.27\pm 0.05$ at relative electron-ion energies around $0$ eV, is found to increase to $3.7\pm 0.5$ at $~$5 eV.

Figure 1

Concept of the EMU detector: A fast, neutralized molecule is dissociating at a distance $S$ from a position-sensitive and energy-sensitive detector. The fragments continue to travel approximately at the velocity of the molecule, but due to the kinetic energy released in the dissociation they acquire macroscopic distances from the transversal center-of-mass ($x_{\mathrm{cm}}$,$y_{\mathrm{cm}}$) when hitting the detector. The position ($x_i$,$y_i$) and the energy $E_i$ of each fragment is recorded.

Figure 2

Some breakup geometries and corresponding hit pattern occurring in the DR of ${\mathrm{D}}_2$H${}^{+}$. The lines mark the responding readout strips in vertical ($x_i$) and horizontal ($y_i$) directions; the numbers at the end are the mass numbers $A_{x_i}$ and $A_{y_i}$ derived with Eq. (4). The breakup geometries shown in (a) and (c) result in the same readout pattern (hit pattern) as the geometries shown in (b) and (d), respectively. But while the hit pattern resulting from the geometries shown in (a) and (b) still allow unique identification of the fragmentation channel, the pattern resulting from the breakup geometries depicted in (c) and (d) is ambiguous with respect to the fragmentation channels D $+$ D $+$ H and HD $+$ D.

Figure 3

Schematic cross section through the EMU detector, a double-sided, fully depleted Si-strip detector of $10\times 10$ cm${}^2$ active surface area. The $p^{+}$Si and the Al layers of the junction contact, through which the particles enter the detector, were made as thin as possible to minimize the energy losses of the DR fragments in these dead layers.

Figure 4

Schematic view of the EMU setup, which is located in the BAMBI beamline downstream of the electron target of the TSR.

Figure 5

A schematic view of the electronic setup for the EMU detector.

Figure 6

Pulse height spectra obtained in the DR of 2.39 MeV ${\mathrm{D}}_2$H${}^{+}$ ions at a detuning energy of $E_d=0$ eV. The figure shows the sum of the calibrated pulse height spectra (a) of all $x$ strips on the front and (b) of all $y$ strips on the back side. Above channel 550, counts are multiplied by a factor of 10.

Figure 7

The distribution of the H (black) and ${\mathrm{D}}_2$ (gray; red online) positions observed for 5000 ${\mathrm{D}}_2$ $+$ H events following the DR of ${\mathrm{D}}_2$H${}^{+}$ at $E_d$ $=$ 0 eV. Note that H events within the red (gray) area are partly shadowed by the ${\mathrm{D}}_2$ events due to the plotting procedure.

Figure 8

The horizontal and vertical distribution of the center-of-mass coordinates of DR events of ${\mathrm{D}}_2$H${}^{+}$ taken after 30 s of phase-space cooling at collision energies of $E_d$ $=$ 0 eV. The blue (smooth) lines are fits by a Gaussian distribution.

Figure 9

(a) Energy dependence of the DR rate coefficient of ${\mathrm{D}}_2$H${}^{+}$  as measured by Lammich et al. [4]. (b) Fragment channel branching ratios determined in the present work as a function of the collision energy with only statistical uncertainties. In the absence of isotope effects one expects to find $B$(HD $+$ D) $=$ 2$B$(D${}_2$ $+$ H).

Figure 10

Normalized projected distance distributions for the two-fragment channels (a) ${\mathrm{D}}_2$ $+$ H and (b) HD $+$ D observed in the DR of ${\mathrm{D}}_2$H${}^{+}$ at $E_d=0$ eV; the solid lines are the result of a fit with simulated distributions (dashed lines) corresponding to the population of different vibrational levels ${\mathrm{D}}_2(v)$ and HD($v$), respectively. The resulting population probabilities are shown in the inserts; for histogram bins spanning more than one unit in $v$, the relative populations were forced to be equal within the bin.

Figure 11

Normalized distribution of weighted transverse distances $D^2$ observed in the DR of ${\mathrm{D}}_2$H${}^{+}$ at $E_d=0$ eV for the fragment channel H $+$ D $+$ D after 30 s of phase-space cooling. The solid line is the result of a simulation assuming an isotropic breakup geometry and an initial rotational excitation of the ${\mathrm{D}}_2$H${}^{+}$ ions corresponding to a rotational temperature of 100 K.

Figure 12

Dalitz ratio plot of the three-body fragmentation channel following the DR of ${\mathrm{D}}_2$H${}^{+}$ at $E_d=0$ after 30 s of electron cooling. The triangles depict projected dissociation geometries for a sample of points in the $(Q_1,Q_2)$ plane; the white dot shows the position of the H, the black points the positions of the Ds.