Journal of Molecular Spectroscopy最新文献

The accuracy of the potential energy curves for diatomic molecules determined from experimental data is confirmed empirically with numerous examples in the literature. Usually PECs are determined from a limited set of experimental data and this in turns limits the range of internuclear distances where the shape of the potential is unambiguously fixed. While the uncertainty for interpolation could be assessed, the extrapolation is usually questionable and needs careful analyses. The Morse/Long-Range potential (Le Roy and Henderson, 2007) has been reported to have a built in long-range asymptotic behavior and therefore it is plausible to expect that one can expect good extrapolation properties and even possibility to determine important molecular parameters like $D_e$ or/and $C_m$ from limited set of experimental data. In this contribution we undertake a systematic study which confirms these expectations in the case of Ca${}_2$ ground state.

We present new high-resolution photoabsorption spectra of ammonia spanning the region between 59,000 cm⁻¹ and 93,000 cm⁻¹ that were recorded by using the Fourier Transform Spectrometer at the Synchrotron SOLEIL. This region extends from just above the Franck-Condon envelope for the $\tilde{A}$ ¹$A_2″$ ← $\tilde{X}$ ¹$A_1\prime$ transition to well above the NH₃⁺ ${\tilde{X}}^{+}$ ²$A_2″$ ionization threshold. The spectra were recorded at a measured resolution of 0.23 cm⁻¹ in both a room-temperature cell (293 K) and in a slit-jet supersonic expansion (∼70 K). The absolute photoabsorption cross section with an uncertainty of ± 5% is also reported for the room-temperature spectrum. The present resolution is a factor of 10 – 100 times higher than in other recently reported broad band spectra of ammonia, and many of the observed bands show partially resolved rotational structure. We have attempted to assign this structure for a number of these bands. The oscillator strengths extracted from the data are in good agreement with previous measurements but, in the case of structured bands, the present higher resolution measurements return higher peak absorption cross sections, that increase further when the sample is cooled. The present higher resolution spectra suggest that a number of previous vibronic band assignments that were based on quantum defect considerations may require some revision. Finally, we discuss the substantial differences between the photoabsorption and photoionization data just above the first ionization threshold.

Ab initio calculations of the PN potential and electric dipole moment in the $X^1{\Sigma }^{+}$ ground electronic state were performed at short bond lengths, $r=0.2$–0.8 Å, and semi-empirical analytical potential-energy and dipole-moment functions were constructed based on all available experimental and theoretical information. The analytical forms for the potential-energy functions include the Extended Morse Oscillator and the Extended Hulburt-Hirshfelder potential. The dipole-moment function of PN was presented by our irregular and rational functions previously used for CO. The potential-energy and dipole-moment model functions were fitted simultaneously to the experimental line positions and permanent dipoles at $v=0$-2, as well as to the ab initio data from our present and previous studies. With these new functions, the improved line list for the ground electronic state of ³¹P¹⁴N was calculated. We show that the new analytic representations of the potential and dipole moment functions help significantly reduce the numerical noise in the intensities of high overtones as well as the associated saturation at high wavenumbers leading to the so-called “overtone plateaus” in spectra of diatomic molecules (see Medvedev et al., J. Mol. Spectrosc., 330, 36 (2016)) and thus provide reliable transition intensities at very high transition frequencies. The 3-0 band is identified as vibrational anomaly, and rotational anomalies inside this and some other bands are found.

The accurate potential energy surface of magnesium monohydroxide, MgOH, in its ground electronic state $\tilde{X}{}^2{\Sigma }^{+}$ has been determined from ab initio calculations using the coupled-cluster approach in conjunction with the correlation-consistent basis sets up to septuple-zeta quality. The core-electron correlation, higher-order electron correlation, scalar relativistic, and adiabatic effects were taken into account. The equilibrium configuration of the MgOH molecule was confirmed to be linear, although with the bending potential energy function being very flat near the minimum. The vibration–rotation-spin energy levels of the MgOH, MgOD, ²⁵MgOH, and ²⁶MgOH isotopologues were predicted using a variational approach. The spectroscopic constants of these isotopologues were determined to high accuracy.

Ammonia is the object of extensive investigations due to the peculiar pattern of its interacting vibration states and its central role in astronomical sciences. ¹⁴ND₃ is of interest in astrochemistry because the determination of deuterium fractionation ratios in ammonia and its deuterated isotopologues contributes to a deeper knowledge of the chemistry in the cold, dense cores of the interstellar medium. Here, the ground, v₂ = 1, 2 and v₄ = 1, a, s states of ¹⁴ND₃ are analyzed thanks to new spectra recorded with the Canadian Light Source synchrotron, from 60 to 1500 cm⁻¹ at a resolution ranging from 0.00096 to 0.003 cm⁻¹. Overall, 7765 inversion, rotation-inversion, and vibration–rotation-inversion transitions in ν₂, 2ν₂, ν₄, 2ν₂ ← ν₂, 2ν₂ ← ν₄, ν₄ ← ν₂, ν₂ ← ν₂, 2ν₂ ← 2ν₂, and ν₄ ← ν₄ have been fitted simultaneously to characterize the s and a levels of the excited states. 4021 and 2388 transitions were assigned in the cold and hot bands, respectively, and 150 inversion and 1206 inversion-rotation transitions in v₂ = 1, 2 and v₄ = 1. 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 118 spectroscopic parameters, determined with high precision. In addition, a new analysis of the ground state has been performed. 9256 data have been fitted, 78 inversion and 837 rotation-inversion transitions, and 8341 ground state combination differences. An improved set of parameters and term values are derived thanks to the increased precision of the newly assigned rotation-inversion transitions with J/ K values up to 31/30 and to the larger number of the ground state combination differences.

We have extended our previous work (Torres et al., 2022) by performing absorption laser spectroscopy of I${}_2$ in the 14400–14600 cm⁻¹ range, using a narrow linewidth diode laser, whose frequency is actively measured using a calibrated wavelength meter. In this range, we have observed 1204 reference lines, 417 more lines than Salami and Ross (2005). The remaining lines are in good agreement with the works carried out by Gerstenkorn and Luc (1979) and by Salami and Ross (2005). The spectrum predicted by the software Iodine Spec5 reproduced our results very accurately.

The infrared spectrum of gaseous methyl bromide (CH₃Br) in natural abundance has been analyzed with high resolution, between 1150 and 1700 cm⁻¹. In this spectral region, two fundamental bands, ν₂ (A₁) and ν₅ (E), an overtone 2ν₃ (A₁) and a combination band ν₃ + ν₆ (E) occur. The novelty of the present study consists in considering, besides the strong Coriolis and α-interactions coupling the v₂ = 1 and v₅ = 1 levels, a large variety of anharmonic and rovibrational interactions involving also the v₃ = 2 and v₃ = v₆ = 1 levels. For this latter level, the data set belonging to the ν₃ + ν₆ combination band has been extended with respect to previous work of Ouahman et al. [Spectrochim. Acta 45A (1989) 175–179], to about thousand assigned transitions with $-6\le K·\Delta K\le +16$. Thanks to the completeness of the theoretical model, the global standard deviation of the reproduction of the experimental wavenumbers in the ν₂/ν₅ system was greatly improved with respect to the previous high-resolution study of Kwabia Tchana et al. [J. Mol. Spectrosc. 228 (2004) 441–452]. For this Coriolis-interacting band system, two different reduction schemes, according to the theory of Stříteská et al. [J. Mol. Spectrosc. 256 (2009) 135–140], were applied and were proved to be equally successful.

The intensity of the atmospheric diagnostic fine-structure line of the O${}_2$ molecule at 118.75 GHz is revised based on multiple measurements with a resonator spectrometer in pure O${}_2$ and O${}_2$–N${}_2$ mixture at pressures ranging from 250 to 1500 Torr and temperatures within 278–327 K. The resulting intensity of 1.000(3)$\cdot 10^{-25}$ cm/molec confirms the earlier measurements and allows decreasing the uncertainty down to $\sim$0.3%. An excellent agreement with the result of calculations presented in HITRAN is demonstrated, suggesting that the 10%–20% uncertainty recommended by the database for all fine structure lines is too large.