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15 NH 3 is the object of extensive investigation due to the central role of ammonia in astronomical sciences and to the complexity of modeling its interacting vibrationally excited states. Of major interest in astrochemistry is the determination of the 14 N/ 15 N ratio in space, characterized by unexpected variability among different solar system objects and reservoirs. Recently, the spectroscopic analysis of ground and v 2 = 1 a, s states of 15 NH 3 has been completed at experimental accuracy. Here, the characterization of the a, s inversion symmetry levels of v 2 = 1, 2 and v 4 = 1 states is presented. New spectra of 15 NH 3 have been recorded from 325 to 2000 cm -1 at a resolution ranging from 0.00096 cm -1 to 0.003 cm -1 , using the Canadian Light Source synchrotron at CLS. 7518 transitions covering nine bands, ν 2 , 2ν 2 , ν 4 , 2ν 2 ← ν 2 , ν 4 ← ν 2 , 2ν 2 ↔ ν 4 and the inversion-rotation transitions in the excited states, have been fitted simultaneously. The effective Hamiltonian adopted includes all symmetry allowed interactions between and within the studied excited states, according to the most recent results on ammonia. The transitions have been reproduced at experimental accuracy using 185 spectroscopic parameters, determined with high precision. The leading diagonal parameters, G v , B, C, D's, compare well with those of 14 NH 3 . The wavenumbers of the assigned transitions are compared with their theoretically predicted values. An improved set of ground state parameters is also derived. These results noticeably improve the wavenumber line list in the high-resolution transmission molecular absorption (HITRAN) database.

The ν2 (A1)/ν5 (E)/ν3+ν6 (E) band system of H3(28)SiI was investigated using Fourier transform infrared spectra recorded from 820 to 1100 cm(-1) at a resolution of 2.0×10(-3) cm(-1). In total, 11,903 transitions were assigned. Additional 1466 transitions reaching the v3=v6=1 state were obtained from the ν3+ν6-ν6 and ν3+ν6-ν3 hot bands near 360 and 590 cm(-1), respectively. Moreover, 30 highly accurate CO2 laser sideband transitions of the (r)Q0 branch of ν5 (J.M. Frye, W. Schupita, and G. Magerl, J. Mol. Spectrosc. 128, 427 (1988)) were implemented in the data set with J(max)″=140 and K(max)″=21. To adequately reproduce the complex pattern of interacting levels the Hamiltonian employed included 14 off-diagonal terms. These comprise x,y Coriolis ro-vibration resonances, between ν2/ν5, ν2/ν3+ν6 and ν5/ν3+ν6, and the anharmonic Fermi resonance between ν5/ν3+ν6. All these resonances strongly perturb the v2=1, v5=1, and v3=v6=1 excited states whose rounded deperturbed vibrational term values are 904.5, 941.1, and 953.7 cm(-1), respectively. In addition, the Δl=Δk=±2 l-resonance was found to be active within the v3=v6=1 state and between v5=1 and v3=v6=1; the Δl=±2,Δk=∓1 l-resonance within the v5=1 state and between v5=1 and v3=v6=1 was established, as well as the Δl=±1,Δk=∓2 α resonance between v2=1 and v5=1. A standard deviation of the fit, 0.48×10(-3) cm(-1), resulted which is ca. three times the estimated precision of experimental wavenumbers. Improved J-dependent ground state parameters of H3SiI were obtained by fitting 5420 combination differences, σ(fit)=0.22×10(-3) cm(-1)., (Copyright © 2016 Elsevier B.V. All rights reserved.)

The empirical equilibrium structure of acetylene has been derived by exploiting the very precise experimental rotational constants available in the literature for the 10 isotopologues relative to all the possible combinations of H, D, (12)C and (13)C atoms. The geometry obtained when data for all species are fitted together is: re(CH) = 106.167(14) pm and re(CC) = 120.2866(72) pm. This determination shows some systematic residuals due to the singly D-substituted isotopologues. If we exclude such species from the fit, we obtain our most precise evaluation: re(CH) = 106.1689(23) pm and re(CC) = 120.2817(12) pm. The possibility of a breakdown of the Born-Oppenheimer approximation has also been tested.

The infrared spectrum of the perdeuterated acetylene, (12)C2D2, has been recorded from 900 cm(-1) to 5500 cm(-1) by Fourier transform spectroscopy at a resolution ranging between 0.004 and 0.009 cm(-1). Ninety-two bands involving the ν1, ν2, and ν3 stretching modes, also associated with the ν4 and ν5 bending vibrations and 9 bands involving pure bending transitions have been observed and analysed. In total, 8345 transitions for the stretching-bending, and 862 for the pure bending modes have been assigned in the investigated spectral region. All the transitions relative to each stretching mode, i.e. the fundamental, its first overtone, and associated hot and combination bands involving bending states up to v4 + v5 = 2, were fitted simultaneously. The Hamiltonian adopted for the analysis is that appropriate to a linear molecule and includes vibration and rotation l-type interactions. The Darling-Dennison interaction between v4 = 2 and v5 = 2 levels associated with the various stretching states was also considered. The standard deviation for each global fit is smaller than 0.0006 cm(-1), of the same order of magnitude of the measurement precision.

Stannane, SnH(4), has been studied both theoretically, using high-level ab initio methods, and experimentally, using high-resolution spectroscopy to analyze the Sn-H stretching fundamental band of the H(120)SnD(3) isotopologue. The geometry and the anharmonic force field of the molecule have been calculated ab initio, using the coupled-cluster with single, double, and perturbative triple excitations level of theory. H(120)SnD(3), present as minor isotopologue in (120)SnD(4), has been studied by Fourier transform spectroscopy at an effective resolution of ca. 0.005 cm(-1) near 1900 cm(-1) and the ν(1) band was identified. About 360 ro-vibration transitions with J(') up to 18 and K up to 10 have been assigned. Since the spectrum evidenced the existence of some perturbations, the transitions were analyzed either neglecting, or including in the model A(1)∕E Coriolis-type interactions with nearby dark states. The standard deviation of the fits, ca. 1.5 × 10(-3) cm(-1), is about one order of magnitude larger than the estimated experimental precision, and is only slightly dependent on the adopted model. The spectroscopic parameters obtained from this and from previous analyses of stannane isotopologues have been compared with the theoretical results. The theoretical force field is also used to obtain semi-experimental harmonic frequencies and the equilibrium geometry.

We report the first experimental observation of homonuclear propensity in collisional relaxation of a polyatomic molecule. A pump-probe stimulated Raman setup is used to pump population to a single rotational level of the v(2) = 1 vibrationally excited state in (13)C(12)CD(2) and then monitor the redistribution of the rotational population that has taken place after a fixed delay. The Q-branch of the 2ν(2)-ν(2) band shows a pattern of intensity alternation between the even and the odd rotational components, with the greater intensities always corresponding to the rotational levels with the same parity as the one where all the population was initially deposited. The effect can be explained by the existence of a propensity rule that favors collisional relaxation between rovibrational levels of the same parity., (© 2011 American Institute of Physics)

All 18,219 vibration-rotation absorption lines of (13)CH(12)CH published in the literature, accessing substates up to 9400 cm(-1) and including some newly assigned, were simultaneously fitted to J-dependent Hamiltonian matrices exploiting the well-known vibrational polyad or cluster block-diagonalization, in terms of the pseudo quantum numbers N(s) = v(1) + v(2) + v(3) and N(r) = 5v(1) + 3v(2) + 5v(3) + v(4) + v(5), also accounting for k = l(4) + l(5) parity and e/f symmetry properties. Some 1761 of these lines were excluded from the fit, corresponding either to blended lines, for about 30% of them, or probably to lines perturbed by Coriolis for the remaining ones. The dimensionless standard deviation of the fit is 1.10, and 317 vibration-rotation parameters are determined. These results significantly extend those of a previous report considering levels below only 6750 cm(-1) [Fayt, A.; et al. J. Chem. Phys. 2007, 126, 114303]. Unexpected problems are reported when inserting in the global fit the information available on higher-energy polyads, extending from 9300 to 10 120 cm(-1). They are tentatively interpreted as resulting from a combination of the relative evolution of the two effective bending frequencies and long-range interpolyad low-order anharmonic resonances. The complete database, made of 18,865 vibration-rotation lines accessing levels up to 10 120 cm(-1), is made available as Supporting Information.

Perchloryl fluoride, FClO(3), has been studied both experimentally and theoretically, using high-resolution Fourier transform spectroscopy and high-level ab initio methods. The geometry, dipole moment, and anharmonic force field of the molecule have been calculated ab initio, using the coupled-cluster with single, double, and perturbative triple excitations [CCSD(T)] level of theory. The infrared spectra of monoisotopic species have been recorded. The nu(1) + nu(2) bands of F(35)Cl(16)O(3), F(37)Cl(16)O(3), F(35)Cl(18)O(3), and F(37)Cl(18)O(3), the nu(2) + nu(3) and nu(2) + nu(3) - nu(3) bands of F(35)Cl(16)O(3) and F(37)Cl(16)O(3), and the 2nu(3) - nu(3) band of F(37)Cl(16)O(3) have been analyzed. The spectroscopic parameters obtained from these and from previous analyses have been compared with the theoretical results.

All known vibration-rotation absorption lines of 13CH12CH accessing levels up to 6750 cm-1 were gathered from the literature. They were fitted simultaneously to J-dependent Hamiltonian matrices exploiting the well known vibrational polyad or cluster block diagonalization, in terms of the pseudo-quantum-numbers Ns=v1+v2+v3 and Nr=5v1+3v2+5v3+v4+v5, and accounting also for l parity and ef symmetry properties. The anharmonic interaction coupling terms known to occur from a pure vibrational fit in this acetylene isotopologue [Robert et al., J. Chem. Phys. 123, 174302 (2005)] were included in the model. A total of 12 703 transitions accessing 158 different (v1v2v3v4v5,l4l5) vibrational states was fitted with a dimensionless standard deviation of 0.99, leading to the determination of 216 vibration-rotation parameters. The experimental data included very weak vibration-rotation transitions accessing 18 previously unreported states, some of them forming Q branches with very irregular patterns.

In (12)C(13)CH(2) 129 vibrational term values up to 10,000 cm(-1) are merged, about 60% of which are newly reported. They are fitted using an effective Hamiltonian with a standard deviation of 0.22 cm(-1). The vibrational assignments and vibrational constants are listed and discussed. The energy pattern is found to be very similar to the one in (12)C2H2 with additional anharmonic resonances arising from the lack of u/g character in the asymmetric isotopolog.

We report frequency measurements for relatively weak H(2)O and (16)O(3) rotational transitions in the ground state and in the nu(2) = 1 vibrationally excited state. We obtained the frequency measurements by using the laboratory technique of tunable far-infrared spectroscopy with the objective of improving H(2)O and O(3) line parameters required for modeling the important atmospheric spectral window near 119 cm(-1). New sets of molecular constants are calculated from the (16)O(3) data, and improved values are reported for the frequencies of the H(2)O lines. The improvement in atmospheric simulations obtained with the new results is illustrated by comparison with recent high-resolution balloon-based atmospheric measurements. These new data significantly improve simulations of high-resolution atmospheric emission spectra.

We report the first detection of the pure rotational spectra of H79 Br and H81 Br in the first excited vibrational state at room temperature. We recorded the spectra with a tunable far-infrared spectrometer equipped with an optimized Ge:Ga photodetector and an improved multiple-pass gas cell to enhance the overall sensitivity. In particular, a minimum detectable absorption of better than 10-7 cm-1 was achieved. We measured a total of 60 frequencies for the two isotopomers with an accuracy of a few parts in 108 . The hyperfine structure was observed at the lower J values. The derived spectroscopic parameters, which include nuclear and quadrupole hyperfine constants, reproduce the assigned transitions within their experimental errors.