We report electron diffraction of pyrene nanoclusters embedded in superfluid helium droplets. Using a least-squares fitting procedure, we have been able to separate the contribution of helium from those of the pyrene nanoclusters and determine the most likely structures for dimers and trimers. We confirm that pyrene dimers form a parallel double-layer structure with an interlayer distance of 3.5 Å and suggest that pyrene trimers form a sandwich structure but that the molecular planes are not completely parallel. The relative contributions of the dimers and trimers are ~6:1. This work is an extension of our effort of solving structures of biological molecules using serial single-molecule electron diffraction imaging. The success of electron diffraction from an all-lightatom sample embedded in helium droplets offers reassuring evidence of the feasibility of this approach.

We report experimental observations of Coulomb explosion using a nanosecond laser at 532 nm with intensities less than 10 12 W/cm 2 . We observe multiply charged atomic ions Ar n+ (1 ≤ n ≤ 7) and C n+ (1 ≤ n ≤ 4) from argon clusters doped with molecules containing aromatic chromophores.The yield of Ar n+ depends on the size of the cluster, the number density, and the photostability of the dopant. We propose that resonant absorption of Ar N + achieves a high degree of ionization, and the highly positively charged cluster has the capability to strip electrons from the evaporating Ar + on the surface of the cluster prior to Coulomb explosion, forming Ar n+ .

We report the effect of two molecular species, fluorene (C13H10) and 1, 3, 5-trichlorobenzene (C6H3Cl3, 3ClB), embedded in atomic argon clusters, on the generation of multiply charged atomic ions (MCAI) in moderately intense nanosecond laser fields at 532 nm. The near resonant-enhancement of two photon absorption in the two aromatic species produces only a few low charge state (+2) atomic ions in a neat molecular cluster, but enclosure of the same cluster with layers of Ar can significantly increase the charge state of MCAI. The yields of singly charged atomic ions from the molecular species, such as H+, C+, and Cl+, are positively correlated to the number of molecules inside an Ar cluster, but the yields of the MCAI and Ar+ demonstrate opposite behaviors. A higher number of aromatic molecules is actually detrimental to the production of Ar+ and of MCAI. Results of exponential fittings of the yields of MCAI at different laser intensities reveal a systematic change for the exponent of Ar+: with increasing concentrations of 3ClB in Ar clusters, the exponent decreases and eventually reaches the same value as those of MCAI. These results are consistent with our previous hypothesis that the formation mechanism of MCAI may be different from that of singly charged species, and that the strong resonance of Ar3+ may play an important role in the overall energy absorption. Moreover, the effect of the molecular core seems to change the formation mechanism of Ar+ to that of MCAI.

We report the laser intensity dependence of multiply charged atomic ions (MCAIs) Arn+ with 2 ≤ n ≤ 8 from argon clusters in focused nanosecond laser fields at 532 nm. The laser field, in the range of 1011–1012 W/cm2, is insufficient for optical field ionization but is adequate for multiphoton ionization. The MCAI sections of the mass spectra for clusters containing 3700 and 26 000 atoms are dominated by Arn+ with 7 ≤ n ≤ 9, extending to Ar14+. While the distributions of the MCAIs remain largely constant throughout the intensity range of the laser, the abundance of Ar+ relative to the abundances of the MCAIs increases dramatically with increasing laser intensity. Consequently, exponential fittings of the yields result in a larger exponent for Ar+ than for MCAIs, and the exponents of MCAIs with 2 ≤ n ≤ 8 are similar, with only slight variations for different charge states. The width of the arrival time and, hence, the corresponding kinetic energy of Ar+ also increases with increasing laser intensities, while the width of the arrival time of MCAIs remains constant throughout the range of measurements. These results call for more detailed theoretical investigations in this regime of laser–matter interactions.

We report detailed measurements of velocities and sizes of superfluid helium droplets produced from an Even–Lavie pulse valve at stagnation pressures of 20–60 atm and temperatures between 5.7 and 18.0 K. By doping neutral droplets with Rhodamine 6G cations produced from an electrospray ionization source and detecting the positively charged droplets at two different locations along the beam path, we determine the velocities of the different groups of droplets. By subjecting the doped droplet beam to a retardation field, size distributions can then be analyzed. We discover that at stagnation temperatures above 8.0 K, a single group of droplets is observed at both locations, but at 8.0 K and below, two different groups of droplets with different velocities are detectable. The slower group, considered from fragmentation of liquid helium, cannot be deterred by the retardation voltage at 9 kV, implying an exceedingly large size. The faster group, considered from condensation of gaseous helium, has a bimodal distribution when the stagnation temperatures are below 12.3 K at 20 and 40 atm, or 16.1 K at 60 atm. We also report similar size measurements using low energy electrons for impact ionization, and this latter method can be used for facile in situ characterization of pulsed droplet beams. The mechanism of the bimodal size distribution of the condensation group and the reason for the coexistence of both the condensation and fragmentation groups remain elusive.