Sample exchange by beam scanning with applications to noncollinear pump-probe spectroscopy at kilohertz repetition rates.

In laser spectroscopy, high photon flux can perturb the sample away from thermal equilibrium, altering its spectroscopic properties. Here, we describe an optical beam scanning apparatus that minimizes repetitive sample excitation while providing shot-to-shot sample exchange for samples such as cryostats, films, and air-tight cuvettes. In this apparatus, the beam crossing point is moved within the focal plane inside the sample by scanning both tilt angles of a flat mirror. A space-filling spiral scan pattern was designed that efficiently utilizes the sample area and mirror scanning bandwidth. Scanning beams along a spiral path is shown to increase the average number of laser shots that can be sampled before a spot on the sample cell is resampled by the laser to ∼1700 (out of the maximum possible 2500 for the sample area and laser spot size) while ensuring minimal shot-to-shot spatial overlap. Both an all-refractive version and an all-reflective version of the apparatus are demonstrated. The beam scanning apparatus does not measurably alter the time delay (less than the 0.4 fs measurement uncertainty), the laser focal spot size (less than the 2 µm measurement uncertainty), or the beam overlap (less than the 3.3% measurement uncertainty), leading to pump-probe and autocorrelation signal transients that accurately characterize the equilibrium sample.

Interferometrically stable, enclosed, spinning sample cell for spectroscopic experiments on air-sensitive samples.

In experiments with high photon flux, it is necessary to rapidly remove the sample from the beam and to delay re-excitation until the sample has returned to equilibrium. Rapid and complete sample exchange has been a challenge for air-sensitive samples and for vibration-sensitive experiments. Here, a compact spinning sample cell for air and moisture sensitive liquid and thin film samples is described. The principal parts of the cell are a copper gasket sealed enclosure, a 2.5 in. hard disk drive motor, and a reusable, chemically inert glass sandwich cell. The enclosure provides an oxygen and water free environment at the 1 ppm level, as demonstrated by multi-day tests with sodium benzophenone ketyl radical. Inside the enclosure, the glass sandwich cell spins at ≈70 Hz to generate tangential speeds of 7-12 m/s that enable complete sample exchange at 100 kHz repetition rates. The spinning cell is acoustically silent and compatible with a ±1 nm rms displacement stability interferometer. In order to enable the use of the spinning cell, we discuss centrifugation and how to prevent it, introduce the cycle-averaged resampling rate to characterize repetitive excitation, and develop a figure of merit for a long-lived photoproduct buildup.

Enhanced interferometric detection in two-dimensional spectroscopy with a Sagnac interferometer.

An intrinsically phase-stable Sagnac interferometer is introduced for optimized interferometric detection in partially collinear two-dimensional (2D) spectroscopy. With a pump-pulse pair from an actively stabilized Mach-Zehnder interferometer, the Sagnac scheme is demonstrated in broadband, short-wave IR (1-2 µm), 2D electronic spectroscopy of IR-26 dye.

Absolute measurement of femtosecond pump-probe signal strength.

The absolute femtosecond pump-probe signal strength of deprotonated fluorescein in basic methanol is measured. Calculations of the absolute pump-probe signal based on the steady-state absorption and emission spectrum that use only independently measured experimental parameters are carried out. The calculation of the pump-probe signal strength assumes the pump and probe fields are both weak and includes the following factors: the transverse spatial profile of the laser beams; the pulse spectra; attenuation of the propagating pulses with depth in the sample; the anisotropic transition probability for polarized light; and time-dependent electronic population relaxation. After vibrational and solvent relaxation are complete, the calculation matches the measurement to within 10% error without any adjustable parameters. This demonstrates quantitative measurement of absolute excited state population.

Bulklike hot carrier dynamics in lead sulfide quantum dots.

Hot electronic dynamics in lead sulfide nanocrystals is interrogated by degenerate pump-probe spectroscopy with 20-25 fs pulses over a broad frequency range around three times the nanocrystal band gap. For each nanocrystal diameter, an initial reduction in absorption is seen only at the peak of the quantum confined E1 transition, while increased absorption is seen at all other wavelengths. The signals from the nanocrystals are approximately 300 times weaker than expected for a two-level system with the same absorbance and molar extinction coefficient and are weaker near time zero. These results appear to be inconsistent with quantum confinement of the initially excited high energy states. Arguments based on carrier scattering length, the wave packet size supported by the band structure, and effective mass are advanced to support the hypothesis that, for many direct-gap semiconductor quantum dots, the carrier dynamics at three times the band gap is localized on the 1-2 nm length scale and essentially bulklike except for frequent collisions with the surface.

Bilateral ring scotomas following laser in situ keratomileusis.

A 53-year-old white man who had a history of transient ocular hypertension had bilateral laser in situ keratomileusis (LASIK) for myopia. Subsequent computerized static perimetry revealed bilateral, persistent, repeatable midperipheral ring scotomas. The pre-LASIK visual fields were within normal limits, and the optic discs appeared stable and not diagnostic for glaucomatous optic neuropathy. A comprehensive baseline data set before laser refractive surgery aids subsequent assessment of individuals at high risk for developing glaucoma.

New approaches to the analysis of high connectivity materials: design frameworks based upon 4(4)- and 6(3)-subnet tectons.

Coordination framework polymers derived from lanthanide metal ions with N,N'-dioxide ligands (4,4'-bipyridine-N,N'-dioxide, pyrazine-N,N'-dioxide, 1,2-bis(pyridin-4-yl)ethane-N,N'-dioxide, trans-1,2-bis(pyridin-4-yl)ethene-N,N'-dioxide) exhibit such intricate architectures that a new strategy is required to appreciate and understand their structures. Rather than analyzing the overall structure in terms of the connectivity of individual metal nodes, which can lead in some cases to extremely complex topological treatments, our new strategy is based on the visualization of the structures as combinations of interconnected layered 2-D sheets or subnet tectons. Despite the diversity and relative complexities of many of the structures discussed here, they can all be described by the interconnection of just two types of 2-D subnet tectons, 4(4) square grids or 6(3) hexagonal grids. The interconnection of these layered sheets with bridging N,N'-dioxide molecules gives rise to both 2-D bilayer and 3-D network extended structures depending upon the relative dispositions of the interconnecting N,N'-dioxide ligands. Thus, 2-D bilayers result when the N,N'-dioxide ligands that bridge two subnet tectons are located on the same side of the sheet, while 3-D networks are formed when the bridging N,N'-dioxide ligands are located on both sides of the sheet. This analysis allows ready identification and interpretation of some of the most highly connected and complex architectures yet observed in materials chemistry.

Anion control over interpenetration and framework topology in coordination networks based on homoleptic six-connected scandium nodes.

Reaction of ScX3 (X=NO3-, CF3SO3-, ClO4-) with 4,4'-bipyridine-N,N'-dioxide (L) affords topologically distinct six-connected three-dimensional coordination frameworks, {[Sc(L)3](NO3)3}(infinity) (1), {[Sc(L)3](CF3)SO3)3(CH3OH)2.7(H2O)3}(infinity) (2), {[Sc(L)3](ClO4)3}(infinity) (3) and {[Sc(L)4(H2O)2](ClO4)3}(infinity) (4). Compounds 1, 2 and 3 are networks based on octahedrally co-ordinated ScO6 centres bound through six oxygen atoms from six separate N-oxide ligands L. Compounds 1 and 3 are doubly interpenetrated and have alpha-polonium-type structures of 4(12)6(3) topology based upon three intersecting (4,4) nets. The structure of 2 is unusual and shows parallel, co-planar layers of (4,4) nets connected in a criss-crossed fashion to afford a new 4(8)6(6)8 topology. In 4 only four ligands L bind to each Sc(III) centre with two additional water molecules bridging metal nodes. Significantly, the bridges formed by L do not sit in a plane and if connections through L are considered alone the resultant structure is a diamondoid array typically based upon a tetrahedral connecting node at Sc. Five interpenetrating diamondoid networks are observed that are cross-bridged by water molecules to form a single three-dimensional array of 4(8)6(7) topology. Compound 4 can also be viewed as incorporating two intersecting (4,4) grids based upon two ligands L and two bridging waters. Thus, variation of anion, solvent and conditions critically affects the structures of products formed, and the series of polymers reported herein illustrates how tectons based upon (4,4) grids can be combined and distorted to form non-NaCl topologies and even cross-bridged, multiply interpenetrated diamondoid materials. Both compounds 2 and 4 represent unusual examples of self-penetrated coordination frameworks.

Unprecedented bilayer topologies in 5- and 6-connected framework polymers.

Three bilayer structures with unprecedented 6-connected topology and a single example of a bilayer structure with 5-connected topology have been generated by co-ordination of 4,4'-bipyridine-N,N'-dioxide at La(iii), Yb(iii) or Er(iii) nodes.