Resonant structure of low-energy ${\mathrm{H}}_3^{+}$ dissociative recombination

High-resolution dissociative recombination rate coefficients of rotationally cool and hot ${\mathrm{H}}_3^{+}$ in the vibrational ground state have been measured with a 22-pole trap setup and a Penning ion source, respectively, at the ion storage-ring TSR. The experimental results are compared with theoretical calculations to explore the dependence of the rate coefficient on ion temperature and to study the contributions of different symmetries to probe the rich predicted resonance spectrum. The kinetic energy release was investigated by fragment imaging to derive internal temperatures of the stored parent ions under differing experimental conditions. A systematic experimental assessment of heating effects is performed which, together with a survey of other recent storage-ring data, suggests that the present rotationally cool rate-coefficient measurement was performed at 380${}_{-130}^{+50}$ K and that this is the lowest rotational temperature so far realized in storage-ring rate-coefficient measurements on ${\mathrm{H}}_3^{+}$. This partially supports the theoretical suggestion that temperatures higher than assumed in earlier experiments are the main cause for the large gap between the experimental and the theoretical rate coefficients. For the rotationally hot rate-coefficient measurement a temperature of below 3250 K is derived. From these higher-temperature results it is found that increasing the rotational ion temperature in the calculations cannot fully close the gap between the theoretical and the experimental rate coefficients.

Figure 1

(a) Measured 2D distributions of the kinetic energy released in the DR of ${\mathrm{H}}_3^{+}$ into $\mathrm{H}+\mathrm{H}+\mathrm{H}$. Distribution for the ${\mathrm{H}}_3^{+}$ beam produced with para-${\mathrm{H}}_2$ in the 22-pole trap, IR22p, is shown by (blue) squares; distribution for the ${\mathrm{H}}_3^{+}$ beam produced with the Penning source, Ipenn-09, is shown by (red) triangles. Also shown are the simulated distributions at 0, 380, and 3250 K. All distributions are normalized to unit area. (b) Semilogarithmic zoom of the tail of IR22p (blue squares) and the previously measured I22p-09 (green circles). Simulations shown are at temperatures of 0, 200, and 400 K.

Figure 2

${\mathrm{H}}_3^{+}$ DR rate coefficients, R22n and Rpenn, as measured for the ion beam produced with the 22-pole trap and the Penning source, respectively, on a double-logarithmic scale.

Figure 3

Reduced DR rate coefficients from the rotationally cool ${\mathrm{H}}_3^{+}$ beam R22n compared with previously reported rates on a semilogarithmic scale. For clarity, error bars are only shown for every fifth point. Solid curves denote the experiments performed with the 22-pole trap, with the line thickness increasing with decreasing year of publication.

Figure 4

${\mathrm{H}}_3^{+}$ DR rate coefficients as predicted for 15, 100, 300, and 3000 K, with the line thickness increasing with temperature. The 15 K rate is calculated using a 1:1 ortho-to-para ratio.

Figure 5

(a) The reduced ${\mathrm{H}}_3^{+}$ DR rate coefficient versus collision (detuning) energy, as measured using the beam from the 22-pole-trap (R22n), is compared with the theoretical rate coefficient calculated for a source of thermal ions at 300 K, over the range from 0 to 0.06 eV. The relevant excited rotational thresholds that can produce an infinite Rydberg series of $\mathit{np}$ resonances just below each of them are shown as vertical lines. Numerical values of these threshold energies and their corresponding quantum numbers are listed in Table . For clarity, error bars are only shown for every fifth point. (b) Over the same energy range, theoretical spectra for the reduced DR rate coefficient are shown at four temperatures (100, 300, 500, and 700 K), with the line thickness increasing with temperature.

Figure 6

(a) The reduced ${\mathrm{H}}_3^{+}$ DR rate coefficient versus collision (detuning) energy, as measured using the beam from the 22-pole-trap (R22n), is compared with the theoretical rate coefficient calculated for a sample of thermal ions at 300 K, over the range from 0.05 to 0.5 eV. For clarity, error bars are only shown for every fifth point. (b) Over the same energy range, theoretical convolved spectra for the reduced DR rate are shown at four different temperatures (100, 300, 500, and 700 K), with the line thickness increasing with temperature.

Figure 7

(a) The reduced ${\mathrm{H}}_3^{+}$ rate coefficient versus collision (detuning) energy, as measured using the beam from the Penning source (Rpenn), is compared with the theoretical rate calculated for a source of thermal ions at 1000 K (purple curve) and 3000 K (red curve), over the range from 0 to 0.06 eV. For clarity, error bars are only shown for every fifth point. (b) The same, for the energy range of 0.05 to 0.5 eV. The line thickness increases with temperature.