Anisotropy and Molecular Rotation in Resonant Low-Energy Dissociative Recombination

Angular fragment distributions from the dissociative recombination (DR) of ${\mathrm{HD}}^{+}$ were measured with well directed monochromatic low-energy electrons over a dense grid of collision energies from 7 to 35 meV, where pronounced rovibrational Feshbach resonances occur. Significant higher-order anisotropies are found in the distributions, whose size varies along energy in a partial correlation with the relative DR rate from fast-rotating molecules. This may indicate a breakdown of the nonrotation assumption so far applied to predict angular DR fragment distributions.

Figure 1

Transverse distance distributions $F(D)$ for $E_d=25\mathrm{meV}$ (a) and $E_d=27\mathrm{meV}$ (b) with fits by Eq. (1) (solid line) requiring sizable contributions of Legendre polynomials of order $n=2$ and 4. For comparison, the distributions expected for isotropic emission are indicated by dotted lines ($D<17\mathrm{mm}$). For $D>17\mathrm{mm}$ the contributions to the fit from initial levels $J\le 1$ (short-dashed) and $J\ge 2$ (long-dashed) are shown. Arrows indicate the maximum $D$ values for $J=1$ and 5, respectively.

Figure 2

(a) Scaled, rotationally averaged DR rate coefficient $\tilde{\alpha }(E_d)E_d^{1/2}$, (b) low-$J$ fraction $b_{01}$ (symbols connected to guide the eye), and (c),(d) anisotropy coefficients $a_2$ and $a_4$ as functions of the detuning energy $E_d$. The photocathode was used except for the gray symbols indicating use of the thermionic cathode. The dotted lines indicate the values of $a_n={\chi }_n(E_d){\tilde{a}}_n$ for pure $d$ symmetry of the transition matrix.