Dynamical Spectroscopy of Transient He2 Molecules

The velocities of atoms can be manipulated by near-resonant laser light. After many absorption-spontaneous emission cycles momentum is transferred to the atoms in the direction of the laser beam. In this way atoms can be slowed down and cooled to milliKelvin temperatures and below. In three mutually orthogonal pairs of counter-propagating laser beams in combination with a magnetic field atoms can be cooled and trapped. In this so-called magneto-optical trap (MOT) two atoms can be excited to a molecular state by absorption of a photon. This technique is called photoassociation spectroscopy. In our case we study metastable helium atoms, which have a large internal energy. When two metastable helium atoms approach each other, one of the atoms ionizes and the ions are detected by a microchannelplates detector. When a probe laser beam is directed onto the MOT cloud and scanned in frequency, atom pairs can be excited to several ro-vibrational states. In the excited state the metastable helium atoms are accelerated towards each other and ionize at short internuclear distance. The ro-vibrational states appear as peaks in the ionization signal and from the position of these states information on the excited state interaction potential can be extracted