Inversion of the Ka = 0 and 1 rotational levels in the lowest excited vibrational state of HNCS

Fairly strong, regularly spaced absorption lines have been observed in the microwave spectrum of HNCS and assigned to b-type, K_a = 0 ← 1, Q-branch transitions arising from molecules in the lowest excited vibrational state. The Fortrat diagram of these lines has the appearance of a c-type Q branch, which is impossible in HNCS because of its symmetry. This anomalous b-type Q-branch spectrum is caused by strong a-type Coriolis interactions among the three low-lying bending modes; the K_a = 1 levels of the lowest excited vibrational state are perturbed and shifted lower in energy than the K_a = 0 levels for each J. This interpretation has been confirmed by the observation of P- and R-branch transitions associated with this Q branch. The band origin has been determined to be ?40 104.287 MHz (?1.3377 cm^?1). The inversion of the K_a = 0 and 1 energy levels is consistent with the interpretation of HNCS as a quasi-linear molecule.     

The High-Resolution Spectra of the ν11 Band of Triacetylene near 622 cm−1: Revised Assignments for Hot Bands

The rovibrational spectra of triacetylene (C₆H₂) were recorded for the ν₁₁ band in high resolution by FTIR spectroscopy. The assignments for the hot-band system (v₁₁, v₁₃) = (1, 1) ← (0, 1) have been fully revised, and reliable molecular parameters have been determined including the rotational and vibrational l-type doubling constants. The assignments have been confirmed by a simulation of the absorption profile in the Q-branch region. The intensity perturbation caused by the l-type resonance in the (v₁₁, v₁₃) = (1, 1) state has been observed and discussed.     

Submillimeter spectrum and spectroscopic constants of the arsine molecule in the ground vibrational state

The submillimeter spectrum of the arsine molecule, AsH₃, of both the allowed R-branch transitions (J + 1 ← J, J = 0, 1, 2, 3) and forbidden transitions of the Q branches, |K| = 4 ← 7, 5 ← 8, 6 ← 9, is investigated in the frequency range from 220 to 900 GHz. Weak absorption lines were observed by using the spectrometer RAD with sensitivity increased by a nontunable cavity cell. On the basis of the results obtained and microwave data available in the literature the rotational spectrum of the arsine molecule in the ground state is analyzed.     

Sub-Doppler and Doppler spectroscopy of DCN isotopomers in the terahertz region: ground and first excited bending states (v1v2v3)=(0 1 0)

The rotational spectra of the deuterium cyanide isotopic species DCN, D¹³CN, DC¹⁵N, and D¹³C¹⁵N were recorded in the vibrational ground and first excited bending state (v₂=1) up to 2 THz. The R-branch transitions from J=3←2 to J=13←12 were measured with sub-Doppler resolution. These very high resolution (∼70 kHz) and precise (±3-10 kHz) saturation dip measurements allowed for resolving the underlying hyperfine structure due to the ¹⁴N nucleus in DCN and D¹³CN for transitions as high as J=10←9. Additional high JR-branch (J=25←24 to J=28←27) transitions around 2 THz and direct l-type (ΔJ=0, J=19 to J=25) transitions from 66 to 118 GHz were recorded in Doppler-limited resolution. For the ground state of D¹³C¹⁵N, the J=1←0 transition was measured for the first time. The transition frequency accuracies for the other deuterated species were significantly improved. These new experimental data, together with the available infrared rovibrational data and previously measured direct l-type transitions, were subjected to a global least squares analysis for each isotopomer. This yielded precise sets of molecular constants for the ground and first excited vibrational states, including the nuclear quadrupole and magnetic spin-rotation coupling constants of the ¹⁴N nucleus for DCN and D¹³CN. The hyperfine structure due to the D, ¹³C, and ¹⁵N nuclei have not been resolved, but led to a broadening of the observed saturation dips.     

Infrared-microwave double resonance in CH3OH using a Zeeman-tuned 3.5-μm HeXe laser

Infrared microwave double resonance signals have been observed for CH₃OH using the 3.5-μm HeXe laser line. When microwave transitions in the ground vibrational state are pumped, the double resonance signals are obtained on two infrared transitions v = 1 ← 0 of ν^CH(a′); v = 1, J, K, μ = 4, 2, 1 ← v = 0, J, K, μ = 3, 2, 1, and 4, 3, 1 ← 3, 3, 1. Three weak double resonance signals are due to the collision-induced transitions. Their relative intensities have been explained successfully by using the rate constants of collision-induced transitions which are proportional to the dipole matrix elements between the states involved in the transitions.     

R-branch head of the ν3 band of CO2 at elevated temperatures

The R-branch head of the 00⁰1 ← 00⁰0 band of ¹²C¹⁶O₂ has been recorded with Doppler-limited resolution using a tunable laser difference-frequency spectrometer. J values to 140 were measured at temperatures to 985 K. The data have been combined with extremely precise transition frequencies for 0 ≤ J ≤ 76 measured with a Fourier transform interferometer to obtain an improved set of spectral constants for this ν₃ band of CO₂.     

Microwave spectrum of CH3OD

The microwave spectrum of CH₃OD has been observed in the frequency region between 14 and 92 GHz. All the ground-state transitions with J ≤ 8 and J = 2 ← 1, a-type transitions in the excited torsional states (v = 1 and v = 2) have been observed. The spectrum has been analyzed and rotational constants, torsional constants, torsion-vibration-rotation interaction constants, and centrifugal distortion constants have been evaluated. The Stark effect measurements have been made and the dipole moment components have been determined as μ_a = 0.833 ± 0.008 D and μ_b = 1.488 ± 0.015 D.     

Millimeter-wave spectrum of the C4v molecule iodine oxygen pentafluoride IOF5: K-doubling of the |k| = 2 transitions in the ground state spectrum

Measurements are reported for the rotational spectrum of the C_4v molecule IOF₅ in the ground vibrational state in the range 30–75 GHz (J7 ← 6 to J17 ← 16). The K-doubling of |k| = 2 transitions due to an off-diagonal centrifugal distortion interaction of the type (Δl, Δk) = (0, ±4) has been observed. The centrifugal distortion constants D_J, D_JK, and R₆ have been determined as 0.139(2) kHz, 0.107(4) kHz, and 21(2) Hz, respectively.     

Microwave spectrum of silyl fluoride: Coriolis resonance between ν2 and ν5 states

The J = 1 ← 0 and J = 2 ← 1 microwave rotational transitions of SiH₃F and SiD₃F have been measured for the ground and the v₂ = 1, v₃ = 1, v₅ = 1, and v₆ = 1 vibrational states, for which the various rotational and vibration-rotation interaction constants have been obtained. Both molecules show an X-Y Coriolis resonance between the ν₂ and ν₅ vibrational states, whose separation are 29 and 8 cm^?1, respectively. In the case of SiD₃F the resonance is very strong and an exact numerical diagonalization of the energy matrix was employed.     

Λ-Doubling in the v = 2 ← 0 overtone band in the infrared spectrum of NO

Λ-doubling in the first overtone, v = 2 ← 0, vibration-rotation band of ¹⁴N¹⁶O has been measured with Doppler-limited resolution using a tunable difference-frequency laser spectrometer. P- and R-branch Λ-splittings for $\text{1}\text{2}\text{<- J ≤}\text{53}\text{2}$ in the ${}^2\text{Π}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ electronic state and for $\text{47}\text{2}\text{≤ J ≤}\text{57}\text{2}$ in the ${}^2\text{Π}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ state have been determined by least squares fitting of the spectral profiles to Gaussian doublets with a precision of better than ±1.5 MHz for the stronger, well-resolved lines. The Λ-doubling parameters obtained from a least square Hamiltonian analysis of the data for the v = 2 level are p₂ = 349.542 (60) MHz and q₂ = 2.754 (5) MHz with the error limits representing three standard deviations. These precise v = 2 parameters are combined with excellent v = 1 constants available in the literature to calculate the Λ-splittings expected in the v = 2 ← 1 hot band of NO for comparison with recently reported measurements. The effect of nuclear hyperfine structure on the lineshapes of some of the low J overtone transitions has also been observed.     

Vibrational-rotational Einstein coefficients for HF/DF and HCl/DCl

The RKR potential functions have been combined with the best experimentally based dipole functions to calculate the Einstein coefficients for HF/DF and HCl/DCl. Calculations were done for the Δν = 1,2,3 transitions for a wide range of ν′ (?15 for HF/DF and ?8 for HCl/ DCl) and J′ (?25). Experimental tests involving comparison of P- and R-branch line intensities for high-J′ transitions of the HF (ν₁ → ν₀, ν₂ → ν₁ and ν₃ → ν₂ bands) and the Δν = 2 and Δν = 1 transitions for HF/DF and HCl are done to examine the reliability of the Einstein coefficients for the rotational and vibrational levels, respectively. Good agreement is obtained for HF/DF even for the high-J′ R-branch intensities which vary markedly with J′. But the data suggest improvement is needed in the dipole functions for HCl (and DCl).     

Stark spectroscopy of 13CD3OH with the HCN laser

The laser Stark spectrum of ¹³CD₃OH has been studied using the 311 μm line of the HCN laser. An extensive series of absorption lines has been observed and assigned to the J = 3 to 11 members of the K = 2 ← 3E₁, Q-branch transition in the v_t = 1 excited torsional state. Zero-field frequencies for all the assigned transitions are given with improved accuracy over those calculated from available molecular constants. For the Q-branch series, the branch origin and the series expansion coefficients are also presented.     

M-dependence of collisional transfer of rotational energy in OCS mixtures

Collision-induced transitions between rotational levels of OCS in the ground vibrational state have been investigated by steady-state microwave double resonance, with the M sublevels separated by a Stark field. The (2 ← 1)_P-(1 ← 0)_S, (3 ← 2)_P-(1 ← 0)_S, and (4 ← 3)_P-(1 ← 0)_S systems have been studied for pure OCS and for mixtures with excess CH₃OH, He, and H₂. For four-level systems having dipolar connections (ΔJ = 1; ΔM = 0, ± 1; parity ± ? ?) between pump and signal levels, it is found for OCS and the OCS-CH₃OH mixture that the dipole-type ΔJ = 1 transitions always dominate the collisional transfer, but for the OCS-He and OCS-H₂ mixtures that ΔJ = 2 quadrupole-type transitions are dominant. For all four collision partners, significant ΔJ = 2 and ΔJ = 3 collisional transfer is observed in some systems, indicating the presence of high-order terms in the collisional interaction.     

The two-dimensional anharmonic oscillator: The microwave spectrum of silyl isocyanate,SiH3NCO

The J + 1 ← J transitions (J = 2, 3, 4, 5, and 6) in the microwave spectrum of SiH₃NCO have been assigned for the vibrational ground state and for the vibrational states v₁₀ = 1, 2, and 3. The results for v₁₀ = 0 confirm earlier work. The vibration-rotation constants show a remarkable variation with v₁₀ and l₁₀. To a large extent the anomalous behavior of these constants has been explained in terms of a strongly anharmonic potential function for the ν₁₀ vibrational mode.     

The microwave spectrum of IF5 in excited vibrational states: Accidental degeneracy and avoided crossing of the kl = 3 and kl = −1 levels in the state v9(E) = 1

Measurements of the microwave spectrum of the C_4v molecule IF₅ in the excited vibrational states v₉(E) = 1 and v₅(B₁) = 1 are reported for the transitions J12 ← 11 and J13 ← 12 (65–72 GHz). The considerable spectral perturbations produced by an accidental degeneracy and avoided crossing of the ψ^? (kl = 3) and ψ^? (kl = ?1) levels of the v₉(E) = 1 state have been measured and analyzed. A spectroscopic determination of the axial rotation constant C₉ is reported and its implications for the molecular structure of IF₅ discussed.     

High-Resolution Laser Spectroscopy of the AΠ1←XΣ System of IBr with a Titanium:Sapphire Ring Laser

The vibrotational absorption spectra of the A←X electronic transition of I^79/81Br were measured in the 11 330- to 13 220-cm⁻¹ region using a Ti:sapphire ring laser. The P-, Q-, and R-branch lines of the rotational states from J=10 to 100 belonging to the v′←v″=(3∼20)′,←(1∼6)″ bands were assigned. The P- and R-branch lines, unlike the Q-branch lines, were split into the doublet by the nuclear quadrupole coupling effect of the I atom. The quadrupole coupling constants of eQq₀ and eQq₂ in the A state were estimated to be −0.030±0.018 and −0.062±0.018 cm⁻¹, respectively, by using the first order perturbation theory. The unperturbed line positions for the rotational lines higher than J=20 were determined. The Dunham coefficients of the X state were determined by the least squares fitting method using the pseudo vibrotational transition wavenumbers obtained by calculating the combination differences between the electronic spectral lines assigned and the far infrared vibrotational lines reported by Nelander et al. (7). The spectroscopic constants of Tv′, Bv′, Dv′, and Hv′ of the A state were determined suitable for the vibrational states from v′=3 to 20 by using a least squares fitting procedure.     

Rydberg states with a 2Π core: The b3Πi and C1Π states of DCl

The rotational structure in the lowest Rydberg complex of hydrogen chloride, [X²Π]4sσ, was reinvestigated. The study is limited to the spectrum of D³⁵Cl, the HCl bands being too diffuse for a detailed analysis of second-order effects. The Λ-type doubling in both component states, b³Π_i and C¹Π, is small since it arises from the uncoupling of the core rather than Rydberg orbital angular momentum. It can be interpreted in terms of pure precession relations that are known to exist between the ground and first excited states of DCl⁺. By contrast, the spin-orbit interactions, also originating in the core, are strong. In addition to distorting the triplet splitting in b³Π, they lead to an avoided crossing between the nearly coinciding levels b³Π₀ (v = 1) and C¹Π₁ (v = 0). They are also responsible for anomalies in the b³Π₀ ← X¹Σ⁺R-branch intensities of DCl as well as of HBr, DBr, and HI. From the J values at the observed R-branch minima we have estimated the ratio $\text{μ}{}_{\mathrm{\parallel }}\text{μ}{}_{\mathrm{\perp }}$ of the transition moments associated with the excitation of a 3pσ or 3pπ core electron to the 4sσ Rydberg orbital of DCl and, correspondingly, of the other hydrogen halides.     

Study of the torsion-rotation Hamiltonian for symmetric tops using the millimeter wave spectrum of methyl silane

The torsion-rotation Hamiltonian for symmetric tops has been tested in methyl silane by combining recent anticrossing molecular beam measurements in the ground torsional state (v = 0) with pure rotational spectra taken for v as high as 4. The earlier microwave data set which consisted of J = 1 ← 0 and 2 ← 1 has been greatly extended by studying millimeter transitions for J = 4 ← 3, 5 ← 4, and 13 ← 12. An analysis of the 72 rotational frequencies for v ≤ 2 and the 15 anticrossing data for v = 0 yielded an excellent fit using 14 rotational, torsional, and distortion constants including the effective values for the A rotational constant and the barrier height V₃. No satisfactory fit could be obtained when the data set was extended to include measurements for (v = 3) or (v = 4). For each of these higher torsional levels, the difference between the observed frequencies and the predictions based on the best (v ≤ 2) constants can be expressed in terms of a shift δB_v in the B rotational constant, where δB_v is a smooth function of the torsional energy. This disagreement is of particular interest because it may result from the fact that the molecule passes from hindered to free rotation as v is increased from 2 to 4. The possibility of perturbation by a low-lying vibrational level is considered briefly. The information contained in the different types of spectra is discussed; the redundancy relations are treated and a Fourier expansion of the diagonal torsional matrix elements is introduced. For ¹²CH₃²⁹SiH₃, ¹²CH₃³⁰SiH₃, and ¹³CH₃²⁸SiH₃ pure rotational spectra for v = 0 were studied briefly in natural abundance. The results were combined with existing data for two deuterated symmetric rotors to obtain a structure based only on symmetric top rotational constants.     

The infrared spectrum of 31P14N near 1300 cm−1

The infrared absorption spectrum of the PN molecule has been measured at temperatures between 800 and 1050°C with a tunable diode laser. The transitions measured ranged from J″ = 3 to J″ = 53 and included the vibrational transitions v = 1 ← 0, 2 ← 1, 3 ← 2, and 4 ← 3. These measurements are combined with microwave measurements made by others to yield a consistent set of ten Dunham ro-vibrational constants and their uncertainties.     

The Infrared Spectrum of CCH2: The Bending States up to v4+v5=4

The vibration-rotation spectra of ¹³C monosubstituted acetylene, ¹²C¹³CH₂, have been recorded in the region between 450 and 3200 cm⁻¹ with an effective resolution ranging from 0.004 to 0.006 cm⁻¹. A total of about 5300 rovibrational transitions have been assigned to 53 bands involving the bending states up to v_t=v₄+v₅=4, allowing the characterization of the ground state and of 30 vibrationally excited states. All the bands involving states up to v_t=3 have been analyzed simultaneously by adopting a model Hamiltonian which takes into account the vibration and rotation l-type resonances. The derived spectroscopic parameters reproduce the transition wavenumbers with a RMS value of the order of the experimental uncertainty. Using the same model larger discrepancies between observed and calculated values have been obtained for transitions involving states with v_t=4. These could be satisfactorily reproduced by only adopting, in addition to the previously determined parameters which were constrained in the analysis, a set of effective constants for each vibrational manifold.