Turn-up effects at low momentum for the highest occupied molecular orbital of oxygen at various impact energies by electron momentum spectroscopy

The unexpected higher intensity at low momentum region in (e,2e) ionization cross sections, as called turn-up effect, was observed recently. This work systemically investigated this effect for the highest occupied molecular orbital of oxygen at various impact energies of 400, 600, 800, 1200, 1800, and 2400 eV by electron momentum spectroscopy. This turn-up became higher as the impact energy declined, which can be qualitatively predicted by the distorted wave explanation. However, it may be also due to some other dynamic interaction, such as the dynamic correlation. Since the oxygen molecule is the simplest multicenter system with such effects, this experimental result could be a test-bed for the accurate calculation of this effect in the molecular system.