This paper reports the results of an international interlaboratory study led by the National Institute of Standards and Technology (NIST) on the measurement of high-pressure surface excess methane adsorption isotherms on NIST Reference Material RM 8850 (Zeolite Y), at 25 °C up to 7.5 MPa. Twenty laboratories participated in the study and contributed over one-hundred adsorption isotherms of methane on Zeolite Y. From these data, an empirical reference equation was determined, along with a 95% uncertainty interval ( U k =2 ). By requiring participants to replicate a high-pressure reference isotherm for carbon dioxide adsorption on NIST Reference Material RM 8852 (ZSM-5), this interlaboratory study also demonstrated the usefulness of reference isotherms in evaluating the performance of high-pressure adsorption experiments.

The original version of this article was published open access. Unfortunately, due to a technical issue, the copyright holder name in the online version (HTML and XML) is incorrectly published as “Springer Science+Business Media, LLC, part of Springer Nature 2018”. Instead, it should be “The Author(s) 2018”.

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Challenging applications in trace gas measurements require low uncertainty and high acquisition rates1,2,3,4. Many cavity-enhanced spectroscopies exhibit significant sensitivity and potential5,6, but their scanning rates are limited by reliance on either mechanical or thermal frequency tuning7. Here, we present frequency-agile, rapid scanning spectroscopy (FARS) in which a high-bandwidth electro-optic modulator steps a selected laser sideband to successive optical cavity modes. This approach involves no mechanical motion and allows for a scanning rate of 8 kHz per cavity mode, a rate that is limited only by the cavity response time itself. Unlike rapidly frequency-swept techniques8,9,10,11, FARS does not reduce the measurement duty cycle, degrade the spectrum's frequency axis or require an unusual cavity configuration. FARS allows for a sensitivity of ∼2 × 10−12 cm−1 Hz−1/2 and a tuning range exceeding 70 GHz. This technique shows promise for fast and sensitive trace gas measurements and studies of chemical kinetics. Frequency-agile, rapid scanning spectroscopy requires no mechanical motion and provides a scanning rate of 8 kHz per cavity mode at a sensitivity of ∼2 × 10-12 cm-1 Hz-1/2, with a scanning range that exceeds 70 GHz. This technique is promising for fast and sensitive trace gas measurements and chemical kinetic studies.

We report a method for rapidly scanning a cw-laser through optical cavity resonances. This technique involves a microwave source and an electro-optic phase modulator, giving high sensitivity, wide tunability and fast acquisition rates.

Primary gas standards, gas chromatography, and frequency-stabilized cavity ring-down spectroscopy measurements have been used to assess the effect of variations in the argon mixing ratio on the CO2 mixing ratios reported by commercial cavity ring-down spectroscopy sensors. Supporting calculations demonstrate that the use of argon-free, synthetic air standards can lead to a bias of approximate to 0.7 mol mol(-1) at atmospheric concentration levels of CO2 as a result of pressure-broadening effects. This bias is an order of magnitude greater than the precision of the best commercial sensors and significantly exceeds the World Meteorological Organization's target compatibility goal.

We present a low uncertainty measurement technique for determining molecular transition frequencies. This approach is complementary to sub-Doppler saturation spectroscopies and is expected to enable new frequency measurements for a wide variety of molecular species with uncertainties at the kHz-level. The technique involves measurements of Doppler broadened lines using cavity ring-down spectroscopy whereby the probe laser is actively locked to the ring-down cavity and the spectrum frequencies are linked directly to an optical frequency comb that is referenced to an atomic frequency standard. As a demonstration we have measured the transition frequency of the (30012) ← (00001) P14e line of CO2 near 1.57 μm with a combined standard uncertainty of ~9 kHz. This technique exhibits exceptional promise for measurements of transition frequencies and pressure shifting parameters of many weak absorbers, and indicates the potential for substantially improved measurements when compared to those obtained with conventional spectroscopic methods.

We present ultrasensitive measurements of molecular absorption using frequency-agile rapid scanning, cavity ring-down spectroscopy with an external-cavity diode laser. A microwave source that drives an electro-optic phase modulator with a bandwidth of 20 GHz generates pairs of sidebands on the probe laser. The optical cavity provides for high sensitivity and filters the carrier and all but a single, selected sideband. Absorption spectra were acquired by stepping the tunable sideband from mode-to-mode of the ring-down cavity at a rate that was limited only by the cavity decay time. This approach allows for scanning rates of 8 kHz per cavity resonance, a minimum detectable absorption coefficient of 1.7 9 10 cm after only 20 ms of averaging, and a noiseequivalent absorption coefficient of 1.7 9 10 cm Hz. By comparison with cavity-enhanced laser absorption spectrometers reported in the literature, the present system is, to the best of our knowledge, among the most sensitive and has by far the highest spectrum scanning rate.

We present ultrasensitive measurements of molecular absorption using frequency-agile rapid scanning, cavity ring-down spectroscopy with an external-cavity diode laser. A microwave source that drives an electro-optic phase modulator with a bandwidth of 20 GHz generates pairs of sidebands on the probe laser. The optical cavity provides for high sensitivity and filters the carrier and all but a single, selected sideband. Absorption spectra were acquired by stepping the tunable sideband from mode-to-mode of the ring-down cavity at a rate that was limited only by the cavity decay time. This approach allows for scanning rates of 8 kHz per cavity resonance, a minimum detectable absorption coefficient of 1.7 × 10−11 cm−1 after only 20 ms of averaging, and a noise-equivalent absorption coefficient of 1.7 × 10−12 cm−1 Hz−1/2. By comparison with cavity-enhanced laser absorption spectrometers reported in the literature, the present system is, to the best of our knowledge, among the most sensitive and has by far the highest spectrum scanning rate.

Ultraviolet photoemission spectroscopy, inverse photoemission spectroscopy, and two-photon photoemission spectroscopy are techniques commonly used to examine the electronic structure of surfaces and interfaces. We have used all three to study self-assembled monolayers of 4,4′-bis(phenylethynyl)benzenethiol on gold, a system often studied for potential application in molecular electronic junctions. We compare the results of these measurements and discuss them in relation to electronic structure calculations. The result is a better understanding of the valence electronic structure for this system, of the spectroscopic methods involved, and of previous measurements.

Photoelectron spectroscopy was used to explore changes in Fermi level alignment, within the pi-pi* gap, arising from modifications to the coupling chemistry of conjugated phenylene ethynylene oligomers to the Au surface. Self-assembled monolayers were formed employing either thiol (4,4'-ethynylphenyl-1-benzenethiol or OPE-T) or isocyanide (4,4'-ethynylphenyl-1-benzeneisocyanide or OPE-NC) coupling. The electronic density of states in the valence region of the two systems are nearly identical with the exception of a shift to higher binding energy by about 0.5 eV for OPE-NC. Corresponding shifts appear in C(1s) spectra and in the threshold near E(F). The lack of change in the optical absorption suggests that a rigid shift of the Fermi level within the pi-pi* gap is the major effect of modifying the coupling chemistry. Qualitative consideration of bonding in each case is used to suggest the influence of chemisorption-induced charge transfer as a potential explanation. Connections to other theoretical and experimental work on the effects of varying coupling chemistries are also discussed.

Vibrationally resonant sum-frequency generation (VR-SFG) and spectroscopic ellipsometry (SE) have been used to characterize self-assembled monolayer films of unsubstituted and mononitro-substituted oligo(phenylene-ethynylene) molecules on vapor-deposited An substrates. When combined with quantum chemical calculations of the relevant transition moment directions, orientation distributions and electronic excitation spectra are obtained. The orientation distribution from VR-SFG is in good agreement with previous IR reflection studies, indicating both molecules are tilted from the surface normal by similar to30degrees. The calculated resonant hyperpolarizabilities are in good agreement with experimental spectra. The optical polarizability extracted from SE suggests strong intermolecular interactions, consistent with molecular exciton theory.

One- and two-photon photoelectron spectroscopies were used to determine the electronic structure around the Fermi level for self-assembled monolayers of a prototypical "molecular wire", 4,4'-(ethynylphenyl)-1-benzenethiol (C6H5-CdropC-C6H4-CdropC-C6H5-SH), on An. One-photon ultraviolet photoelectron spectroscopy indicated a separation between the Fermi level and the peak of the occupied delocalized pi levels of 1.9 eV, thus providing a representative value for the hole injection barrier. Two states were identified in two-photon photoelectron spectroscopy measurements corresponding to excitation to the lowest exciton and excitation to an unoccupied final state derived from the e(2u) levels of benzene. The separation between the Fermi level and the corresponding unoccupied pi* states is estimated to be 3.2 eV, giving a transport gap of similar to1.9 + 3.2 = 5.1 eV. Occupied states associated with Au-S interactions are observed near the Fermi level for comparison studies on benzenethiol monolayers. Charge transfer associated with the formation of these levels, and their unoccupied counterparts, is suggested to produce the approximately 0.7 eV shift of the Fermi level toward the highest occupied orbitals on the oligomer.

This communication reports measurement of one-photon (21.2 eV) and one-color, two-photon (3.2-4.5 eV) photoemission spectra of 4,4'-bis-(phenylethynyl)benzenethiol chemisorbed on gold. Four features are observed in these spectra: two occupied, predominantly molecular levels below the Fermi level and two unoccupied, predominantly molecular levels above the Fermi level. The occupied and unoccupied bands closest to the Fermi level are assigned to delocalized pi-bands, and the other occupied and unoccupied bands, to localized pi-bands. With this assignment, the hole- and electron-injection barriers and the transport gap for those levels are deduced.

This paper reports the electroless deposition of copper, from a basic solution of copper sulfate, sodium hydrogentartrate, and formaldehyde, onto 4-mercaptobenzoic acid self-assembled on gold. The copper was found to be approximate to180 nm thick after a; approximate to30-min immersion. Deposition did not occur on bare gold or self-assembled layers of 1-octadecanethiol or 3-mercaptobenzoic acid. This latter observation suggests that the carboxylic acid functional group and its position play a role in the deposition process. Infrared absorption spectroscopy was used to evaluate these layers. On surfaces that were microcontact printed with 4-mercaptobenzoic acid, copper deposited only on the stamped areas.

The mechanism of the reaction CH4+O(D-1(2))--> CH3+OH was investigated by ultrafast, time-resolved and state-resolved experiments. In the ultrafast experiments, short ultraviolet pulses photolyzed ozone in the CH4.O-3 van der Waals complex to produce O(D-1(2)). The ensuing reaction with CH4 was monitored by measuring the appearance rate of OH(v=0,1;J,Omega,Lambda) by laser-induced fluorescence, through the OH A <--X transition, using short probe pulses. These spectrally broad pulses, centered between 307 and 316 nm, probe many different OH rovibrational states simultaneously. At each probe wavelength, both a fast and a slow rise time were evident in the fluorescence signal, and the ratio of the fast-to-slow signal varied with probe wavelength. The distribution of OH(v,J,Omega,Lambda) states, P-obs(v,J,Omega,Lambda), was determined by laser-induced fluorescence using a high-resolution, tunable dye laser. The P-obs(v,J,Omega,Lambda) data and the time-resolved data were analyzed under the assumption that different formation times represent different reaction mechanisms and that each mechanism produces a characteristic rovibrational distribution. The state-resolved and the time-resolved data can be fit independently using a two-mechanism model: P-obs(v,J,Omega,Lambda) can be decomposed into two components, and the appearance of OH can be fit by two exponential rise times. However, these independent analyses are not mutually consistent. The time-resolved and state-resolved data can be consistently fit using a three-mechanism model. The OH appearance signals, at all probe wavelengths, were fit with times tau (fast)approximate to0.2 ps, tau (inter)approximate to0.5 ps and tau (slow)approximate to5.4 ps. The slowest of these three is the rate for dissociation of a vibrationally excited methanol intermediate (CH3OH*) predicted by statistical theory after complete intramolecular energy redistribution following insertion of O(D-1(2)) into CH4. The P-obs(v,J,Omega,Lambda) was decomposed into three components, each with a linear surprisal, under the assumption that the mechanism producing OH at a statistical rate would be characterized by a statistical prior. Dissociation of a CH4O* intermediate before complete energy randomization was identified as producing OH at the intermediate rate and was associated with a population distribution with more rovibrational energy than the slow mechanism. The third mechanism produces OH promptly with a cold rovibrational distribution, indicative of a collinear abstraction mechanism. After these identifications were made, it was possible to predict the fraction of signal associated with each mechanism at different probe wavelengths in the ultrafast experiment, and the predictions proved consistent with measured appearance signals. This model also reconciles data from a variety of previous experiments.While this model is the simplest that is consistent with the data, it is not definitive for several reasons. First, the appearance signals measured in these experiments probe simultaneously many OH(v,J,Omega,Lambda) states, which would tend to obfuscate differences in the appearance rate of specific rovibrational states. Second, only about half of the OH(v,J,Omega,Lambda) states populated by this reaction could be probed by laser-induced fluorescence through the OH A <--X band with our apparatus. Third, the cluster environment might influence the dynamics compared to the free bimolecular reaction.