Overtone Transitions Research Articles

Phase shift cavity ring down at low temperatures: Vibration–rotation overtone absorption of H–D (Δ v = 4) at 297 and 105 K

Phase shift cavity ring down (PS-CRD) spectroscopy has been used for the detection of a vibrational–rotational overtone transition ( v = 0 → 4) of H–D at 297 and 105 K. The PS-CRD technique at low temperatures involves an optical cavity attached to a cryostat where the static cell and mirrors of the optical cavity are all at the same temperature. A novel design allows the alignment of the mirrors of the optical cavity at low temperatures without disrupting the vacuum in the chamber surrounding the cell. The peak absorption lines were corrected for distortion because the bandwidth (fwhm) of the laser in our experiment exceeds the width of the absorption lines. The calculated line strengths of the rotational transitions are in good agreement with literature values.

Second OH overtone excitation and statistical dissociation dynamics of peroxynitrous acid

The second OH overtone transition of the trans-perp conformer of peroxynitrous acid (tp-HOONO) is identified using infrared action spectroscopy. HOONO is produced by the recombination of photolytically generated OH and NO(2) radicals, and then cooled in a pulsed supersonic expansion. The second overtone transition is assigned to tp-HOONO based on its vibrational frequency (10 195.3 cm(-1)) and rotational band contour, which are in accord with theoretical predictions and previous observations of the first overtone transition. The transition dipole moment associated with the overtone transition is rotated considerably from the OH bond axis, as evident from its hybrid band composition, indicating substantial charge redistribution upon OH stretch excitation. The overtone band exhibits homogeneous line broadening that is attributed to intramolecular vibrational redistribution, arising from the coupling of the initially excited OH stretch to other modes that ultimately lead to dissociation. The quantum state distributions of the OH X (2)Pi (nu=0) products following first and second OH overtone excitation of tp-HOONO are found to be statistical by comparison with three commonly used statistical models. The product state distributions are principally determined by the tp-HOONO binding energy of 16.2(1) kcal mol(-1). Only a small fraction of the OH products are produced in nu=1 following the second overtone excitation, consistent with statistical predictions.

First-principles computation of the anharmonic vibrational spectra of sulfuryl halides SO 2X 2 (X = F, Cl, Br)

Equilibrium geometries and anharmonic vibrational spectra of sulfuryl halides SO 2X 2 (X = F, Cl, Br) have been computed on MP2(frozen core) level of electronic structure theory in 6-31G(df) and effective core potential (ECP) Stevens–Basch–Krauss–Jasien–Cundari (SBKJC) SBKJC(3df) basis sets. Anharmonic spectra have been determined directly from MP2 potential energy surfaces using vibrational self-consistent field (VSCF) and correlation corrected vibrational self-consistent field (CC-VSCF) methods. Computed anharmonic corrections vary from 1.5 cm −1 for low frequency δ(O S O) and δ(X–S–X) deformation modes to 12–20 cm −1 for ν a(S O), ν s(S O), and ν a(S–X) stretching modes. Wavenumbers of fundamental transitions of SO 2F 2 and SO 2Cl 2 calculated in SBKJC(3df) basis are within 2–12 cm −1 of their experimental counterparts with RMSD of 7.7 and 7.3 cm −1, respectively. Anharmonic spectra of SO 2F 2 and SO 2Cl 2 computed in 6-31G(df) basis are farther apart from experiment having RMSDs of 17.2 and 15.0 cm −1. Overtone transitions with observable infrared intensity (above 0.5 km/mol) are attributable to 2 ν 1(2 ν a(S O)) and 2 ν 2(2 ν s(S–F)) modes of sulfuryl fluoride or 2 ν 2(2 δ(O S O)) mode of SO 2Cl 2 and SO 2Br 2.

Dynamic nuclear polarization and nuclear magnetic resonance in the vicinity of edge states of a 2DES in GaAs quantum wells

Nuclear magnetic resonance is detected via the in-plane conductivity of a two-dimensional electron system at unity Landau level filling factor in the regime of the quantum Hall effect in narrow and wide quantum wells. The NMR is spatially selective to nuclei with a coupling to electrons in the current carrying edge states at the perimeter of the 2DES. Interpretation of the electron-nuclear double resonance signals is facilitated by numerical simulations. A new RF swept method for conductivity-detected NMR is introduced which offers more efficient signal averaging. The method is applied to the study of electric quadrupole interactions, weakly allowed overtone transitions, and evaluation of the extent of electron wave function delocalization in the wide quantum well.

Lineshape study of argon broadened water vapour overtone transitions in the 818–834 nm wavelength region

Argon-broadened lineshape parameters of the water vapour rovibrational transitions belonging to the (2 1 1) ← (0 0 0) overtone band in the 818–834 nm wavelength region are measured by using a diode laser spectrometer. The first derivative signals of the overtone transitions have been recorded at different pressures of the perturber keeping the sample at its saturated vapour pressure at room temperature. Use of a balanced detector improves the S/ N ratio of the spectra. The recorded lineshapes are fitted with standard Voigt line profiles in order to determine the linestrength and collisional broadening coefficient of 12 water vapour transitions.

A simple scanning semiconductor diode laser source and its application in wavelength modulation spectroscopy around 825 nm

A very simple and inexpensive tunable semiconductor diode laser controller is designed for stable operation of the diode laser. The diode laser controller is stable within +/−8 μA and +/−10 mK, respectively. The noise spectrum of the current controller is studied by FFT analysis. We have used our home-made diode laser system in a tunable diode laser absorption spectrometer (TDLAS) to probe weak overtone transitions of water vapour molecule. The diode laser wavelength is coarsely tuned by changing the operating temperature to probe (2, 1, 1)←(0, 0, 0) band overtone transitions of water vapour within 818–835 nm. To demonstrate line shape study, seven transitions are scanned by ramping the drive current of the diode laser (at constant operating temperature) under different perturber (laboratory air) pressures within the sample cell. A balanced detector and a lock-in amplifier are used for phase sensitive detection purpose. Small current modulation amplitude, balanced detection and proper adjustment of the lock-in amplifier help to obtain a S/ N ratio ranging from ∼100 to ∼7 using a small sample path length of 1.5 m. Experimentally obtained derivative spectrum is numerically integrated to reveal the original line shape and fitted with a nonlinear least squares fitting program to extract air broadening coefficients and line strength parameters. The spectroscopic line parameters are compared with the results from HITRAN database.

CH Stretching Vibrational Overtone Spectra of 1,3,5,7-Cyclooctatetraene and 1,1,1-Trichloroethane

Local mode frequencies, omega, and anharmonicities, omegax, are obtained from the delta upsilon(CH) = 2-7 spectral regions of 1,3,5,7-cyclooctatetraene (COT) and 1,1,1-trichloroethane. In 1,1,1-trichloroethane omega and omega x are used in conjunction with ab initio potential energy surfaces to calculate local mode anharmonicity-torsion coupling terms, delta(omega x), and frequency-torsion coupling terms, delta(omega). Blue-shifting of sterically hindered CH oscillators in 1,1,1-trichloroethane indicates nonbonded, through-space intramolecular interactions with Cl. Multiple, complex Fermi resonances are observed in 1,1,1-trichloroethane and in COT between local mode states and local mode/normal mode combination states. Intensities of vibrational overtone transitions are calculated in the range delta upsilon(CH) = 3-9 using ab initio dipole moment functions and the harmonically coupled anharmonic oscillator (HCAO) model. HCAO intensities are compared to experimental intensities at delta upsilon(CH) = 3.

Fluorescence-dip infrared spectroscopy and predissociation dynamics of OH AΣ+2 (v=4) radicals

Fluorescence-dip infrared spectroscopy, an UV-IR double-resonance technique, is employed to characterize the line positions, linewidths, and corresponding lifetimes of highly predissociative rovibrational levels of the excited A (2)Sigma(+) electronic state of the OH radical. Various lines of the 4 <--2 overtone transition in the excited A (2)Sigma(+) state are observed, from which the rotational, centrifugal distortion, and spin-rotation constants for the A (2)Sigma(+) (v = 4) state are determined, along with the vibrational frequency for the overtone transition. Homogeneous linewidths of 0.23-0.31 cm(-1) full width at half maximum are extracted from the line profiles, demonstrating that the N = 0 to 7 rotational levels of the OH A (2)Sigma(+) (v = 4) state undergo rapid predissociation with lifetimes of < or =23 ps. The experimental linewidths are in near quantitative agreement with first-principles theoretical predictions.

Effective One-Dimensional Dipole Moment Function for the OH Stretching Overtone Spectra of Simple Acids and Alcohols

To gain insight on the absorption intensities, as well as the direction of the transition moment for the OH stretching vibration in alcohols and acids, we performed detailed analyses for nitric acid, acetic acid, methanol, tert-butyl alcohol, water, and OH radical. We obtained both the potential energy surface and the dipole moment function (DMF) by the B3LYP method and performed quantum mechanical vibrational calculation using the grid variational method based on the local mode model. In this work, we employed the sum rule of the absorption intensities for the one-dimensional (1-D) vibrational Hamiltonian to construct an effective 1-D DMF, which is responsible for the total sum of the overtone intensities. The direction of this effective DMF was found to be tilted away from the OH bond by about 30 degrees for the polyatomic molecules. The nonlinearity of the DMFs in the directions parallel and perpendicular to the OH bond is discussed to rationalize the tilting. Furthermore, we analyzed the effective 1-D DMFs with the vibrational wave function expansion method and derived the effective portion of the 1-D DMF that is responsible for the overtone transition moment.

CH Stretching Vibrational Overtone Spectra of tert-Butylbenzene, tert-Butyl Chloride, and tert-Butyl Iodide

The vapor phase CH stretching vibrational overtone spectra of tert-butylbenzene and tert-butyl chloride are measured in the Delta upsilon(CH) = 2-7 region, while the spectrum of tert-butyl iodide is recorded in the Delta upsilon(CH) = 2-6 region. The overtone spectrum of tert-butylbenzene is too complex to make detailed spectral assignments. Local mode frequencies, omega, and anharmonicities, omegax, are obtained for tert-butyl chloride and tert-butyl iodide. The torsional dependencies of the local mode frequency, delta(omega), and anharmonicity, delta(omega)(x), are calculated for the tert-butyl halides. Nonbonded, through-space intramolecular interactions are observed in the blue-shifting of sterically hindered CH oscillators. Scaling factors are presented for relating ab initio calculated local mode parameters to experimental values for alkyl CH oscillators. Fermi resonances are observed between local mode states and local mode/normal mode combination states in tert-butyl chloride and tert-butyl iodide. Vibrational overtone transition intensities are calculated in the range Delta upsilon(CH) = 3-9 using the harmonically coupled anharmonic oscillator (HCAO) model and ab initio dipole moment functions. The resultant HCAO intensities are compared to experimental intensities at Delta upsilon(CH) = 3.

Infrared, Raman, and Inelastic Neutron Scattering Spectra of Dodecahedrane: an Ih Molecule in Th Site Symmetry

The Raman spectrum of crystalline dodecahedrane, C20H20, a species of nominal I(h) symmetry, exhibits splitting of the H(g) Raman active modes. The Raman inactive gerade vibrations of G(g), T(1g), and T(2g) symmetry are found to have weak Raman activity. The IR forbidden vibrations of T(2u), G(u), and H(u) type have moderate IR activity. All of this is consistent with the T(h) site symmetry. A treatment of the structure and vibrations of dodecahedrane using a periodic lattice DFT method results in a slightly distorted T(h) structure with six C-C bonds that are 0.001 A longer than the other 24. The vibrational spectrum computed for this structure exhibits splittings of the H(g) modes that are consistent with the observed spectra, but the computed splittings are larger than observed in room-temperature data. A complex pattern observed in the C-H stretching region is assigned. The inelastic neutron scattering spectrum calculated from the computed normal modes for the T(h) molecule in the lattice agrees quantitatively with experiment when overtone and combination transitions are included and allowance is made for anharmonicity of the C-H stretch motion. Finally, it is argued that the existing crystallographic determination of the average C-C bond length of 1.544 A is shortened by disorder and should be revised upward to agree with the computed value of 1.558 A.

Anharmonic Vibrational Motions In C60: A Potential Energy Surface Derived From Vibrational Self-Consistent Field Calculations

A potential energy surface (PES) is developed for C60 designed to describe vibrational motions valid in the anharmonic limit. The PES is based on a previously existing one that is fit to the harmonic fundamentals and is then modified to generate anharmonicity of all orders and in all terms, but without additional fitted parameters. The resulting Cartesian vibrational motions are decomposed into normal modes, and the anharmonic expansion coefficients are calculated including 2-mode couplings and up to 4th order. The resulting PES is used in a vibrational self-consistent field (VSCF) algorithm to calculate the anharmonically corrected fundamental frequencies. The parameters are then fit to fundamental infrared and Raman frequencies. While it is not possible to assign combination and overtone transitions with sufficient experimental accuracy, conclusions about the effects of anharmonic vibrational coupling in C60 are described.

Infrared overtone spectroscopy and unimolecular decay dynamics of peroxynitrous acid

Peroxynitrous acid (HOONO) is generated in a pulsed supersonic expansion through recombination of photolytically generated OH and NO(2) radicals. A rotationally resolved infrared action spectrum of HOONO is obtained in the OH overtone region at 6971.351(4) cm(-1) (origin), providing definitive spectroscopic identification of the trans-perp (tp) conformer of HOONO. Analysis of the rotational band structure yields rotational constants for the near prolate asymmetric top, the ratio of the a-type to c-type components of the transition dipole moment for the hybrid band, and a homogeneous linewidth arising from intramolecular vibrational energy redistribution and/or dissociation. The quantum state distribution of the OH (nu=0,J(OH)) products from dissociation is well characterized by a microcanonical statistical distribution constrained only by the energy available to products, 1304+/-38 cm(-1). This yields a 5667+/-38 cm(-1) [16.2(1) kcal mol(-1)] binding energy for tp-HOONO. An equivalent available energy and corresponding binding energy are obtained from the highest observed OH product state. Complementary high level ab initio calculations are carried out in conjunction with second-order vibrational perturbation theory to predict the spectroscopic observables associated with the OH overtone transition of tp-HOONO including its vibrational frequency, rotational constants, and transition dipole moment. The same approach is used to compute frequencies and intensities of multiple quantum transitions that aid in the assignment of weaker features observed in the OH overtone region, in particular, a combination band of tp-HOONO involving the HOON torsional mode.

Near-IR photodissociation of peroxy acetyl nitrate

Abstract. Measurements of the C-H overtone transition strengths combined with estimates of the photodissociation cross sections for these transitions suggest that near-IR photodissociation of peroxy acetyl nitrate (PAN) is less significant (Jnear-IR≈3x10-8s-1 at noon) in the lower atmosphere than competing sinks resulting from unimolecular decomposition and ultraviolet photolysis. This is in contrast to the photochemical behavior of a related peroxy nitrate, pernitric acid (PNA), that undergoes rapid near-IR photolysis in the atmosphere with Jnear-IR≈10-5s-1 at noon (Roehl et al., 2002). This difference is attributed to the larger binding energy and larger number of vibrational degrees of freedom in PAN, which make 4νCH the lowest overtone excitation with a high photodissociation yield (as opposed to 2νOH in PNA).

Theoretical study of anharmonic vibrational spectra of HNO 3, HNO 3–H 2O, HNO 4: Fundamental, overtone and combination excitations

Vibrational frequencies are computed for the fundamental, OH stretching overtone and combination transitions of HNO 3, HNO 4 and the HNO 3–H 2O complex. The frequencies are computed directly from ab initio MP2 potential surface points, using the correlation corrected vibrational self-consistent field (CC-VSCF) method, which includes anharmonic effects. The results are compared with experimental data. The computed fundamental transitions are in accord with experiment. The main improvement over the harmonic approximation is for the OH stretching frequencies. The OH overtone excitations (up to the 3rd overtone) of HNO 3, HNO 4 are also in good accord with experiment. For overtone levels near the dissociation threshold, the deviations from experiment are larger, as the VSCF method is unsatisfactory for the extremely large anharmonicities in these cases. Finally, very satisfactory results are obtained for the combination mode transitions. The main conclusions are (1) CC-VSCF seems to work well also for low overtone excitations and for combination transitions. (2) The MP2/TZP potential surfaces, used in the CC-VSCF calculations, are by the test of spectroscopy adequate for these species. The results are encouraging for VSCF calculations of larger, related systems such as HNO 3–(H 2O) n , n > 1.

Overtone-Induced Decarboxylation: A Potential Sink for Atmospheric Diacids

Atmospheric photochemistry induced by solar excitation of vibrational overtone transitions has recently been demonstrated to be of importance in cleaving weak bonds (in HO(2)NO(2)) and inducing intramolecular rearrangement followed by reaction (in H(2)SO(4)). Here, we propose another potentially important process: the decarboxylation of organic acids. To demonstrate this possibility, we have calculated the decarboxylation pathways for malonic acid and its monohydrate. The barrier to the gas-phase decarboxylation was calculated to be in the range 26-28 kcal/mol at the B3LYP/6-311++G(3df,3pd) level of theory, in good agreement with previous results. The transition state is a six-membered ring structure which is accessed via concerted O-H and C-C stretches; excitation of v(OH) > or = 3 of either one of the OH stretching modes is sufficient to supply the energy needed for the decarboxylation. A low-energy isomer of the malonic acid-water complex forms an eight-membered, multiply hydrogen bonded structure, bound by 3-6 kcal/mol, somewhat less stable than the lowest energy, six-membered ring isomer. Decarboxylation of such complexes uses water as a catalyst; the water accepts an acidic proton from one malonic acid group and transfers a proton to the carbonyl of the other acid group. The barrier for this process is 20-22 kcal/mol, suggesting that complexes excited to v(OH) > or = 2 possess sufficient energy to react. Using estimated absorption cross sections for the OH overtone transitions, we suggest that the overtone-induced decarboxylation of malonic acid and its water complex is competitive with wet deposition of the acid and with gas-phase reaction with OH for removal of the acid.

Sunlight initiated atmospheric photochemical reactions

The Sun is the light source driving atmospheric chemistry. The wavelengths dependent photon flux is controlled by solar emission modulated by absorption of atmospheric gases, aerosols and clouds. The factors determining the characteristics of this light source, its altitude, latitude and zenith angle dependence are discussed to explain the effectiveness of the Sun in driving chemical reactions. Examples of chemical reactions occurring on the excited and the ground electronic state potential energy surfaces of molecules and radicals are used to illustrate the complexity of atmospheric photochemistry. Specifically, the near ultraviolet (UV) photochemistry of chlorine dioxide is used to exemplify electronic state reactions occurring in the atmosphere. The near infrared (IR) photochemistry of nitric and sulfuric acids are discussed to illustrate reactions important in the atmosphere which occur with solar pumping of vibrational overtone transitions in the ground electronic state of these molecules.

Saturated absorption in acetylene and hydrogen cyanide in hollow-core photonic bandgap fibers

Saturated absorption is studied in overtone transitions of C2H2 and H13CN molecules confined in the hollow core of a photonic bandgap fiber. The dynamics of filling and venting the fiber is markedly different for the two molecules owing to the presence of a permanent dipole moment in one of them. Saturation is observed for input power down to 10 mW, and well resolved Lamb dips limited by transit time broadening across the 10 ?m core diameter are observed with a counter-propagating probe beam.

Calculated absorption and vibrational circular dichroism spectra of fundamental and overtone transitions for a chiral HCCH molecular fragment in the hypothesis of coupled dipoles

We establish the general behavior of absorption and vibrational circular dichroism spectra (VCD) of a chiral HCCH fragment lacking all symmetry elements; the study is limited to CH-stretching modes, and the Hamiltonian employed is written in terms of normal-mode coordinates and momenta and approximates two different Morse oscillators interacting through a harmonic coupling term; rotational strengths are evaluated within a hypothesis of coupled electric dipoles. Van Vleck contact transformations written in terms of raising and lowering operators are used to calculate spectra up to the manifold Deltav = 4. Three transformations are necessary to obtain fourth-order terms in the relevant operators, namely, the electric and magnetic dipole moments. The dynamics of the system exhibits 1:1 resonance terms in addition to the Darling-Dennison coupling term. We discuss the importance of coupling between CH stretches with respect to differences in their local mechanical characteristics in determining the aspect of the absorption and VCD fundamental and overtone spectra of increasing quantum number.

Numerical study of the efficiency of a CO-laser oscillation on overtone or fundamental transitions depending on the lasing spectrum width

A numerical model taking into account the multiquantum vibrational—vibrational exchange processes is developed for simulating a CO laser on a CO—N2 mixture. The energy characteristics of a CO laser operating either on overtone or fundamental transitions, depending on the number of vibrational—rotational transitions falling within the band of a selective resonator, are studied theoretically. Particular emphasis is placed on forecasting the efficiency of narrow-band lasing (on one, three, and five transitions) depending on the position of the centre of the filter's spectral window and the resonator Q factor. It is shown that using the approximation of the single-quantum vibrational—vibrational exchange introduces a considerable error in the calculation of the characteristics of a CO laser with a narrow-band filter (1–5 transitions). The law of the increase in the lasing energy with increasing the spectral-filter bandwidth (from one to 35 transitions), the centre frequency of which corresponds to the 23→22 and 24→22 transitions for the fundamental and overtone bands, respectively, is studied. It is shown that, even for lasing on five neighbouring transitions, the output energy exceeds 50% of the energy achieved in the free-running mode.