Continetti Group

Machine C

Machine C has undergone several changes since it was first built. Initially, the machine was used to perform coincidence studies of dissociative photodetachment (DPD) of anions, a technique pioneered in this lab. Experiments involving dissociative recombination were also attempted on this machine. The most recent experimental work on Machine C involved dissociative charge exchange which has led to several important insights into dissociation mechanisms and dynamics of highly excited electronic states of small molecules.

This machine is currently set up to study dissociative photodetachment with a twist. For basic information how the DPD experiments are carried out see the Machine A page. In this case a cryogenic (10K) electrostatic ion beam trap has been added to the detection region. Anions are trapped for several seconds at the full beam energy, up to 16keV, and radiatively cool to ground vibrational and low rotational states, free from re-excitation by warm blackbody photons. This enables energetics and dynamics in DPD experiments to be determined more accurately and can lead to studies of cooling processes of molecules on long (>millisecond) time scales, particularly through dipole forbidden transitions. Additionally, this trap could also lead to studies of the dissociation of state-prepared ions, and will enable laser-based coherent control and alignment experiments to be performed on ions with well-characterized quantum states.

Most recently, we have included a cryogenic octopole accumulation trap (COAT) for enhanced control of internal temperature of anions prepared for PPC experiments. A variety of precursor anions are generated via molecular expansion from a piezo electric pulsed valve through a discharge and are then extracted via a Wiley McLaren time of flight spectrometer and focused into COAT. COAT has been used to cool internal energy states of anions, such as O₃, as well as stabilizing higher energy anions that have not been made previously.

This machine features the same unique quadrant crossed delay line neutral detector as before for detection of up to 8 neutral fragments from a single dissociation event, as well as a wedge and strip anode-based velocity mapping electron detector within the trap cavity for detection of electrons ejected over the full 4pi solid angle. The interaction of the laser pulse and ion packe occurs at the center of the trap inside the electron detector, and the same cold packet of ions will repeatedly be probed until a majority of the ions are lost due to dissociation or scattering/charge exchange with background gas at ~1E-11 torr. Ions are automatically bunched synchronized to the kHz repetition rate of the laser using a phaselocked RF voltage on an electrode in the trap.

More photos of Machine C can be found in the photo gallery.