Quantum control of cold molecular collisions using stark-induced adiabatic raman passage

Molecular scattering experiments interrogate the forces that govern interactions at the quantum level, but the amount of detailed information that can be extracted from a scattering experiment is limited by how precisely the input and output quantum states are defined. Experimental efforts to simplify the baffling variety of different quantum states found in room temperature gases have an illustrious history extending over more than one hundred years, and yet have only recently begun to achieve the level of control over the quantum world necessary for direct detailed interrogations of molecular interaction potentials. We present here our small contribution to this long history, where we have used optical adiabatic passage to prepare internal molecular quantum states, and then studied the rotationally inelastic scattering of these state-prepared molecules at very low collision energies. Under these conditions, nearly complete control over the quantum states was achieved, allowing us to experimentally derive insight into the dynamics of molecular scattering. To prepare the internal quantum states of molecules prior to the scattering even, we have made use of the Stark-induced adiabatic Raman passage (SARP) technique. As we describe here, SARP makes use of the time-varying adiabatic eigenstates of the light-matter system to smoothly transfer population from the initially populated ground state to a selected rovibrationally eigenstate with orientational specificity. Because of SARP's adiabatic nature, the population transfer is insensitive to fluctuations in the laser power, frequency, and arrival time, making it ideal for use in scattering experiments, which require data collection over long periods of time. Additionally, SARP is capable of vibrationally exciting large numbers of simple homonuclear diatomic molecules like H2, which are important targets for scattering experiments due to their theoretical tractability, but are nearly impossible to state prepare using other techniques. Here, we expand both theoretically and experimentally on the prior work developing SARP to show that this process is capable of population transfer to highly excited vibrational levels, thus opening up the study of the dynamics of these exotic systems. We have designed and constructed an apparatus capable of generating cold (low energy) scattering of the molecules state prepared by SARP. To do so, we make use of the small relative velocity present between two different gas components in a single supersonically expanded molecular beam. In order to demonstrate the principle of state prepared scattering in the mixed molecular beam, we have first used SARP to prepare HD in a variety of different magnetic or orientational sublevels of the (v = 1, j = 2) rovibrational energy eigenstate, where v and j give the vibrational and rotational quantum numbers. We then studied the rotational relaxation of these excited HD molecules by collision with H2, D2, and He coexpanded in the same supersonic beam, which led to a wealth of stereodynamic insights into these interaction processes. In particular, nearly complete control was achieved in the HD-He scattering case, allowing us to fully realize the potential of these experiments and directly access information about the scattering matrix.

Keywords: Chemistry.
Advisor: Zare Richard N. 
Dai Hongjie
Fayer Michael D.