Dynamics of Excited Sodium Atoms Attached to Helium Nanodroplets Evgeniy Loginov † and Marcel Drabbels* Laboratoire de Chimie Physique Mole ́ culaire, Ecole polytechnique Fe ́ de ́ rale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland ABSTRACT: The dynamics of laser-excited sodium atoms at the surface of helium nanodroplets has been investigated as a function of quantum state. For all cases, excitation of the system leads to desorption of the sodium atom from the droplet surface. The mean kinetic energy of the desorbed atoms scales linearly with excitation frequency, indicative of an impulsive desorption process. The energy partitioning between the helium and the desorbing sodium atom depends on the quantum state and appears to be related to the size and shape of the electron orbital. The speed distributions of desorbed NaHe exciplexes point toward a direct formation process of an exciplex with no internal energy. Photoelectron spectroscopy reveals an increasing importance of helium-induced relaxation with increasing quantum state, which is tentatively attributed to curve crossing between different NaHe N interaction potentials during the desorption process. 1. INTRODUCTION Helium nanodroplets are fascinating quantum systems that are nowadays routinely used as matrix for spectroscopic studies. 1-4 Many of the droplets’ properties have been elucidated using atomic and molecular probes. 5-7 Alkali atoms play a special role in this, as they are one of the few species known to reside on the surface of the droplets and thus able to probe the boundary region. 8,9 Consequently, the np 2 P ← ns 2 S transitions of alkali atoms attached to helium nanodroplets have attracted the interest of theoreticians and experimentalist alike. 9-31 Recently the spectroscopy of higher excited states has been ex- plored. 32-42 From these studies some general trends could be identi fied. The spectra involving states with low and intermediate principal quantum number are characterized by broad resonances that are blue-shifted with respect to the corresponding atomic resonances, reflecting the repulsive interaction of the excited valence electron of the atom with the helium. The spectra can be successfully reproduced using the so-called pseudodiatomic model, in which the internal degrees of freedom of the helium droplet are ignored and the system is treated as a diatomic molecule in which the helium droplet plays the role of the second atom. 9,21 The interaction between the alkali atom and the helium droplet is reduced to a one-dimensional potential calculated as the sum of alkali-He pair potentials. This simple model progressively fails to reproduce the experimental spectra involving higher excited states as helium-induced configuration interactions are not taken into account. 34,43 For states with large principal quantum number where the mean radius of the electron orbit is comparable or larger than the size of the droplet, the interaction of the valence electron with the helium decreases and a Rydberg-like system results. 33,36 While the role of the excited state on the spectrum is now fairly well understood, its influence on the ensuing dynamics is still largely unexplored. Experiments concentrating on the np 2 P ← ns 2 S transitions have found that the excited atoms desorb from the droplets on a picosecond time scale, either as bare atoms or as alkali-helium exciplexes. 10-15 The heavy alkalis Rb and Cs are an exception, they remain attached to the helium droplet when excited close to the gas phase D1 transition. 37,44 Only recently the desorption dynamics and the exciplex formation involving higher excited states was investigated. 38,45 A combined experimental and theoretical study revealed that the desorption of the excited atoms is a direct, rather than a statistical, process in which the energy is partitioned between the helium droplet and the alkali atom. 45 This allows the desorption process to be accurately described by an impulsive model, analogous to the dissociation of molecules. Another study revealed that the exciplexes formation and desorption is a direct process and that the helium induces relaxation of the excited atoms. 38 Similar processes have been identified in experiments on barium atoms which are also located at the surface of helium droplets. 46 In order to establish how the different processes depend on the excited state we have investigated the dynamics of Na atoms located on helium nanodroplets as a function of the excitation energy. The dynamics of the 4s state has been reported recently. 45 Here we discuss the result for states ranging from the 3d state all the way up to high Rydberg states close to the ionization threshold. Received: December 13, 2013 Revised: March 27, 2014 Published: March 27, 2014 Article pubs.acs.org/JPCA © 2014 American Chemical Society 2738 dx.doi.org/10.1021/jp4121996 | J. Phys. Chem. A 2014, 118, 2738-2748