The optical spectra of hydrogen-passivated diamond clusters (diamondoids) precisely defined in size and shape have been measured in the gas phase, i.e., under an environment similar to boundary conditions typically assumed by theory. Characteristic optical properties evolve for these wide band-gap semiconductor nanocrystals as a function of size, shape, and symmetry in the subnanometer regime. These effects have not previously been theoretically predicted. The optical response of the tetrahedral-shaped C_{26}H_{32} diamond cluster [1(2,3)4] pentamantane is found to be remarkably similar to that of bulk diamond.

The ionization and charge separation processes of nanoplasmas created by resonant excitation of atomic clusters in intense soft x-ray pulses have been investigated. Through irradiation with femtosecond pulses from the FLASH free electron laser (FEL) at λ = 13.7 nm and power densities exceeding 10 14 W cm −2 the clusters are highly ionized with transient atomic charge states up to 9+. Variation of the cluster composition from pristine to doped and core-shell systems allows tracking of the spatial origin and charge states of the fragments yielding insight into the nanoplasma dynamics. The data give evidence for efficient charge redistribution processes leading to a Coulomb explosion of the cluster outer part and recombination of the nanoplasma core. The experiments show qualitatively different processes for (soft) x-ray produced nanoplasmas from the optical (IR) strong-field regime where the clusters disintegrate completely in a Coulomb explosion. (Some figures in this article are in colour only in the electronic version) Understanding the interaction of light with matter has been a central theme of physics over the past century, starting with the concept of the photon and the inception of quantum theory. The invention of the laser and the continuing advance in laser technologies has made it possible to explore regimes of nonlinear light-matter interaction leading to novel laser-based concepts for particle accelerators, plasma formation and nuclear fusion [1]. Currently, we are witnessing the advent of intense lasers in the x-ray regime. One of the most exciting prospects of research with x-ray lasers is direct imaging of nonperiodic nanoscale objects, such as biomolecules, nanocrystals, living cells and viruses [2]. Even though it is crucial for the success of the imaging experiments, understanding the interaction of intense x-ray pulses with atomic systems and the underlying dynamics is still in its infancy. To date, virtually all studies about the ionization as well as nuclear dynamics of nanometer-sized structures in intense (soft) x-ray pulses are of theoretical nature [3-6] and no experimental data are available. For the experimental investigations of matter in intense light pulses atomic clusters are ideal because their size can be tuned from the molecular to the bulk-like regime and there is no energy dissipation into surrounding media. The ionization dynamics of clusters in intense laser pulses depend considerably on the radiation wavelength. In the infrared spectral regime the cluster is ionized by the optical field and the resulting transient nanoplasma is efficiently heated by the external laser field via inverse bremsstrahlung (IBS) and collective effects, leading to a Coulomb explosion of the cluster [7]. Because the ponderomotive energy scales with ω −2 (ω-laser frequency) and thus, the direct effect of the laser field on the electron movement is small, it was a big surprise when experiments in the vacuum-ultraviolet spectral regime at 100 nm and intensities up to 10 13 W cm −2 reported unexpectedly...

The ionization potentials of size- and isomer-selected diamondoids (nanodiamond containing one to five crystal cages) have been measured by means of total-ion-yield spectroscopy. We find a monotonic decrease of the ionization potential with increasing diamondoid size. This experimental result is compared to recent theoretical predictions and comparable investigations on related carbon clusters, the fullerenes, which show isomer effects to be stronger than size dependence.

We present photoluminescence spectra and excited state decay rates of a series of diamondoids, which represent molecular structural analogues to hydrogen-passivated bulk diamond. Specific isomers of the five smallest diamondoids (adamantane-pentamantane) have been brought into the gas phase and irradiated with synchrotron radiation. All investigated compounds show intrinsic photoluminescence in the ultraviolet spectral region. The emission spectra exhibit pronounced vibrational fine structure which is analyzed using quantum chemical calculations. We show that the geometrical relaxation of the first excited state of adamantane, exhibiting Rydberg character, leads to the loss of T d symmetry. The luminescence of adamantane is attributed to a transition from the delocalized first excited state into different vibrational modes of the electronic ground state. Similar geometrical changes of the excited state structure have also been identified in the other investigated diamondoids. The excited state decay rates show a clear dependence on the size of the diamondoid, but are independent of the particle geometry, further indicating a loss of particle symmetry upon electronic excitation.

We investigated the changes in electronic structures induced by chemical functionalization of the five smallest diamondoids using valence photoelectron spectroscopy. Through the variation of three parameters, namely functional group (thiol, hydroxy, and amino), host cluster size (adamantane, diamantane, triamantane, [121]tetramantane, and [1(2,3)4]pentamantane), and functionalization site (apical and medial) we are able to determine to what degree these affect the electronic structures of the overall systems. We show that unlike, for example, in the case of halobenzenes, the ionization potential does not show a linear dependence on the electronegativity of the functional group. Instead, a linear correlation exists between the HOMO-1 ionization potential and the functional group electronegativity. This is due to localization of the HOMO on the functional group and the HOMO-1 on the diamondoid cage. Density functional theory supports our interpretations.