Spectroscopy of molecules in superfluid He droplets at low photon energy

Spectroscopy and helium droplets have, in recent years, combined to produce some remarkable results, such as the observation of superfluidity manifestations on the atomic length scale, as well as the synthesis and investigation of highly unstable atomic and molecular complexes that can only be formed in this cold (0.37 K) liquid environment. As helium itself is not easily amenable to laser excitation, atoms and molecules (dopants) individually captured by helium droplets were established as very effective probes of the properties of the droplets, most notably superfluidity. In synthetic applications, with helium now just an ultracold substrate, high resolution spectroscopy (primarily infrared) provided the structural information on novel aggregates formed in the droplets. Contrary to initial expectations, the width of individual lines was not lifetime-limited, but rather indicated a complex interaction of the dopant with the droplet. The spectral blurring was investigated, by simple experiments and models, as a mean to learn about such interaction; at the same time various schemes have been tried to minimize blurring, and thus increase the amount of information obtainable from a spectrum. One possible approach, at the price of a strong reduction in sensitivity, is to reduce the energy of the probing photon, thus closing some interaction channels between the excited dopant and the droplet. This naturally takes us to electron (and, in perspective, nuclear) spin resonance spectroscopy as the method of choice: This proposal is centered on the development of a new experimental method: optically-detected magnetic resonance (ODMR) in helium droplets. It takes the energy of the pump photon down to the microwave region while maintaining good sensitivity (ideally comparable to that of single-laser experiments) thanks to the visible probe photon. A spin-polarized sample is indispensable, and several experimental schemes will be explored to produce it. ODMR will allow us to address both of the important issues mentioned above, namely: a) To explore the He droplets themselves, and their interaction with the dopants. Of particular interest in the context of superfluidity and elementary excitations in a He droplet is the fact that the energy of the pump photon is comparable to that of surface excitations of the droplet and can in principle be tuned via the magnetic field strength. The latter excitations are believed to be responsible for energy and angular momentum dissipation inside a droplet: a basic problem still awaiting to be addressed experimentally. b) To probe the structure of complexes formed in He droplets with higher resolution. In this context the experiments proposed here will be a proof-of-principle for magnetic resonance in He droplets, and have been chosen with simplicity in mind. In a broader perspective, they shall be the first step towards application of one of the most standard and versatile tools of the synthetic chemist, that is: nuclear magnetic resonance, to molecules and complexes in He droplets. This type of experiments is entirely new, and a lot of surprises should be expected. We endeavor to anticipate the most likely scenarios and their early ramifications.