Several computational methods are presented for the rapid extraction of decay time constants from discrete exponential data. Two methods are found to be comparably fast and highly accurate. They are corrected successive integration and a method involving the Fourier transform (FT) of the data and the application of an expression that does not assume continuous data. FT methods in the literature are found to introduce significant systematic error owing to the assumption that data are continuous. Corrected successive integration methods in the literature are correct, but we offer a more direct way of applying them which we call linear regression of the sum. We recommend the use of the latter over FT-based methods, as the FT methods are more affected by noise in the original data.

Evanescent-wave cavity ring-down spectroscopy was used to monitor the adsorption of human hemoglobin to a fused-silica surface from aqueous solution. An uncoated dove prism was situated in a ring-down cavity such that the beam entered and exited with a normal-incidence geometry. This afforded ring-down times as high as 5 mus and values of sigma(tau)/tau as low as 0.3%. Normal-incidence geometry permits the detection of both S- and P-polarized light, yielding some information of the orientation of adsorbates. The orientation of the adsorbed hemoglobin molecules is found to change as adsorption progresses, but with a different time profile than adsorption itself. The adsorption kinetics from a quiescent solution is consistent with a reaction-diffusion model that includes both reversible and irreversible adsorption operating in parallel. Systems behaving according to this model also seem to display adsorption isotherms, although the increased adsorption from more concentrated solutions is only a consequence of the system being under kinetic control. In some cases, this may be sufficient to explain the paradox of protein adsorption systems which seem to be both irreversible and consistent with isotherm models as well.

The ionization of nitric acid on the surface of crystalline ice was examined from 130 to 150 K using FTIR transmission spectroscopy. A spectral feature of the hydronium ion, H 3 O + , was monitored as a function of time. The results are best understood when they are separated into (a) low and (b) high nitric acid exposure, depending upon the amount of nitric acid adsorbed on the ice surface. (a) For low nitric acid exposure ((∼2-20) × 10 15 molecules/cm 2 ), the absorbance of H 3 O + can be fit to a single exponential (i.e., first-order expression). The resulting rate constant, k ) (4.9 ( 0.7) × 10 -3 s -1 , is attributed to the dissolution of the ions in the ice surface layer (i.e., a reactive layer on the ice surface). The thickness of this ice surface layer is estimated to be 10 nm. (b) For high nitric acid exposure ((∼20-200) × 10 15 molecules/cm 2 ), the absorbance of H 3 O + can be fit to a double exponential expression that is composed of the first-order rate constant above, along with another first-order rate constant, k′ ) (5.2 ( 0.7) × 10 -4 s -1 . This rate constant is attributed to the dissolution of ions in an acid-rich ice surface layer. Both rate constants are independent of temperature, indicating a small activation energy (E a ) 0 ( 2 kcal/mol).

The growth of Ruddlesden–Popper perovskite thin films of organic lead halides is complicated by the existence of multiple crystallization pathways available to precursors in solution. During thin-film growth processes, such as spin-coating or blade-coating, solvents can evaporate too quickly to clearly resolve different reaction intermediates and products that form during crystallization. Here, we resolve multiple reaction products and intermediates that form during growth of (C4H9NH3)2(CH3NH3) n−1Pb n I3n+1 Ruddlesden–Popper compounds by studying drop-cast precursor solutions through the evolution of X-ray diffraction, photoluminescence, and optical micrographs in situ over long timescales in a thin-film geometry. We found that methylammonium-rich solvate intermediates play a crucial role in directing the bulk optical properties of the films and form simultaneously with smaller regions of Ruddlesden–Popper phases during growth. The microstructure and optical properties of these sub-phases were characterized during growth and after annealing, revealing that discrepancies between thin-film and single-crystal optical properties originate from solvate intermediates. These lower-band-gap minority phases dominate the optical emission spectrum by means of rapid energy migration and contribute to sub-band-gap electronic states in photovoltaic devices. Processing routes to yield thin films with optical properties similar to single crystals of Ruddlesden–Popper phases were developed by tuning the precursor stoichiometry and deposition kinetics.

The nature of the surface of crystalline water ice is investigated by monitoring isotope exchange in the first few bilayers. Near-monolayer amounts of D2O are deposited on thin films of H2O ice and isotope exchange at 145 K is monitored with Fourier-Transform infrared spectroscopy as a function of time. No exchange occurs on the surface of pure ice, however, exchange is readily observed on the surface of ice doped with small amounts of hydrogen chloride (HCl). The lack of exchange at the surface of pure ice stands in contrast to similar experiments performed of D2O embedded in the bulk. This suggests a depletion of mobile defects on the surface of pure crystalline ice at 145 K. This relative depletion may cause a significant difference between reactivity on the ice surface and in the ice bulk for other systems.