Vibrational Overtone Research Articles

Dinitrogen Coupling to a Terpyridine-Molybdenum Chromophore Is Switched on by Fermi Resonance

Summary The traditional view of a chemical change is inherently local and classical, and such a change relies on a mix of thermodynamic and kinetic parameters to control reactivity. Often, the thermodynamic stability of chemical bonds necessitates significant energy input for activation. One fundamental question is potentially transformative: can quantum mechanics enable selective bond activation? A possible approach involves strategic input of energy to reaction-specific vibrational levels. Toward this goal, our work describes the coupling of vibrational motions in a terpyridine-molybdenum complex hosting a nonreactive substrate—dinitrogen. Ultrafast coherence spectroscopies revealed a Fermi-resonance coupling mechanism connecting in-plane breathing motion of the light-harvesting terpyridines with the stretching motion of the spatially disparate dinitrogen bridge. Notably, the coupling is significantly enhanced in the photoexcited state. This Fermi resonance indicates an energy conduit that drives the two motions in sync and thereby amplifies vibrational energy exchange. Achieving selective bond activation by bridging vibrations could present a quantum-inspired design principle in synthetic chemistry.

Prospects for a polar-molecular-ion optical probe of varying proton-electron mass ratio

Molecules with deep vibrational potential wells provide optical intervals sensitive to variation in the proton-electron mass ratio ($\mu$). On one hand, polar molecules are of interest since optical state preparation techniques have been demonstrated for such species. On the other hand, it might be assumed that polar species are unfavorable candidates, because typical molecule-frame dipole moments reduce vibrational state lifetimes and cause large polarizabilities and associated Stark shifts. Here, we consider single-photon spectroscopy on a vibrational overtone transition of the polar species TeH$^+$, which is of practical interest because its diagonal Franck-Condon factors should allow rapid state preparation by optical pumping. We point out that all but the ground rotational state obtains a vanishing low-frequency scalar polarizability from coupling with adjacent rotational states, because of a fortuitous relationship between rigid rotor spacings and dipole matrix elements. We project that for good choices of spectroscopy states, demonstrated levels of field control should make possible uncertainties of order $1 \times 10^{-18}$, similar to those of leading atomic ion clocks. The moderately long lived vibrational states of TeH$^+$ make possible a frequency uncertainty approaching $1 \times 10^{-17}$ with one day of averaging for a single trapped ion. Observation over one year could probe for variation of $\mu$ with a sensitivity approaching the $1 \times 10^{-18}/\textrm{yr}$ level.

Photothermal CH vibrational overtone detection by two‐color laser absorption

AbstractIn this experiment, the thermal lens technique is used with a modified arrangement of three lasers to induce a two‐color absorption. Two‐pump lasers, a variable wavelength orange dye laser (588–617 nm), a single line blue laser (488 nm), and a single line probe yellow laser (568 nm) are employed. A comparison is made between the magnitude of the thermal lens signal obtained with a one pump laser versus two pump lasers. The absorbing molecules are benzene and naphthalene in liquid n‐Hexane. The CH vibrational overtone spectra are obtained at room temperature for several concentrations. The molecules are excited to a high vibrational state (Δυ = 6) with the first laser and to an electronic level with a second laser (two‐color absorption). Using two pump lasers, the limit of detection of the molecule is several orders of magnitude more sensitive than using one pump laser. A nonlinear behavior of the integrated signal versus concentration is shown for the two‐color laser process. Linear behavior is shown for the one pump laser experiment. A model of signal amplification for a nonlinear absorption is presented to explain the results. The separation and identification of CH overtone bands in molecules and the sensitivity of the technique is emphasized to convey the potential use of CH overtone spectroscopy for imaging in thermal lens microscopy.

High vibrational overtone excitation‐induced conformational isomerization of glycolic acid in solid argon matrix

AbstractThe high overtone‐induced isomerization of glycolic acid in a low‐temperature argon matrix was investigated using Raman spectroscopy. The Raman spectrum of glycolic acid is presented, and the spectral assignment is supported by vibrational anharmonic calculations. The high overtone excitation of the lowest energy conformer (SSC) at 532 nm induced direct conformational isomerization to higher energy conformers (GAC and AAT). Furthermore, upon excitation at 532 nm GAC and AAT isomerized back to SSC and photo‐induced equilibrium was observed between the conformers. Kinetic model was used to describe the observed isomerization and isomerization rate constants were obtained. No experimental evidence of isomerization between GAC and AAT nor other higher energy conformers were observed.

Investigating the photodissociation of H2O2 using frequency modulation laser absorption spectroscopy to monitor radical products

The photodissociation of H2O2 dilute in argon at 193 nm has been investigated using near infrared frequency modulation laser absorption spectroscopy on selected rotational lines of the OH vibrational overtone bands between 6500 and 7100 cm−1 to detect the OH and HO2 radical products with the same laser. Sequential OH and HO2 time-dependent profiles have been measured at room temperature and total pressures ranging from 3 to 56 Torr. Kinetic analysis gives the product distribution following H2O2 photolysis of (82 ± 1) % OH and (18 ± 1) % HO2 under the present conditions, in excellent agreement with previously reported measurements.

Tunable mid-infrared fiber laser source based on acetylene-filled hollow-core fiber

We report here a 3.1 µm band tunable fiber gas laser source based on acetylene-filled nodeless hollow-core fibers with low losses at both the pump and laser wavelengths. The gas is pumped in the first-order rotational–vibrational overtones near the 1.5 µm region by a modulated, amplified narrowband and accurately tunable diode laser; then 3 µm lasing transmissions are generated in the single-pass amplified spontaneous emission regime. The laser wavelength is step-tunable in the range of 3.09–3.21 µm when the pump laser is precisely tuned to different absorption lines of the P-branch of acetylene molecules. Furthermore, this system can be extended to other selected gases to generate tunable output in the spectral band up to 5 µm.

Relaxation of the H2O Overtone Bending Vibration in the Water Dimer···Hydroxyl Radical Complex

The relaxation mechanism of the overtone bending vibration in the collision of the water dimer with the vibrationally excited hydroxyl radical is studied by use of trajectory procedures. The transfer of the OH(v = 1) energy to the dimer stretches is followed by a near-resonant first overtone transition to the donor monomer. Nearly a quarter of the trajectories undergo a complex-mode collision, forming the (H2O)2···OH complex bound by a hydrogen bond with the lifetime ranging from a subpicosecond scale to >100 ps. The overtone vibration relaxes to the ground state, transferring approximately half of its energy to the dimer hydrogen-bonding (H2O···H2O) and the remaining half to the complex hydrogen-bonding (H2O)2···OH, via near-resonant pathways, each consisting of a series of intermolecular low-frequency vibrations.

He-broadening and shifting coefficients of HCl lines in the (1←0) and (2←0) infrared transitions

ABSTRACTWe report experimental results on the broadening and shifting coefficients in a large interval of J quantum numbers in the fundamental and first overtone vibration–rotation bands of the HCl molecule in mixtures with helium. Many of the values, especially for the overtone transition, are novel. The uncertainties for the isotope-averaged broadening coefficients amount to 2% on average for the fundamental and 1.5% for the overtone bands. Shifts have a typical uncertainty of 3%.

Tuning the Near-Infrared Absorption of Aromatic Amines on Tapered Fibers Sculptured with Gold Nanoparticles

We discover an unexpected enhancement of the absorption of near-infrared light by aromatic amine overtones on photonic microfibers sculptured with gold nanoparticles. The adsorbed nanoparticles make the near-infrared spectroscopy of aromatic amines on microfibers feasible despite the small absorption cross-section of the molecular vibration overtones. We demonstrate that in the presence of gold nanomediators, the absorption of light by weak overtone transitions in N-methylaniline as a model analyte is dramatically enhanced. We attribute this effect to the increase of the mean trajectory of light in a microfiber due to its resonant scattering on metallic nanoparticles. The spectrally integrated transmittance scales with the concentration of nanoparticles to the power 1/6—the phenomenon of diffusive propagation of light. Practical applications of the discovered effect will include the detection of aromatic amines for efficient treatment of metabolic disorders resulting from the amino acids deficiency, resea...

Cyanamide as an Infrared Reporter: Comparison of Vibrational Properties between Nitriles Bonded to N and C Atoms.

Infrared (IR) probes based on terminally blocked β-cyanamidoalanine (AlaNHCN) 1 and p-cyanamidophenylalanine (PheNHCN) 2 were synthesized, and the vibrational properties of their CN stretch modes were studied using Fourier transform infrared (FTIR) and femtosecond IR pump-probe spectroscopies in combination with quantum chemical calculations. From FTIR studies, it is found that the transition dipole strengths of the cyanamide (NHCN) group in 1 and 2 are much larger than those of the nitrile (CN) group but comparable to those of the isonitrile (NC) and azido (N3) groups in their previously studied analogs. The CN stretch frequencies in 1 and 2 are red-shifted from those in their nitrile analogs but more blue-shifted from the NC and N3 stretch frequencies in their isonitrile and azido analogs. The much larger transition dipole strength and the red-shifted frequency of the cyanamide relative to nitrile group originates from the n → π* interaction between the N atom's nonbonding (n) and CN group's antibonding (π*) orbitals of the NHCN group. Unlike aliphatic cyanamide 1, aromatic cyanamide 2 shows a complicated line shape of the CN stretch spectra. Such a complicated line shape arises from the Fermi resonance between the CN stretch mode of the NHCN group and one of the overtones of the phenyl ring vibrations and can be substantially simplified by deuteration of the NHCN into NDCN group. From IR pump-probe experiments, the vibrational lifetimes of the CN stretch mode in 1 were determined to be 0.58 ± 0.04 ps in D2O and 0.89 ± 0.09 ps in H2O and those in 2 were determined to be 1.64 ± 0.13 ps in CH3OD/dimethyl sulfoxide and 0.30 ± 0.05 and 2.62 ± 0.26 ps in CH3OH. The short time component (0.30 ± 0.05 ps) observed for 2 in CH3OH is attributed to the vibrational relaxation through Fermi resonance. These vibrational lifetimes are close to those of the nitrile and azido groups but shorter than those of the isonitrile group. Consequently, cyanamide behaves like an apparent vibrational hybrid of nitrile and isonitrile in that cyanamide is similar to nitrile in vibrational frequency and lifetime but to isonitrile in transition dipole strength. It is believed that cyanamide has the potential to be a strongly absorbing IR reporter of the conformational and environmental structure and dynamics of biomolecules in comparison to nitrile, a weak absorber.

Linear and Nonlinear Thermal Lens Signal of the (Δν = 6) C-H Vibrational Overtone of Naphthalene in Liquid Solutions of n-Hexane.

The thermal lens technique is applied to vibrational overtone spectroscopy of solutions of naphthalene (C10H8) in liquid hexane. The C-H fifth vibrational (Δν = 6) overtone spectrum of C10H8 is detected at room temperature for mole fractions from 0.08 to 19 × 10-6 using n-C6H14 as solvent. By detecting the absorption band in a 19 ppm (parts per million) solution, the peak absorption of the signal is approximately (2.2 ± 0.3) × 10-7 cm-1. A plot of normalized integrated intensity as a function of the mole fraction of naphthalene in solution reveals a dependence of the magnitude of the signal with the probe laser wavelength. If the wavelength of the probe laser is 568 nm, the thermal lens signal (TLS) is linear as a function of the mole fraction of the solution. When the wavelength of the probe laser is 488 nm, the TLS is nonlinear as a function of the concentration. Three different models of nonlinear absorption are discussed. A two-color absorption model that includes the simultaneous absorption of the pump and probe lasers could explain the enhanced magnitude and the nonlinear behavior of the TLS for solutions of mole fraction < 0.1%.

Si Nanostrip Optical Waveguide for On-Chip Broadband Molecular Overtone Spectroscopy in Near-Infrared.

The ability to probe the molecular fundamental or overtone (high harmonics) vibrations is fundamental to modern healthcare monitoring techniques and sensing technologies since it provides information about the molecular structure. However, since the absorption cross section of molecular vibration overtones is much smaller compared to the absorption cross section of fundamental vibrations, their detection is challenging. Here, a silicon nanostrip rib waveguide structure is proposed for label-free on-chip overtone spectroscopy in near-infrared (NIR). Utilizing the large refractive index contrast (Δ n > 2) between the silicon core of the waveguide and the silica substrate, a broadband NIR lightwave can be efficiently guided. We show that the sensitivity for chemical detection is increased by more than 3 orders of magnitude when compared to the evanescent-wave sensing predicted by the numerical model. This spectrometer distinguished several common organic liquids such as N-methylaniline and aniline precisely without any surface modification to the waveguide through the waveguide scanning over the absorption dips in the NIR transmission spectra. Planar NIR Si nanostrip waveguide is a compact sensor that can provide a platform for accurate chemical detection. Our NIR Si nanostrip rib waveguide device can enable the development of sensors for remote, on-site monitoring of chemicals.

Формирование периодических колебаний несинусоидальной формы с ангармонизмом обертонов

Damped bending vibrations of a high-Q string with fixed ends are analyzed. The model of a thin elastic homogeneous string with an electromagnetic pickup placed near the string is used to construct a second-order wave equation in partial time and coordinate derivatives that takes into account the elastic properties of string material, its length, tension and damping, the excitation intensity and location, and the pickup position with respect to the string. The solution as a function of time has the form of a series of higher harmonics (overtones) of fundamental-frequency vibration. It is shown that free oscillations have a nonsinusoidal shape slowly changing during the transient. To obtain a correct solution, several dozens of overtones having commensurable amplitudes should be taken into account. It is found that the oscillations are characterized by inharmonicity of overtones, which manifests itself in a progressive increase in the overtone eigenfrequency with the growth of its number in comparison with a value that is an integer multiple of the fundamental frequency. A nondamped process is accompanied by a periodically varying waveform of the carrier oscillation, and as the vibration decays, its initial nonsinusoidal shape approaches a harmonic waveform. To measure the process quantitative parameters, calibrated arrays of sound records of about 100 strings of a professional grand piano were used in the range of the fundamental tone frequencies exceeding five octaves. It is found that the equivalent Q factor of such oscillating systems varies from a few hundred to several thousand. To determine the inharmonicity parameter value, a modified cepstral transformation method is used. The basic idea of the modified method lies in finding a calibrated pre-distortion of the frequency axis after the first spectral transformation of the process time history samples at which the cepstral response is best located in the region of repiods. Measurements of the inharmonicity parameter for the above-mentioned array of records showed that its values are from a few hundredths to several percent, depending on the combination of the vibrating system parameters. It is shown that the presence of tens of higher inharmonic overtones propagating along the vibration system with different velocities leads to noticeable periodic variations in the process shape and magnitude, also in the case of undamped oscillation. The obtained results can be used for improving the quality of electronic synthesizers of quasi-periodic signals through bringing the musical synthesizer vibration structure and sound closer to those obtained in expensive natural instruments. A training laboratory work has been developed and introduced into the education course, in which the above-mentioned quasi periodic oscillation of a distributed high-Q vibration system with inharmonicity of overtones is synthesized, and a cepstral analysis of the obtained signals is carried out.

"Divide and conquer" semiclassical molecular dynamics: A practical method for spectroscopic calculations of high dimensional molecular systems.

We extensively describe our recently established "divide-and-conquer" semiclassical method [M. Ceotto, G. Di Liberto, and R. Conte, Phys. Rev. Lett. 119, 010401 (2017)] and propose a new implementation of it to increase the accuracy of results. The technique permits us to perform spectroscopic calculations of high-dimensional systems by dividing the full-dimensional problem into a set of smaller dimensional ones. The partition procedure, originally based on a dynamical analysis of the Hessian matrix, is here more rigorously achieved through a hierarchical subspace-separation criterion based on Liouville's theorem. Comparisons of calculated vibrational frequencies to exact quantum ones for a set of molecules including benzene show that the new implementation performs better than the original one and that, on average, the loss in accuracy with respect to full-dimensional semiclassical calculations is reduced to only 10 wavenumbers. Furthermore, by investigating the challenging Zundel cation, we also demonstrate that the "divide-and-conquer" approach allows us to deal with complex strongly anharmonic molecular systems. Overall the method very much helps the assignment and physical interpretation of experimental IR spectra by providing accurate vibrational fundamentals and overtones decomposed into reduced dimensionality spectra.

Nuclear spin/parity dependent spectroscopy and predissociation dynamics in vOH = 2 ← 0 overtone excited Ne-H2O clusters: Theory and experiment.

Vibrationally state selective overtone spectroscopy and state- and nuclear spin-dependent predissociation dynamics of weakly bound ortho- and para-Ne-H2O complexes (D0(ortho) = 34.66 cm-1 and D0(para) = 31.67 cm-1) are reported, based on near-infrared excitation of van der Waals cluster bands correlating with vOH = 2 ← 0 overtone transitions (|02-〉 and |02+〉) out of the ortho (101) and para (000) internal rotor states of the H2O moiety. Quantum theoretical calculations for nuclear motion on a high level potential energy surface [CCSD(T)/VnZf12 (n = 3, 4)], corrected for basis set superposition error and extrapolated to the complete basis set (CBS) limit, are employed to successfully predict and assign Π-Σ, Σ-Σ, and Σ-Π infrared bands in the spectra, where Σ or Π represent approximate projections of the body-fixed H2O angular momentum along the Ne-H2O internuclear axis. IR-UV pump-probe experimental capabilities permit real-time measurements of the vibrational predissociation dynamics, which indicate facile intramolecular vibrational energy transfer from the H2O vOH = 2 overtone vibrations into the VdWs (van der Waals) dissociation coordinate on the τprediss = 15-25 ns time scale. Whereas all predicted strong transitions in the ortho-Ne-H2O complexes are readily detected and assigned, vibrationally mediated photolysis spectra for the corresponding para-Ne-H2O bands are surprisingly absent despite ab initio predictions of Q-branch intensities with S/N > 20-40. Such behavior signals the presence of highly selective nuclear spin ortho-para predissociation dynamics in the upper state, for which we offer a simple mechanism based on Ne-atom mediated intramolecular vibrational relaxation in the H2O subunit (i.e., |02±〉 → {|01±〉; v2 = 2}), which is confirmed by the ab initio energy level predictions and the nascent OH rotational (N), spin orbit (Π1/2,3/2), and lambda doublet product distributions.

Dynamics of Changes in the Infrared Spectrum of Distilled and Heavy Water upon Exposure to an Electron Beam of Nanosecond Duration

The absorption spectra of distilled and heavy water upon exposure to an electron beam of nanosecond duration are investigated. The changes in the absorption spectrum of distilled water after exposure to an electron beam are that the absorption band of valence vibrations of the OH groups becomes broadened, the amplitude of the valence asymmetric band increases, and a shift of the absorption maximums toward shorter wavelength frequencies and an increase in the amplitudes with insignificant band broadening are observed for absorption bands comprised of the sum of deformation and libration vibrations of water molecules and the deformation band of water. The changes in the absorption spectrum of heavy water after its irradiation with an electron beam consist in that the band of valence vibrations of the OH groups of the H2O and HDO molecules is broadened, while the absorption band of the OH overtones of the deformation vibrations of the HDO molecule becomes stronger and the absorption range is preserved. A shift of the absorption maximum toward higher frequencies is observed for the deformation vibrations of the H2O and HDO molecules of heavy water, and the maximum of deformation vibrations of the D2O molecules remains unchanged.

Fluorescence emission and image patterning from selective photocyclic aromatization of cis–cisoid helical poly(phenylacetylene)s in situ in a film via top-down photodegradation

Selective photocyclic aromatization (SCAT) of cis–cisoid helical poly(phenylacetylene) derivatives was investigated during the in situ photodegradation in a film. An infrared absorption peak appeared at ∼1700 cm−1 owing to the overtone vibration of the 1,3,5-trisubstituted benzene and the peak at 1060 cm−1 due to the cis C=C bending of the main chain decreased, indicating occurrence of SCAT. Simultaneously, the polymer film became emissive in visible region during SCAT. The fluorescence (FL) quantum efficiency was approximately 0.92% in the film and not so much high relative to usual fluorophores. The FL emission disappeared in an instant when the decomposition product dissolved in a solvent. The FL lifetime of the polymer film increased with photodegradation. These all indicated the occurrence of excimer emission. A finely resolved FL image pattern was readily obtained using a photomask.

Line shape parameters of helium-broadened 12C16O transitions in the 3 → 0 overtone band near 1.57 µm

Frequency-stabilized cavity ring-down spectroscopy was used to determine helium-broadened 12C16O line shape parameters for eleven lines in the 3 → 0 vibrational overtone band, including helium pressure broadening and pressure shifting. The spectra provided evidence of two non-Voigt mechanisms affecting line shape: collisional (Dicke) narrowing and speed dependence of the line broadening. Multispectrum fits of the speed dependent Nelkin-Ghatak profile (with the quadratic approximation for speed dependence) enabled precise determinations of the line shape parameters associated with these two mechanisms. The measured line shape parameters are presented and are compared to past work for helium-broadened 12C16O transitions in the 3 → 0 vibrational band (broadening coefficients) and in the 1 → 0 fundamental vibrational band (broadening and shifting coefficients). Significant reductions in the uncertainty of the broadening coefficients are demonstrated, along with the first determinations of helium pressure shifting and narrowing parameters for this transition. Analysis of the spectra at elevated pressure reveals evidence of line mixing with fitted first-order line mixing parameters that are consistent with literature values.

Torsion-rotation structure and quasi-symmetric-rotor behaviour for the CH3SH asymmetric CH3-bending and C-H stretching bands of E parentage

High-resolution Fourier transform spectra of the asymmetric methyl-bending and methyl-stretching bands of CH3SH have been recorded employing synchrotron radiation at the FIR beamline of the Canadian Light Source. Analysis of the torsion-rotation structure and relative intensities has revealed the novel feature that for both bend and stretch the in-plane and out-of-plane modes behave much like a Coriolis-coupled l-doublet pair originating from degenerate E modes of a symmetric top. As the axial angular momentum K increases, the energies of the coupled “l=±1” modes diverge linearly, with effective Coriolis ζ constants typical for symmetric tops. For the methyl-stretching states, separated at K=0 by only about 1cm−1, the assigned sub-bands follow a symmetric top Δ(K−l)=0 selection rule, with only ΔK=−1 transitions observed to the upper l=−1 in-plane A′ component and only ΔK=+1 transitions to the lower l=+1 out-of-plane A″ component. The K=0 separation of the CH3-bending states is larger at 9.1cm−1 with the l-ordering reversed. Here, both ΔK=+1 and ΔK=−1 transitions are seen for each l-component but with a large difference in relative intensity. Term values for the excited state levels have been fitted to J(J+1) power-series expansions to obtain substate origins. These have then been fitted to a Fourier model to characterize the torsion-K-rotation energy patterns. For both pairs of vibrational states, the torsional energies display the customary oscillatory behaviour as a function of K and have inverted torsional splittings relative to the ground state. The spectra show numerous perturbations, indicating local resonances with the underlying bath of high torsional levels and vibrational combination and overtone states. The overall structure of the two pairs of bands represents a new regime in which the vibrational energy separations, torsional splittings and shifts due to molecular asymmetry are all of the same order, creating a challenging and complex vibration-torsion-rotation coupling environment.

Near-Infrared Promoted Wettability Recovery of Superhydrophilic ZnO

Superhydrophilic and superhydrophobic surfaces exhibit numerous industrial applications such as self-cleaning, antifogging, anti-icing, electrowetting, and photocatalysis. A surface with reversible wetting behaviors that can be controlled by external field is both scientifically and technologically of importance. Light is a major external trigger for many natural processes. We report here an intelligent remote technology based on alternative UV and near-infrared (NIR) irradiation that enables dynamical wettability tuning of ZnO wafers. Notably, NIR irradiation with a moderate power density of 500 ± 5 mW/cm2 can rapidly recover patterned ZnO from the UV-induced superhydrophilic state (<5°) to the original state (125 ± 3°) in just 2 h. We discover that the chemical environments of O 1s and Zn 2p electrons exhibit a back-and-forth shift upon UV and NIR irradiation, respectively. The NIR-promoted wettability recovery is attributed to selective actuation of vibrational overtone transitions of hydroxyl (OH) band and background absorption of water in the NIR region. Our results highlight that oxygen and water in air are two crucial factors in determining the wettability recovery process, which could be selectively activated or deactivated by remote optical sources. We expect that this discovery could help us to understand many natural processes and shed light on the design of novel devices.