Radiofrequency spectroscopy of AsH3 inside a CO2N2O laser cavity

The method of radiofrequency spectroscopy inside a CO₂N₂O laser cavity has been applied to the observation of pure nuclear quadrupole resonance, A₁-A₂ transitions and two-photon transitions of AsH₃. From the assignment of these transitions some coincidences between laser lines and Δk = 3 transitions in the ν₂ band and Δ|k ? l|; = 3 transitions in the ν₄ band have been assigned. The quadrupole coupling constant eqQ and its rotational dependence χ_J, χ_K and χ_d, and the spin-rotation constants C_N and C_K for the As nucleus have been determined for the ground state. The effective values of eqQ has been determined for the ν₂ and the ν₄ states.     

Variation of Br quadrupole coupling with isotopic substitution in methyl bromide

The J = 0 ← 1 transitions in CH₃⁷⁹Br (I), CH₃⁸¹Br (II), CD₃⁷⁹Br (III), and CD₃⁸¹Br (IV) were measured using a Stark-cell spectrometer constructed from C-band waveguide. High-resolution spectra yielded precise values for the bromine quadrupole coupling strength. Values obtained were eqQ(I) = ?577.08 ± 0.15 MHz, eqQ(II) = ?482.18 ± 0.15 MHz, eqQ(III) = ?575.66 ± 0.15 MHz, and eqQ(IV) = ?480.89 ± 0.15 MHz. The observed center frequencies for the J = 0 ← 1 transitions are ν₀(I) = 19136.35 ± 0.03 MHz, ν₀(II) = 19063.62 ± 0.03 MHz, ν₀(III) = 15429.23 ± 0.03 MHz, and ν₀(IV) = 15362.41 ± 0.03 MHz. A 0.26 ± 0.02% decrease in bromine quadrupole coupling is observed when the methyl group is fully deuterated. This is in agreement with, and supports interpretations given for, previous results on methyl chloride.     

Laser-radiofrequency double-resonance spectroscopy of methyl chloride: Hyperfine structure of the ground and ν6 states

Pure quadrupole resonances of methyl chloride have been observed using the highly sensitive method of infrared-radiofrequency double resonance inside a laser cavity. Quadrupole resonances have been observed for the ground and ν₆ states for both the CH₃Cl³⁵ and CH₃Cl³⁷ isotopes using the direct double-resonance as well as collisionally induced satellites. Most of the six hyperfine constants: the quadrupole coupling constant, eqQ; its rotational dependence, χ_J and χ_K; Hougen's coefficient, χ_D; and the spin-rotation constants, C_N and C_K, have been determined for the two states for both isotopes.     

The ν2 and ν4 bands of AsH3

The infrared absorption of arsine, AsH₃, between 750 and 1200 cm^?1 has been recorded at a resolution of 0.006 cm^?1. Altogether 2419 transitions, including nearly 700 “perturbation allowed” transitions with Δ∥k ? l∥ = ±3, ±6, and ±9, have been assigned to the ν₂(A₁) and ν₄(E) bands. Splitting of the transitions for K″ = 3, 6, and 9 was also observed. To fit the rotational pattern of the v₂ = 1 and v₄ = 1 vibrational states up to J = 21, all the experimental data were analyzed simultaneously on the basis of a rovibrational Hamiltonian which took into account the Coriolis interaction between ν₂ and ν₄ and also included several essential resonances within them. The derived set of 38 significant spectroscopic parameters reproduced the 2328 transition wavenumbers retained in the final fit within the accuracy of the experimental measurements.     

The infrared bands ν2 and ν5 of CH3Br with Coriolis interaction

The infrared bands ν₂ and ν₅ of CH₃Br have been measured at a resolution of 0.015 cm^?1. The lines due to the isotopic species CH₃⁷⁹Br and CH₃⁸¹Br were resolved and altogether about 3000 lines were assigned. The bands were analyzed simultaneously by taking into account the xy-Coriolis resonance between the states v₂ = 1 and v₅ = 1. A local perturbation observed in ν₅ could be explained in terms of a (1,?2) resonance between v₂ = 1 and v₅ = 1. Some perturbation allowed transitions could be assigned and they give an equation between A₀ and D₀^K.     

Analysis of the ν6 band of 12CH3D at 8.6 μm

The ν₆(E) fundamental vibration-rotation band of monodeuteromethane (¹²CH₃D) has been recorded in the spectral range 1033–1270 cm^?1 with a resolution of approximately 0.04 cm^?1. Of the 669 transitions with J′ ≤ 17 identified, 633 have been retained for the determination of the rotational levels in the upper state v₆ = 1. The Coriolis interaction between the v₆ = 1(E) and v₃ = 1 (A₁) vibrational states of ¹²CH₃D results in large A₁A₂ splittings of levels with v₆ = 1 and |K ? l₆| = 0 or 3; the mixing in K and l₆ also gives rise to some ten forbidden transitions observed in the spectra. These effects have been very well explained within the formulation based on the contact transformation method. Values of 15 molecular structure constants of the v₆ = 1 state have been determined from a least-squares analysis of the 633 retained transitions. These constants can be used to estimate values of the upper-state energies up to fourth order, and through them the spectral positions of the 633 retained transitions are reproduced with an overall standard deviation of 0.013 cm^?1, which is within experimental uncertainties.     

Spectroscopic constants of the ground and lower vibrational states of CH2BrF: A combined high resolution infrared and microwave study

The enriched ⁸¹Br isotopic species of bromofluoromethane has been investigated in the infrared and microwave regions. The rovibrational spectrum of the ν₅ fundamental has been studied by high resolution FTIR spectroscopy, while the rotational spectra of the ground and v₆ = 1 states have been observed by means of microwave spectroscopy. More than 2700 transitions have been assigned in the ν₅ band and the analysis of the rovibrational structure reveals a first-order c-type Coriolis resonance with the v₆ = 2 state. The present study improves the ground state constants available in the literature and enables the determination of further centrifugal distortion parameters together with the full bromine quadrupole coupling tensor. A set of spectroscopic parameters up to the sextic distortion terms for the vibrational excited states has been accurately evaluated for the first time.     

Microwave spectra of CH314NC and CH315NC in the 4ν8 state

The J → J + 1 transitions (J = 4, 5, 6, 7, 8) in the microwave spectra of methyl isocyanide and its ¹⁵N derivative have been obtained and analyzed in the 4ν₈ degenerate vibrational state. Theoretical analytical expressions are given for the rotational frequencies in a 4ν_E state, separately for the l = 0, ±2, ±4 values. These formulas could only be used as a starting point for the assignment and analysis, because of the complexity of the spectrum and the number of accidental resonances appearing in many l = 0 and ±2 lines for low K values. A detailed analysis was obtained through a diagonalization of the energy matrix. Many types of A₁A₂ doublings could be localized; in particular for CH₃¹⁵NC the K, l = ±1, ±4 doubling allowed the calculation of the g₆ coefficient of the 〈K, l|H|, l ± 6〉 term. As in the 3ν₈ state, some lines seem to undergo the effects of a vibrational resonance. A set of constants is given for both species, and a comparison is made with the other states.     

Microwave spectrum of methyl fluoride in excited vibrational states: Coriolis interaction and anomalous l-doubling transitions

The microwave spectrum of CD₃F in the v₂ = 1, v₃ = 1, v₅ = 1, and v₆ = 1 states was observed, including the direct l-doubling transitions in the v₅ = 1 and v₆ = 1 states. The Coriolis interaction between the v₂ = 1 and v₅ = 1 states was analyzed in detail. An anomaly in the Stark effect was noticed in some of the direct l-doubling transitions in the v₆ = 1 states, and was explained in terms of accidental degeneracy between the Kl₅ = 1 and the Kl₅ = ? 2 levels. Molecular constants associated with the vibrational and rotational motions were determined through an analysis of these spectra.The J = 1 ← 0 transitions of ¹²CH₃F in the v₂ = 1, v₃ = 1, v₅ = 1, and v₆ = 1 states and of ¹³CH₃F in the v₃ = 1 and v₆ = 1 states were observed, as well as the direct l-doubling transitions of ¹²CH₃F in the v₅ = 1 state. A preliminary analysis was carried out on the strong Coriolis interaction between the v₂ = 1 and v₅ = 1 states.     

High resolution infrared spectrum of CH3Br: The ν4 band near 3100 cm−1

A Fourier transform spectrum (resolution 0.005 cm^?1) of the ν₄ band of methyl bromide has been analyzed. More than one thousand lines have been assigned for each isotopic species CH₃⁷⁹Br and CH₃⁸¹Br. A perturbation is apparent mainly near KΔK = 5. It was interpreted as due to a Coriolis interaction with ν₃ + ν₅ + ν₆. The whole spectrum is reproduced with a standard deviation smaller than 10^?3 cm^?1. Finally, the “hot” band ν₄ + ν₃ ? ν₃ has been assigned and analyzed.     

Laser Stark spectroscopy of the coriolis coupled ν2 and ν5 bands of CD3Cl

About 940 Stark resonances for CD₃³⁵Cl and 610 resonances for CD₃³⁷Cl have been measured by using a CO₂ laser with the 9- and 10-μm regions. They were assigned to 59 rovibrational transitions of ν₂ (J′ ≤ 37, K ≤ 14) and 200 of ν₅ (J′ ≤ 47, ?14 ≤ k′l′ ≤ +10) for ³⁵Cl, and 31 of ν₂ (J′ ≤ 12, K ≤ 10) and 175 of ν₅ (J′ ≤ 46, ?14 ≤ k′l′ ≤ +9) for ³⁷Cl. These data, combined with the microwave and FIR data in the ν₂ and ν₅ states, were analyzed by taking account of the Coriolis interaction between ν₂ and ν₅, and the (2, 2) and (2, ?1) interactions in ν₅. Several ΔK = +2 transitions of the ν₅ band were observed in the Stark spectra, and the ground state constants, A₀ and D_K0, were determined precisely for both ³⁵Cl and ³⁷Cl species. Also, the vibrationally induced dipole moments were obtained. The molecular constants and the zero-field transition frequencies of the ν₂ and ν₅ bands were determined.     

Contribution to the study of the 2ν5 (A1) rovibrational band of CH3Br near 2860 cm−1

This band was studied on a Fourier Transform spectrum (resolving power of the apparatus: 0.005 cm^?1). For each isotopic species CH₃⁷⁹Br and CH₃⁸¹Br, about 800 lines were assigned. The band is well explained by a x-y type Coriolis interaction with ν₂ + ν₅. Several local resonances occur for low K values as well as a doubling of the K = 3 levels. Their study provides interesting information on neighboring bands, especially 3ν₆(E), ν₂ + ν₃ + ν₆, and 2ν₅(E).     

Determination of A0 of CH3Br from its Raman spectrum

A value of A₀ = 5.1800 ± 0.0010 cm^?1 for CH₃Br has been determined from an analysis of the ν₄ Raman band, based both on a direct fit of Q-branch frequencies and on ground state combination differences. The constants ν₄, (Aζ)₄, η₄₄, and A_e = 5.2442 ± 0.0015 cm^?1 were also determined. The equilibrium distance of the H atoms from the figure axis is calculated as 0.32077 ± 0.00005 Å. All the fundamental Raman bands of CH₃Br were observed, and experimental results for the ν₁, ν₂ and ν₅ bands are included.     

10-μm sub-doppler laser-Stark spectroscopy of methyl chloride: Analysis of the ν6 bands of the CH335Cl and CH337Cl

About 900 Stark transitions from 70 vibration-rotation transitions in CH₃³⁵Cl and about 400 transitions from 38 transitions in CH₃³⁵Cl in the ν₆ band have been assigned. These data were analyzed simultaneously with previously published microwave data on the ν₆ = 1 state. The fit has a standard deviation of about 2 MHz for the data for both isotopes. The isoptopic shift ν₆35 ? ν₆37 = 0.3766(6) cm^?1. Rotational dependence of the dipole moment was also just apparent at about μ_J = μ_K = 1 × 10^?5 D, and a complete set of molecular constants is given.     

Analysis of the ν9 and ν10 bands of cyclopropane

The infrared spectrum of gaseous cyclopropane has been measured in the regions 980–1080 and 1400–1500 cm^?1, containing the ν₁₀ and ν₉E′ fundamental bands, respectively, using a high-resolution Fourier transform instrument. Deconvolution was used to enhance the resolution in the crowded parts of the spectrum to ～0.0020–0.0025 cm^?1. Apart from the rotational l-resonance affecting mainly the low-K subbands, the ν₉ band is strongly perturbed by Fermi resonance with the 2ν₁₄² state lying ～41 cm^?1 above. The effects of the Fermi resonance are most pronounced in the high-KΔK = +1 subbands as the 2ν₁₄^?2 levels would cross the ν₉⁺ levels near K = 30. ^rR lines of 2ν₁₄^?2 for K = 24 to 36, enhanced by the resonance, have been identified in the region 1469–1475 cm^?1 of the spectrum. Two extra perturbation-enhanced subbands are found adjacent to the K = 16?17 and K = 17?16 subbands of ν₉: these have been ascribed to the K = 18?17 transitions in 2ν₁₄^?2 and to the K = 19?16 transitions in $\text{ν}{}_2\text{2ν}{}_{14}{}^0$, respectively, as their upper states coincide with the corresponding levels ν₉^?(K = 16) and ν₉⁺(K = 17). A combination of l-resonance and Fermi resonance is mainly responsible for the interactions causing the perturbations and appearance of the extra subbands, but a direct rotational interaction 〈ν₉^?(K)|h₃|2ν₁₄^?2(K + 2)〉 also had to be introduced to accurately account for the observations. In contrast, the ν₁₀ band is not appreciably perturbed by other states and merely exhibits effects of strong l-resonance in the low-K subbands, and K-doubling of the high-J lines of the K = 2–3 subband. A detailed analysis of the spectrum and of the perturbations is described and sets of accurate spectroscopic constants are reported for ν₁₀ and ν₉ as well as for the perturbing state 2ν₁₄², which reproduce 3020 observed lines of the ν₁₀ band with a standard deviation of 0.0008 cm^?1 and 1810 lines of ν₉ with a standard deviation of 0.0010 cm^?1.     

External cavity tunable diode laser spectra of the ν1 + 2ν4 stretch-bend combination bands of NH3 and NH3

The ammonia ν₁ + 2ν₄ perpendicular stretch-bend combination band has been investigated in spectra of ¹⁴NH₃ and ¹⁵NH₃ recorded in the 6400-6800 cm⁻¹ region with an external cavity tunable diode laser (ECTDL) spectrometer. For ¹⁴NH₃, new assignments were determined initially by extrapolating from published low-J jet-cooled beam results up to transitions of higher J and K. Corresponding ν₁ + 2ν₄ transitions for the ¹⁵NH₃ species were then found by identifying similar patterns of lines with a characteristic downshift of approximately 9.7 cm⁻¹. Assignments were confirmed employing ground-state combination differences. Term values, a-s inversion splittings, l-doubling energies and parameter estimates from simple single-state fits are reported for the two ammonia species.     

Diode laser spectrum of the ν6 band of CH3I using a novel étalon as a calibration scale

The ΔK = Δl = +1 transitions of the ν₆ band of the CH₃I molecule were measured by a tunable diode laser spectrometer. A newly designed Fabry-Perot étalon with a free spectral range of 0.00945 cm^?1 was used to calibrate the line position very precisely. The present data were analyzed together with the previously reported transition wavenumbers, and precise molecular constants have been obtained.     

The vibration-internal rotation-overall rotation interactions in the symmetric-top molecules: The microwave spectrum of methyl silane

Laurie's theory on the coupling between internal rotation and a degenerate vibration in a symmetric-top molecule is examined by investigating the microwave spectra of methyl silane and methyl silane-d₃ in the ν₁₂ and ν₆ + ν₁₂ states. Two sets of l-type doublets are observed in the J = 2 ← 1, K = 1 transitions; however, the observed frequencies as well as the Stark effects of the l-type doublets indicate definitely that Laurie's expressions for the l-type doublets are to be revised. It is shown theoretically that Coriolis couplings remove discrepancy between theory and experiment; when A₁ and A₂ denote the rotational constants of CH₃ and SiH₃ about the symmetry axes and ζ₁ and ζ₂ measure the magnitudes of angular momenta induced by the ν₁₂ vibration in the two tops, experimental results require that (−A₁ζ₁ + A₂ζ₂) is 104 950-107 048 MHz and 45 214-47 626 MHz for CH₃SiH₃ and CH₃SiD₃, respectively.     

New analysis of the Coriolis-interacting v2 and v5 bands of CH3Br and CH3Br

The and fundamental bands of CH₃⁷⁹Br and CH₃⁸¹Br have been studied by Fourier transform infrared spectroscopy with an unapodized resolution of 0.004 cm⁻¹, corresponding to an improvement of one order of magnitude compared to previous studies. For both isotopomers, some 2427 (2239) lines were newly assigned for the parallel and the perpendicular bands and, in addition, 80 perturbation-allowed transitions were also added. The ground-state axial rotational constants A₀ were redetermined from allowed and perturbation-allowed infrared transitions observed in the v₂ and v₅ bands around the local crossing. The A₀ values obtained for both isotopomers are more accurate but fully compatible with those obtained previously. Using those results, and the variation of the rotational constants with vibration, new accurate equilibrium constants A_e and B_e have been also determined for CH₃⁷⁹Br and CH₃⁸¹Br. The excited states v₂=1 and v₅=1 are coupled by Coriolis-type interactions (Δl=±1,ΔK=±1) and (Δl=?1,ΔK=±2), while the l₅=±1 levels of v₅ interact also through “l(2,2)”-type interaction (Δl=±2,ΔK=±2). The Coriolis coupling term was determined to be for CH₃⁷⁹Br and for CH₃⁸¹Br. All interaction parameters have been determined with higher accuracy, compared to previous studies. A total of 4213 (3704) line positions with J?68(64) and K?16(11) including all available data was fitted using 20 (18) parameters with a root-mean-square deviation of 0.0007 (0.0006) cm⁻¹ for CH₃⁷⁹Br and CH₃⁸¹Br, respectively. Two different but equivalent forms of reduced Hamiltonians with two different sets of constrained constants were successfully applied according to Lobodenko's reduction [J. Mol. Spectrosc. 126 (1987) 159]. The ratio of the transition moments, |d₂/d₅|=1.65, and a positive sign of the Coriolis intensity perturbation d₂×ζ₂₅×d₅ were determined. Therefore, it has been possible to generate an accurate prediction of the whole spectrum between 1200 and 1650 cm⁻¹, including Q branches.     

Vibration-rotation spectra of ethane and deuteroethanes: The ν2 band of CH3CD3

The ν₂ band of CH₃CD₃ has been measured under an effective resolution of 0.04 cm^?1. About 400 transitions observed in the region from 2130 to 2060 cm^?1 have been identified as due to the ν₂ fundamental band. The least-squares analysis of these transitions yields the band constants: ν₀ = 2089.957, B′ = 0.548937, D_J′ = 6.97 × 10^?7, D_JK′ = 1.92 × 10^?6, A′ - A″ = ?0.01158, and D_K′ - D_K″ = 1.30 × 10^?6 cm^?1. The ground-state constants B″, D_J″, and D_JK″ are fixed to the values obtained from microwave spectroscopy.