Sub-Kelvin Stereodynamics of the $\mathrm{Ne}({{}^{3}P}_2)+{\mathrm{N}}_2$ Reaction

We present an experimental study of the low-energy stereodynamics of the $\mathrm{Ne}({{}^{3}P}_2)+{\mathrm{N}}_2$ reaction. Supersonic expansions of the two reactants are superposed in a merged beam experiment, where individual velocity control of the two beams allows us to reach average relative velocities of zero, yielding minimum collision energies around 60 mK. We combine the merged beam technique with the orientation of the metastable neon atoms and measure the branching between two reaction channels, Penning ionization and associative ionization, as a function of neon orientation and collision energy, covering the range 0.06–700 K. We find that we lose the ability to orient Ne below $\approx 100\mathrm{K}$ due to dynamic reorientation. Associative ionization products $\mathrm{Ne}\text{−}{\mathrm{N}}_2^{+}$ predissociate with a probability of 30%–60% and that associative ionization is entirely due to reactions of the $\mathrm{\Omega }=2$ state, where the singly occupied $p$ orbital of the ${\mathrm{Ne}}^{*}$ is oriented along the interatomic axis.

Figure 1

Sketch of the interaction region. Two parallel molecular beams collide with the relative velocity $v_{\mathrm{rel}}$ in the center of a pair of solenoids, arranged at 90 deg, which produces the magnetic field $\overrightarrow{B}$ at an angle ${\theta }_{kB}$.

Figure 2

Samples of raw TOF spectra at ${\theta }_{kB}=85°$ for (a) $E_{\mathrm{coll}}/k_B=68\mathrm{mK}$ and (b) $E_{\mathrm{coll}}/k_B=616\mathrm{K}$. (c),(d) Ion yields as a function of ${\theta }_{kB}$ for the same collision energies. BG is short for background.

Figure 3

Experimental (a) and fitted (b) reactivity as a function of collision angle and energy. In both cases, the left side shows the energy range 0.06–30 K, and the right side shows the range 30–700 K. (Left) Raw data. (Right) The true AI/PI obtained through a fit of Eq. (4).

Figure 4

Reaction cross section ratios for $\mathrm{\Omega }=0$, 1, and 2 (a)–(c). Error bars are one standard deviation of the cross section histograms as obtained from the fit. (d) The steric effect as a function of energy. The pink-shaded area shows the range where transverse and longitudinal velocities are comparable. (Left) Results are from fits to the raw reactivity $R_{\mathrm{low}\_E}$. (Right) The corrected reactivity $R$ was used.

Figure 5

The probability $P_{\text{pd}}$ for predissociation of the AI products $\mathrm{Ne}\text{−}{\mathrm{N}}_2^{+}$, obtained by fitting Eq. (4) to the experimental data.