Abstract
The combination of supersonic expansions with IR action spectroscopy techniques is the basis of many successful approaches to study cluster structure and dynamics. The effects of temperature and temperature evolution are important with regard to both the cluster synthesis and the cluster dynamics upon IR excitation. In the past the combination of the sodium doping technique with IR excitation enhanced near threshold photoionization has been successfully applied to study neutral, especially water clusters. In this work we follow an overall examination approach for inspecting the interplay of cluster temperature and cluster structure in the initial cooling process and in the IR excitation induced heating of the clusters. In molecular simulations, we study the temperature dependent photoionization spectra of the sodium doped clusters and the evaporative cooling process by water molecule ejection at the cluster surface. We present a comprehensive analysis that provides constraints for the temperature evolution from the nozzle to cluster detection in the mass spectrometer. We attribute the IR action effect to the strong temperature dependence of sodium solvation in the IR excited clusters and we discuss the effects of geometry changes during the IR multi-photon absorption process with regard to application prospects of the method.
Highlights
Draws the conclusion that the phase equilibrium between the liquid and crystalline solid state appears in this size range through heterophasic oscillations in time, a process without analog for macroscopic water
At elevated temperatures evaporative cooling may play an important role. To analyse these competitive processes we have examined the dependence of the ion signal as a function of the delay time between the IR and ionizing UV laser pulse and we simulated the solvation of the Na atom in (H2O)[7] clusters and the evaporative cooling process over a broad range of cluster temperatures
In this work we have examined the temperature evolution of water clusters in IR action spectroscopy experiments using continuous supersonic expansions
Summary
Draws the conclusion that the phase equilibrium between the liquid and crystalline solid state appears in this size range through heterophasic oscillations in time, a process without analog for macroscopic water. The photoionization is fragmentation free under these conditions.[29] Second, the positive IR enhanced signal is obtained simultaneously for each cluster size of the distribution and third, the IR excitation can be fragmentation free at least for certain stable cluster configurations.[30,31] Molecular simulations have shown that heating increases the abundance of Na atoms in configurations with low ionization energy explaining the positive IR signal.[27,32] at elevated temperatures evaporative cooling may play an important role To analyse these competitive processes we have examined the dependence of the ion signal as a function of the delay time between the IR and ionizing UV laser pulse and we simulated the solvation of the Na atom in (H2O)[7] clusters and the evaporative cooling process over a broad range of cluster temperatures. Detailed descriptions of the method are found in ref. 15 and 32, more details on the experiments of this work are given in the ESI.†
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