Femtosecond Rotational Dynamics of D_{2} Molecules in Superfluid Helium Nanodroplets.

Rotational dynamics of D_{2} molecules inside helium nanodroplets is induced by a moderately intense femtosecond pump pulse and measured as a function of time by recording the yield of HeD^{+} ions, created through strong-field dissociative ionization with a delayed femtosecond probe pulse. The yield oscillates with a period of 185 fs, reflecting field-free rotational wave packet dynamics, and the oscillation persists for more than 500 periods. Within the experimental uncertainty, the rotational constant B_{He} of the in-droplet D_{2} molecule, determined by Fourier analysis, is the same as B_{gas} for an isolated D_{2} molecule. Our observations show that the D_{2} molecules inside helium nanodroplets essentially rotate as free D_{2} molecules.

Rotational Coherence Spectroscopy of Molecules in Helium Nanodroplets: Reconciling the Time and the Frequency Domains.

Alignment of OCS, CS_{2}, and I_{2} molecules embedded in helium nanodroplets is measured as a function of time following rotational excitation by a nonresonant, comparatively weak ps laser pulse. The distinct peaks in the power spectra, obtained by Fourier analysis, are used to determine the rotational, B, and centrifugal distortion, D, constants. For OCS, B and D match the values known from IR spectroscopy. For CS_{2} and I_{2}, they are the first experimental results reported. The alignment dynamics calculated from the gas-phase rotational Schrödinger equation, using the experimental in-droplet B and D values, agree in detail with the measurement for all three molecules. The rotational spectroscopy technique for molecules in helium droplets introduced here should apply to a range of molecules and complexes.

Heterogeneous and hyperfine interactions between valence states of molecular iodine correlating with the I((2)P1/2) + I((2)P1/2) dissociation limit.

Detailed analysis of interactions between all 0g (+), 1u, and 0u (-) weakly bound states of iodine molecule correlating with the I((2)P1/2) + I((2)P1/2) (bb) dissociation limit has been performed. For this purpose, the 0u (-) (bb) state has been described using analysis of rotationally resolved excitation spectra of luminescence from the g0g (-) state populated in a three-step three-color perturbation facilitated excitation scheme via the 0u (-) state. Energies of 41 rovibrational levels, molecular constants, and potential energy curve have been determined. Energy gaps between closest rovibrational levels of the 0u (-) and 0g (+), 1u (bb) states are found to be large, ∼6 cm(-1). However, interaction of all three 0g (+), 1u, and 0u (-) (bb) states has been observed. It has been found that the 0u (-) and 1u electronic states are mixed by heterogeneous interactions, while their mixing with the 0g (+) one is due to hyperfine interactions predominantly. Admixture coefficients and electronic matrix elements of the coupling between the 0g (+) ∼1u, 0g (+)∼0u (-), and 0u (-) ∼1u states have been estimated.