Spectroscopy and Dynamics of K Atoms on Argon Clusters

Abstract

We present a combined experimental and simulation study of the 4s → 4p photoexcitation of the K atom trapped at the surface of ArN clusters made of a few hundred Ar atoms. Our experimental method based on photoelectron spectroscopy allows us to firmly establish that one single K atom is trapped at the surface of the cluster. The absorption spectrum is characterized by the splitting of the atomic absorption line into two broad bands, a Π band associated with p orbitals parallel to the cluster surface and a Σ band associated with the perpendicular orientation. The spectrum is consistent with observations reported for K atoms trapped on lighter inert gas clusters, but the splitting between the Π and Σ bands is significantly larger. We show that a large amount of K atoms are transiently stuck and eventually lost by the Ar cluster, in contrast with previous observations reported for alkaline earth metal systems. The excitation in the Σ band leads systematically to the ejection of the K atom from the Ar cluster. On the contrary, excitation in the Π band leads to the formation of a bound state. In this case, the analysis of the experimental photoelectron spectrum by means of nonadiabatic molecular dynamics simulation shows that the relaxation drives the system toward a basin where the coordination of the K atom is 2.2 Ar atoms on the average, in a poorly structured surface.

Absorption Spectroscopy, a Tool for Probing Local Structures and the Onset of Large-Amplitude Motions in Small KArn Clusters at Increasing Temperatures

“…The Π-band reported by Awali et al . in the large cluster experiment is quantitatively very close to that obtained here for KAr 6 at 21 K. It features an asymmetric band peaking slightly on the red side of the atomic line, with a significant shoulder extending as a tail to the blue side of the same line.…”

“…The discussion above concerning the floppy character of the KAr 10 clusters appears as the onset of such a surface exploration when the cluster size increases. Actually, such a surface motion in the large cluster experiment was successfully simulated in the theoretical part of the latter work for, for example, KAr 560 clusters …”

“…The only available information concerns much larger clusters, broadly distributed about the KAr n ≈800 average size (note that this work combines experimental information with numerical simulations, adapted to the large cluster environment). The cluster temperature was not measured in the latter experiment by Awali et al where the K atom is deposited collisionally on the Ar clusters using the pick-up technique. Nevertheless, it can be estimated by comparison with the temperature of 30–34 K measured by Torchet and co-workers in the 1980s for pure Ar clusters of similar size. , Since the K–Ar binding energy is approximately half that of Ar–Ar (41.7 vs 99.53 cm –1 ) only clusters of ≤30 K temperature can accommodate a K atom.…”

“…The simulation of the photoelecton spectra reported by Awali et al . requests the optimization of the excited states, which is under study and will be the object of a forthcoming paper.…”

“…Nevertheless, it can be estimated by comparison with the temperature of 30–34 K measured by Torchet and co-workers in the 1980s for pure Ar clusters of similar size. , Since the K–Ar binding energy is approximately half that of Ar–Ar (41.7 vs 99.53 cm –1 ) only clusters of ≤30 K temperature can accommodate a K atom. Awali et al have discussed that because of this effect, many pick-up events result into the late evaporation of the K atom. This result is consistent with the evaporation observed in the present calculations at temperatures above 25 K for the larger clusters.…”