Vibrational cooling of ${\mathrm{H}}_2^{+}$ and ${\mathrm{D}}_2^{+}$ in a storage ring studied by means of two-dimensional fragment imaging

The vibrational cooling of ${\mathrm{H}}_2^{+}$ and ${\mathrm{D}}_2^{+},$ having no dipole moment, has been studied at the storage ring TARN II. A two-dimensional imaging technique was used to measure the kinetic-energy release (KER) in the dissociative recombination of ${\mathrm{H}}_2^{+}$ and ${\mathrm{D}}_2^{+}$ as a function of the storage time at a relative energy of 0 eV. The vibrational-level populations of the ${\mathrm{H}}_2^{+}$ and ${\mathrm{D}}_2^{+}$ beams were estimated from the KER data. The estimation shows that the relative population of ${\mathrm{H}}_2^{+}$ ions in their vibrational ground states reaches more than 90% at a storage time of 26 s. This result is in agreement with the recent result in TARN II [T. Tanabe et al., Phys. Rev. Lett. 83, 2163 (1999)]. The relative population of ${\mathrm{D}}_2^{+}$ ions in their vibrational ground states is estimated to be more than 90% for a storage time of 10 s.