Vibrational Transition Moments Research Articles

Rovibronic molecular line list for the N[formula omitted] second positive system

A line list for the N2 second positive system, B3Πg - C3Πu, has been compiled using the PGOPHER spectral simulation software. The line list extends the number of vibrational states of the B3Πg up to v= 29 and a maximum rotational state of J= 150 for simulation temperatures up to 7000 K. New electronic–vibrational transition moments were calculated using refined potential energy curves and a transition dipole moment with the DUO software. Comparisons to experimental data and the SPECAIR software have been used to validate the new line list. The results are available in ASCII ExoMol .state and .trans files and as a PGOPHER input file for use in spectral analysis.

FTIR synchrotron spectroscopy of lower modes of methyl-D3 mercaptan (CD3SH)

The infrared Fourier transform spectrum of the lower vibrational modes of the CD3SH isotopologue of methyl mercaptan has been recorded using synchrotron radiation at the FIR beamline of the Canadian Light Source. Torsion-rotation structure has been analyzed for a strong parallel band centred at 644 cm−1 and a weaker perpendicular band centred at 727 cm−1, identified respectively with a mixed {in-plane-CD3-rocking/CSH-bending} mode and the out-of-plane CD3 rock. The torsional energies for the latter mode follow well-behaved curves, inverted with respect to the ground state, but those for the former do not display a systematic pattern. Surprisingly, no clear evidence was seen for the C-S stretching fundamental band, in contrast to CH3SH and the analogous CD3OH and CH3OH methanol species for which the corresponding C-S or C-O stretching bands are major features of the spectrum. This mystery prompted an ab initio calculation to compare the CD3SH and CH3SH vibrational frequencies and transition moments. A mode lying in the expected position was indeed predicted for CD3SH but with almost vanishing intensity. The calculated reduced mass and effective force constant for this mode corresponded closely to those for the C-S stretch of normal CH3SH, indicating that an anticipated strong C-S stretching band is in fact almost completely extinguished for CD3SH.

Line-by-line intensity measurements of methyl fluoride in the ν2 and ν5 bands

Systematic measurements of line intensities in the ν2 and ν5 bands of CH3F were performed for the first time for 2040 transitions using a mono-spectrum fitting technique utilizing a Voigt and Rautian profiles. The spectra are recorded at room temperature for eight pressures of CH3F from 0.1247 to 2.5080 Torr using the Bruker IFS125HR Fourier Transform spectrometer located at the LISA facility in Créteil. Strong intensity perturbations were observed and analyzed accounting for the strong Coriolis coupling ν2 and ν5 bands and l-type interactions in the ν5 band. The transition dipole moments squared were determined for each line, allowing determination of the vibrational transition moment and the Herman–Wallis coefficients of each band using the Aliev and Watson models. Comparison with data available in the literature permits to test the validity of these models applied to the ν2/ν5 dyad system strongly affected by resonances.

Line intensity parameters, He-broadening and line shift coefficients in the 2v 2 0 and 3v 2 1 − v 2 1 bands of OCS

Using Fourier transform spectrometer, He-broadening coefficients in the and bands of carbonyl sulfide (16O12C32S) have been measured for the first time in the P- and R-branches. Measurements have been performed at room temperature (295 K) for 200 ro-vibrational lines with rotational quantum number ranging from 1 ≤ J ≤ 64, using a mono-spectrum non-linear least squares fitting of Voigt profiles. The measured values were discussed as a function of the rotational quantum number and compared to previous data. These broadening coefficients were also calculated on the basis of Robert and Bonamy's formalism using an ab-initio potential energy surface (PES). In addition, we have measured pressure shift coefficients for 92 transitions in both bands, these results were compared with those of the 2ν3 band. The line intensities in the and bands were also retrieved using the same spectra. The analysis of these intensities allows us to derive a consistent set of line intensity parameters such as vibrational transition moments, band intensities as well as Herman-Wallis (HW) coefficients.

Cavity-ring-down spectroscopy of the heavy ozone isotopologue 18O3: Analysis of a high energy band near 95% of the dissociation threshold

The absorption spectrum of the 18O3 isotopologue of ozone was recorded by cavity-ring-down spectroscopy between 7920 and 7985 cm−1 with a routine noise equivalent absorption, αmin, on the order of a few 10−11 cm−1. This permitted to detect and analyze an extremely weak vibrational band assigned as 7ν1+ν3 following predictions from the ab initio potential energy surface reported in J Chem Phys 2013;139:134307. This is the most excited rovibrational band of the 18O3 isotopologue detected so far. In total, 259 ro-vibrational transitions were measured and modeled using an effective Hamiltonian model. The vibrational dipole transition moment parameter of the band deduced from the analysis and the fit to 177 experimental line intensities was evaluated as 2.6 × 10−6 Debye. The absorption coefficient of the recorded pure 18O3 spectrum was found significantly smaller compared to that of the 16O3 in the same range. The set of 171 upper state energies experimentally determined cover the range from 92.6% (at J = 1) to 97% (at J = 26) of the first dissociation threshold D0. The comparison with ab initio predictions for the band origin and ʋ-dependent rotational constants is discussed. An absorption line list containing 477 transitions is provided as Supplementary material for the 7ν1+ν3 band.

The rotationally resolved infrared spectrum of TiO and its isotopologues

In this study, we present the ro-vibrationally resolved gas-phase spectrum of the diatomic molecule TiO around 1000 cm−1. Molecules were produced in a laser ablation source by vaporizing a pure titanium sample in the atmosphere of gaseous nitrous oxide. Adiabatically expanded gas, containing TiO, formed a supersonic jet and was probed perpendicularly to its propagation by infrared radiation from quantum cascade lasers. Fundamental bands of 46-50TiO and vibrational hotbands of 48TiO are identified and analyzed. In a mass-independent fitting procedure combining the new infrared data with pure rotational and electronic transitions from the literature, a Dunham-like parameterization is obtained. From the present data set, the multi-isotopic analysis allows to determine the spin-rotation coupling constant γ and the Born–Oppenheimer correction coefficient ΔU10Ti for the first time. The parameter set enables to calculate the Born–Oppenheimer correction coefficients ΔU02Ti and ΔU02O. In addition, the vibrational transition moments for the observed vibrational transitions are reported.

Line intensity parameters in the ν3 and 2ν3-ν3 bands of methyl chloride in the 13 µm region

We performed the systematic measurements of line intensities of ro-vibrational absorption transitions of the ν3 and 2ν3-ν3 parallel bands of 12CH335Cl and 12CH337Cl isotopes. The spectra were recorded in the spectral region between 650 and 800 cm−1 with a high-resolution Fourier transform spectrometer.A mono-spectrum fitting technique was used to fit a series of seven spectra at pressures of CH3Cl ranging from 1.02 to 10.24 mbar to retrieve line intensities of 2385 transitions of the two isotopic species. The measured line intensities are discussed in relation to rotational quantum numbers, and used to derive the square of the transition dipole moments for each line. The analysis of these moments allows us to derive a consistent set of line intensity parameters, such as vibrational transition moments, band intensities as well as Herman-Wallis coefficients for both bands. The results of the ν3 band are in satisfactory agreement with previous works and present databases, but a larger discrepancy (≈ 30%) is observed for the 2ν3-ν3 hot band.

Determination of dielectric axes and transition moment directions in β-Ga2O3 from the polarization dependence of vibrational spectra

β-Ga2O3 is a promising ultrawide bandgap semiconductor for high power and extreme environment applications. The dominant O—H center in Ga2O3 has been assigned to a Ga(1) vacancy–2H (VGa(1)-2H) complex. An analysis of the polarization dependence of the vibrational absorption of the VGa(1)-2D center in monoclinic β-Ga2O3 provides a unique strategy for the determination of both the orientation of the principal dielectric axes in the near infrared and the direction of the vibrational transition moment of the defect.

Dissociative recombination of N2H+ ions with electrons in the temperature range of 80-350 K.

Recombination of N2H+ ions with electrons was studied using a stationary afterglow with a cavity ring-down spectrometer. We probed in situ the time evolutions of number densities of different rotational and vibrational states of recombining N2H+ ions and determined the thermal recombination rate coefficients for N2H+ in the temperature range of 80-350 K. The newly calculated vibrational transition moments of N2H+ are used to explain the different values of recombination rate coefficients obtained in some of the previous studies. No statistically significant dependence of the measured recombination rate coefficient on the buffer gas number density was observed.

Line intensities and self-broadening coefficients of methyl chloride in the 10 µm region

We performed the first systematic measurements of line intensities and pressure broadening coefficients of ro-vibrational absorption transitions of the ν6 perpendicular band of CH335Cl and CH337Cl isotopologues. The spectra were recorded in the spectral region between 920 and 1130 cm−1 with a high-resolution Fourier transform spectrometer. A multi-pressure fitting technique was used to fit a series of seven spectra at pressures of CH3Cl ranging from 1.02 to 10.24 mbar to derive these parameters for about 2132 transitions with 1 ≤ J ≤ 55 and 0 ≤ K ≤ 12. The line intensities were discussed as a function of the rotational quantum numbers, and were used to derive the transition dipole moments squared for each line. The analysis of these moments allows us to derive a consistent set of line intensity parameters such as vibrational transition moments, band intensity as well as Herman-Wallis coefficients. The results were found to be in satisfactory agreement with databases and with low resolution measurements of band intensity.The rotational dependencies of the self-broadening coefficients have been clearly observed and modeled using an empirical polynomial law. The average accuracies have been estimated to be around 4 and 5% for line intensities and self-broadening coefficients respectively.

High-resolution sub-Doppler infrared spectroscopy of atmospherically relevant Criegee precursor CH2I radicals: CH2 stretch vibrations and "charge-sloshing" dynamics.

The combination of a pulsed supersonic slit-discharge source and single-mode difference frequency direct absorption infrared spectroscopy permit first high resolution infrared study of the iodomethyl (CH2I) radical, with the CH2I radical species generated in a slit jet Ne/He discharge and cooled to 16 K in the supersonic expansion. Dual laser beam detection and collisional collimation in the slit expansion yield sub-Doppler linewidths (60 MHz), an absolute frequency calibration of 13 MHz, and absorbance sensitivities within a factor of two of the shot-noise limit. Fully rovibrationally resolved direct absorption spectra of the CH2 symmetric stretch mode (ν2) are obtained and fitted to a Watson asymmetric top Hamiltonian with electron spin-rotation coupling, providing precision rotational constants and spin-rotation tensor elements for the vibrationally excited state. Analysis of the asymmetric top rotational constants confirms a vibrationally averaged planar geometry in both the ground- and first-excited vibrational levels. Sub-Doppler resolution permits additional nuclear spin hyperfine structures to be observed, with splittings in excellent agreement with microwave measurements on the ground state. Spectroscopic data on CH2I facilitate systematic comparison with previous studies of halogen-substituted methyl radicals, with the periodic trends strongly correlated with the electronegativity of the halogen atom. Interestingly, we do not observe any asymmetric CH2 stretch transitions, despite S/N ≈ 25:1 on strongest lines in the corresponding symmetric CH2 stretch manifold. This dramatic reversal of the more typical 3:1 antisymmetric/symmetric CH2 stretch intensity ratio signals a vibrational transition moment poorly described by simple "bond-dipole" models. Instead, the data suggest that this anomalous intensity ratio arises from "charge sloshing" dynamics in the highly polar carbon-iodine bond, as supported by ab initio electron differential density plots and indeed consistent with observations in other halomethyl radicals and protonated cluster ions.

Spectroscopic studies of ozone in cryosolutions: FT-IR spectra of 16O3 in liquid nitrogen, oxygen, argon and krypton

We have measured and interpreted the IR spectra of ozone dissolved in liquid nitrogen, oxygen, argon, and krypton in the 650–4700cm−1 spectral region at 79–117K. Frequency shifts, band intensities and bandshapes of 22 spectral features of soluted ozone were analyzed. The bands of the А1 symmetry have a complex contour and possess an excess intensity with respect to the value of the purely vibrational transition moment. It was found that this effect is related to the manifestation of the Coriolis interaction. The bandshape distortion manifests itself as an additional intensity from the side of the В1 symmetry band being an intensity source in the case of the Coriolis interaction.

Time-evolution study of photoinduced charge-transfer in tertiary amine-fluorophore systems

Photoinduced charge transfer in a fluorescent sensor of organophosphorus nerve agents is examined by solution of the time dependent Schrödinger equation, under the influence of an external electromagnetic field and employing potential energy curves obtained by density functional theory (DFT) and time dependent DFT (TDDFT) calculations for the ground and the excited states, respectively. The systems of interest here consist of a tertiary amine-pyrene molecule, with different numbers of CH2 spacer units between the t-amine and the pyrene. Quenching of pyrene emission has been observed in these systems, to different extent depending on the number of spacer units, which is due to photoinduced electron transfer from the t-amine to pyrene. Here, we present a new approach based on time-evolution for the explanation of the experimental observation of the quenching of pyrene emission. In this work the inversion coordinate at nitrogen is considered as a generalized coordinate for the charge-transfer process. Vibrational levels, dipole moments and dipole transition moments of the ground, excited, and charge transfer electronic states along the inversion coordinate are computed by DFT and TDDFT calculations. The time evolution of the excitation probability between the vibrational levels of the ground state and the first excited state and the charge transfer state has been calculated for different excitation frequencies. The results are in agreement with experimental observations regarding the photoinduced electron transfer and decreasing probability for charge transfer with increasing number of CH2 spacer units between the t-amine and the pyrene.

A global ab initio dipole moment surface for methyl chloride

A new dipole moment surface (DMS) for methyl chloride has been generated at the CCSD(T)/aug-cc-pVQZ(+d for Cl) level of theory. To represent the DMS, a symmetry-adapted analytic representation in terms of nine vibrational coordinates has been developed and implemented. Variational calculations of the infrared spectrum of CH3Cl show good agreement with a range of experimental results. This includes vibrational transition moments, absolute line intensities of the ν1, ν4, ν5 and 3ν6 bands, and a rotation–vibration line list for both CH335Cl and CH337Cl including states up to J=85 and vibrational band origins up to 4400cm−1. Across the spectrum band shape and structure are well reproduced and computed absolute line intensities are comparable with highly accurate experimental measurements for certain fundamental bands. We thus recommend the DMS for future use.

A global potential energy surface and dipole moment surface for silane.

A new nine-dimensional potential energy surface (PES) and dipole moment surface (DMS) for silane have been generated using high-level ab initio theory. The PES, CBS-F12(HL), reproduces all four fundamental term values for (28)SiH4 with sub-wavenumber accuracy, resulting in an overall root-mean-square error of 0.63 cm(-1). The PES is based on explicitly correlated coupled cluster calculations with extrapolation to the complete basis set limit, and incorporates a range of higher-level additive energy corrections to account for core-valence electron correlation, higher-order coupled cluster terms, and scalar relativistic effects. Systematic errors in computed intra-band rotational energy levels are reduced by empirically refining the equilibrium geometry. The resultant Si-H bond length is in excellent agreement with previous experimental and theoretical values. Vibrational transition moments, absolute line intensities of the ν3 band, and the infrared spectrum for (28)SiH4 including states up to J = 20 and vibrational band origins up to 5000 cm(-1) are calculated and compared with available experimental results. The DMS tends to marginally overestimate the strength of line intensities. Despite this, band shape and structure across the spectrum are well reproduced and show good agreement with experiment. We thus recommend the PES and DMS for future use.

Ab Initio Quantum Approaches to the IR Spectroscopy of Water and Hydrates.

This Perspective highlights progress in ab initio quantum approaches to IR spectroscopy of water and hydrates. Here, "ab initio" refers to many-body potentials and dipole moment surfaces for flexible water and hydrates. Specifically, these are mathematical representations of two-body and three-body interactions based on permutationally invariant fitting of tens of thousands of ab initio electronic energies, a spectroscopically accurate one-body monomer potential, and four- and higher-body interactions described by the long-range interactions incorporated into, for example, the TTM3-F family of potentials. There are currently two such potentials of this type, denoted WHBB and MB-pol, which are being used in expanding applications. Here, the focus is on infrared spectroscopy, using the WHBB potential and dipole moment surface, with an embedded, local monomer quantum method to obtain vibrational energies and dipole transition moments. Comparisons are also made with the popular q-TIP4P/F potential. Brief mention is made of an application to small HCl-H2O clusters.

Studies on the electric dipole moment function and line parameters for high overtone bands of NO

An accurate electric dipole moment function (EDMF) has been obtained by fitting the best available data including the pure rotational band 0–0 for individual ro-vibrational transitions for nitric oxide. Line intensities calculated using the fitted EDMF agree better with the measurements than the current HITRAN database. Moreover, the accurate ab initio potential energy curve (PEC) and EDMF were found using the multi-reference averaged quadratic coupled-cluster (AQCC) approach with the basis set aug-cc-pV6Z (aV6Z). The good agreement of the vibrational transition moments, computed using the Rydberg–Klein–Rees (RKR) potential and the fitted EDMFs, with experiments shows the present fitted EDMF could be more accurate than other fitted ones at the valid range. Our ab initio vibrational transition moments agree better with experimental data than the ones calculated using the previous theoretical PEC and EDMF, especially at the high overtones. We expect that the present study will be helpful for the transitions including high υ and high overtone bands of NO.

IR spectroscopy of pyridine–water structures in helium nanodroplets

We present the results of an IR spectroscopic study of pyridine-water heterodimer formation in helium nanodroplets. The experiments were carried out in the frequency range of the pyridine C-H stretch region (3055-3100 cm(-1)) and upon water deuteration in the D-O stretch region (2740-2800 cm(-1)). In order to come to an unambiguous assignment we have determined the angle between the permanent dipole and the vibrational transition moment of the aggregates. The experiments have been accompanied by theoretical simulations which yielded two minimum structures with a 16.28 kJ mol(-1) energy difference. The experimentally observed bands were assigned to two structures with different H-bonds: an N···H bond and a bifurcated O···H-C bond.

Studies on electric dipole moment function and line parameters for high overtone bands of NO

An accurate electric dipole moment function (EDMF) has been obtained by fitting the best available data including the pure rotational band 0–0 for individual ro-vibrational transitions for nitric oxide (NO). Line intensities calculated using the fitted EDMF agree very well with the measurements. Moreover, an accurate ab initio EDMF was also found using the highly accurate MRCI approach with the basis set aug-cc-pV6Z using a finite field. The ab initio EDMF is more consistent with the fitted ones than found in previous theoretical results of Langhoff et al. The good agreement of the vibrational transition moments, computed using the Rydberg–Klein–Rees (RKR) potential and fitted EDMFs, with experiments shows our fitted EDMF could be more accurate than other fitted ones at the valid range. Our ab initio vibrational transition moments agree well not only with experimental data but also with our fitted results for experimentally unobserved bands. By combining RKR potential and the ab initio EDMF, we further predict the Einstein coefficients, which can help measure in the transitions including high υ and high overtone bands of NO.

High resolution spectroscopy of HCl–water clusters: IR bands of undissociated and dissociated clusters revisited

We report a detailed study on the IR spectroscopy of HCl-water complexes in superfluid helium nanodroplets in the frequency range from 2660 to 2675 cm(-1). We have recorded spectra of HCl-H2(16)O as well as of HCl-H2(18)O complexes and compared these results with theoretical predictions. In addition, we have carried out mass-selective intensity measurements as a function of partial pressure of HCl as well as of H2(18)O (pick-up curves). The results support a scenario where the IR-absorption in this part of the spectrum contains contributions from undissociated as well as from dissociated clusters with Cl(-)(H2O)3(H3O)(+) being the smallest dissociated complex. These findings are corroborated by additional electric field measurements yielding the orientation of the vibrational transition moment with respect to the permanent dipole moment. As a result we are able to assign a broad absorption band starting at 2675 cm(-1) to dissociated HCl-water clusters (HCl)1(H2O)n with n ≥ 4. The two narrow absorption lines at 2667.9 cm(-1) and 2670 cm(-1) are assigned to an undissociated cluster, in agreement with previous studies.