Abstract
Single molecules (SF6, OCS, Glyoxal, Tetracene, Phthalocyanine) are captured inside liquid He droplets with ≃5000 atoms in a molecular beam and studied via depletion laser spectroscopy in the infrared and visible ranges. The spectral lines are in general very sharp (Δv ≃ 0.01–0.1 cm−1 ) and show rotational structure. Rotational structure of the IR-spectra of SF6 and OCS provides first information on the droplet temperature,1 which is 0.37 K for4 He and 0.14 K for3 He droplets, respectively. Several different SF6 van der Waals complexes of defined composition have been constructed inside the droplet via the sequentional capture of the constituents and have been investigated.2 Spectra of the electronic excitation (Glyoxal S1 ← S0) consist of sharp vibronic lines each accompanied by a phonon sideband.3 The structure of the weak phonon sideband provides a first experimental evidence for the "roton" gap in the surrounding He medium and indicates that the He-droplets are superfluid as predicted by theory. The experiments have been extended to larger molecules as tetracene and phthalocyanine. These show sharp zero phonon lines and some additional structures in the phonon wings that are currently ascribed to localized vibrations of a solid He "snowball" over the molecule. These experiments demonstrate the unique advantages of He-droplets as an ultracold, homogeneous, and very gentle matrix for molecular spectroscopy.
© 1997 Optical Society of America
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