High resolution quantum cascade laser studies of the ν3 band of methyl fluoride in solid para-hydrogen

Spectra of solid para-H(2) doped with CH(3)F at 1.8 K are studied in the ν(3) region (~1040 cm(-1)) using a quantum cascade laser source. As shown previously, residual ortho-H(2) in the sample (~1000 ppm) gives rise to distinct spectral features due to clusters of the form CH(3)F-(ortho-H(2))(N), with N = 0, 1, 2, 3, etc. Brief annealing at 7 K is found to give narrower spectral lines (≥0.006 cm(-1)) than conventional (5 K) annealing, and causes the N = 3 and 4 lines to fragment into two or more components. The N = 3 line is observed to be particularly stable and persistent. The N = 0 line (no ortho-H(2) neighbors) is resolved into two closely spaced (≈0.007 cm(-1)) components which are assigned to the K = 0 and 1 states of CH(3)F rotating around its C(3v) symmetry axis (ortho- and para-CH(3)F, respectively). Similar K-structure is also evident for other lines. Weak but persistent features ("N = 1/2 lines") are observed mid way between N = 0 and 1.     

Comparison and assessment of procedures for calculating the R(12) line strength of the ν1 + 2 ν2 + ν3 band of CO2

Recently, results for the CO(2) R(12) line strength parameter have been reported, which differ significantly and are inconsistent with respect to quoted uncertainties. We investigate to what extent this inconsistency might be caused by the chosen data analysis methods. To this end, we assess and compare a parametric fitting procedure and a non-parametric approach. We apply the methods to simulated and measured line spectra, and we specify the conditions required for the safe application of the two procedures. For our present data, the corresponding conditions are satisfied for both methods, and consistent results are obtained. However, the simulations reveal that the fitting procedure can show shortcomings when the uncertainty in the wavenumber is large.     

Study of the ν2 + ν3 infrared band of ONCl

Two hundred and sixty A type rovibrational lines of the ν₂ + ν₃ vibrational band of ¹⁶O¹⁴N³⁵Cl, around 925 cm⁻¹, have been assigned; a least-squares calculation with a r.m.s. deviation of 0.0006 cm⁻¹ has made it possible to measure several constants of the (011) vibrational level.     

High-resolution Fourier transform infrared absorption spectroscopy of the ν6 band of c-C3H2

The gas-phase high-resolution absorption spectrum of the ν(6) band of cyclopropenylidene (c-C(3)H(2)) has been observed using a Fourier transform infrared spectrometer for the first time. The molecule has been produced by microwave discharge in an allene (3.3 Pa) and Ar (4.0 Pa) mixture inside a side arm glass tube. The observed spectrum shows a pattern of c-type ro-vibrational transitions in which the Q-branch lines strongly and distinctly stand out in the spectrum. A combined least-squares analysis of the observed 216 ro-vibrational transitions together with 28 millimeter-wave rotational transitions from the previous study has resulted in an accurate determination of the molecular constants in the ν(6) state. The band center is found to be at 776.11622(13) cm(-1) with one standard deviation in parentheses, which is 2.3% lower than the matrix isolation value. The intensity ratio I(3)(ν(3))/I(6)(ν(6)) obtained from the observed ν(3) and ν(6) bands, 1.90(9), is somewhat lower than the ratio estimated from ab initio (2.4-2.6) and DFT (2.8) calculations.     

Infrared laser spectroscopy of the fundamental band of a 3π CO

Many rotational components of the fundamental band of metastable a ³Π CO have been measured in absorption using diode laser spectroscopy with concentration modulation detection. Line positions are in good agreement with predictions from optically derived parameters. Resolved or partially resolved splittings arising from lambda-doubling appear for the three Ω components. Splittings in the Ω = 1 and 2 spectra agree satisfactorily with molecular beam (rf) and microwave results while those in the Ω = 0 fundamental deviate by several linewidths (up to 0.015 cm-⁻¹) from calculated values.     

Absolute line intensities of the ν9 band of propyne at 15.5 μm

A tunable diode laser spectrometer was used to perform measurements of absolute line intensities for 20 transitions in the ν₉-fundamental of propyne centered at 15.5 μm. From these data, the vibrational band-strength S^v_o was significantly determined to be S^v_o = 196.8 ± 6.0 cm⁻² atm⁻¹ at 297 K. These results will be used for further applications of astrophysical interest such as a refined contribution for modelling Titan's atmosphere.     

Note: Deperturbation of the ν3 band of BeD2

High rotational levels of the 001 (Σ(u)) state of BeD(2) are perturbed by the nearby 03(3)0 (Φ(u)) state. Deperturbation analysis results in an experimental value for the vibrational energy of the 030 level.     

Infrared studies in the ν1 of ONCI: CO laser line coincidences and pressure broadening data

46 Spectral quasi-coincidences between CO laser lines and ClNO absorption lines are given. The ONCl ν₁ spectra have been obtained using a Fourier transform spectrometer. In four cases the discrepancies between the frequencies are smaller than the Doppler width. Pressure broadening parameters resulting from collisional perturbations by Ar for two rotational transitions in the ν₁ band of ONCl have been measured using a diode laser spectrometer.     

Anomalous temperature dependence of the OH− absorption band in stoichiometric LiNbO3 crystals

Infrared absorption spectra of the stretching vibration of OH⁻ and OD⁻ ions in stoichiometric LiNbO₃ crystal have been measured in the temperature range 10–310 K. The band parameters, halfwidth and position, have been determined with high accuracy by assuming quasi-Voigt line shapes. Anomalous behaviour of the OH⁻ band position has been observed and interpreted by phonons coupled to the stretching vibration with coupling constants of alternate signs.     

Infrared diode laser spectroscopy of the ν2(2+ ← 1−) band of H3O+

Rotational lines in the ν₂ = 2⁺ ← 1⁻ “hot” band of the inversion mode of the oxonium (H₃O⁺) ion have been recorded by diode laser absorption spectroscopy. The ion was generated in low pressure gas discharges and detected using both velocity modulation and modulated hollow cathode techniques. Analysis of the spectra using a simple oblate symmetric top model has allowed the rotational parameters describing the 2⁺ inversion state to be determined for the first time. The band origin lies at 521.4383(52) cm⁻¹. These data will be useful in refining the oxonium ion inversion potential function and should aid in the analysis of other bands involving or perturbed by the 2⁺ level.     

Temperature dependence of broadening and shifts of methane lines in the ν4 band

We have recorded high-resolution absorption spectra of methane broadened by dry air and by N₂ at temperatures from −63 to 41°C using a Fourier transform spectrometer. These spectra have been analyzed to determine pressure broadening and line-shift coefficients, along with their temperature dependences, for 148 lines in the ν₄ fundamental band of ¹²CH₄. The experimental uncertainties for lines with J″≤10 are generally <2% for the broadening coefficient b⁰_L, 6–12% for its temperature dependence exponent n, 6–20% for the line-shift coefficient δ⁰, and 20–40% for its temperature dependence coefficient δ′; for J″> 10 the experimental uncertainties are somewhat larger. These results, especially for N₂-broadening, are in excellent agreement with other recent measurements. Since the present results cover a wide range of rotational quantum numbers (J″ up to 14), the variation of the temperature dependence of the half-widths and shifts from line to line within the ν₄ band is also examined.     

Diode laser i.r. spectrum and rovibrational analysis of CF3Br in the ν2 + ν3 region

The gas phase i.r. spectrum of CF₃Br, with natural isotopic abundance, has been investigated in the ν₂+ ν₃ region near 1120 cm⁻¹ using a tunable diode laser spectrometer. The measurements have been carried out at low temperature (⋍ 200 K) to minimize the effects due to the “hot” band absorptions. The K structure of many P(J) and R(J) manifolds has been resolved and analyzed: the maximum J value reached for individual lines was 62 and 70 for CF₃⁷⁹Br and CF₃⁸¹Br, respectively. The identified transitions have been used in a least-squares fit to the energy expression up to the quartic terms and molecular parameters for the ν₂ + ν₃ combination have been obtained. Residual weak features due to “hot” bands of ν₃ and ν₆ have been assigned; the J structure has been analyzed by means of a polynomial procedure and spectroscopic constants for both the isotopomers have been derived.     

Direct measurement of the picosecond decay time of the J aggregate of pseudoisocyanine iodide using an optically compressed frequency-doubled continuous wave Nd—YAG laser and a synchronously operating streak camera

Using the shorter than 4 ps pulses at 532 nm from a frequency-doubled optically compressed continuous wave Nd—YAG laser, in conjunction with a synchronously operating streak camera, the flourescence decay of the J aggregates of 1,1′-diethyl-2,2′-cyanine (pseudoisocyanine) iodide has been measured with picosecond resolution to be 24 ps. The power and concentration dependence are also described.     

Computational determination of the à state absorption spectrum of NH3 and of ND3 using a new quasi-diabatic representation of the X and à states and full six-dimensional quantum dynamics

A recently developed method to represent adiabatic electronic states coupled by conical intersections has been used to construct a full six-dimensional quasi-diabatic representation of the 1(1)A and 2(1)A states of NH(3). This representation is expected to be appropriate to simulate the photodissociation of ammonia when it is excited to the 2(1)A electronic state. In this work, the electronic structure aspects of this quasi-diabatic representation are analyzed. This representation is then used as the basis for a simulation of the ? ← X absorption spectrum, dominated by a progression in the v(2) mode, using a full six-dimensional quantum mechanical treatment of the nuclear motion. Results are reported for both NH(3) and ND(3). This simulation provides the most accurate computational determination of this absorption spectrum reported to date. These results serve to validate the quasi-diabatic representation and set the stage for subsequent studies of vibrationally mediated photodissociation of NH(3).     

Water adsorption on tin hydrodioxide and its effect on the contour of the infrared absorption band ν(OH)

Tin hydrodioxide SnO₂ · nH₂O (THO, n = 1.5) pellets in potassium bromide were studied by IR absorption spectroscopy. Water adsorption by tin hydrodioxide was shown to give rise to a prominent strong and broad band of stretching vibrations ν(OH) with a peak at 3430 cm^?1. Absorption intensity of this band decreases with distance from the peak rapidly toward higher frequencies and very slowly toward lower frequencies; therefore, the contour is distinguished by very high asymmetry. Analysis of the reasons for this asymmetry taking into account the computer decomposition of the contour into components implies that the unresolved bands from two types of water molecules in THO are superimposed onto the weak bands from two types of hydroxide groups. First type molecules are involved in physisorption to form, with one another, hydrogen bonds that are similar to weak bonds in zeolite and liquid water. Second type molecules are involved in chemisorption and are coordinated to tin ions. Coordination enhances the strengthening of acidic properties and promotes the appearance of strong H-bonds. The peak intensity of the THO ν(OH) band depends primarily on the contribution of vibrational transitions of first type molecules and to a lesser extent on the contribution of vibrational transitions of the first type hydroxide groups. The vibrational transitions in second type molecules and second type groups influence the curvature of the contour on the low-frequency side of the peak.     

Conformational analysis and interpretation of ν(OH) band in the IR spectrum of o-vinylphenol: a DFT study

Two-dimensional cyclic potential energy surface for internal rotation of vinyl and hydroxyl substituents in o-vinylphenol molecule was constructed by the B3LYP/6-311G(d) method. It was shown that o-vinylphenol molecule exists in the gas phase as a mixture of seven rotamers denoted as A (A′), B (B′), C (C′) and D. The B3LYP/cc-pVTZ calculated percentage of the rotamers A and A′ in which OH…π intramolecular interaction occurs, is at most 24%. The height of barriers t interconversions between o-vinylphenol rotamers varies from 0.1 to 5.2 kcal mol⁻¹. According to B3LYP/cc-pVTZ calculations, the inclusion of solvent effect in the framework of the polarizable continuum model for a solution of o-vinylphenol in CCl₄ leads to a decrease in theoretical values of ν(OH) frequencies by about 4–9 cm⁻¹ and to an increase in the percentage of the rotamers without intramolecular hydrogen bond by about 4.3% compared to the corresponding gas-phase values. The simulated IR spectral contours of ν(OH) bands are in good agreement with experimental one Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 499–508, March, 2008.     

Infrared diode laser spectroscopy of the Δυ = 2 band of NaBr using spectrum processing by Fourier transformation

The IR diode laser spectrum of the Δυ = 2 band of NaBr monomer has been observed with a heat-pipe high-temperature cell of a White cell type. Fringes due to optical reflections inside the high-temperature White cell were inherent in highly-sensitive observation of the spectrum. However, they were eliminated, as were high-frequency noises, by Fourier manipulation of the observed diode laser spectrum. The υ = 2-0 up to 6–4 vibration—rotation bands of both Na⁷⁹Br and Na⁸¹Br, 199 lines in total, were assigned in the range between 550 and 600 cm⁻¹. These data, combined with 21 mm wave data from the literature, were analysed with a least squares fit to nine Dunham Y _ij coefficients. Y₁₀ and Y₂₀ for Na⁷⁹Br were determined to be 298.73648(66) and −1.21058(19) cm⁻¹, respectively.     

Rotational spectrum of SiC2 in the ν3 excited state

The rotational spectra of SiC₂ in the vibrationally excited states of the ring deformation mode (υ₃ = 1, 2) were observed in the frequency region of 140–400 GHz by using a source-modulated microwave spectrometer combined with a free space absorption cell. SiC₂ was produced in the cell by discharging a mixture of SiH₄, C₂H₂ and CO. Least-squares analysis of the observed spectral lines yielded the rotational constants and the centrifugal distortion constants precisely. Sextic, octic and decatic centrifugal distortion constants were required in the least-squares fit in order to get a good fit of the observed frequencies to those calculated within experimental errors. The inertial defects for the υ₃ = 1 state and the υ₃ = 2 state do not show a linear dependence on the vibrational quantum number. The quartic centrifugal distortion constants, Δ_JK, Δ_K and δ_K, are abnormally large, and show a large change on the vibrational states. These abnormal behaviours are interpreted in terms of a large amplitude motion of the ν₃ mode.     

The complex spectrum of a "simple" free radical: the Ã-X̃ band system of the jet-cooled boron difluoride free radical

The near-ultraviolet band system of the jet-cooled boron difluoride free radical has been studied by a combination of laser-induced fluorescence and single vibronic level wavelength resolved emission spectroscopies. The radical was produced in a supersonic discharge jet using a precursor mixture of 1%-3% of BF(3) or (10)BF(3) in high pressure argon. A large number of bands were found in the 340-286 nm region and assigned as transitions from the X?(2)A(1) ground state to the lower Renner-Teller component of the A?(2)Π excited state, based on our previous ab initio potential energy surface predictions, matching the emission spectra Franck-Condon profiles of (11)BF(2) and (10)BF(2), and comparison of observed and calculated boron isotope effects. Several bands have been rotationally analyzed providing ground state structural parameters of r(0)(') (BF) = 1.3102(9) ? and θ(0)(') (FBF) = 119.7(6)°. The ground state totally symmetric vibrational energy levels of both boron isotopologues have also been measured and assigned up to energies of more than 8000 cm(-1). Although BF(2) might be considered to be a "simple" free radical, understanding the details of its electronic spectrum remains a major challenge for both theory and experiment.     

Molecular dynamics simulations for CO(2) absorption spectra. I. Line broadening and the far wing of the ν(3) infrared band

Classical molecular dynamics simulations (CMDS) have been carried out for gaseous CO(2) starting from the intermolecular potential energy surface. Through calculations for a large number of molecules treated as rigid rotors, various autocorrelation functions (ACFs) are obtained together with probabilities of rotational changes. Those used in the present paper are the ACFs of the center of mass velocity and of the molecular orientation, and the conditional probability of a change of the angular speed. They enable calculations, respectively, of the mass diffusion coefficient, of the infrared (dipolar) band shape including the wings, and of individual line-broadening coefficients. It is shown that these calculations, free of any adjustable parameter, lead to good agreement with measured values. This is expected from previous studies for the mass diffusion coefficient and line-broadening coefficients, but it is, to our knowledge, the first demonstration of the interest of CMDS for the prediction of band wings. The present results thus open promising perspectives for the theoretical treatment of the difficult problem of far wings profiles.