Photoinitiated Dynamics in Amorphous Solid Water via Nanoimprint Lithography

Laser pulses that act on fragile samples often alter them irreversibly, motivating single-pulse data collection. Amorphous solid water (ASW) is a good example. In addition, neither well-defined paths for molecules to travel through ASW nor sufficiently small samples to enable molecular dynamics modeling have been achieved. Combining nanoimprint lithography and photoinitiation overcomes these obstacles. An array of gold nanoparticles absorbs pulsed (10 ns) 532 nm radiation and converts it to heat, and doped ASW films grown at about 100 K are ejected from atop the irradiated nanoparticles into vacuum. The nanoparticles are spaced from one another by sufficient distance that each acts independently. Thus, a temporal profile of ejected material is the sum of about 10⁶ “nanoexperiments,” yielding high single-pulse signal-to-noise ratios. The size of a single nanoparticle and its immediate surroundings is sufficiently small to enable modeling and simulation at the atomistic (molecular) level, which has not been feasible previously. An application to a chemical system is presented in which H/D scrambling is used to infer the presence of protons in films composed of D₂O and H₂O (each containing a small amount of HDO contaminant) upon which a small amount of NO₂ has been deposited. The pulsed laser heating of the nanoparticles promotes NO₂/N₂O₄ hydrolysis to nitric acid, whose protons enhance H/D scrambling dramatically

Amorphous Solid Water: Pulsed Heating of Buried N₂O₄

Molecular transport and morphological change were examined in films of amorphous solid water (ASW). A buried N₂O₄ layer absorbs pulsed 266 nm radiation, creating heated fluid. Temperature and pressure gradients facilitate the formation of fissures through which fluid travels to (ultrahigh) vacuum. Film thickness up to 2400 monolayers was examined. In all cases, transport to vacuum could be achieved with a single pulse. Material that entered vacuum was detected using a time-of-flight mass spectrometer that recorded spectra every 10 μs. An ASW layer insulated the N₂O₄ layer from the high-thermal-conductivity MgO substrate; this was verified experimentally and with heat-transfer calculations. Laser-heated fluid strips water from fissure walls throughout its trip to vacuum. Experiments with alternate H₂O and D₂O layers reveal efficient isotope scrambling, consistent with water reaching vacuum via this mechanism. It is likely that ejected water undergoes collisions just above the film surface due to the high density of material that reaches the surface via fissures, as evidenced by complex temporal profiles extending past 1 ms. Little material enters vacuum after cessation of the 10 ns pulse because cold ASW near the film surface freezes material that is no longer being heated. A proposed model is in accord with the data

Results from the cloning and sequencing of DNA removed from the microarray.

<p>Results from the cloning and sequencing of DNA removed from the microarray.</p

Example of a portion of an array with a box pattern of electrodes used for deprotection and synthesis with varying concentrations of base during the deprotection step.

<p>Note that the central electrode and electrodes surrounding the box are not active during deprotection and so should not have any synthesis occurring over them.</p

EGA Reation Scheme

<p>EGA Reation Scheme</p

Example of a portion of an array with a box pattern of electrodes used for deprotection and synthesis at 0.4, 0.7, 1.0 and 1.4 micro amp/electrode.

<p>Example of a portion of an array with a box pattern of electrodes used for deprotection and synthesis at 0.4, 0.7, 1.0 and 1.4 micro amp/electrode.</p

Example of a portion of an array demonstrating a voltage study from 1.2 to 2.4 V.

<p>Example of a portion of an array demonstrating a voltage study from 1.2 to 2.4 V.</p

The design of the linker used in this experiment

<p>The design of the linker used in this experiment</p

DNA Synthesis Cycle

<p>DNA Synthesis Cycle</p