We report the cooling of an atomic ensemble with light, where each atom scatters only a single photon on average. This is a general method that does not require a cycling transition and can be applied to atoms or molecules that are magnetically trapped. We discuss the application of this new approach to the cooling of hydrogenic atoms for the purpose of precision spectroscopy and fundamental tests.

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We propose a general method to cool the translational motion of molecules. Our method is an extension of single photon atomic cooling which was successfully implemented in our laboratory. Requiring a single event of absorption followed by a spontaneous emission, this method circumvents the need for a cycling transition and can be applied to any paramagnetic or polar molecule. In our approach, trapped molecules would be captured near their classical turning points in an optical dipole or RF-trap following an irreversible transition process.Cooling the translational motion of molecules has been one of the grand challenges of physics and chemistry for many years, motivated by the wide range of fundamental problems that could be addressed. For example, the study of chemical reactions at ultra low temperatures would open a new and unexplored regime of cold chemistry, where reactions might proceed through resonance states [1] and the reaction rate and outcome could be controlled by externally applied fields [2]. Cold molecules would enable precision molecular spectroscopy, possibly allowing the observation of a variation in time of the fundamental constants [3-5]. Ultimately, cooling of molecules could enable the study of quantum degenerate polar molecular gases [6,7]. The established methods of molecular cooling work down to a temperature of tens to hundreds of mK. These include cooling by collisions with a cold buffer gas either in a cryogenic cell or during an adiabatic (supersonic) expansion. Buffer gas cooled paramagnetic molecules have been magnetically trapped [8] whereas cold supersonic molecular beams have to be first decelerated to trapping velocities by interaction with time dependent electric [9-11] or magnetic fields [12-16].To reach lower temperatures and higher phase space densities, a different method must be used. A natural candidate is evaporative cooling, however this method is not likely to work for molecules in a lowfield seeking state due to inelastic collisions. Application of standard laser cooling to molecules has been investigated by Di Rosa [17], however the method is not general due to the complicated internal structure.More general cooling methods have been suggested, including cavity cooling [18,19] and the ``optical shaker'' [20].A new approach, single-photon cooling, was recently proposed and demonstrated by us for magnetically trapped atoms [21,22]. We show in this Letter that this method can be naturally extended to molecules. We start with an example of a straightforward application of the single photon cooling technique on a molecular radical and propose a new

Dedicated to my parents, Anne and Neal Price, and to Elizabeth, my lovely bride to be, whose love and support keeps me going each day. AcknowledgmentsFirst, I would like to thank my supervisor, Dr. Mark G. Raizen. Never before have I met an individual with so many wonderful and inspiring ideas.His ability to see directly to the heart of physical problems and identify their fundamental issues is astounding. Whenever the project ran into what seemed to be serious trouble, Mark would give us a multitude of ideas to circumvent it. He has served as an endless source of ideas, inspiration, and motivation (when needed). I could not have asked for a better advisor and am thankful that he allowed me to join the group in the summer of 2003.This brings me to the wonderful group of people that I have had the pleasure of working alongside throughout these years. A particular thanks and acknowledgment goes to Travis Bannerman. He joined the group a little before my qualifier and has worked on this project with me for almost its entire duration. He is an extremely smart individual with keen physical instincts.He worked very hard, contributed many ideas and helped to make the project a success. I have no doubt that he will bring the experiment to the next level successfully and will continue to do great work beyond this lab, in whatever field he chooses. Next, I would like to thank Kirsten Veiring. She originally joined this lab as a Würzburg master student and did excellent work determining "magic wavelengths" for Sodium and Rubidium. Apparently she enjoyed v her time, and the lab's students, so much that she decided to return to do her doctoral work. When she did so, she joined me in the Rubidium experiment and helped to conclude much of the work discussed in this dissertation. She has a real eye for detail and a tremendously bright future.I would also like to thank other students in the lab who have made my time here so enjoyable. Hrishi has been in the lab the entire time I have.We both became the senior students on our respective projects around the same time. He has always been extremely friendly and helpful. On several occasions when I did not have a needed piece of equipment and he did, he gladly loaned it to me so that I could precede. Tongcang Li has tremendous physical intuition and is always up for an entertaining discussion on one topic or another. Whenever I was stressed, he always seemed to know how to help me put thing into perspective. Adam Libson is a great experimentalist that I have gotten to know well over the years through countless lunches together. His love of physics will take him far. David Medellin is fantastic with equations and has helped many people in the lab, including myself, by clarifying derivations.Isaac Chavez began as an undergraduate in our lab and made the transition into graduate student. He is an extremely hard worker from whom I expect great things. Tom Mazur is the newest member of our group and, in addition to being a really nice person, has started his graduate studies strong.Of cou...