Knowledge Bank

Vibrationally inelastic cross sections have been measured with a crossed beam apparatus at different well-defined collision energies for the $I2\mathrm{<mrow\; data-mjx-texclass="ORD">\ast </mrow>}+$ He system. By controlling the temperature of the He target beam, we could vary the collision energy from 40 mev to 190 mev. $I2$ molecules were initially laser pumped to the level $v4=35$ or 15 of the $BOu\mathrm{<mrow\; data-mjx-texclass="ORD">+</mrow>}$ state. Electronic fluorescence spectroscopy revealed the state-to-state vibrational cross sections. At the higher collision energies, the relative cross sections are very symmetric for $\Delta v$ UP versus $\Delta v$ DOWN scattering. This symmetry is much reduced when the collision energy more closely approaches the vibrational quantum size. The higher collision energies also create more rotational excitation and it is governed by angular momentum kinematics. Classical trajectory calculations fit the data by assuming a simple repulsive interaction between $I2\mathrm{<mrow\; data-mjx-texclass="ORD">\ast </mrow>}$ and He.