We have performed microwave–infrared double resonance measurements on OCS in pure He4 and mixed He4/He3 clusters. In contrast to our previous study on OCS in pure He4 cluster, where an asymmetric inhomogeneous linewidth increasing with rotational quantum number j was observed [S. Grebenev, M. Havenith, F. Madeja, J. P. Toennies, and A. F. Vilesov, J. Chem. Phys. 113, 9060 (2000)], we find the lines in the mixed helium cluster to be much narrower, showing a symmetric, probably homogeneous line width. The spectra indicate a significant difference in the rotational relaxation of OCS in mixed clusters as compared to pure He4 cluster. We have applied the model of Lehmann [K. K. Lehmann, Mol. Phys. 97, 645 (1999)], describing the interaction of a molecule with the helium cluster, and modified it in order to describe particle in a box states of OCS in mixed helium clusters. The calculations indicate while in the mixed cluster the OCS is confined to the inner He4 core, the energy of the particle in the box states is increased as compared to the pure He4 cluster. Relaxation mechanisms via these particle in the box states and surface excitations (ripplons) of the cluster are discussed in the text.
He Droplets / Cage Effect / SF 6The cage effect is a well known phenomenon e.g. in solid matrix systems where molecular complexes cannot dissociate upon excitation due to the surrounding matrix acting as a cage. Also high density gases or liquids can produce a cage effect as has e.g. been investigated by Troe and his (former) collaborators [1Ϫ3].In the last years a very interesting and extraordinary matrix has stepped into the limelight: the superfluid He droplet [4]. Molecules and atoms trapped in these droplets are surrounded by an ultracold (T Ϸ 0.4 K), homogeneous and inert environment. Since the droplets are liquid, even superfluid, it could have been expected that molecular complexes can dissociate after excitation. In the following we present our findings on the excitation of a SF 6 dimer trapped in a He droplet by a powerful CO 2 laser (maximum power output approximately about 60 W); evidence of the cage effect has been obtained also for this case.The experimental set up will be described in detail elsewhere. Here, only a summary is presented. The He droplets are produced in a supersonic beam expansion, where a source pressure of 25 bar is applied; the nozzle is cooled to temperatures between 14 and 19 K and has a diameter of 5 µm. The He droplets pass through a skimmer and a pick-up cell which is filled with the gas of interest at 10 Ϫ4 Ϫ10 Ϫ5 mbar. On their way through the cell,
Ethylene and ethylene clusters embedded in superfluid He4 droplets have been spectroscopically investigated with continuous wave line tunable CO2 and N2O lasers, utilizing the strong absorption of the ν7 mode of C2H4. The monomer and the different clusters of ethylene absorb in the same spectral region and show significant overlap of their spectra. By measuring the dependence of the signal on the pick-up pressure and fitting the resulting curves with a Poisson distribution and by further studying the spectral dependence on the droplet size, it was possible to assign two peaks. The origin of the 10 cm−1 broad gas phase dimer spectrum has been the subject of much discussion; we show that the broad peak is not due to homogeneous broadening, because the low temperature of the He droplet leads to a dimer full width at half maximum width of about 1 cm−1.
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