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.     

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.     

The v6 = 1 and v6 = 2 vibrational states of DCF3

The high-resolution infrared spectra of DCF₃ were reinvestigated in the ν₆ fundamental band region near 500 cm⁻¹ and around 1000 cm⁻¹ with the aim to assign and analyze the overtone level of the asymmetric CF₃ bending vibration v₆ = 2.The present paper reports on the first study of both its sublevels (A₁ and E corresponding to l = 0 and ±2, respectively) through the high-resolution analysis of the overtone band and the hot and bands.The well-known “loop method”, applied to and , yielded ground state energy differences Δ(K, J) = E₀(K, J) − E₀(K − 3,J) for the range of K = 6 to 30.In the final fitting of molecular parameters, we used the strategy of fitting all upper state data together with the ground state rotational transitions.This is equivalent to that calculating separately the and coefficients of the K-dependent part of the ground state energy terms from the combination loops.All rotational constants of the ground state up to sextic order could be refined in the calculation.This led to a very accurate determination of C₀ = 0.18924413(25) cm⁻¹, , and also .In the course of analyzing simultaneously the overtone band together with the and ν₆ bands, the original assignment of the fundamental ν₆ band [Bürger et al., J. Mol. Spectrosc. 182 (1997) 34-49] was found to be incompatible with the present one. Assignments of the (k + 1, l₆ = +1)/(k − 1,l₆ = −1) levels had to be interchanged, which changed the value of Cζ₆ = −0.14198768(26) cm⁻¹ and the sign of the combination of constants C − B − Cζ in the v₆ = 1 level to a negative value.     

Relaxor behavior of K0.5La0.5Bi2Ta2O9 ceramics

Potassium lanthanum bismuth tantalate (K_0.5La_0.5Bi₂Ta₂O₉), a new relaxor ferroelectric was synthesized via the solid-state reaction route. X-ray structural studies along with Rietveld refinement confirmed it to be an n=2 member of the Aurivillius family of oxides and the refined lattice parameters are , and . The appearance of 1/2{h00} and 1/2{hk0} type superlattice reflections in the electron diffraction patterns reflected the presence of ordered polar regions. A broad dielectric peak associated with frequency dependent dielectric maximum temperature was observed. The value of the diffuseness parameter γ=1.93, obtained from the fit of a modified Curie-Weiss law established the relaxor nature of the title compound. The dielectric relaxation obeyed the Vogel-Fulcher relation wherein and . The relaxor behavior was attributed to the local polar ordering on A-sites.     

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.     

Sub-Doppler high-resolution excitation spectroscopy of the S1 ← S0 transition of dibenzofuran

Rotationally resolved ultrahigh-resolution fluorescence excitation spectra of the S₁ ← S₀ transition of dibenzofuran have been observed using the technique of crossing a collimated molecular beam and the single-mode UV laser beam. 3291 rotational lines of the band and 3047 rotational lines of the band have been assigned. The band has been found to be a b-type transition, in which the transition moment is along the twofold symmetry axis of this molecule, and only the ΔK_a = ± 1 transitions were observed. The excited state is identified to be the S₁¹A₁(ππ^∗) state. In contrast with this, the band has been found to be an a-type transition in which the transition moment is along the long axis in plane. It indicates that the intensity of this vibronic band arises from vibronic coupling with the S₂¹B₂(ππ^∗) state. We determined the accurate rotational constants and the molecule have been shown to be planar both in the ground and excited states.     

4-benzyl pyridinium dihydrogenmonophosphate C6H5CH2C5H4NHH2PO4 : Single-crystal structure and its conductivity in polycrystalline form

The salt 4-benzyl pyridinium dihydrogenmonophosphate is monoclinic P2₁/c with the following unit cell dimensions: ; ; ; and β=97.328(11). Also, , D_x=1.403, , F(000)=560; ; and R=0.0495 and R_w=0.0964 for 3733 independent reflections. The structure consists of infinite parallel two-dimensional planes built of H₂PO₄⁻ anions and C₆H₅CH₂C₅H₄NH⁺ cations mutually connected by strong O-H ?O and N-H ?O hydrogen bonding. There are no contacts other than the normal Van der Waals interactions between the layers. The conductivity relaxation parameters associated with some H⁺ conduction have been determined from an analysis of the spectrum measured in a wide temperature range.     

Combined experimental and theoretical investigation of the CO2 adsorption on LaMnO3+y perovskite oxide

The surface interaction of CO₂ with the perovskite-type oxide LaMnO_3+y has been investigated by means of density functional theory calculations and experimental measurements of adsorption isotherms in the temperature range 298-473 K. A (1 0 0) oriented slab of the cubic structure was used for modeling CO₂ adsorption. The reference unit cell contains alternating LaO⁺ layers and layers; one slab is LaO⁺-terminated and the opposite surface is terminated. A Freundlich isotherm fitted the experimental data satisfactorily. Analysis of the isosteric heat revealed an energetically heterogeneous character for the lanthanum manganite oxide surface, mainly due to the degree of heterogeneity of the adsorption center and due to the adsorbate-adsorbate lateral interactions. Considering theoretical calculations and thermodynamical approaches, two types of active sites were found to be responsible for irreversible and reversible adsorption of CO₂ as a function of surface coverage and O₂ treatment. Strong adsorption takes place on the surface containing La cations. The strongest adsorption is associated with surface oxygen vacancies, center. The next strongest adsorption, a flat adaptation of CO₂ molecules with respect to the surface sites, with a strong binding to a surface oxygen, leads to chemisorbed carbonate species. These adsorption modes are chiefly indicative of a high basic character of the lanthanum manganite oxide surface. Several cationic sites formed by lanthanum and manganese cations are able to weakly adsorb CO₂ molecules in perpendicular or bridged forms. The latter adsorption modes suggest a weak acidic character of the manganite adsorbent.     

Generalized pressure-volume equations mimicking the Stacey reciprocal K-primed equation of state

Two generalized polynomial expressions, one based on a logarithmic form and the other on an exponential form have been set up that give pressure-volume (P-V) relationship and higher derivative properties mimicking the Stacey reciprocal K-primed equation of state. The results have been obtained for pressure P, bulk modulus K and its pressure derivative K′ for six metals viz. Ag, Al, Au, Cu, Mo and W at different values of compression down to V/V₀=0.5. The zero-pressure values of input parameters K₀ and have been taken from the literature, whereas and have been fixed to match the Stacey reciprocal K-primed equation. The polynomial equations thus formulated can be used as a substitute for the Stacey equations of state (EOS) for determining P-V relationship and higher derivative properties such as K and K′.     

High-resolution diode laser absorption spectroscopy of the O-H stretch overtone band (2,0,0)←(0,0,0) of the HO2 radical

The O-H stretching overtone (2ν₁) of the HO₂ radical was observed between 6603.2 to by using tunable diode laser absorption spectroscopy (TDLAS). About 1000 lines were observed in this region of which 491 transitions could be definitively assigned to the 2ν₁. The spectrum is observed to be an A/B hybrid band with band features of both a perpendicular and parallel nature. Transitions of the A-type bands with K_a^′=0-3, N^′?16 and transitions of the B-type bands with K_a^′=0,1, N^′?15 were assigned. The origin calculated from the best fit to the present spectrum is at which is ∼ higher than previously reported. The overtone spectrum is observed to be heavily perturbed, possibly by Fermi resonance with energy levels of the nearby (ν₂+5ν₃) state.     

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 .