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High intensity XUV radiation from a free-electron laser (FEL) was used to create a nanoplasma inside ammonia clusters with the intent of studying the resulting electron–ion interactions and their interplay with plasma evolution. In a plasma-like state, electrons with kinetic energy lower than the local collective Coulomb potential of the positive ionic core are trapped in the cluster and take part in secondary processes (e.g. electron-impact excitation/ionization and electron–ion recombination) which lead to subsequent excited and neutral molecular fragmentation. Using a time-delayed UV laser, the dynamics of the excited atomic and molecular states are probed from −0.1 ps to 18 ps. We identify three different phases of molecular fragmentation that are clearly distinguished by the effect of the probe laser on the ionic and electronic yield. We propose a simple model to rationalize our data and further identify two separate channels leading to the formation of excited hydrogen. [ABSTRACT FROM AUTHOR]

The dynamics following the photoexcitation of Ag atoms in 4He nanodroplets via the 5p 2P1/2← 5s 2S1/2 and 5p 2P3/2← 5s 2S1/2 transitions has been investigated in a joint experimental and theoretical effort. It has been experimentally found that upon excitation to the 2P1/2 state, the Ag atoms are ejected with a speed distribution peaking at about 55 m s−1. When Ag is excited to the 2P3/2 state, a rich phenomenology is found. While a fraction of the impurities remains solvated, the impurities that are ejected from the droplets either as Ag or AgHe have speed distributions similar, but not identical, to those found for excitation to the 2P1/2 state. The experimental findings are qualitatively analyzed within a three-dimensional, time-dependent density functional approach for the helium droplet. The dynamics of the Ag–4He1000 system has been followed for several tens of picoseconds, long enough to observe AgHe exciplex formation and the departure of the photoexcited Ag atom from the helium droplet. [ABSTRACT FROM AUTHOR]

High-resolution absorption spectra of adenine, 9-methyladenine and 2-aminopurine in helium nanodroplets have been recorded. In contrast to molecular beam experiments, large variations in linewidths are observed for adenine and 9-methyladenine. At the same time, the spectrum of 2-aminopurine remains sharp upon solvation in helium droplets. The line broadening observed for adenine and 9-methyladenine is attributed to a significant decrease of the lifetime of the 1Lb(ππ*) state and of 1nπ* levels vibronically coupled to this state. The origin of the lifetime reduction is argued to be related to the increased accessibility of the 1nπ*/1Lb(ππ*) conical intersection upon solvation of these molecules in liquid helium. [ABSTRACT FROM AUTHOR]

The dynamics following the photoexcitation of Ag atoms in (4)He nanodroplets via the 5p (2)P1/2 ← 5s (2)S1/2 and 5p (2)P3/2 ← 5s (2)S1/2 transitions has been investigated in a joint experimental and theoretical effort. It has been experimentally found that upon excitation to the (2)P1/2 state, the Ag atoms are ejected with a speed distribution peaking at about 55 m s(-1). When Ag is excited to the (2)P3/2 state, a rich phenomenology is found. While a fraction of the impurities remains solvated, the impurities that are ejected from the droplets either as Ag or AgHe have speed distributions similar, but not identical, to those found for excitation to the (2)P1/2 state. The experimental findings are qualitatively analyzed within a three-dimensional, time-dependent density functional approach for the helium droplet. The dynamics of the Ag-(4)He1000 system has been followed for several tens of picoseconds, long enough to observe AgHe exciplex formation and the departure of the photoexcited Ag atom from the helium droplet.

The dynamics following the photoexcitation of Na and Li atoms located on the surface of helium nanodroplets has been investigated in a joint experimental and theoretical study. Photoelectron spectroscopy has revealed that excitation of the alkali atoms via the (n + 1)s ←ns transition leads to the desorption of these atoms. The mean kinetic energy of the desorbed atoms, as determined by ion imaging, shows a linear dependence on excitation frequency. These experimental findings are analyzed within a three-dimensional, time-dependent density functional approach for the helium droplet combined with a Bohmian dynamics description of the desorbing atom. This hybrid method reproduces well the key experimental observables. The dependence of the observables on the impurity mass is discussed by comparing the results obtained for the (6)Li and (7)Li isotopes. The calculations show that the desorption of the excited alkali atom is accompanied by the creation of highly non-linear density waves in the helium droplet that propagate at supersonic velocities.

A new method is presented for recording excitation spectra of molecules embedded in helium nanodroplets. The method relies on the complete evaporation of the droplets following excitation of a dissolved molecule and the subsequent detection of the remaining unsolvated molecule by mass spectrometry. The technique has been successfully applied to record the S1 1B(2u) <-- S0 1A(1g) transition in benzene. The transition frequencies determined by this new method, beam depletion spectroscopy and REMPI spectroscopy have been found to differ slightly from each other. It is argued that these differences in transition frequency are related to the different droplet sizes probed by the spectroscopic techniques.