| The cold molecule systems are attractive for a wide range of physics research fields including ultra-cold chemical reactions,quantum information and computation,and preci-sion measurement.Laser-cooling of molecules is a new method of obtaining cold molecules that has only been proposed in the last decades.This method establishes a cycling transition necessary for laser cooling in a kind of molecules with highly diagonal FCFs(Franck-Con-don factors),making it possible for laser-cooling of these molecules.There are currently three molecules SrF,CaF,and YO that have achieved 3D-MOT,and it is expected that on this basis,further production of large samples of molecules at quantum degenerate regions should be possible.Inspired by this expectation,our group started a laser cooling experi-ment of a brand-new molecule—the BaF molecule.BaF molecules have highly diagonal FCFs,and the lifetime of the excited state is about 56ns which are friendly to laser cool-ing.Its cooling wavelength is around 850nm,so in experiments,inexpensive and practical semiconductor lasers can be used as laser sources.This thesis will mainly introduce our research on laser cooling of BaF molecules.We first point out through detailed theoretical calculations that the FCFs of the BaF molecule are highly diagonal.Combining the selection rules with sideband modulation techniques,only 3 lasers—860nm,896nm,898nm—are needed to establish the cycling transition of BaF molecules,which promising the achievement of laser cooling.Then we introduced in detail the experimental system based on this theory,which mainly includes:we have made 3 sets of ECDL and TA to obtain high-power lasers of about 1.5W;we have designed a transfer cavity frequency locking system to realize the lock of the 3 lasers at linewidths of about MHz;we have designed and built The cryogenic-vacuum system which realized the generation of a BaF cold molecular beam with a speed of 200m/s and a temperature of 4K,and the spectrum that distinguishes the fine splitting of BaF molecules is detected.At the end of the experimental part,describing that with the laser cooling scheme proposed by our theory and the system we built,we successfully realized the deflection of the BaF molecule beam by the lasers,the results show that we can achieve the scattering of up to 600 photons of BaF molecules so far,which provided preliminary evidence for the cycling transition.At the end of the thesis,we put forward the problems faced by future research and proposed solutions.All in all,the research work contained in this thesis explored the feasibility of laser cooling of BaF molecules and preliminarily confirmed that BaF molecules may be cooled by lasers,opening the door for further studies on the realization of 3D-MOT of BaF molecules in the future. |
