Interatomic Coulombic decay in helium nanodroplets

Abstract

Interatomic Coulombic decay (ICD) is induced in helium (He) nanodroplets by photoexciting the n = 2 excited state of He + using XUV synchrotron radiation. By recording multiple coincidence electron and ion images we find that ICD occurs in various locations at the droplet surface, inside the surface region, or in the droplet interior. ICD at the surface gives rise to energetic He + ions as previously observed for free He dimers. ICD deeper inside leads to the ejection of slow He + ions due to Coulomb explosion delayed by elastic collisions with neighboring He atoms, and to the formation of He + k complexes.Isolated atoms or molecules excited by energetic radiation typically decay through intramolecular processes such as the emission of an electron or photon. In contrast, in weakly bound complexes, locally generated electrons can additionally interact with neighboring atoms or molecules, leading to new interatomic or intermolecular interactions. Interatomic Coulombic decay (ICD) is a particularly interesting decay process which occurs when local electronic decay is energetically forbidden [1]. Thus, ICD offers a new, ultrafast decay path where energy is exchanged with a neighboring atom leading to its ionization. Since its discovery, ICD has been observed in a wide variety of weakly-bound systems from He dimers [2,3] and rare-gas clusters to biologically relevant systems such as water clusters; for reviews see [4,5]. Today, the focus is on condensed-phase systems where ICD is involved in complex relaxation mechanisms [6][7][8], which can generate genotoxic low-energy electrons and radical cations [9]. Recently, it was suggested to utilize this property of ICD for cancer treatment [10,11].Here we present the first study of ICD in helium (He) nanodroplets. He nanodroplets are generally considered as an ultracold, inert spectroscopic matrix for embedded, isolated molecules and clusters [12,13]. Upon ionization by intense or energetic radiation, however, He droplets turn into a highly reactive medium, inducing reactions and secondary ionization processes of the embedded species [14]. Their homogeneous quantum liquid density profile, and the simple structure of atomic constituents, make He droplets particularly beneficial as benchmark systems for elucidating correlated decay processes. Recent examples include the collective autoionization of multiply excited pure He droplets [15,16] and the creation of doubly charged species by one-photon ionization of doped He droplets [17]. In this work we fully characterize the product states generated by ICD and secondary processes in He nanodroplets using coincidence imaging techniques.The experiments were performed using a He droplet machine attached to a velocity map imaging photoelectron-photoion coincidence (VMI-PEPICO) detector at the GasPhase beamline of Elettra-Sincrotrone Trieste, Italy. The apparatus has been described in detail elsewhere [18,19]. Briefly, a beam of He nanodroplets is produced by continuously expanding pressurized He (50 bar) of high purity He out of a c...

Penning Ionization of Acene Molecules by Helium Nanodroplets

“…The setup used for the present experiments has been described previously. 8,18 Briefly, a beam of He droplets with an average diameter of 6 nm is produced by continuously expanding The EUV light beam at the Gasphase beamline of Elettra Sincrotrone, Trieste, is narrowband (ν/∆ν > 10 3 ) and tunable over the discrete absorption bands of He nanodroplets up to the He ionization threshold. 29 In the photon energy range 19-23 eV we use a 0.2 µm thick tin filter to suppress higher order radiation.…”

The influence of retardation and dielectric environments on interatomic Coulombic decay

“…Furthermore, while a simple dielectric background or extended host molecule can be taken into account with molecular dynamics techniques 15 , 16 or via numerical quantum chemistry 17 – 19 , the full effects of dispersion, absorption, relativistic retardation and sharp boundaries remain computationally beyond reach. This means that such approaches are not able to account for the effects of a complex environment, such as that of a helium nanodroplet 20 – 23 or those found in biological settings 10 , 24 , 25 .…”

Interatomic and Intermolecular Coulombic Decay

“…In larger clusters collisions, fragmentation into complexes that include neutrals, and postdecay nuclear dynamics all lead to a blurring of the sought-for ion signal. 258 , 259 Pure spectroscopy of energetic ions is capable to detect ICD only in some favorable cases and has mostly been superseded by powerful coincidence methods, which will be described in the next section.…”

“…Works on ICD and ETMD that have used VMI electron spectroscopy are covered in refs ( 53 , 228 , 258 , 261 , and 269 − 274 ).…”