Interrogation and Manipulation of Coherences in Superfluid Helium

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In this project, funded by the Experimental Physical Chemistry Program of the Chemistry Division, Apkarian will conduct an experimental research program aimed at fundamental studies of bulk superfluid helium (He-II). The general goals are to investigate microscopic implications of the system, principally its quantum collective dynamics in order to manipulate coherences through multilinear spectroscopy. Superfluid helium streams were recently developed as the carrier fluid in a universal molecule injector, but their properties are not yet sufficiently understood. The properties of the neat stream, namely of a suddenly expanded Bose condensate, are of fundamental and practical interest. The streams will be characterized as a function of the pulsed expansion conditions, such as reservoir temperature, pressure, pulse length, and nozzle design. Light scattering will be used to characterize transitions from droplet to continuous flow; interferometric imaging will be used to quantify density and divergence of the stream; and second harmonic generation will be use to characterize ordered vorticity. Molecular impurities entrained in the stream will be studied spectroscopically. Among additional studies there will be investigations of stream-embedded hydrogen molecules by means of time/frequency resolved CARS spectroscopy, and the coherent rotations of hydrogen will be used to investigate the size dependent liquid-solid transition of hydrogen clusters and the possible superfluidity of these clusters. Graduate and undergraduate students as well as postdoctorate researchers will gain valuable experience through participation in this research, which involves cryogenic engineering, optical engineering, and modern laser techniques, as well as the use of techniques involved in molecular manipulations at 1 K. Nearly all core areas of the sciences, both classical and quantum mechanical, will be used in this research, with the result that the participants will have an unusually rich learning and training experience.