Electron-impact rotational excitation of H3+: relevance for thermalization and dissociation dynamics.

Electrons are known to be efficient in rotationally exciting molecular ions in low-density astrophysical plasmas. Rotational excitation of molecular ions has also been shown to affect the measured values of dissociative recombination (DR) rate coefficients. Thus, electron collisions with H3+ are expected to play a significant role in thermalization and dissociation dynamics of this ion, both in the laboratory and in space. Using the molecular R-matrix method combined with the adiabatic-nuclei-rotation approximation, we have computed new rate coefficients for the rotational excitation of H3+ by electrons at temperatures from 10 to 10,000 K. De-excitation rates are found to amount to a few 10(-7) cm3 s(-1) below 1000 K, i.e. comparable in magnitude to that of DR. In astrophysical environments where the electron fraction exceeds 10(-4), electron collisions are thus expected to contribute to the non-thermal rotational distribution of H3+. The competition between electron and neutral collisions is discussed in the context of recent observations of H3+ towards Galactic centre sources.