Large liquid helium clusters (He L , n+10) produced in a supersonic jet are doped with alkali atoms (Li, Na, K) and characterized by means of laser induced fluorescence. Each cluster contains, on average, less than one dopant atom. Both excitation and emission spectra have been recorded. The observed excitation spectra are analyzed, calculating the transitions within an approach based on the hypothesis that the chromophores are trapped in a dimple on the cluster's surface as predicted by the theoretical calculations of Ancilotto et al. [9]. The results of the model calculations are in good qualitative agreement with the experimental findings. In spite of the very weak binding energy (a few cm\), some of the excited atoms remain bound to the surface, provided the excitation occurs at frequencies not too far from the alkali's gas phase absorptions. These bound-bound excitations produce very broad, red shifted emission spectra. At other, blue shifted frequencies, the excited atoms desorb from the cluster's surface, giving rise to unshifted, free atom, emission spectra. The heavier alkali metals (Na, K) show, compared to the calculations, an additional broadening which is attributed to surface excitations on the helium droplet.

Helium cluster isolation spectroscopy is a recently developed spectroscopic method that involves the formation of a beam of large helium clusters (104 atoms per cluster), the capture by the clusters of the atoms or molecules of interest in a low-pressure pick-up cell, and the spectroscopic study of the isolated species. Here we exploit the unique feature of this method of allowing the selective preparation of high-spin molecular species (e.g., triplet dimers) over their low-spin (singlet) counterparts to measure the spectra of several alkali dimers in their triplet manifold. By probing via laser-induced fluorescence their lowest triplet-to-triplet transitions, Li2, Na2, K2, and NaK are found to reside on the surface of the helium clusters. Since the spectroscopic shifts induced by the helium cluster are minimal, vibrational analysis of the electronic transitions produces transition frequencies that can be compared to previous ab initio and experimental values. Both bound−bound and bound−free transitions have been observed. Emission spectra reveal the presence of vibrational relaxation and nonadiabatic intersystem crossings of the excited dimers that result from the proximity of the helium cluster surface. Through this study we improve our understanding of triplet alkali dimer potential energy curves, we test an efficient analytical model to represent them, and we provide input information for the study of nonadditive effects present in quartet (spin-polarized) alkali trimers which can be formed using the same method.

Nanometer-sized helium droplets, each containing about 10(4) helium atoms, were used as an inert substrate on which to form previously unobserved, spin-3/2 (quartet state) alkali trimers. Dispersed fluorescence measurements reveal that, upon electronic excitation, the quartet trimers undergo intersystem crossing to the doublet manifold, followed by dissociation of the doublet trimer into an atom and a covalently bound singlet dimer. As shown by this work, aggregates of spin-polarized alkali metals represent ideal species for the optical study of fundamental chemical dynamics processes including nonadiabatic spin conversion, change of bonding nature, and unimolecular dissociation.

Rubidium atoms have been deposited on helium nanodroplets and optical excitation and emission spectra of the 5 2P–5 2S transition have been measured and interpreted. After laser excitation of the 5 2P3/2 state, fluorescence from a Rb*He exciplex is observed while pumping the D1 (5 2P1/2–5 2S) line yields only emission from free rubidium atoms. This observation is in agreement with the predictions from a recent model by Reho et al. [J. Chem. Phys. 113, 9694 (2000)] about the extraction of an alkali–helium exciplex from the doped helium nanodroplet surface. A high barrier along the Hen–He–Rb* axis of the 1 2Π1/2 potential prevents desorption of Rb*He within the excited state lifetime, whereas the 1 2Π3/2 potential permits the exciplex extraction. The excitation spectrum, on the other hand, reflects the structure of the excited states 1 2Π1/2, 1 2Π3/2, and 2 2Σ1/2 of the HenRb complex whose potential surfaces will depend strongly on the alkali–He interaction dynamics near the droplet surface. For a heavy surface dopant like Rb or Cs the droplet surface will be strongly distorted upon vibrational excitation of the dopant. Some of the consequences for the potentials are discussed for the example of the 1 2Σ1/2 ground state.

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In this paper we describe an application of reversed time-correlated single photon counting to the time-resolved spectroscopy of impurity atoms and molecules bound to large quantum clusters. The photo-induced dynamics of Na atoms on the surface of He and clusters have been studied by follow-H 2 ing the time dependence of their emission at selected excitation and emission wavelengths. Collection of atomic (16 980 ^145 cm~1) Ñuorescence arising from Na atoms excited on the cluster surface and immediately desorbed from it yields a Ðnite (ca. 70 ps) rise time and a decay time of 16.3 ^0.1 ns, equal to the known lifetime of the 3P ] 3S transition of atomic Na. The frequency distribution of the red emission due to atoms that do not leave the cluster immediately after excitation is shown to be due to a desorbed Na*ÈHe exciplex by obtaining quantitative agreement with predictions derived from available ab initio NaÈHe potentials. Formation of this excimer can occur along either the or excited state surface. 2% 1@2 2% 3@2 " Slow Ï (700 ps) and " fast Ï (ca. 70 ps) components of the rise time of the red emission (15 800 ^125 cm~1) are assigned, respectively, to the two formation channels. Introducing spinÈorbit coupling e †ects into the long range ab initio pair potential for an isolated Na*ÈHe generates a small barrier on the potential curve, which is linked to the observed slow exciplex forma-2% 1@2 tion time.

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We report on the investigation of mixed alkali metal (Ak) -alkaline earth metal (Ake) molecules on the surface of helium nanodroplets (HeN ). These molecules have recently attracted considerable attention as candidates for the formation of ultracold molecules with a magnetic and an electronic dipole moment a . In our experiments, LiCa and RbSr molecules are formed in a sequential pick-up process in their X 2 Σ + ground state and cool down rapidly to the droplet temperature of 0.38 K b . Excitation spectra of LiCa and RbSr were recorded by using resonance enhanced multi-photon ionization time-of-flight (REMPI-TOF) spectroscopy and laser induced fluorescence (LIF) spectroscopy. On the helium droplet, vibronic transitions in Ak-Ake molecules are broadened and show a characteristic asymmetric peak form, which is caused by the interaction between the molecule and the superfluid HeN environment. For the lower electronic transitions in LiCa and RbSr progressions of vibrational bands excited from the X 2 Σ + (ν ′′ = 0) state are observed. The LiCa spectra can be compared to molecular beam experiments c , which enables the assignment of three band systems near 15260 cm −1 , 19300 cm −1 and 22120 cm −1 as 2 Σ + , 2 ΠΩ and 2 Π band, respectively. In the RbSr excitation spectrum we observe a vibrationally resolved band system near 14020 cm −1 . Upon electronic excitation, a fraction of the molecules desorb from the droplet surface and dispersed fluorescence spectra allow to study the X 2 Σ + ground state and excited states of free Ak-Ake molecules.