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            We have two main results from the research supported by the Petroleum Research Fund. They are (1) Measurement of electric field affected collisions between cold ammonia molecules and ultracold rubidium atoms, (2) Creation and deceleration of velocity controlled beams of rotationally excited ammonia molecules.     

Electric field affected cold collisions between molecules and atoms

          One of the tantalizing aspects of creating systems of cold molecules is the possibility of controlling the microscopic interactions with external electric and magnetic fields. Controlling the interactions could lead to a better understanding of the role of quantum mechanics in molecular collisions and in the fundamental processes of breaking and forming a chemical bond. There have been considerable theoretical investigations of how long-range dipole-dipole interactions between two polar molecules can be affected by the application of an electric field to control the relative orientation of the molecules.  However, there has been only one system that has reached the densities of polar molecules required to study these effects. We show that even in the absence of the electric-dipole interaction the space-orienting  nature of electric fields can change collision dynamics. We measure elastic and inelastic collision cross-sections between rubidium atoms (Rb) and ammonia molecules (ND₃) in the presence of an electric field in the cold temperature regime (~ 100 mK).

            We use a co-trapped geometry for these experiments where the molecules are decelerated and subsequently trapped using static electric fields and the rubidium atoms are laser cooled and then trapped using magnetic fields. Once each species is trapped in separate regions of the vacuum system, the coils forming the magnetic trap are translated 60 cm to overlap the two clouds initiating the interactions. We then monitor the dynamics of the ND₃  density by measuring the number of ions created via resonance enhanced multi-photon ionization. We can also measure the temperature and density of the Rb using resonant absorption imaging. Using these measurements and associated Monte Carlo simulations to model some of the dynamics, we can extract the inelastic collision cross-sections and place upper limits on the elastic cross-section (Fig. 1).

            The inelastic cross-sections are nearly at the Langivin capture limit and factors of 5-10 above initial theoretical calculations done by the J. Hutson group at Durham, UK. However, the initial calculations were done without an electric field present. We have worked with the Hutson group and they have now included the affect of electric fields. They see an increase in the inelastic cross-sections with increasing electric fields.

            One can understand this increase using a semi-classical picture of the atom-molecule interactions. Based on the calculated potential surface, the atom always wants to approach the molecule from the nitrogen end.  At low electric fields, ND₃ will adiabatically rotate such that it hits nitrogen end first and then comes back out on the same adiabat with its internal state unchanged. For incredibly large electric fields, the ammonia molecule is essentially fixed in space and the Rb atom will collide at a random angle.  For intermediate electric fields, there will be a distance where the energy of the Stark shift become comparable to the energy to rotate the molecule. There is now a frustration between the orientation due to the electric field and the orientation due to the incoming Rb atom.  In this situation, an avoided crossing between two different states in ammonia appears and thus causes an increased probability of the molecule changing state during the collision. Thus, even without dipole-dipole interactions, externally applied electric fields can change the microscopic collision dynamics. We are currently preparing a manuscript describing this work.