We have measured the laser-induced fluorescence excitation spectra of the 3 1P10←3 1S0 transition of Mg atoms solvated in helium nanodroplets. The observed blue shifts and line broadenings mirror the shifts and broadenings observed in studies of Mg atoms solvated in bulk liquid helium. This similarity allows us to conclude that Mg atoms reside in the interior of the helium droplet. The 3 1P10←3 1S0 transition shows a splitting which we attribute to a quadrupolelike deformation of the cavity which forms around the solute atom after excitation. Temporal evolution of the fluorescence from the solvated 3 1P10 Mg yields a longer lifetime (2.39±0.05 ns) than found in vacuum (1.99±0.08 ns). This difference can be accounted for quantitatively by evaluation of the anisotropic distribution of the helium density in the neighborhood of the excited Mg atom. The question of solvation vs surface location for the guest atoms is also discussed in light of the model of Ancilotto et al. [F. Ancilotto, P. B. Lerner, and M. W. Cole, J. Low Temp. Phys. 101, 1123 (1995)], of existing metal atom–helium potential energy functions, and of our own calculations for the MgHe and CaHe ground states. While the Ancilotto model successfully predicts solvation (or lack of it) if the solvation parameter of the guest atom is not too near the threshold of 1.9, the present knowledge of the interatomic potentials is not precise enough to test the model in the neighborhood of the critical value.

The laser-induced fluorescence excitation spectrum in the vicinity of the 32D ← 32P transition of Al atoms solvated in superfluid helium nanodroplets has been measured. While this transition has not been studied in bulk liquid helium, the observed blue shifts and line broadenings, compared with other transitions for which such measurements do exist, allow us to conclude that Al atoms reside in the interior of the helium droplet. The observed transition shows a splitting that we attribute to quadrupole-like deformations of the cavity formed in the helium droplet around the Al atom. Time-resolved studies of wavelength-selected emission from solvated Al atoms excited to the 32D state reveal a rapid (≤50 ps) and seemingly total nonradiative population transfer to the 42S state, the decay of which is found to account for all of the emission and to have a lifetime (6.4 ± 0.1 ns) that matches that of gas-phase Al.

We have used infrared-infrared double resonance spectroscopy to record a rovibrational eigenstate resolved spectrum of benzene in the region of the CH stretch first overtone. This experiment is the first of a series aimed at investigating intramolecular vibrational energy redistribution ͑IVR͒ in aromatic molecules. The experiment has been carried out in a supersonic molecular beam apparatus using bolometric detection. A tunable resonant cavity was used to enhance the on-beam intensity of the 1.5 m color center laser used to pump the overtone, and a fixed frequency ͓R(30)͔ 13 CO 2 laser was used to saturate the coinciding 18 r Q(2) transition of benzene. After assigning the measured lines of the highly IVR fractionated spectrum to their respective rotational quantum number J, analysis of the data reveals that the dynamics occurs on several distinct time scales and is dominated by anharmonic coupling with little contribution from Coriolis coupling. After the fast ͑ϳ100 fs͒ redistribution of the energy among the previously observed ''early time resonances'' ͓R. H.

The first eigenstate resolved, near the infrared spectrum of benzene in the region of the first C-H stretch overtone ͑6000 cm Ϫ1 ͒ has been obtained with an IR-IR double-resonance molecular beam optothermal spectrometer. Using a hierarchical tree analysis and level spacing statistics, we show that the intramolecular vibrational relaxation occurs nonergodically over at least seven different time scales ranging from 100 fs to 2 ns.

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