Over the past two decades, nanomaterials have advanced from the realm of fundamental science to having a presence in our daily lives, due to emergent phenomena associated with their inherent spatial scales. Nanomaterials are being used in a wide range of applications, including biofuels, biomedical technology, quantum and energy science, and aerospace technologies. Particularly, there is great interest in the photophysical properties of nanomaterials due to their broad tunability, by adjusting their composition, size, or geometry. However, a complete understanding of these properties is currently lacking, in part due to the complex environments often surrounding these nanomaterials. Here, we simplify the situation by studying the photophysical properties of isolated, ligand-free nanoparticles in vacuum, thereby avoiding any effects from the surrounding environment. We accomplish this by combining a magnetron-sputtering nanoparticle source with a velocity map imaging photoelectron spectrometer. Using this novel technique, we drive strong-field effects—including above-threshold-ionization and channel-closings—in nanoparticles at much lower intensities than required to observe similar phenomena in atomic and molecular systems. Additionally, we conduct time-resolved photoelectron measurements on isolated, ligand-free nanoparticles, thereby probing isolated nanoparticle photophysics on femtosecond time scales. We use an 800/400 nm pump-probe geometry to disentangle the contribution of different ionization mechanisms, each with a distinctive timescale ranging from femtoseconds to picoseconds. We find that the transition between mechanisms depends upon the nanoparticle composition, and that these pathways can be influenced by varying the intensities of the pump or probe pulse, to access different regions in the band structure. Moreover, this approach can be extended to EUV probes of excited state dynamics and orbitals. Our unique apparatus, and the ligand-free nanoparticles that it produces, enables the application of methods from strong-field physics and attosecond science to study nanoparticles. This creates opportunities to examine ultrafast imaging of nano-scale potentials, state-resolved photoionization measurements, and surface plasmon dynamics/control.

Time-resolved strong-field photoemission dynamics of isolated, ligand-free nanoparticles using UV, visible, and mid-infrared laser fields

2019 Fall National Meeting

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