Momentum vector correlation maps for three-particle fragmentation of neutral triatomic hydrogen near its ionization threshold

Three-particle dissociation of high-lying Rydberg states of ${\mathrm{H}}_3$, in the immediate vicinity of the ionization threshold, is induced by an external electric field. We observe that the momentum vector correlation map of the center-of-mass motion of the fragments changes rapidly with electronic energy. Near- and above-threshold substantial contributions of fragment orientations which are characteristic of near-linear geometry appear, i.e., two atoms with opposing momenta, the third atom being nearly at rest. This finding is similar to observations by Strasser et al. [Phys. Rev. A 66, 032719 (2002)], who studied dissociative recombination of cold ${\mathrm{H}}_3{}^{+}$ ions with slow electrons. We discuss the likely dissociation paths responsible for the observed correlation.

Figure 1

Unimolecular dissociation of ${\mathrm{H}}_3^{*}$ is detected over the region from the aperture to the beam flag (length 115 mm), when no voltage is applied. Electric-field-induced dissociation is localized to the 3 mm region of the Stark electrodes which are shown on a greatly expanded scale here.

Figure 2

Measured kinetic energy release $W$ for the three-body decay at different applied Stark voltages. The energy values are calculated on the assumption that the distance from the point of dissociation to the detector is 1170 mm. The arrows indicate the energetic location of vibrational levels of metastable $2p\hspace{0.5em}{A}_2^{″}$ molecules which are induced to dissociate by the Stark field. Vibrational excitations are given in the form ($v_1$,$v_2$), where the $v_i$ give the quanta of excitation in the symmetric breathing and the degenerate bending vibration, respectively.

Figure 3

The simulation of the spectrum is shown by the full line. Experimental data are shown with the statistical error bar. The spectrum was accumulated in an 11 h run. A potential energy difference of 7 kV was used. The ionization limits shown correspond to the lowest three rotational levels of the lowest two vibrational states of ${\mathrm{H}}_3{}^{+}$. The top axis gives the Rydberg series with zero quantum defect, converging to the lowest rotational level of ${\mathrm{H}}_3{}^{+}$.

Figure 4

Characteristic distributions of momentum vectors in a Dalitz map are shown on the left. The dimensionless coordinates are those of Eq. (5). The experimental result for the metastable molecule $2p\hspace{0.5em}{A}_2^{″}(0,0)(N=0,K=0)$ is shown at the center. On the right the Dalitz map for all events in the energy interval 3.5–5.0 eV at zero electric field is shown. The gray scale is given in Fig. 5.

Figure 5

Dalitz plots of fragments in specific energy intervals: $W=(\mathrm{a})4.3$–4.4 eV ($n=6$), (b) 4.4–4.5 eV ($n=7$), (c) 4.6–4.7 eV ($n=8$–16), (d) 4.7–4.8 eV (around the ionization threshold), and (e) 4.8–5.0 eV (region of vibrational autoionization). The relative signal in each segment of a Dalitz plot is given in the lower row, together with the respective statistical error bars.

Figure 6

Comparison of the result from Figs. 5c (left) and 5d (center) with the result of Strasser et al. [6] (right). The gray scale is that of Fig. 5.

Figure 7

Cuts through the potential surfaces of ${\mathrm{H}}_3$. Left: $D_{\infty h}$ symmetry; center: transition conserving $C_{2v}$ from $D_{3h}$ to $D_{\infty h}$ (see text); right: $D_{3h}$. Full lines (in $D_{\infty h}$,$C_{2v}$,$D_{3h}$) ${\Sigma }_g^{+}$, $A_1$, and $A_1^{\prime }$; dashed ${\Sigma }_u^{+}$, $B_2$, and $E^{\prime }$; dot-dashed ${\Pi }_u$, $B_1$, and $A_2^{″}$. The vertical lines indicate coordinates common to the middle panel. Figure taken from Ref. [9].