Overtone Transitions Research Articles

Excitation and correlation of 14N overtone transitions and measurement of heteronuclear dipolar coupling using DAPT

For studying systems containing nitrogen, limited use of 14N NMR spectroscopy has been made because of the large quadrupolar interaction experienced by the 14N nucleus and the absence of a central transition. To overcome the above problem, use of overtone spectroscopy has been suggested. Though this approach has limited applicability for powder samples due to second order quadrupole broadening, it is useful for studying oriented samples and single crystals. Here, we demonstrate the use of the recently proposed dipolar assisted polarization transfer (DAPT) pulse scheme for exciting the overtone transitions. The pulse sequence may also be utilized as a two-dimensional experiment to obtain 1H– 14N dipolar couplings and 1H chemical shifts.

Diminished cage effect in solid p-H2: Infrared absorption of CH3S observed from photolysis in situ of CH3SH, CH3SCH3, or CH3SSCH3 isolated in p-H2 matrices

We report the infrared absorption spectrum of the methylthio (or thiomethoxy) radical, CH(3)S (X (2)E(3/2)), produced via photodissociation in situ of three precursors CH(3)SH, CH(3)SCH(3), and CH(3)SSCH(3) isolated in solid p-H(2). The common absorption features observed with similar intensity ratios in each experiment are assigned to CH(3)S. The wavenumbers of these features agree satisfactorily with those predicted with a spin-vibronic Hamiltonian accounting for the anharmonic effects and the Jahn-Teller effects to the quartic term [A. V. Marenich and J. E. Boggs, J. Chem. Theory Comput. 1, 1162 (2005)]. In addition to an absorption line at 724.2 cm(-1), corresponding to a transition of 3(1) previously determined to be 727 cm(-1) from fluorescence spectra of gaseous CH(3)S, we identified fundamental transitions 6(1)(a(1)) at 771.1, 6(1)(e) at 1056.6, 5(1)(a(1)) at 1400.0, 4(1)(a(1)) at 2898.4 cm(-1), and several combination and overtone transitions. In contrast, photolysis of CH(3)SSCH(3) isolated in solid Ar produces mainly H(2)CS, CH(3)SH, and CS(2), but no CH(3)S. These results demonstrate the feasibility of using photolysis in situ of precursors isolated in solid p-H(2) to produce free radicals by taking advantage of the diminished cage effect of the matrix.

Ab initio molecular geometry and anharmonic vibrational spectra of thiourea and thiourea‐d4

Potential energy surface (PES) of thiourea, SC(NH(2))(2), has been searched for stable conformers under C(1), C(s), C(2), and C(2v) symmetry constraints by post-Hartree-Fock ab initio methods with electron correlation level varying from second-order Moeller-Plesset perturbation theory (MP2) to quadratic configuration interaction with single and double excitations (QCISD) and basis sets of double- and triple-zeta quality within 6-31+G(d,p) to aug-cc-pVTZ range. Thiourea conformers of C(2) and C(s) symmetry have been found as stationary points on the PES with no imaginary frequencies at MP2/6-31+G(d,p) level, whereas only the C(2) conformer seems as true minimum when basis sets containing more polarization and/or diffuse functions were used. At QCISD/cc-pVTZ level, only the C(2) thiourea conformer has been found as true minimum on the PES. Anharmonic vibrational spectra of C(2) conformers of thiourea and thiourea-d(4) have been computed by vibrational self-consistent field (VSCF) and correlation-corrected VSCF methods using quartic force field approximation at MP2/TZV+(2d,2p), and MP2/6-311+G(3df,2p) level and direct approach at MP2/6-31+G(2d,p) level. Both quartic force field and direct VSCF methods used PES expansion in curvilinear (internal) coordinates. Wavenumbers of fundamental, first overtone, and combination transitions of C(2) symmetry conformer have been calculated for natural abundance thiourea and thiourea-d(4) isotopomer. Anharmonic corrections originating from mean field and mode coupling effects vary from 5 to 60 cm(-1), whereas mode-mode correlation contribution seems significant in the case of ν(N-H) stretching and δ(NH(2)) deformation modes (15-5 cm(-1)). Application of internal coordinates in the VSCF calculation results in slight underestimation of δ(NH(2)) deformation mode fundamentals and correct description of out-of-plane large-amplitude τ(SCNH) modes.

Cavity-enhanced resonant photoacoustic spectroscopy with optical feedback cw diode lasers: A novel technique for ultratrace gas analysis and high-resolution spectroscopy

Cavity-enhanced resonant photoacoustic spectroscopy with optical feedback cw diode lasers (OF-CERPAS) is introduced as a novel technique for ultratrace gas analysis and high-resolution spectroscopy. In the scheme, a single-mode cw diode laser (3 mW, 635 nm) is coupled into a high-finesse linear cavity and stabilized to the cavity by optical feedback. Inside the cavity, a build-up of laser power to at least 2.5 W occurs. Absorbing gas phase species inside the cavity are detected with high sensitivity by the photoacoustic effect using a microphone embedded in the cavity. To increase sensitivity further, coupling into the cavity is modulated at a frequency corresponding to a longitudinal resonance of an organ pipe acoustic resonator (f=1.35 kHz and Q approximately 10). The technique has been characterized by measuring very weak water overtone transitions near 635 nm. Normalized noise-equivalent absorption coefficients are determined as alpha approximately 4.4x10(-9) cm(-1) s(1/2) (1 s integration time) and 2.6x10(-11) cm(-1) s(1/2) W (1 s integration time and 1 W laser power). These sensitivities compare favorably with existing state-of-the-art techniques. As an advantage, OF-CERPAS is a "zero-background" method which increases selectivity and sensitivity, and its sensitivity scales with laser power.

Cavity Ring Down and Fourier Transform Infrared Spectroscopy at Low Temperatures (84−297 K): Fermi Resonance and Intensities of the C−H Fundamental and Overtone (Δυ = 1−6) Transitions of CHD3

The C-H stretch fundamental and overtone absorptions of CHD(3) have been obtained using Fourier transform infrared (FTIR), near-infrared, and phase shift and pulsed cavity ring down (CRD) techniques at temperatures between 84 and 297 K. The partially resolved rotational-vibrational spectra of CHD(3) that included the fundamental transition nu(1), the overtone transitions, and combination bands 5nu(1) and 4nu(1) + 2nu(5) and 6nu(1) and 5nu(1) + 2nu(5) were obtained and compared with the simulated spectra at the corresponding temperature. The strength of the Fermi resonance between levels upsilonnu(1) and (upsilon - 1)nu(1) + 2nu(5) with upsilon = 2-6 was calculated using the experimental perturbed energies and relative peak intensities. The integrated absorption was calculated as a function of the density of the gas samples and used to obtain the cross section and the band strength of the Deltaupsilon = 5 and 6 transitions. Ab initio molecular orbital calculations were done to obtain the dipole moment function (DMF) as a function of the C-H internuclear distance. Intensity calculations with the DMF correctly predict the order of magnitude of the experimental band strengths.

Calculated Spectroscopy and Atmospheric Photodissociation of Phosphoric Acid

Detection of phosphine (PH(3)) gas in the upper troposphere suggests that the biogeochemical P cycle also includes an atmospheric component that consists of volatile phosphorus-containing molecules. A reasonable end product for the oxidation of PH(3) in the atmosphere is phosphoric acid (H(3)PO(4)). We propose that H(3)PO(4) may be photodissociated into HOPO(2) and H(2)O in the stratosphere, where H(3)PO(4) is likely to be present in gaseous form. We have calculated the energy barrier of this reaction and show that in addition to electronic transitions, OH-stretching overtone transitions can also provide the necessary energy. OH-stretching fundamental and overtone transitions were calculated with the use of an anharmonic oscillator local mode model. The probability of overtone induced photodissociation was estimated with molecular dynamical reaction coordinate simulations. Electronic transitions were calculated with the equation of motion coupled cluster singles doubles method. We have calculated the photodissociation rate constants for absorption of visible, UV, and Lyman-alpha radiation at altitudes from 20 to 100 km. We show that at altitudes between 30 and 70 km, the photodissociation of H(3)PO(4) is likely to proceed via absorption in the UV region by electronic transitions.

XH-stretching overtone transitions calculated using explicitly correlated coupled cluster methods

We have calculated XH-stretching (where X=O, C, F, Cl) fundamental and overtone transitions for three diatomics and a few small molecules using a local mode model. The potential energy curves and dipole moment functions are calculated using the recently developed explicitly correlated coupled cluster with single doubles and perturbative triples theory [CCSD(T)-F12] with the associated VXZ-F12 (where X=D, T, Q) basis sets. We find that the basis set convergence of calculated frequencies and oscillator strengths obtained with the explicitly correlated method is much more rapid than with conventional CCSD(T) and the Dunning type correlation consistent basis sets. Furthermore, CCSD(T)-F12 frequencies and oscillator strengths obtained with the VTZ-F12 and VQZ-F12 basis sets are found to be in excellent agreement with the CCSD(T) complete basis set limit. We find that comparison of CCSD(T)-F12 frequencies with experiment is less good. The inclusion of explicit correlation exposes the inherent error of the CCSD(T) method to overestimate vibrational frequencies, which is normally compensated by basis set incompleteness error. As a consequence, we suggest that conventional CCSD(T) in combination with the aug-cc-pVTZ or aug-cc-pVQZ basis sets is likely to yield calculated XH-stretching frequencies in closest agreement with experiment.

The structure of the NO(X Π2)−N2 complex: A joint experimental-theoretical study

We report the first measurement of the spectrum of the NO-N(2) complex in the region of the first vibrational NO overtone transition. The origin band of the complex is blueshifted by 0.30 cm(-1) from the corresponding NO monomer frequency. The observed spectrum consists of three bands assigned to the origin band, the excitation of one quantum of z-axis rotation and one associated hot band. The spacing of the bands and the rotational structure indicate a T-shaped vibrationally averaged structure with the NO molecule forming the top of the T. These findings are confirmed by high level ab initio calculations of the potential energy surfaces in planar symmetry. The deepest minimum is found for a T-shaped geometry on the A(")-surface. As a result the sum potential also has the global minimum for this structure. The different potential surfaces show several additional local minima at slightly higher energies indicating that the complex most likely will perform large amplitude motion even in its ground vibrational state. Nevertheless, as suggested by the measured spectra, the complex must, on average, spend a substantial amount of time near the T-shaped configuration.

An exchange-Coulomb model potential energy surface for the Ne–CO interaction. II. Molecular beam scattering and bulk gas phenomena in Ne–CO mixtures

Four potential energy surfaces are of current interest for the Ne-CO interaction. Two are high-level fully ab initio surfaces obtained a decade ago using symmetry-adapted perturbation theory and supermolecule coupled-cluster methods. The other two are very recent exchange-Coulomb (XC) model potential energy surfaces constructed by using ab initio Heitler-London interaction energies and literature long range dispersion and induction energies, followed by the determination of a small number of adjustable parameters to reproduce a selected subset of pure rotational transition frequencies for the (20)Ne-(12)C(16)O van der Waals cluster. Testing of the four potential energy surfaces against a wide range of available experimental microwave, millimeter-wave, and mid-infrared Ne-CO transition frequencies indicated that the XC potential energy surfaces gave results that were generally far superior to the earlier fully ab initio surfaces. In this paper, two XC model surfaces and the two fully ab initio surfaces are tested for their abilities to reproduce experiment for a wide range of nonspectroscopic Ne-CO gas mixture properties. The properties considered here are relative integral cross sections and the angle dependence of rotational state-to-state differential cross sections, rotational relaxation rate constants for CO(v=2) in Ne-CO mixtures at T=296 K, pressure broadening of two pure rotational lines and of the rovibrational lines in the CO fundamental and first overtone transitions at 300 K, and the temperature and, where appropriate, mole fraction dependencies of the interaction second virial coefficient, the binary diffusion coefficient, the interaction viscosity, the mixture shear viscosity and thermal conductivity coefficients, and the thermal diffusion factor. The XC model potential energy surfaces give results that lie within or very nearly within the experimental uncertainties for all properties considered, while the coupled-cluster ab initio surface gives results that agree similarly well for all but one of the properties considered. When the present comparisons are combined with the ability to give accurate spectroscopic transition frequencies for the Ne-CO van der Waals complex, only the XC potential energy surfaces give results that agree well with all extant experimental data for the Ne-CO interaction.

Duschinsky mixing between four non-totally symmetric normal coordinates in the S1–S0 vibronic structure of (E)-phenylvinylacetylene: a quantitative analysis

(E)-Phenylvinylacetylene was shown previously (C.-P. Liu, J. J. Newby, C. W. Müller, H. D. Lee, T. S. Zwier, J. Phys. Chem. A, 2008, 112, 9454.) to support extensive Duschinsky mixing among its four lowest-frequency out-of-plane normal coordinates Q(45)-Q(48) in the S(1)<-- S(0), i.e. (L(a)) A (1)A'<-- X (1)A', electronic transition. The complexity of this mixing is considerably increased relative to that of its parent styrene due to the longer conjugated side chain. Here we quantitatively analyze this change of the motional character of the four non-totally symmetric vibrations upon electronic excitation. The peak intensities of 182 overtone and combination transitions spread over seven SVLF spectra were fit simultaneously with seven parameters in an automated least-squares fitting procedure in which an unweighted least-squares sum was minimized using a pattern search algorithm. The seven parameters consisted of the six Duschinsky rotation angles and the S(1) frequency of normal mode nu(48). The required four-dimensional Franck-Condon overlap integrals were calculated using previously reported recursion relations between harmonic oscillator wavefunctions. As a consistency check, the intensities of all possible 434 electric dipole allowed overtone and combination bands of normal modes nu(45)-nu(48) up to individual vibrational quantum numbers of v = 4 were simulated. The comparison with the experimental intensities revealed with few exceptions very good agreement. The results of the Duschinsky analysis are discussed in light of the pi-pi* electronic excitation as represented by different ab initio (HF, CIS, CASSCF), density functional (B3LYP and BP86) and time-dependent density functional (TD-B3LYP and TD-BP86) methods. Our Duschinsky mixing analysis reveals a challenging complexity that is not quantitatively reproduced by widely used excited state quantum chemical methods. The sensitivity of Duschinsky mixing coefficients to both excited state equilibrium geometries and force fields thus provides a valuable benchmark for the improvement of excited state quantum chemical methods.

Lasers on overtone transitions of carbon monoxide molecule

The experimental and theoretical study of lasers working on the first vibrational overtone of the CO molecule is reviewed. A theoretical model of the active medium of the CO laser and its verification are presented. The CO laser operation in a wide spectral range (2.5–4.2 μm) that spans the atmospheric transparency window is demonstrated in the review of experimental works. It is shown that the overtone CO laser can serve as an effective mid-IR source for atmospheric spectroscopy, long-distance transportation of laser radiation, remote laser sensing, etc.

Vibrational overtone excitation in electron mediated energy transfer at metal surfaces

Vibrational overtone excitation is, in general, inefficiently stimulated by photons, but can under some circumstances be efficiently stimulated by electrons. Here, we demonstrate electron mediated vibrational overtone excitation in molecular collisions with a metal surface. Specifically, we report absolute vibrational excitation probabilities to ν = 1 and 2 for collisions of NO(ν = 0) with a Au(111) surface as a function of surface temperature from 300 to 985 K. In all cases, the observed populations of vibrationally excited NO are near those expected for complete thermalization with the surface, despite the fact that the scattering occurs through a direct “single bounce” mechanism of sub-ps duration. We present a state-to-state kinetic model, which accurately describes the case of near complete thermalization (a regime we call the strong coupling case) and use this model to extract state-to-state rate constants. This analysis unambiguously shows that direct vibrational overtone excitation dominates the production of ν = 2 and that, within the context of our model, the intrinsic strength of the overtone transition is of the same order as the single quantum transition, suggesting a possible way to circumvent optical selection rules in vibrational pumping of molecules. This result also suggests that previous measurements of vibrational relaxation of highly vibrationally excited NO exhibiting highly efficient multi-quantum jumps (Δν ∼ −8) are mechanistically similar to vibrational excitation of NO(ν = 0).

Spectroscopic and Kinetic Properties of HO2 Radicals and the Enhancement of the HO2 Self Reaction by CH3OH and H2O

The line center absorption cross sections and the rate constants for self-reaction of hydroperoxy radicals (HO(2)) have been examined in the temperature range of 253-323 K using pulsed laser photolysis combined with tunable diode laser absorption in the near-IR region. The transition probed was in the 2nu(1) OH overtone transition at 1506.43 nm. The temperature dependence of the rate constant (k) for the HO(2) + HO(2) reaction was measured relative to the recommended value at 296 K, giving k = (3.95 +/- 0.45) x 10(-13) x exp[(439 +/- 39)/T] cm(3) molecule(-1) s(-1) at a total pressure of 30 Torr (N(2) + O(2)). After normalizing our determination and previous studies at low pressure, we recommend k = (2.45 +/- 0.50) x 10(-13) x exp[(565 +/- 130)/T] cm(3) molecule(-1) s(-1) (0 < P < 30 Torr, 95% confidence limits). The observed rate coefficient, k(obs), increases linearly with CH(3)OH concentration, and the enhancement coefficient (k'), defined by k(obs) = k + k'[CH(3)OH], is found to be (3.90 +/- 1.87) x 10(-35) x exp[(3849 +/- 135)/T] cm(6) molecule(-2) s(-1) at 30 Torr. The analogous water vapor enhancement coefficient (k'') is (1.16 +/- 0.58) x 10(-36) x exp[(4614 +/- 145)/T] cm(6) molecule(-2) s(-1). The pressure-broadened HO(2) absorption cross section is independent of temperature in the range studied. The line center absorption cross sections at 1506.43 nm, after correction for instrumental broadening, are (4.3 +/- 1.1) x 10(-19), (2.8 +/- 0.7) x 10(-19), and (2.0 +/- 0.5) x 10(-19) cm(2)/molecule at total pressures of 0, 30, and 60 Torr, respectively (95% confidence limits).

IR spectrum of the O–H⋯O hydrogen bond of phthalic acid monomethylester in gas phase and in CCl 4 solution

The absorption spectrum of the title compound in the spectral range of the hydrogen-bonded OH-stretching vibration has been investigated using a five-dimensional gas phase model as well as a QM/MM classical molecular dynamics simulation in solution. The gas phase model predicts a Fermi-resonance between the OH-stretching fundamental and the first OH-bending overtone transition with considerable oscillator strength redistribution. The anharmonic coupling to a low-frequency vibration of the hydrogen bond leading to a vibrational progression is studied within a diabatic potential energy curve model. The condensed phase simulation of the dipole–dipole correlation function results in a broad band in the 3000 cm −1 region in good agreement with experimental data. Further, weaker absorption features around 2600 cm −1 have been identified as being due to motion of the hydrogen within the hydrogen bond.

Photoacoustic detection of gases using microcantilevers

We describe a new technique for measuring the infrared absorption spectra of gases using atomic force microscope microcantilevers. This photoacoustic system is demonstrated for a dilute acetylene/helium mixture by recording the acetylene ν1+ν3 infrared overtone transitions using a wavelength modulated tunable diode laser as the infrared light source. The technique presents significant advantages over existing methods in terms of size, simplicity, speed and insensitivity to ambient vibrations. The maximum achievable signal-to-noise for resonant and non-resonant photoacoustic excitation of the microcantilever is examined and is found to be limited by the microcantilever’s Brownian noise.

Observation of a Zundel-like transition state during proton transfer in aqueous hydroxide solutions.

It is generally accepted that the anomalous diffusion of the aqueous hydroxide ion results from its ability to accept a proton from a neighboring water molecule; yet, many questions exist concerning the mechanism for this process. What is the solvation structure of the hydroxide ion? In what way do water hydrogen bond dynamics influence the transfer of a proton to the ion? We present the results of femtosecond pump-probe and 2D infrared experiments that probe the O-H stretching vibration of a solution of dilute HOD dissolved in NaOD/D(2)O. Upon the addition of NaOD, measured pump-probe transients and 2D IR spectra show a new feature that decays with a 110-fs time scale. The calculation of 2D IR spectra from an empirical valence bond molecular dynamics simulation of a single NaOH molecule in a bath of H(2)O indicates that this fast feature is due to an overtone transition of Zundel-like H(3)O(2)(-) states, wherein a proton is significantly shared between a water molecule and the hydroxide ion. Given the frequency of vibration of shared protons, the observations indicate the shared proton state persists for 2-3 vibrational periods before the proton localizes on a hydroxide. Calculations based on the EVB-MD model argue that the collective electric field in the proton transfer direction is the appropriate coordinate to describe the creation and relaxation of these Zundel-like transition states.

Anharmonic overtones quenching in Er 3+ complexes

The organic complexes of the erbium ion Er 3+ emitting in the near-infrared at 1.5 μm are widely investigated as potential active materials for a new generation of low-cost optical amplifiers for telecommunications. The limiting factor in these systems is the very low quantum yield of the lanthanide ion, generally ascribed to the presence of nearby high-energy vibrational centers as OH and CH bonds which are present as residual coordination water and as functional groups of the organic ligands. To make a quantitative prediction of the extent of the non-radiative decay originating from Förster transfer, the transition moments of the vibrational overtones in resonance with the ion emission have been evaluated from first principle calculations to compute the intensity of the fundamental normal modes; the overtone transitions intensities with energy falling in the 1.5 μm region have been evaluated by recursive analytical expressions obtained by using the Morse oscillator approximation.

A phase-stabilized carbon nanotube fiber laser frequency comb

A frequency comb generated by a 167 MHz repetition frequency erbium-doped fiber ring laser using a carbon nanotube saturable absorber is phase-stabilized for the first time. Measurements of the in-loop phase noise show an integrated phase error on the carrier envelope offset frequency of 0.35 radians. The carbon nanotube fiber laser comb is compared with a CW laser near 1533 nm stabilized to the nu(1) + nu(3) overtone transition in an acetylene-filled kagome photonic crystal fiber reference, while the CW laser is simultaneously compared to another frequency comb based on a Cr:Forsterite laser. These measurements demonstrate that the stability of a GPS-disciplined Rb clock is transferred to the comb, resulting in an upper limit on the locked comb's frequency instability of 1.2 x 10(-11) in 1 s, and a relative instability of <3 x 10(-12) in 1 s. The carbon nanotube laser frequency comb offers much promise as a robust and inexpensive all-fiber frequency comb with potential for scaling to higher repetition frequencies.

The near IR spectrum of the NO(XΠ2)–CH4 complex

We report the first measurement of the near IR spectrum of the NO-CH(4) complex in the region of the first vibrational NO overtone transition in an IR-resonance enhanced multiphoton ionization double resonance experiment. The origin band is located at 3723.26 cm(-1), i.e., redshifted by 0.59 cm(-1) from the corresponding NO monomer frequency. The observed spectrum consists of two bands assigned to the origin band and the excitation of hindered rotation of the NO monomer in the complex similar to z-axis rotation. The spacing and the relative intensity of the bands are consistent with a structure in which NO resides preferentially in a position perpendicular to the intermolecular axis. The deviation from the linear configuration with C(3v) symmetry can be regarded as a Jahn-Teller (JT) distortion. Each band is dominated by two broad peaks with a few resolved rotational structures. The large spacing between the two peaks is indicative of significant angular momentum quenching, possibly another manifestation of the JT effect. The delay dependence between the IR and UV laser pulses reveals a lifetime of about 10 ns for the vibrationally excited complex due to vibrational predissociation. On the other hand, the linewidth of the narrowest spectral features indicates a much shorter excited state lifetime of about 100 ps, most likely due to intramolecular vibrational redistribution.

Fundamental and Overtone Vibrational Spectra of Gas-Phase Pyruvic Acid

Pyruvic acid (CH(3)COCOOH) is an important keto acid present in the atmosphere. In this study, the vibrational spectroscopy of gas-phase pyruvic acid has been investigated with special emphasis on the overtone transitions of the OH-stretch, with Delta v(OH) = 2, 4, 5. Assignments were made to fundamental and combination bands in the mid-IR. The two lowest energy rotational conformers of pyruvic acid are clearly observed in the spectrum. The lowest energy conformer possesses an intramolecular hydrogen bond, while the next lowest rotational conformer does not. This difference is clearly seen in the spectra of the OH vibrational overtone transitions, and it is reflected in the anharmonicities of the OH-stretching modes for each conformer. The spectra of the OH-stretching vibration for both conformers were investigated to establish the effect of the hydrogen bond on frequency, intensity, and line width.