Excitation of Sodium Atoms Attached to Helium Nanodroplets: The 3p ← 3s Transition Revisited

Abstract

The dynamics of Na atoms on the surface of helium nanodroplets following excitation via the 3p ← 3s transition has been investigated using state-specific ion-based detection of the products. Excitation of the system to the 3p (2)Π states is found to lead to the desorption of both bare Na and NaHe exciplexes. The associated speed distributions point to an impulsive desorption process for Na products and a thermally driven process for the NaHe exciplexes. In contrast, excitation of the 3p (2)Σ state leads exclusively to the impulsive desorption of Na atoms. In this case, the desorption is accompanied by a helium-induced relaxation process, as evidenced by the large fraction of detected Na (2)P1/2 atoms. The relaxation process is thought to be related to a crossing between the (2)Π1/2 and (2)Σ potential energy curves at large distance.

Density functional theory of doped superfluid liquid helium and nanodroplets

“…This has been attributed to a curve crossing taking place between the pseudo-diatomic states at long range. 110 Similar curve crossings have also been reported for other alkali metal-rare gas systems. 111…”

“…It was shown that Π state excitation produces both bare Na and NaHe n exciplexes. 110 Based on their measured velocity distributions, the bare Na atoms appear to be produced by an impulsive mechanism whereas exciplex production is thermally driven. The Σ state is very repulsive and leads to impulsive desorption of bare Na.…”

“…The Σ state is very repulsive and leads to impulsive desorption of bare Na. 110 Based on the spin-adiabatic approximation, these bare atoms should only be produced in the 2 P 3/2 state. However, the experiment measured population in both the 2 P 3/2 and the 2 P 1/2 states.…”

Dynamics of solvation and desolvation of rubidium attached to He nanodroplets

“…Both the 6pΣ and the 6pΠ-states of the RbHe diatom feature potential wells which sustain bound vibrational states that can be directly populated by laser excitation out of the groundstate of the RbHe N complex [14,29]. Thus, exciplexes are directly created in a process akin to photoassociation, in contrast to previously observed Na * He and K * He exciplexes which were formed by an indirect tunneling process upon excitation of the lowest pΠ-states [27,28]. Exciplex formation is the only route to producing Rb + He ions by photoionization using continuous-wave or nanosecond lasers, where ionization takes place at long delay times when the dynamics of exciplex formation and desorption off the droplets is long complete.…”

“…A clear manifestation of the breakdown of the simple pseudo-diatomic model is the formation of Rb * He exciplexes which we discuss in the following section. Recently, M. Drabbels and coworkers suggested that the pseudo-diatomic potentials of the excited Na * He N complex may be transiently shifted and even intersect [17,28]. Detailed three-dimensional simulations including the full spectrum of properties of He droplets are needed to provide an accurate description of this kind of dynamics [16,20,21,41].…”