Etalon-Assisted Study of the Strong CO Ligand Vibrations of the fac-[Re(CO)3(bpy)(CH3CN)]+ Octahedral Complex.

The strong CO ligand vibrations of an octahedral complex, fac-[Re (CO)3(bpy)(CH3CN)]+, in acetonitrile are observed at 2040 and 1932 cm-1. Facial rhenium tricarbonyl systems offer very strong and isolated CO vibrations with the potential for interactions between these vibrations. This work first identifies the dominant ion-pair species using attenuated total reflection infrared (ATR-IR) absorption spectra on a dilution series and then determines the strength of these CO ligand vibrations (as isolated vibrations) with a combination of ATR-IR and etalon-based measurements that determine the absolute complex index of refraction of the solution. Finally, the etalon experiments are modeled to study the interaction between vibrations, which is a property not embedded in the solution's complex index of refraction. The ATR-IR spectra are accomplished on a dilution series as well as a larger set of spectra as these solutions evaporated. The A'(1) CO ligand band at 2040 cm-1 is fit with a sum of three Lorentzian functions characterizing the distribution of free, solvent-separated, and contact ion pairs of this octahedral complex vs concentration. The other CO ligand band at 1932 cm-1 is broader and complicated by the dynamics of vibrational interactions, the unresolved splitting of the A'(2) and A″ CO vibrations, and ion-pair speciation. The etalon transmission measurements vs angle were on a 0.029 M solution, and Rabi splittings of 19 and 38 cm-1 were observed for the A'(1) CO vibration and the unresolved A'(2) + A″ CO vibrations, respectively. The great strength of the CO ligand vibrations is evident despite the use of a dilute solution. Integrated band intensities are reported in comparison to hybrid density functional calculations for isolated vibrations. Then, the observed Rabi splittings are modeled to obtain the coupling strength of the CO ligand vibration with etalon cavity modes and with each other. In summary, this work develops a method to determine the concentration of these solutions from the ATR-IR spectrum, characterizes the ion-pairing, shows that the index of refraction is not constant in the IR spectral region of interest, and develops an interaction Hamiltonian that characterizes cavity-vibration and vibration-vibration coupling.

Etalon-Assisted Determination of the Complex Index of Refraction of a Solution for the Study of Strong Cavity-Vibrational Coupling of PF6- in Acetonitrile.

A new method is established using an etalon cavity to assist in the determination of the wavelength-dependent complex index of refraction of a solution throughout the mid-infrared range. The results are used to study the cavity-vibration polaritons of PF6- in acetonitrile. Mixed states are formed by placing solution inside a pair of parallel plate mirrors with a wavelength-scale spacing, i.e., within an etalon, such that there are cavity states that are angle-tuned into resonance with the strong P-F vibrations. The dominant ν3 vibrations of PF6- consist of nearly triply degenerate oscillations of the partial-positively charged phosphorous against antisymmetric concerted motions of different sets of fluorine atoms with partial negative charges. These vibrations are dominant even though the solute is 29 times less concentrated than the solvent on a molar basis. The first part of the paper describes the method of determining the complex index of refraction of the solution from a combination of etalon transmission maxima and the attenuated total reflection (ATR) absorption spectrum of the solution. The results are presented as an analytical function including a sum of 37 vibrational contributions. Absolute integrated isolated band intensities were determined to be 463 ± 4, 462 ± 7, and 266 ± 4 km/mol for the three ν3 PF6- vibrations at 841.4, 847.4, and 854.0 cm-1, respectively, which sum to 1191 ± 9 km/mol for the ν3 band. Then, the results are used to simulate the measured etalon transmission using the transfer matrix (TM) method with and without the ν3 target vibrations. The etalon transmission simulations reconstruct the position of cavity modes in the absence of target vibrations. They provide input data for the testing of simple quantum mechanical models for the interaction of vibrations with cavity modes and the interactions of vibrations with other vibrations within the molecule and between solute and solvent. The model shows that the nearly degenerate ν3 vibrations interact with each other with a vibration-vibration coupling of 33 ± 5 cm-1. This is comparable to the cavity-vibration coupling of 30.4 ± 2.9 cm-1 of the two strongest vibrations of PF6-.

Effective mass of cavity-vibration polaritons formed in etalons with liquid carbon tetrachloride.

Etalons are pairs of parallel plate mirrors with wavelength-scale spacing that exhibit cavity modes, giving transmission maxima (fringes) due to constructive interference. Infrared transmission measurements as a function of angle were used to determine the effective mass of etalon cavity modes using a gap filled with air and then liquid carbon tetrachloride. The air-filled etalon gives results in agreement with pure photon expectations established herein. Liquids with vibrations having strong infrared transition intensity (vibrational strong coupling mode) can strongly perturb the pattern of transmission resonances, creating mixed states of infrared cavity modes and molecular vibrations, i.e., cavity-vibration polaritons. The effective mass of one cavity-vibration polariton close to the strong vibration of carbon tetrachloride is 4.36 times heavier than the pure photon cavity mode expectation, i.e., the mass factor vs pure light. The mass factors are largest when closest to the strong vibrational frequency, and they converge to the one far away from the strong vibration. This work gives quantitative values of the effective mass of cavity-vibration polariton states and is a diagnostic for the mixing of vibrations with etalon transmission.

Changing Vibration Coupling Strengths of Liquid Acetonitrile with an Angle-Tuned Etalon.

This work is the first report on nonzero molecular vibration-vibration coupling in an infrared cavity-vibration experiment. Vibration-vibration coupling strength is determined as a cavity mode of parallel spaced mirrors (etalon mode or fringe) is angle-tuned in the region between two vibrations of liquid acetonitrile which are Fermi coupled, namely, a CN stretch dominated vibration and a nearby combination band dominated by the symmetric CH3 bend and C-C stretch. All other infrared cavity-vibration work to date involving more than one vibration has used a value of zero for vibration-vibration coupling; however, this work starts with Fermi coupled vibrations and reveals that there are changes in the vibration-vibration coupling and cavity-vibration couplings as the cavity mode is angle-tuned between the interacting vibrations. The ability to change fundamental vibrational dynamics within a cavity is an exciting result which helps to build a foundation for understanding molecular vibrational dynamics in parallel plate etalon cavities.

Effect of Strongly Coupled Vibration-Cavity Polaritons on the Bulk Vibrational States within a Wavelength-Scale Cavity.

A Rabi splitting of 43.0 ± 1.0 cm-1 (95% conf.) was determined for the interaction of the CD3 deformation, the strongest fundamental vibration of the liquid CD3C≡N molecule and fringe modes of a parallel-plate Fabry-Pérot cavity containing this liquid. Note that vibration-cavity polaritons are also called dressed states, hybrid or mixed states. Since the experimental configuration has many orders of magnitude more vibrational oscillators than photons, vibrational oscillators not in dressed states far outnumber those in the dressed states. This work is distinguished from related vibration-cavity work by a method to extract the position, width, phase, and intensity of bulk vibrational signals including reconstruction of the position of the fringe without vibrational contributions. It reveals how the bulk vibrational oscillators are changed by interaction within the cavity even though they are not in dressed states. Although the dressed states are obvious targets for manipulation of chemical response, it is interesting to consider whether the lesser but more prevalent changes of the bulk vibrations can also be used to change the chemical response.

Dust Library of Plasmonically Enhanced Infrared Spectra of Individual Respirable Particles.

This work characterizes collections of infrared spectra of individual dust particles of ∼4 µm size that were obtained from three very different environments: our lab air, a home air filter, and the 11 September 2001 World Trade Center event. Particle collection was done either directly from the air or by placing dust powder from various samples directly on the plasmonic mesh with 5 µm square holes as air is pumped through the mesh. This arrangement enables the recording of "scatter-free" infrared absorption spectra of individual particles of size comparable to the probing wavelengths whose vibrational signatures are otherwise dominated by scattering and dispersive line shape distortions. The spectra are sensitive to the amounts of various infrared active components and analysis using a Mie-Bruggeman model for mixed composition particles provides volume fractions of the components. Inhalation of dust particles of ∼4 µm size has significant health consequences as these are among the largest inhaled into people's lungs. The chemical composition of ∼4 µm respirable particles is of great interest from health, atmospheric, and environmental perspectives as different environments may pose different hazards and spectroscopic challenges.

Extracting Infrared Spectra of Protein Secondary Structures Using a Library of Protein Spectra and the Ramachandran Plot.

Infrared (IR) spectra from 1200 to 1800 cm(-1) of the pure α-helix and ß-sheet secondary structures have been extracted using a covariant least-squares procedure which relates a library of 40 infrared (IR) solution protein spectra from the work of Dong, Carpenter, and Caughey and amino acid fractions of the proteins based on assignments by STRIDE (secondary structure identification) of Eisenhaber and Argos. The excitonic splitting of the ß-sheet structures is determined for this library of solution proteins. The method is extended to find a set of spectral basis functions that analyze IR spectra of protein samples for α-helix and ß-sheet content. A rigorous error analysis including covariance, the correlations between the input library spectra, was used to justify the results and avoid less meaningful results. The utility of the results on α-helix and ß-sheet regions is demonstrated by detecting protein changes due to cancer in imaging Fourier transform IR (FTIR) spectra of liver tissue slices. This work ends with a method to extract IR spectra of less prominent torsional angle distributions.

Generalizing thermodynamic properties of bulk single-walled carbon nanotubes.

The enthalpy and Gibbs free energy thermodynamical potentials of single walled carbon nanotubes were studied of all types (armchairs, zig-zags, chirals (n>m), and chiral (n

Infrared spectral model for subwavelength particles of mixed composition based on the spectra of individual particles with calibration data for airborne dust.

A Mie-Bruggeman spectral model is presented which predicts the orientationally averaged, infrared spectra of individual mixed-composition particles or the average spectrum of collections of such particles. The model uses parameters extracted from sets of individual particle spectra of pure materials known to be in subject mixtures. The spectra of both calibrants and subject particles were recorded by trapping size-selected particles in the holes of plasmonic metal mesh. Calibrating data is presented for quartz, calcite, dolomite, three clays, gypsum, polyethylene, and living organic material (yeast cells). The individual particle spectra of these calibrants are averaged to account for crystal orientation effects, fit by a Mie theory model, and tabulated herein as dielectric functions of each component. The component dielectric functions are combined in this model with Bruggeman effective medium theory producing a spectral prediction for mixed-composition particles. The Mie-Bruggeman model was used to analyze the composition of dust from our lab air [K. E. Cilwa et al. J. Phys. Chem. C 2011, 115, 16910] based on the average spectrum of the dust particles. The model does a reasonable job of characterizing the dust in our laboratory air exhibiting promise for future applications. This work presents the model and illustrates potential; however, much more work will be required before its accuracy as a quantitative analytical method is established.

Infrared metrics for fixation-free liver tumor detection.

Infrared (IR) spectroscopic imaging of human liver tissue slices has been used to identify and characterize liver tumors. Liver tissue, containing a liver metastasis of breast origin (mucinous carcinoma), was surgically removed from a consenting patient and frozen without formalin fixation or dehydration procedures, so that lipids and water remained in the tissues. A set of IR metrics (ratios of various IR peaks) was determined for tumors in fixation-free liver tissues. K-means cluster analysis was used to tell tumor from nontumor. In this case, there was a large reduction in lipid content upon going from nontumor to tumor tissue, and a well-resolved IR spectrum of nontumor liver lipid was obtained and analyzed. These IR metrics may someday guide work on IR spectroscopic diagnostics on patients in the operating room. This work also suggests utility for these methods beyond the identification of liver tumors, perhaps in the study of liver lipids.

Spectral challenges of individual wavelength-scale particles: strong phonons and their distorted lineshapes.

Beyond our own interest in airborne particulate matter, the prediction of extinction and absorption spectra of single particles of mixed composition has wide use in astronomy, geology, atmospheric sciences, and nanotechnology. Single particle spectra present different challenges than traditional spectroscopic approaches. To quantify the amount of a material in a bulk sample (molecules in solution or the gas phase), one might employ the Beer-Lambert law assuming a simple slab-type assay geometry and averaging over orientation, whereas with single particles one might have a specific orientation and require a nonlinear, Mie-like particle theory. The complicating single particle issues include: strong and broad scattering at wavelengths similar to the particle size, phonon lineshape phase shifting, particle shape effects, distortion of transition lineshapes by strong vibrational bands, bi- and trirefringence, crystal orientation effects including dispersion, and composition mixtures. This work uses a combination of three-dimensional finite difference time domain (3D-FDTD) calculations and experimental infrared spectra on single, crystalline quartz particles to illustrate some of the challenges--in particular the distortion of lineshapes by strong phonons that lie within a range of strong scattering. It turns out that many mineral dust components in the inhalable size range have strong phonons. A Mie-Bruggeman model for single particle spectra is presented to isolate the effects of strong phonons on lineshapes which has utility for analysing the spectra of single, mixed-composition particles. This model will ultimately enable the determination of volume fractions of components in single particles that are mixtures of many materials with strong phonons, as are the dust particles breathed into people's lungs.

Single Airborne Dust Particles using Plasmonic Metal Films with Hole Arrays.

An airborne dust particle is trapped in the hole of a plasmonic metal film with a patterned array of holes (mesh) by pumping air through the mesh. Both scatter-free infrared spectra and scanning electron images are obtained on the same individual airborne dust particle, showing the feasibility of multiple, nondestructive experiments on a single, subwavelength particle. Ultimately, this may help to elucidate the effect of shape, orientation, and crystallinity on IR dust particle spectra.

Developing Plasmonics Under the Infrared Microscope: From Ni Nanoparticle Arrays to Infrared Micromesh.

Microscopes typically collect light over large ranges of angles dispersing plasmonic resonances. While this is an advantage for recording spectra of microscopic particles, it is a disadvantage for sensing by resonance shifts. Adaptations are described herein which enable one to identify, manipulate, and examine narrow plasmonic resonances under a microscope. Noting more general familiarity with metal nanoparticle arrays, a useful perspective is offered by relating the optical transmission of small Ni nanoparticle arrays to that of Ni metal films with microhole arrays, i.e., infrared-active mesh. This perspective also includes the connection to traditional dispersion studies, a new microscope method to measure the propagation length of surface-plasmon-polariton-mediated resonances, and the shifting of resonance positions by latex microspheres in the holes of mesh. A useful perspective is offered by relating the optical transmission of small Ni nanoparticle arrays to that of Ni metal films with microhole arrays, i.e., infrared-active mesh.

Modifying infrared scattering effects of single yeast cells with plasmonic metal mesh.

The scattering effects in the infrared (IR) spectra of single, isolated bread yeast cells (Saccharomyces cerevisiae) on a ZnSe substrate and in metal microchannels have been probed by Fourier transform infrared imaging microspectroscopy. Absolute extinction [(3.4±0.6)×10(-7) cm(2) at 3178 cm(-1)], scattering, and absorption cross sections for a single yeast cell and a vibrational absorption spectrum have been determined by comparing it to the scattering properties of single, isolated, latex microspheres (polystyrene, 5.0 µm in diameter) on ZnSe, which are well modeled by the Mie scattering theory. Single yeast cells were then placed into the holes of the IR plasmonic mesh, i.e., metal films with arrays of subwavelength holes, yielding "scatter-free" IR absorption spectra, which have undistorted vibrational lineshapes and a rising generic IR absorption baseline. Absolute extinction, scattering, and absorption spectral profiles were determined for a single, ellipsoidal yeast cell to characterize the interplay of these effects.

Propagation lengths of surface plasmon polaritons on metal films with arrays of subwavelength holes by infrared imaging spectroscopy.

Metal films with arrays of subwavelength holes (mesh) exhibit extraordinary transmission resonances to which many attribute a role for surface plasmon polaritons (SPPs); others debated this point. Experimental measurements of propagation lengths are presented under conditions that pertain to the use of SPPs for surface spectroscopy. The lateral extent of electromagnetic propagation along the mesh surface is measured by recording absorption spectra of a line of latex microspheres as a function of distance away from the line along the mesh. Measurements reveal an exponential functional form for decay of absorption signal laterally from the absorption source. Results at 697 cm(-1), which are closest to the strongest transmission resonance of the mesh, reveal a 1/e propagation distance along the surface of 17.8+/-2.9 microm. This is 40% larger than the lattice spacing implicating the holes as the SPP damping mechanism, however, this is significantly shorter than smooth metal expectations.

Extraordinary transmission of metal films with arrays of subwavelength holes.

Metal films with patterns of subwavelength holes (grids or meshes) have interesting optical properties including the extraordinary transmission effect. These optically thick metal films transmit more radiation than that incident on the holes owing to the excitation of surface plasmons (SPs). Meshes present a new and simple way to excite SPs at perpendicular incidence (i.e., without the need to vary the angle of the incident beam). This represents a new opportunity to integrate SPs with experiments and devices-a new instrument in the toolbox of SP techniques that may broaden the range of SP applications. This review discusses the discovery, basic optical physics, the role of SPs, and applications of the extraordinary transmission of subwavelength hole arrays.

Interaction of an infrared surface plasmon with an excited molecular vibration.

The interaction of an infrared surface plasmon and an excited molecular vibration was investigated by using a square array of subwavelength holes in a Ni film which supports propagating, surface-plasmon-mediated, transmission resonances. The largest transmission resonance [the (1,0)(-)] was tuned through the rocking vibration of the hexadecane molecule (at 721 cm(-1)) in a hexadecane film on the mesh by varying the thickness of the film. The interaction of the rocking vibration and surface plasmon is characterized spectroscopically by an increase in the intensity of the vibrational band by more than a factor of 2, variation of the vibrational line shape relative to the spectrum on a nonmetallic surface, and shifts in vibrational peak position by as much as 3.0 cm(-1). Relationships are developed between the transmission resonance position and the thickness and dielectric properties of the coating.

Carbon chains and the (5,5) single-walled nanotube: structure and energetics versus length.

Reliable thermochemistry is computed for infinite stretches of pure-carbon materials including acetylenic and cumulenic carbon chains, graphene sheet, and single-walled carbon nanotubes (SWCNTs) by connection to the properties of finite size molecules that grow into the infinitely long systems. Using ab initio G3 theory, the infinite cumulenic chain (:C[double bond]C[double bond]C[double bond]C:) is found to be 1.9+/-0.4 kcal/mol per carbon less stable in free energy at room temperature than the acetylenic chain (.C[triple bond]C-C[triple bond]C.) which is 24.0 kcal/mol less stable than graphite. The difference between carbon-carbon triple, double, and single bond lengths (1.257, 1.279, and 1.333 A, respectively) in infinite chains is evident but much less than with small hydrocarbon molecules. These results are used to evaluate the efficacy of similar calculations with the less rigorous PM3 semiempirical method on the (5,5) SWCNT, which is too large to be studied with high-level ab initio methods. The equilibrium electronic energy change for C(g)-->C[infinite (5,5) SWCNT] is -166.7 kcal/mol, while the corresponding free energy change at room temperature is -153.3 kcal/mol (6.7 kcal/mol less stable than graphite). A threefold alternation (6.866, 6.866, and 6.823 A) in the ring diameter of the equilibrium structure of infinitely long (5,5) SWCNT is apparent, although the stability of this structure over the constant diameter structure is small compared to the zero point energy of the nanotube. In general, different (n,m) SWCNTs have different infinite tube energetics, as well as very different energetic trends that vary significantly with length, diameter, and capping.

Controlling the passage of light through metal microchannels by nanocoatings of phospholipids.

The flow of polarized light through a metal film with an array of microchannels is controlled by the phase of an optically active, phospholipid nanocoating, even though the coating does not cover the open area of the microchannels. The molecular details of the assembly (DPPC phospholipid monolayer/bilayer on a hexadecanethiol monolayer on a copper- or nickel-coated microarray) were determined using the infrared, surface-plasmon-mediated, extraordinary transmission of the metal microarrays. Infrared absorption spectra with greatly enhanced absorptions by comparison to literature were recorded and used as a diagnostic for the phase, composition, and molecular geometry of these nanocoatings. This approach presents new tools for nanoscale construction in constricted microspaces, which may ultimately be useful with individual microchannels.

Photoelectron spectra of hydrated electron clusters: Fitting line shapes and grouping isomers.

The photoelectron spectra of (H2O)(n = 2-69) - and (D2O)(n = 2-23) - are presented, and their spectral line shapes are analyzed in detail. This analysis revealed the presence of three different groupings of species, each of which are seen over the range, n = 11-16. These three groups are designated as dipole boundlike states, seen from n = 2-16, intermediate states, found from n = 6-16, and bulk embryonts, starting at n = 11 and continuing up through the largest sizes studied. Almost two decades ago [J. V. Coe et al., J. Chem. Phys. 92, 3980 (1990)], before the present comprehensive analysis, we concluded that the latter category of species were embryonic hydrated electrons with internalizing excess electrons (thus the term embryonts). Recent experiments with colder expansion (high stagnation chamber pressures) conditions by Neumark and coworkers [J. R. R. Verlet et al., Science 307, 93 (2005)] have also found three groups of isomers including the long-sought-after surface states of large water cluster anions. This work confirms that the species here designated as embryonts are in the process of internalizing the excess electron states as the cluster size increases (for n > or = 11).