C. di Lauro

Coriolis interactions about X-Y axes in symmetric tops

Selection rules and matrix elements are derived for Coriolis interactions between vibrational levels due to rotation about (x, y) axes in symmetric top molecules. The theory is developed in detail for the case of interaction between an A1 and an E species vibrational level in a C3v molecule; perturbations to both the positions and the intensities of the rovibration transitions in the spectrum are considered. A computer program has been written which calculates exactly the perturbed spectrum of two interacting rovibration bands according to this model, the results being presented directly by a graph plotter connected to the computer. This has been used to interpret perturbations observed in two pairs of interacting fundamentals in the spectrum of CH3F (ν2 - ν5 and ν3 - ν6) and one pair in CD3Cl (ν2 - ν5). The resulting analysis of the observed spectrum leads to new values for some vibration-rotation interaction constants and also leads to a unique determination of the sign relationship between the dipole moment derivatives in each pair of interacting normal vibrations. These sign relations are summarized in Figs. 8, 12, and 15.

Rotational analysis of the ν2 + ν6±1, ν5⊣1 + ν6⊣1, ν5⊣1 + ν6±1, and 2ν3 + ν6±1 interacting infrared bands of methyl chloride

Abstract The rotational analysis of the infrared absorption of CH 3 35 Cl and CH 3 37 Cl between 2280 and 2580 cm −1 has been carried out on a Fourier transform spectrum with a resolution of 0.005 cm −1 . The bands ν 2 + ν 6 ( E ), ν 5 + ν 6 ( E + A 1 + A 2 ), and 2 ν 3 + ν 6 ( E ) occurs in this region, giving rise to several widespread and local perturbations. The l -type doubling effects observed in the ν 5 + ν 6 vibrational system involve the mixing of perpendicular and parallel components, and are analyzed in detail. Substantial improvements in the analysis are achieved with respect to previous works (C. Alamichel and C. di Lauro, Mol. Phys. 30 , 73–79 (1979); C. di Lauro, G. Guelachvili, and C. Alamichel, J. Phys. 37 , 355–358 (1976)) and a value of the A ″ rotational constant is given from the analysis of perturbation-allowed transitions.

Vibrational spectra and force constants of symmetric tops: High-resolution spectra of H374Ge35Cl in the ν1ν4 region near 2100 cm−1

Abstract The gas-phase infrared spectrum of monoisotopic H 3 74 Ge 35 Cl has been studied in the ν 1 , ν 4 region near 2100 cm −1 with a resolution of 0.008 cm −1 . Rotational fine structure for Δ J = ±1 branches has been resolved for both fundamentals. ν 1 ( a 1 ), 2119.977 03(19) cm −1 ; and ν 4 ( e ), 2128.484 65(8) cm −1 are weakly coupled by Coriolis x,y resonance, Bξ 1,4 y 2.6 × 10 −3 cm −1 , and l -type resonance within ν 4 , q 4 (+) −8.4 × 10 −6 cm −1 , has been observed. An extended Fermi resonance with ν 5 ±1 + 2 ν 6 ±2 , which mainly affects the kl = −14 and −15 levels of ν 4 , has been detected and analyzed. In addition, several weak and local resonances perturb essentially every K subband of ν 4 and some of ν 1 , and a qualitative model is proposed to account for the features observed in the spectrum. Disregarding the transitions involved in local perturbations, the rms deviation of the fit to the remaining 2021 lines is σ = 1.34 × 10 −3 cm −1 .

A strong Coriolis perturbation in the infrared spectrum of trans-d2 ethylene

The ν7 C-type perpendicular band in the infrared spectrum of trans-C2H2D2 has been analysed, the anomalous rotational structure being explained in terms of Coriolis resonance with ν10. Earlier assignments of the Q-branches (1) have been corrected. The vibrational frequency ν70 = 724.5 ± 0.5 cm−1 has been determined with the aid of a graphical procedure. No definitive results have been established for ν10.

Extended analysis of the high resolution spectrum of C2H6 near 7 μm: the ν6, ν8, ν4 + ν12, 2ν4 + ν9 vibrational system, and associated hot transitions

The room temperature high resolution infrared spectrum of C2H6 between 1330 and 1610 cm−1, the region of the ν6 and ν8 fundamentals, has been re-investigated owing to the relevance of this spectral region in atmospheric and planetary applied research. The assignments of transitions from the ground vibrational state to the upper states ν6, ν8, ν4 + ν12 and 2ν4 + ν9 (4592 in total) and from the lower state ν4 to the upper state ν4 + ν8 (1090 lines) have been considerably extended with respect to our previous work [F. Lattanzi, C. di Lauro and J. Vander Auwera, J. Mol. Spectrosc. 248 (2008) 134–145], especially for the hot transitions. In particular, three new series of perturbation activated transitions were found, with ΔK = ±2, made observable by the resonance of the type l(Δl = ±2, ΔK = ∓1) within ν8. Also, new P-transitions to ν6 were found, made observable by the x,y-Coriolis resonance with ν8, at the values of K from 15 to 18. The extension of the assignments in the high frequency wing of ν8 and their analysis allowed the discovery of two additional resonance interactions. The first interaction, a higher order Coriolis-type between ν8(±K, l = ±1) and ν6(±K ± 2, l = 0), induces a few detectable transitions to ν6 with ΔK = 3. The second interaction, of the type l(Δl = ±2, ΔK = ∓1), comes to resonance between ν8(k = ±19, l = ±1) and 2ν4 + ν9 (k = ±20, l = ∓1). This last resonance and the x,y-Coriolis resonance of ν6 and 2ν4 + ν9 allows one to observe several transitions to the 2ν4 + ν9 vibrational state. The extension of the assignments in the (ν4 + ν8) − ν4 hot band allowed discovery of a resonance interaction of the type l(Δl = ±2, ΔK = ∓1) between the states ν4 + ν8 and 2ν4 + ν12, in addition to their well known Fermi-type interaction. The K-values before and after the level crossing could be determined for both interactions. Better values were also determined for the J-structure parameters B, DJ and DJK in the ν4 state, from 410 selected combination differences. Least squares fit calculations, performed on 2084 upper state energy levels for the cold system (RMS of 3.69 × 10−3 cm−1) and 500 for the hot system (RMS of 6.62 × 10−3 cm−1), required a more sophisticated Hamiltonian model than in our previous work.

Perturbation activated transitions in the high resolution infrared spectrum of C2H6: rotational constants and torsional splitting in the ground state

The l-type perturbation mechanisms that activate vibration–rotation transitions with Δk = ±2, Δ(l modulo 3) = ∓1 in the perpendicular bands of molecules with threefold axial symmetry have been investigated in the case of ethane-like molecules. It is shown that in this class of molecules such transitions, combined with the usual ones with Δk = Δ(l modulo 3) = ±1, give information not only on the values of the K-structure rotational parameters, but also on the torsional splitting of the lower combining state. Transitions of this type have been examined in two perturbed regions of the infrared spectrum of C2H6, in the ν8 fundamental and in the ν4 + ν10 combination. Assuming that the four torsional components in the ground state of C2H6 follow the energy pattern generated when tunneling occurs from one minimum of the torsional potential barrier to the two adjacent minima only, a torsional splitting of 0.00566 ± 0.00015 cm−1 in each rotational level has been evaluated. The values of the ground state rotational...

Rotational constants of C2D6 in the ground and v4=1 vibrational states

Abstract A high-resolution Fourier transform infrared spectrum of C 2 D 6 , in the region of the ν 6 and ν 8 fundamental vibration-rotation bands, has been partially analyzed, and the wavenumbers of the assigned transitions of the ν 8 and ( ν 4 + ν 8 ) − ν 4 bands have been used to determine the rotational parameters B , D JK , and D J of the ground and v 4 = 1 vibrational states. The observation of additional transitions, made detectable by the effect of locally resonant level couplings, allows the establishment of relations between the values of the rotational constants A and D K in the mentioned vibrational states. It is found that the available values of the quartic centrifugal distortion parameters, including D K , determined by a harmonic force field, are highly reliable; therefore accurate values of A 0 and A 4 can be evaluated.

The Zeeman effect in spin-allowed and forbidden doublet-doublet transitions in orthorhombic rotating molecules

Abstract Theoretical predictions of the Zeeman effect on spin-allowed and forbidden doublet-doublet transitions in near-symmetric top molecules are reported, with the assistance of computer simulations of band profiles. Bandwidths in the high-field limit have been determined. Marked differences have been found between the behavior of spin-allowed and forbidden transitions. The intermediate-field effect on spin-allowed transitions is sensitive to the relative energy order of the zero-field spin components of the rotational sublevels in the combining vibronic states. In the high-field limit the rotational components of spin-allowed transitions give rise to a single band, corresponding to Δ M S = 0, whereas the bands with Δ M S = ±1 are also expected in spin-forbidden transitions. The applicability to actual cases is finally discussed.

The Zeeman effect in doublet-quartet transitions in orthorhombic rotating molecules

Abstract Theoretical predictions on the effect of magnetic fields of variable strength on the rotational structure of doublet-quartet transitions in orthorhombic near symmetric top molecules are reported, with the assistance of computer simulations of line profiles. Low-field profiles are discussed, and it is shown that low-field linewidths are approximately equal to 2gβH|ΔJ − ΔN| for transitions between states of any spin multiplicity. In a high field, transitions allowed by direct spin-orbit contamination of the combining states give rise to three lines with ΔMs = 0, ±1, whereas two additional lines, with ΔMs = ±2, are found if the combining states are appreciably contaminated also by indirect coupling to the active states, through intermediate levels. Computed Zeeman profiles are reported and discussed for several typical cases, with emphasis on the effects of different light polarizations. Prospectives of applicability are briefly outlined.

Rotational selection rules for doublet-quartet transitions in asymmetric top molecules

Abstract Selection rules on the rotational structure of doublet-quartet transitions in asymmetric top molecules are discussed on the basis of the spin-orbit coupling mechanism, rather than by symmetry arguments, and some additional rules are found with respect to a previous work. In the case that the combining doublet and quartet states are appreciably mixed only with levels directly connected to them by the spin-orbit operator, it turns out that, except for the very low values of the rotational quantum numbers, branches with |Δ N − Δ J | > 1 are essentially forbidden. Thus, branches with Δ N = ± 3 ( T and N branches) are not expected to be observable. In this case, it is found also that in a high magnetic field transitions with Δ M s = ±2 are forbidden, and only Δ M s = 0, ±1 bands are expected to occur. All these restrictions are removed if the combining states are also appreciably contaminated by states indirectly connected to them by the spin-orbit operator, through intermediate levels.