Absorption spectra of AsH2 radical in 435-510 nm by cavity ringdown spectroscopy

The absorption spectra of jet-cooled AsH₂ radicals were recorded in the wavelength range of 435-510 nm by cavity ringdown spectroscopy. The AsH₂ radicals were produced by pulsed DC discharge in a molecular beam of a mixture of AsH₃, SF₆, and argon. Seven vibronic bands with fine rotational structures have been identified and assigned as the , , and (n = 1-3) bands of the electronic transition. Based on the previous studies of AsH₂ radical, rotational assignments and rotational term values for each band were obtained, and the molecular parameters including vibrational constants, rotational constants, centrifugal distortion constants, and spin-rotation interaction constants were also determined.     

Dispersed fluorescence spectroscopy of S0 vibrational levels in formaldehyde-d

High resolution dispersed fluorescence (DF) spectra of excited vibrational levels in S₀ HDCO up to 10 000 cm⁻¹ energy were recorded in a free-jet expansion. Excitation to the 0₀₀ rotational level in 4⁰ and 4¹ S₁ HDCO yielded pure vibrational spectra that are free from rotational congestion. The 162 transitions (133 unique vibrational levels) assigned in these spectra have been fit to a multiresonant Hamiltonian model, which includes harmonic frequencies , anharmonic constants (x_ij), and resonance constants (K). The assigned vibrational states were fit to the model with a standard deviation of 4.02 cm⁻¹. Extensive vibrational mixing is observed throughout the spectra. Six harmonic constants, eight anharmonic constants, and four resonance constants (K_44,1, K_66,1, K_44,66, and K_33,5) were determined experimentally. The 18 experimentally determined spectroscopic constants, with the exception of and K_66,1, were found to agree within 6 cm⁻¹ of ab initio calculated values.     

Theoretical calculations of vibrational frequencies and rotational constants of the N2O isotopomers

Theoretical values of vibrational frequencies and rotational constants for all the isotopomers of nitrous oxide are reported. The calculations are carried out variationally using an empirical Morse-cosine potential energy surface previously determined for N₂O, and a set of optimal internal vibrational coordinates. The spectroscopic constants obtained are compared to those extracted from spectroscopic measurements for the , , , , , and isotopomers. The agreement between calculated and observed values for these isotopomers is shown to be excellent, especially for the rotational constants. As a result, an unidentified band recently recorded is properly assigned. The spectroscopic constants computed for the rest of the isotopomers, for which observed information is much scarcer, have therefore a predictive character. The vibrational zero point energies for all the N₂O isotopomers are also given.     

Gas-phase detection of discharge-generated DSOD

We report the first spectroscopic detection of perdeuterated 1-oxadisulfane, DSOD, generated in a radio-frequency plasma of D₂S and D₂O. The chain molecule DSOD produces a perpendicular-type spectrum, with well-known spectral features encountered in previous studies of geometrically related molecules, such as compact Q-branches, which are clearly recognizable. The arrangement of the transitions shaping the Q-branches usually provides sufficient proof for a clear-cut detection of a chain molecule such as DSOD. Guided by quantum chemical calculations, we have located the band center of the -branch of DSOD in the frequency region near 466.5 GHz using the Cologne terahertz spectrometer. This -branch displays both b- and c-type spectra, quite analogous to the behavior of the corresponding -branch of HSOH. In addition, we have been able to detect an internal rotational splitting of ∼0.5 MHz for c-type transitions of the -branch, which lends independent support to our present assignment. From the measurements performed on Q-branches, we derive the following differences in rotational constants: A−(B+C)/2=93331.001(15) MHz, and (B−C)/4=172.95923(29) MHz, in excellent agreement with theoretical predictions.     

The millimetre-wave rotational spectrum of phenylacetylene

The room-temperature rotational spectrum of phenylacetylene (C₆H₅CCH), was studied at frequencies up to 340 GHz. Extensive new measurements, covering rotational transitions with quantum number values up to J=140 and K_a=59, allowed determination of precise spectroscopic constants for the ground state and for the lowest two excited vibrational states, v₂₄=1 and v₃₆=1. The two excited states belong to the lowest B₁ and B₂ symmetry normal modes and their rotational transitions are very strongly perturbed by a-axis Coriolis resonance. A successful fit of the resonance is reported, resulting in and , in good agreement with results of ab initio computations.     

Substitution structure of cyanogen, NCCN, from high-resolution far infrared spectra

The lowest lying vibrational bands of the gas-phase spectra of cyanogen, NCCN, and four of its isotopomers, , , , and , were recorded with a Fourier transform interferometer. The resolution was limited by the maximum optical path difference (MOPD) attainable with the interferometer to . Rovibrational transitions of the ν₅ () and also the ν₂-ν₅ () band systems were assigned for all five isotopomers. The use of an effective Hamiltonian for linear molecules to fit the data yielded precise spectroscopic vibrational and rotational constants for the vibrational states (v₁v₂v₃v₄v₅) or (v₄v₅)=(00), (01), (02), (03), and (01000). These data include the first rotationally resolved transitions involving (01000). Complete substitution (r_s) structures of cyanogen, based on both single and double isotopic substitution of the parent species, were calculated. The derived structure is r_CC=138.48(17) pm and r_CN=115.66(13) pm. The two r_s structures coincide within the errors due to remaining contributions of zero-point vibrations.     

Gas phase electronic spectrum of propadienylidene C3H2

The rotationally resolved vibronic bands in the forbidden electronic transition of the cumulene carbene C₃H₂ have been observed in the gas phase by cavity ring down absorption spectroscopy through a supersonic planar plasma with allene as precursor. The band detected in the 16 223 cm⁻¹ region is a result of vibronic interaction and is assigned to a combination of a₁ and b₂ vibrations with a frequency around 2250 cm⁻¹. Another vibronic band near 15 810 cm⁻¹ has an unusual rotational structure because the K_a = 0-1 subband is absent. It is assigned to a combination of a₁ and b₁ vibrations, ∼1850 cm⁻¹, which borrow intensity from the near lying state due to a-type Coriolis coupling. A rotational analysis using a conventional Hamiltonian for an asymmetric top molecule yields molecular constants for the vibrational excited levels of the ùA₂ state, which were used for the determination of the geometry. The stronger transition of C₃H₂, measured in a neon matrix in the 16 161-24 802 cm⁻¹ range, was not detected. The reason for this is a short lifetime of the state, leading to line broadening.     

Stretch-bend combination polyads in the ÃAu state of acetylene, C2H2

Rotational analyses are reported for a number of newly-discovered vibrational levels of the S₁-trans (ùA_u) state of C₂H₂. These levels are combinations where the Franck-Condon active and vibrational modes are excited together with the low-lying bending vibrations, and . The structures of the bands are complicated by strong a- and b-axis Coriolis coupling, as well as Darling-Dennison resonance for those bands that involve overtones of the bending vibrations. The most interesting result is the strong anharmonicity in the combinations of (trans bend, a_g) and (in-plane cis bend, b_u). This anharmonicity presumably represents the approach of the molecule to the trans-cis isomerization barrier, where ab initio results have predicted the transition state to be half-linear, corresponding to simultaneous excitation of and . The anharmonicity also causes difficulty in the least squares fitting of some of the polyads, because the simple model of Coriolis coupling and Darling-Dennison resonance starts to break down. The effective Darling-Dennison parameter, K₄₄₆₆, is found to increase rapidly with excitation of , while many small centrifugal distortion terms have had to be included in the least squares fits in order to reproduce the rotational structure correctly. Fermi resonances become important where the K-structures of different polyads overlap, as happens with the 2¹3¹B¹ and 3¹B³ polyads (B = 4 or 6). The aim of this work is to establish the detailed vibrational level structure of the S₁-trans state in order to search for possible S₁-cis (¹A₂) levels. This work, along with results from other workers, identifies at least one K sub-level of every single vibrational level expected up to a vibrational energy of 3500 cm⁻¹.     

Millimeter-wave spectroscopy of deuterated hydrogen sulfide, SH2D

The monodeuterated isotopologue of the sulfonium ion, the asymmetric species SH₂D⁺, was produced in a magnetically confined negative glow discharge and detected for the first time by means of its rotational spectrum in the region 324-637 GHz. The determined rotational constants, along with those for the symmetric isotopologues and for obtained in previous measurements, enabled to compute a semi-experimental equilibrium structure, where the contribution of the zero-point vibrations is assumed from available ab initio calculations. In addition, thanks to the large body of quartic centrifugal distortion constants now available, the harmonic force field of the sulfonium ion has been refined by a least-squares procedure.     

Ab initio calculations of elastic constants and thermodynamic properties of Li2O for high temperatures and pressures

The elastic constants and thermodynamic properties of Li₂O for high temperatures and pressures are calculated by the ab initio unrestricted Hartree-Fock (HF) linear combination of atomic orbital (LCAO) periodic approach. The lattice constant, elastic constants, Debye temperature, and thermal expansion coefficient obtained are in good agreement with the available experimental data and other theoretical results. It is found that at zero pressure the elastic constants C₁₁, C₁₂ and C₄₄, bulk modulus B and Debye temperature Θ_D decrease monotonically over the wide range of temperatures from 0 to 1100 K. When the temperature , C₁₂ approaches zero, consistently with the transition temperature 1200 K. However, with increasing pressure, they all increase monotonically and the anisotropy will weaken.     

Ab initio anharmonic force field and equilibrium structure of the sulfonium ion

The semi-experimental equilibrium structure of the sulfonium ion, , has been obtained from the experimental ground-state rotational constants available for five isotopologues and the corresponding vibrational corrections computed at the CCSD(T)/cc-pwCVQZ level of theory. This geometry has been found in very good agreement with the pure ab initio equilibrium structure calculated at the CCSD(T) level of theory using a basis set of sextuple-zeta quality and including core correlation corrections. The anharmonic force field has been used for deriving spectroscopic properties: in particular, in addition to the vibrational corrections, the rotational parameters of the SH₂D⁺ isotopic species, not yet experimentally observed, have been predicted to a guessed good accuracy.     

Fourier-transform microwave and submillimeter-wave spectroscopy of chloroiodomethane, CH2ICl

Guided by a previous microwave study (9–35 GHz), the rotational spectrum of both chlorine isotopologues of chloroiodomethane in its vibrational and electronic ground state has been re-investigated in the microwave region and extended to the millimeter/submillimeter-wave region. Weak a-type transitions have been recorded by Fourier transform microwave spectroscopy below 20 GHz whilst strong b-type rotational transitions have been recorded between 15 and 646 GHz, corresponding to energy levels with J″ ≤ 108 and . Molecular constants including those describing the hyperfine structures owing to the two halogen atoms were accurately determined for both species from the least-squares analysis of a total of 1475 distinct transition frequencies (of which 906 belong to the CH₂I³⁵Cl isotopologue). The two sets of rotational constants allowed us to derive an r₀ structure of CH₂ICl.     

Field suppression of the modulated phase of Ce2Pd2Sn

Low temperature magnetic (M) and thermal (C_P) properties of the intermetallic compound Ce₂Pd₂Sn have been investigated at zero and different magnetic fields. Two transitions were recognized at and , with latter nearly coinciding with the extrapolated Curie-Weiss temperature . The Curie factor evaluated from T≥T_M, is ≈2μ_B. The positive value of θ_P, the triangular coordination of the magnetic (Ce) atoms and the weak effect of applied magnetic field, reveal that T_M cannot be considered as a canonic antiferromagnetic transition like claimed in the literature. M(T) measurements under moderate magnetic fields () show T_C(B) increasing while T_M(B) is practically not affected. Both transition merge in a critical point at for , where the intermediate phase is suppressed. At , the cusp of a first order transition is observed in C_P(T). According to the proposed ferromagnetic ground state, it is followed by a C_P(T)∝T^3/2exp(-E_g/T) dependence, with a gap of anisotropy .     

The pure rotational spectra of the lanthanum monohalides, LaF, LaCl, LaBr, LaI

Pure rotational spectra have been measured for all the major isotopomers of the lanthanum monohalides, LaF, LaCl, LaBr, and LaI, in their ground and (except for ) excited vibrational states. The spectra were observed with a cavity pulsed jet Fourier transform microwave spectrometer in the frequency range 5-24 GHz. The molecules were prepared by laser ablation of La metal and allowing the resulting plasma to react with SF₆, Cl₂, Br₂, or CH₃I precursor in an Ar carrier gas of the pulsed jet. For LaBr this is the first reported spectrum of any kind. Rotational constants, centrifugal distortion constants, nuclear quadrupole coupling constants, and nuclear spin-rotation constants have been determined for all the molecules. Accurate equilibrium (r_e) internuclear distances have given an indication of where the Born-Oppenheimer approximation is beginning to fail. From the centrifugal distortion constants and vibration-rotation (α_e) constants good estimates of the harmonic vibration frequencies and bond dissociation energies have been obtained. The halogen nuclear quadrupole coupling constants indicate the molecules to be highly ionic.     

Rotationally resolved electronic spectrum of propadienylidene

The rotationally resolved electronic spectrum of a vibrational band in the transition of the cumulene carbon chain C₃H₂ was measured in the 625 nm region in a supersonic discharge using cw cavity ring down spectroscopy. The rotational structure was analysed using a conventional Hamiltonian for an asymmetric top molecule, and the constants A′, B′, and C′ in the upper state were determined. The observed band is assigned to a combination of a₁ and b₂ vibrations with the frequency around 2000 cm⁻¹. The geometries in the ¹A₁, ¹A₂, ¹B₁ states and the intersection point between the latter two were obtained using ab initio calculations. The effective structure in the measured vibrational level of the ¹A₂ state was inferred from the determined constants.     

Atomic-potential parameters for H2 and D2: quantum corrections in the calculation of second-virial coefficients

First-order quantum corrections were introduced into the computation of the second-virial coefficients of H₂ and D₂. The quantum effects, for the studied two light molecules, are considerable even at the room temperature and become prominent at low temperatures. Atomic potentials, incorporating the quadrupole interactions, were employed in the calculations. Optimum atomic-potential parameters ε_H, σ_H, ε_D and σ_D were obtained from the nonlinear least-squares fit of the experimental second-virial coefficients. The fitted virial coefficients cover the temperature ranges of 173-423 and 153- for H₂ and D₂, respectively.     

Microwave study of the rotational spectrum of oxygen molecule in the range up to 1.12 THz

Microwave study of the rotational transitions of oxygen molecule in its electronic and vibrational ground states is reported. Eight transitions belonging to N=3-1, N=5-3, and N=7-5 groups were investigated. Central line frequencies and pressure broadening parameters for O₂ and N₂ as perturbers were determined. The highest frequency of measured transition (N,J)=(7,6)-(5,6) has been 1.12 THz. Spectrometer with backward wave oscillator (BWO) and acoustic detector (RAD) was used. Since this experiment has more than doubled the number of previously measured rotational lines of oxygen molecule and better accuracy was achieved, the fitting of new set of rotational transition frequencies has been performed and new more accurate molecular constants for in , v=0 state have been obtained.