We report the first experimental confirmation of a metal-atom-He exciplex formation in liquid helium. Following the excitation of the D2 line of an Ag atom, we have observed the atomic D1 line emission and a broadband emission to the red of the D1 line. The band has been assigned to theà 2 P 3͞2 2X 2 S 1 bound-free transition of the He-Ag-He linear exciplex trapped in a microcavity in liquid helium. The assignment has been confirmed by an ab initio calculation on the AgHe 2 molecule. The absence of the atomic D2 emission as well as an emission from AgHe indicates that the 2 P 3͞2 ! 2 P 1͞2 relaxation and the formation of AgHe 2 in theà 2 P 3͞2 , y 0 0 state occur on a subnanosecond time scale. PACS numbers: 67.40.Yv, 33.20.Kf In recent years much interest arose in spectroscopic studies of impurity particles in liquid helium. The interest was stimulated by the development of new experimental techniques for dispersing atoms and molecules in the liquid [1][2][3][4]. The dispersed neutral species are usually trapped in microscopic bubblelike cavities formed by a strong repulsive force between the impurity electron cloud and closed shells of surrounding helium atoms [5]. The coupling of the trapped neutral atoms with the helium bubbles (or soft cages) usually produces a large blueshift and broadening for absorption lines [3,6,7], while emission lines show shifts and broadening on the order of only 1-2 nm or less. Such behavior of the absorption lines has been related to the Franck-Condon principle. The liquid configuration around the impurity atom does not change during the excitation, thus immediately after the excitation, the excited atom finds itself still in the bubble equilibrated for the ground state. The stronger interaction of the liquid with the excited atom, which usually has an electron cloud larger than that of the ground state, induces a large blueshift and broadening for the absorption lines. Such shift and broadening have been extensively studied for Ba, Cs, and Rb in a pressure range of up to 30 atm (solid helium), and reproduced by a calculation based on metal-atom-He pair potentials [8][9][10].Although remarkable progress has recently been made in spectroscopy of neutral atoms in liquid helium [11], there remain a number of phenomena for which no clear interpretation has been given. One of them is the absence of laser induced fluorescence (LIF) from light alkali atoms (Li, Na, and K) [6]. Also for heavy alkalis (Rb and Cs), LIF in the D2 line is missing or very weak despite the fact that the excitation spectra clearly show D2 absorption. An attempt to explain these observations has been made by Dupont-Roc [12]. Using a simple model based on a continuous and local description of liquid helium, he analyzed a configuration of liquid helium around an alkali atom impurity in the lowest p state. According to his model, a light alkali atom in the p excited state attracts 5-6 helium atoms in a nodal plane, forming a helium ring localized near the atomic core (1.9 , R , 3 Å). Such a configuration can produ...

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The rotation-vibration spectra of (32SF6)2 have been studied near the ν3 band of the 32SF6 monomer. The parallel band 14 cm−1 below the monomer band origin shows a well resolved J-structure, while the perpendicular band 8 cm−1 above the origin exhibits several Q-branch peaks as the only resolved strong lines. The structure of (32SF6)2 is consistent with a D2d symmetry from the intensity alternation and the existence of a first-order Coriolis interaction observed in the perpendicular band. The energy difference between the two bands is very close to the value calculated by a dipole–dipole and dipole-induced dipole interaction model, while the location of the two bands is blueshifted from the calculated values by about 2 cm−1. The possible influence of internal rotation is discussed.

The high-resolution infrared absorption spectra of the symmetric (ν1) and the antisymmetric NO stretching (ν4) bands of nitric oxide dimer (NO)2 have been measured for 14NO and 15NO in supersonic free jets. The ν1 and ν4 bands exhibit a dramatic difference in linewidth: approximately 200 MHz [full width at half-maximum (FWHM)] for the ν1 band and approximately 5 GHz (FWHM) for the ν4 band. The predissociation lifetimes deduced from the linewidths are in excellent agreement with those reported in the recent time-resolved measurement for 14NO [Casassa et al., J. Chem. Phys. 89, 1966 (1988)]. There is no systematic dependence of the linewidth on the rotational states of (NO)2. Isotope substitution does not influence the linewidths significantly. However, the ν4 band structure of (15NO)2 is very different from that of (14NO)2, a difference that may be explained by a perturbation from a low-lying singlet vibronic state. All of the experimental results obtained to date may be accounted for if it is assumed that the predissociation of (NO)2 is enhanced by an electronically nonadiabatic transition to a repulsive triplet surface. Vibrational potential coupling between the NO stretching and intermolecular modes, particularly an in-plane NO bending mode, appears to play a key role in the mode specificity.