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Vibrational spectra of CO physisorbed onto well defined NaCl(100) surfaces were studied using a Fourier transform infrared interferometer. Structures of CO starting from the monolayer to multilayers were explored. At 31.5 K and a CO pressure of 1×10-6 mbar only the monolayer is formed. Polarization measurements confirm our earlier study that the monolayer CO molecules are aligned perpendicular to the NaCl(100) surface. Increasing the CO pressure to 7×10-6 mbar produces multilayer adsorption. The multilayer spectra closely resemble that of α-CO absorption previously reported. The near perfect match of crystal structures and lattice constants of α-CO and NaCl is reasoned to force the epitaxial growth of single crystal multilayers in our experiments. At 22 K the monolayer absorption is at 2155.01 cm-1 with a bandwidth (FWHH) of 0.26 cm-1. The two prominent features in the multilayer spectra at 22 K are assigned to the longitudinal optical (LO) mode at 2142.54 cm-1 and the transverse optical (TO) mode at 2138.51 cm-1. Their frequency separation is a consequence of the lowering of the cubic symmetry of the bulk α-CO crystal by the shape, in the form of thin slabs, of our multilayer samples. Their bandwidths depend on the thickness of the sample and are characterized by a bandwidth parameter of 0.25 cm-1 for the LO mode and 0.85 cm-1 for the TO mode. The relative absorbances of these modes depend on the polarization of the infrared radiation. Theoretical formalism to account for the band splitting and absorption profiles of the infrared absorption is reviewed and applied to our measurements. While many features of our data can be explained by the present theory, further work is required to account for all the experimental results. [ABSTRACT FROM AUTHOR]

Electron–photon temporal correlations in electron energy loss spectroscopy (EELS) and cathodoluminescence (CL) spectroscopies have recently been used to measure the relative quantum efficiency of materials. This combined spectroscopy, named cathodoluminescence excitation (CLE) spectroscopy, allows for the identification of excitation and decay channels, which are hidden in average measurements. Here, we demonstrate that CLE can also be used to measure excitations' decay time. In addition, the decay time as a function of the excitation energy is measured, as the energy for each electron–photon pair is probed. We used two well-known insulating materials to characterize this technique, nanodiamonds with NV0 defects and hexagonal boron nitride (h-BN) with 4.1 eV defects. Both also exhibit marked transition radiations, whose extremely short decay times can be used to characterize the instrumental response function. It is found to be typically 2 ns, in agreement with the expected limit of the EELS detector temporal resolution. The measured lifetimes of NV0 centers in diamond nanoparticles (20–40 ns) and 4.1 eV defect in h-BN flakes (<2 ns) match those reported previously. [ABSTRACT FROM AUTHOR]

The negatively charged nitrogen-vacancy defect center, (NV)–, in diamond has been investigated theoretically for its one- and two-photon absorption properties involving the first excited state with the 3A2→3E transition. Time-dependent density functional theory (TD-DFT), configuration interaction with single excitation (CIS), and complete active space self-consistent field (CASSCF) were employed in this investigation along with the 6-31G(d) basis set. Diamond lattice models containing 24–104 carbon atoms were constructed to imitate the local environment of the defect center. TD-DFT calculations in large molecular cluster models (with 85 or more carbon atoms) predicted the vertical excitation energy quite consistent with the experimental absorption maximum. CASSCF calculations were feasible only for small cluster models (less than 50 carbon atoms) but yielded one-photon absorption (OPA) and two-photon absorption (TPA) cross sections somewhat larger than the experimental values obtained with linearly polarized incident light [T.-L. Wee et al., J. Phys. Chem. A 111, 9379 (2007)]. CIS calculations in larger cluster models showed a systematic overestimation of the excitation energy while just slightly underestimated the OPA cross section and overestimated the TPA cross section. The agreements between calculations and measurements suggest that the computational approaches established in this work are applicable to explore the optical properties of related defect centers in diamond as well. [ABSTRACT FROM AUTHOR]

In studying ultrafast electron transfer from a dye molecule to a nanosized semiconductor particle, pump-probe experiments are commonly used. In this system the electron transfer (ET) rate is faster than vibrational relaxation so that the ET rate should be described by a single-level rate constant and the probing signal (often in the form of time-resolved spectra) contains the contribution from the dynamics of both population and coherence (i.e., wave packet). In this paper, we shall present the theoretical treatments for femtosecond time-resolved pump-probe experiment and the dynamics of population and coherence by the density matrix method, and the calculation of single-level ET rate constant involved in a pump-probe experiment. As an application, we show the theoretical results using parameters extracted from experiments on a specific dye/semiconductor system. [ABSTRACT FROM AUTHOR]

This paper describes a systematic study on the clathrate structure of H+(H2O)21 using tandem mass spectrometry, vibrational predissociation spectroscopy, Monte Carlo simulations, and density functional theory calculations. We produced H+(H2O)n from a continuous corona-discharged supersonic expansion and observed three anomalies simultaneously at the cluster temperature near 150 K, including (1) the peak at n=21 is more intense than its neighboring ions in the mass spectrum, (2) the size-dependent dissociation fractions show a distinct drop for the 21-mer, and (3) the infrared spectrum of H+(H2O)21 exhibits only a single feature at 3699 cm-1, corresponding to the free-OH stretching of three-coordinated water molecules. Interestingly, the anomalies appear or disappear together with cluster temperature, indicating close correlation of these three observations. The observations, together with Monte Carlo simulations and density functional theory calculations, corroborate the notion for the formation of a distorted pentagonal dodecahedral (512) cage with a H2O molecule in the cage and a H3O+ ion on the surface for this “magic number” water cluster ion. The dodecahedral cage melts at higher temperatures, as evidenced by the emergence of a free-OH stretching feature at 3717 cm-1 for the two-coordinated water in H+(H2O)21 produced in a warmer molecular beam. Extension of this study to larger clusters strongly suggests that the experimentally observed isomer of H+(H2O)28 is most likely to consist of a distorted protonated pentakaidecahedral (51263) cage enclosing two neutral water molecules. [ABSTRACT FROM AUTHOR]

Fixing a diamond containing a high density of Nitrogen-Vacancy (NV) center ensembles on the apex of a multimode optical fiber (MMF) extends the applications of NV-based endoscope sensors. Replacing the normal MMF with a tapered MMF (MMF-taper) has enhanced the fluorescence (FL) collection efficiency from the diamond and achieved a high spatial resolution NV-based endoscope. The MMF-taper's high FL collection efficiency is the direct result of multiple internal reflections in the tapered region caused by silica, which has a higher refractive index (RI) than the surrounding air. However, for applications involving fluidic environments whose RI is close to or higher than that of the silica, the MMF-taper loses its FL collection significantly. Here, to overcome this challenge and achieve a high spatial resolution NV-based endoscope in a fluidic environment, we conceptually proposed a tapered ultra-high numerical aperture microstructured optical fiber (MOF) whose air capillaries at the tapered end are sealed. Since the end-sealed air capillaries along the tapered MOF (MOF-taper) have isolated the MOF core from the surrounding medium, the core retains its high FL collection and NV excitation efficiency in liquids regardless of their RI values. Replacing the MMF-taper with the MOF-taper will achieve a versatile NV-based endoscope that could potentially find widespread applications in fluidic environments where many biological processes and chemical reactions occur. [ABSTRACT FROM AUTHOR]

We experimentally demonstrate a simple and robust optical fiber based method to achieve simultaneously efficient excitation and fluorescence collection from Nitrogen-Vacancy (NV) defects containing micro-crystalline diamond. We fabricate a suitable micro-concave mirror that focuses scattered excitation laser light into the diamond located at the focal point of the mirror. At the same instance, the mirror also couples the fluorescence light exiting out of the diamond crystal in the opposite direction of the optical fiber back into the optical fiber within its light acceptance cone. This part of fluorescence would have been otherwise lost from reaching the detector. Our proof-of-principle demonstration achieves a 25 times improvement in fluorescence collection compared to the case of not using any mirrors. The increase in light collection favors getting high signal-to-noise ratio optically detected magnetic resonance signals and hence offers a practical advantage in fiber-based NV quantum sensors. Additionally, we compacted the NV sensor system by replacing some bulky optical elements in the optical path with a 1 × 2 fiber optical coupler in our optical system. This reduces the complexity of the system and provides portability and robustness needed for applications like magnetic endoscopy and remote-magnetic sensing. [ABSTRACT FROM AUTHOR]

A sine wave that controls a quadrupole ion trap is generated from a low voltage source, boosted to high voltage through a transformer. Since not even the best transformers can keep a flat amplitude response with respect to frequency, knowing the accurate peak-to-peak value of the sine wave is paramount when the frequency is varied. We have developed an averaging peak-to-peak voltage detector for such measurements and demonstrated that the device is an essential tool to make possible high-precision mass determination of single charged microparticles with masses greater than 1×1011 u. Tests of the detector with sine waves from a FLUKE 5720A standard source in the neighborhood of 1400 Vpp and frequencies ranging from 100 to 700 Hz showed a measurement accuracy better than 10 ppm. The detector settled within 5 s after each reset to 5 digits of DVM rock-steady reading, and the calibration against the same source after 3 weeks of continuous use of the circuit produced a mere overall 1 ppm difference. [ABSTRACT FROM AUTHOR]

The number of negatively charged nitrogen-vacancy centers (N-V)- in fluorescent nanodiamond (FND) has been determined by photon correlation spectroscopy and Monte Carlo simulations at the single particle level. By taking account of the random dipole orientation of the multiple (N-V)- fluorophores and simulating the probability distribution of their effective numbers (Ne), we found that the actual number (Na) of the fluorophores is in linear correlation with Ne, with correction factors of 1.8 and 1.2 in measurements using linearly and circularly polarized lights, respectively. We determined Na=8±1 for 28 nm FND particles prepared by 3 MeV proton irradiation. [ABSTRACT FROM AUTHOR]

This work demonstrates that the gold tip of an atomic force microscope enhances not only the fluorescence of the nitrogen-vacancy centers in nanodiamonds but also improves the optical resolution of the fluorescence image of the particles down to 40 nm in an apertureless near-field scanning optical microscope. With the tip in close contact with 20-30 nm diamonds, the average fluorescence intensity enhancement is 3. By measuring the fluorescence decay lifetime and the saturation intensity, we confirm that the fluorescence enhancement is contributed predominantly by the increase of the radiative decay rate. [ABSTRACT FROM AUTHOR]

A correction to the article "Quantifying the number of color centers in single fluorescent nanodiamonds by photon correlation spectroscopy and Monte Carlo simulation," is presented.