Dispersed Fluorescence Spectroscopy and Transition Dipole Moment of HCO β 2A′-X̄2A′

Dispersed fluorescence spectra of β ²A′ - x? ²A′ transitions are measured on exciting HCO to the vibrational levels (0,0,0), (0,0,1), (0,1,0) and (1,0,0) of β ²A′ in a gas cell. Vibrational energies of bound and resonant states are determined for energy up to 17 500 cm^?1 from the ground vibrational level of the x? ²A′ state. Rotationally resolved spectra of the bands β ²A′ - x? ²A′ 2⁰₃3⁰₁, 2¹₄ and 2⁰₂3¹₂ were recorded and analyzed. Abnormal intensities in the P, Q and R branches are due to the mechanism of “axis switching”. From the ratios of emission intensity of a- and b-type transitions, without taking into account the varied lifetimes of rotational states of the upper electronic state, the directions of transition dipole moments of these bands are determined to be 39 ? 43° to the inertial a axis.     

Studies on the circular dichroism of penam and penam sulfoxides

Optical rotatory properties of models of 6-aminopenicillanic acid (6-AP A) as obtained from earlier molecular orbital and rotatory strength calculations are corroborated by new CD data for 6-AP A. The new spectra extend down to 180 nm, revealing a positive Cotton effect near 185 nm due to carbonyl π→π^* transitions. The pH sensitivity of the optical activity of the β-lactam amide π→π^* and the 203-nm n→π^* transitions is explainable in terms of electrostatic effects and orbital interactions present in α-aminoketones and 4-membered rings which tend to flatten out the NH₂ group. These same interactions are responsible for the unusually low pK_a of 4.75 for the amino group; (the carboxyl group of 6-AP A has a normal pK_a of 2.5). The previous calculational procedures are extended to the sulfoxides of 6-AP A, and the predicted CD spectra are compared to new experimental curves and also to available data for penicillin sulfoxides. The MO's associated with the principal bands are illustrated with the aid of electron density maps. The positive Cotton effect observed and calculated near 230 nm is due to two charge transfer transitions within the asymmetric penam nucleus. Similar to the situation with the unoxidized penams, the sulfoxide of 6-AP A displays a very low pK_a for the amino group (3.8) and a change toward a less positive CD band at 202 nm upon protonation of this group.     

Effect of a transverse magnetic field on the plume emission in laser-produced plasma: An atomic analysis

We have investigated the effect of varying transverse magnetic field on the plasma plume emission of laser-produced lithium plasma. Two atomic transitions for lithium neutral Li (I) and two for Li ion LI (II) are taken for the study. It has been found that for Li (I), the emission from 670.8 nm transition (2s²S_1/2←2p²P_3/2′1/2) shows initial enhancement and then subsequent decrease for higher fields. Of course, the overall intensity is increased for all the fields when compared to the case of without field. On the other hand, for 610.3 nm (2p²P_1/2←3d²P_3/2′5/2), there is continuous decrease in intensity. Interestingly, for Li (II) transitions also, after an initial increase in intensity up to 0.08 Ta decrease is observed. From the atomic analysis, we find that for 670.8 nm line, the cause of initial enhancement is increase in electron impact excitation whereas for decreased intensity, increased field-induced ionization appears to be responsible mechanism. However, for 610.3 nm line, decrease in intensity appears to be due to decreased recombination. For Li (II), 478.8 nm (3p¹P₁←4d¹D₂) and 548.4 nm (2s³S₁←2p³P_2,1,0) transitions, initial increase appears to be due to increased confinement (increase in plasma density) and subsequent decrease in intensity with increase in field due to decreased recombination.     

A Variety of Phase‐Transition Behaviors in a Niccolite Series of [NH3(CH2)4NH3][M(HCOO)3]2

A niccolite series of [bnH₂²⁺][M(HCOO)₃]₂ (bnH₂²⁺=1,4‐butyldiammonium) shows four kinds of metal‐dependent phase transitions, from high temperature para‐electric phases to low‐temperature ferro‐, antiferro‐, glass‐like, and para‐electric phases. The conformational flexibility of bnH₂²⁺ and the different size, mass, and bonding character of the metal ion lead to various disorder‐order transitions of bnH₂²⁺ in the lattice and relevant framework modulations, thus different phase transitions and dielectric responses. The magnetic members display a coexistence or combination of electric and magnetic orderings in the low‐temperature region.     

An A‐Site Mixed‐Ammonium Solid Solution Perovskite Series of [(NH2NH3)x(CH3NH3)1−x][Mn(HCOO)3] (x=1.00–0.67)

The A‐site mixed‐ammonium solid solutions of metal–organic perovskites [(NH₂NH₃)_x(CH₃NH₃)_1?x][Mn(HCOO)₃] (x=1.00–0.67) exhibit para‐ to ferroelectric diffuse phase transitions with lowered transition temperatures from x=1.00 to 0.67. These properties are due to the decreased framework distortion and polarization in their low temperature ferroelectric phases caused by the increased CH₃NH₃⁺ concentration.     

An optical investigation of Ho2(SO4)3 ·8H2O

The optical absorption spectra of Ho³⁺ in single crystals of Ho₂(SO₄)₃·8H₂O have been studied between 2500 A and 9000 A at 4.2 and 77 K. The microscopic crystal structure has been related to the macroscopic shape of the crystals. From the polarization of the optical transitions it is found that the point symmetry of the crystalline electric field (CEF) at the site of the rare earth ions in the sulfate lattice is well described by C_1h. The CEF parameters associated with this symmetry were derived from the observed Stark splitting of the [SL]?J levels and the polarization of the optical transitions. We suggest that the magnetically inequivalent sites observed in Tm₂(SO₄)₃·8H₂O are due to triclinic distortions of the monoclinic CEF.     

Polarization and assignment of the two-photon excitation spectrum of benzene vapor

The two-photon excitation (TPE) of benzene fluorescence in the vapor phase at 60 torr is reported for the total-energy region from 38 086 cm^?1 to 42 441 cm^?1 using both circular and linear polarized light from a nitrogen-pumped dye-laser. The theory of the polarization dependence of the vibronic transitions in benzene is briefly reviewed, and it is seen how transitions involving vibrations of b_1u symmetry are expressly forbidden for this type of TPE experiment in which the two photons are identical. Five vibronic origins with distinctive rotational contours and polarization dependence are identified in the TPE spectrum. The υ₁₄(b_2u) vibronic origin at 1570 cm^?1 (above the electronic origin of the ^IB_2u state) stands out very prominently in the linear polarized spectrum, but nearly disappears in the circular polarized spectrum. This striking polarization dependence indicates a significant contribution of A_2u electronic states to the intermediate states of this TPE vibronic transition. The relatively great strength of the υ₁₄ band may be due to vibronic borrowing by the b_2u mode from the ground electronic state (A_1g).     

Electronic absorption spectra of V2O5

MO calculations of electronic structure and optical transitions for [VO_n]^{5 ?; 2n} (n = 4, 5, 6) clusters in V₂O₅ monocrystals have been carried out by means of the semiempirical CNDO CI method. Using the calculated results, a complete analysis of V₂O₅ optical data as available in the literature and as obtained in the electroreflectivity experiments presented in this paper is performed. An identification of optical transitions in a wide energy range is presented. The optical properties of vanadium pentoxide are shown to be due to the localized charge transfer electronic transitions in the clusters. The fine structure of optical spectra is connected with the covalent splittings of the vanadium 3d and oxygen 2p atomic levels.     

Charge transfer shake-up satellites in X-ray photoelectron spectra of solid compounds of 3d0 ions

Satellite structure has been observed at 8 eV and 12 eV below the main metal 2p peaks for compounds of Sc³⁺ and Ti⁴⁺. These features are attributed to charge transfer “shake-up”: transitions (t_2g → t_2g^* and e_g → e_g^* respectively). The 8 eV satellites are only found in sulphate complexes of Sc³⁺. This is the first observation of satellites which are probably due to t_2g → t_2g^* monopole charge transfer transitions. The probability for this transition is usually low, resulting in the observation of satellites due to the e_g → e_g transition.     

Triplet states of the amide group. Trapped electron spectra of formamide and related molecules

Trapped electron (TE) spectra are obtained using ion cyclotron resonance detection of scavenged electrons. The lowest singlet-triplet transitions, ³(n→π^*), in formamide (HCONH₂) and N,N-dimethyl formamide (HCONMe₂) are found at vertical energies of 5.30 and 5.00 eV, respectively. An unresolved band containing the ³(π→π^*) and ³(n→3s) states appears at higher energies, centered at 6.60 and 6.00 eV, respectively. The TE spectra of formaldehyde (HCHO), acetaldehyde (MeCHO) and acetone (Me₂CO) are obtained for comparison and are used along with results from ab initio theoretical calculations in establishing assignments. Singlet-triplet transitions dominate the spectra of all of these carbonyl containing molecules, to the exclusion of low lying singlet-singlet transitions. This is in agreement with other TE spectra and the expectation that (dσ/dE) will be higher near threshold for singlet-triplet as compared to singlet-singlet transitions.     

Laser-induced fluorescence of high-temperature vapor complexes of ErCl3 with AlCl3, GaCl3 and InCl3

Laser excitation of equilibrium vapor mixtures ErCl₃(s)-ACl₃(g) (A = Al, Ga, In) at 475–1100 K gives rise both to resonance fluorescence from the f → f Er³⁺ transitions of the Er-Cl-A vapor complexes, and to Raman scattering due to the vibrational modes of the ACl₃ vapor. The laser-induced fluorescence from the ⁴F_{$\text{9}\text{2}$}, ⁴S_{$\text{3}\text{2}$} and ²H_{$\text{11}\text{2}$} states has been investigated at different temperatures and excitation.     

Synthesis,crystal structure,magnetism, and absorption spectra of A2UX5 Type Halides (A = K,Rb; X = Cl,Br, I)

The ternary uranium(III) halides A₂UX₅ (A = K, Rb; X = Cl, Br, I) have been prepared from the binary components AX and UX₃ in sealed tantalum containers. According to their Guinier X-ray powder patterns, they all crystallize with the K₂PrCl₅/Y₂HfS₅ type of structure. Lattice constants for ambient temperature are reported. Single-crystal structure refinemens were undertaken for K₂UI₅ and Rb₂UCl₅. Magnetic susceptibility data were recorded with a SQUID magnetometer from liquid helium to room temperature. One-dimensional (intrachain) and three-dimensional antiferromagnetic ordering occur at low temperatures dependent upon the U³⁺? U³⁺ distance. Absorption spectra were recorded between 4 000 and 28 000 cm^?1. They show f—f transitions typical for U³⁺ and, depending on the halide, very strong f—d transitions above 14 000 to 15 000 cm^?1, respectively.     

Exploration of Metal-Metal Charge Transfer (MMCT) and its Effect in Generating New Colored Compounds in Mixed Metal Oxides

Compounds belonging to the palmierite structure, (Zn_3−xM_x)A₂O₈ (M=Co, Ni, Cu and A=V, P) have been prepared employing solid state methods. The transition metal substituted compounds of Zn₃V₂O₈ exhibits colors that are unique varying from mint green to forest green for Co²⁺ ions. The observed colors were understood based on the allowed d-d transitions and metal to metal charge transfer (MMCT) transitions. The MMCT transitions involve partially filled d-orbitals of Co²⁺ (3d⁷), Ni²⁺ (3d⁸), and Cu²⁺ (3d⁹) ions and the V⁵⁺ ions (3d⁰). The spinel compounds, Zn_2−xCo_xMO₄ (M=Ti, Sn) were also prepared to understand the MMCT transitions in the compounds. Band structure calculations were carried out to understand the participating orbitals near the Fermi level and the band gap. The calculations support the idea that the substitution of transition elements in the palmierite structure reduces the overall band gap from 3.18 eV for Zn₃V₂O₈ to 2.61 eV Zn_2.5Co_0.5V₂O₈ compound. This indicates the substitution of transition elements provide a tool towards band gap engineering.     

VUV spectroscopic properties of rare-earth (RE3+ = Sm3+, Eu3+, Tb3+, Dy3+) -activated layered borate Ba6Gd9B79O138

Vacuum ultraviolet (VUV) spectroscopic properties of rare-earth RE³⁺- activated (RE³⁺ = Sm³⁺, Eu³⁺, Tb³⁺ and Dy³⁺) Ba₆Gd₉B₇₉O₁₃₈ borates (BGBO) are investigated. The strong absorption bands in the VUV range of un-doped and RE³⁺-activated BGBO were observed. The band range from 140 to 200 nm with a peak at about 173 nm results from the host lattice absorption. For Sm³⁺-activated BGBO, the charge transfer transition from O^2- to Sm³⁺ was observed at 202 nm. In addition, it exhibits bright red emission originating from the Sm³⁺ f-f transitions of ⁴G_5/2 → ⁶H_J (J = 5/2, 7/2 and 9/2). The O^2--Eu³⁺ charge transfer (CT) at 249 nm is observed in the excitation spectrum for Eu³⁺-doped BGBO. For Tb³⁺-activated BGBO, the broad bands around 208 and 230 nm are due to the spin-allowed and spin-forbidden f-d transitions of Tb³⁺, respectively. In addition, the absence of the f-d transitions of Sm³⁺ and Dy³⁺ in the excitation spectra probably due to the photo-ionization effect. It is demonstrated that there are energy transfers from the BGBO host lattice to the luminescent activators depending on the activators.     

Single crystal circular and linear dichroism spectra and absolute configuration of M(en)3(NO3)2 (M = Zn(II), Cu(II), Ni(II), Co(II), Mn(II) and Ru(II)

The isomorphous single crystals of M(ethylenediamine)₃(NO₃)₂, where M is Zn(II), Ni(II) and Co(II), exhibit macroscopic optical activity as predicted by their acentric space group. Axial circular dichroism measurements on these pure crystals show conclusively that spontaneous resolution has occurred. The axial circular dichroism and orthoaxial linear dichroism spectra of these pure crystals, and of Cu(II), Ni(II), Co(II), Mn(II) and Ru(II) doped into the Zn(en)₃(NO₃)₂ crystal have been measured at ambient and cryogenic temperatures in the range from 7 to 35 kK. The first NO₃^? transition at 32.5 kK is assigned as ¹A ← ¹A based on its linear polarization and sign of rotational strength. The d-d transitions are assigned in the context of D₃ symmetry and reveal a small negative crystal field parameter k, consistent with theoretical prediction. A positive R for all d-d transitions is found to be associated with the Λ configuration for all of the complex ions, by correlation with the crystal and solution circular dichroism of Ru(en)₃²⁺.     

Photofragmentation of SnCl2 at 193 nm

Using photofragment spectroscopy the time of flight spectra and angular distributions for SnCl fragments, Sn and Cl atoms from the fragmentation of SnCl₂ have been measured at 193 nm. From the time, of flight it was found that dissociation takes place into SnCl (X ²Π) and Cl(²P) and that the SnCl fragment is highly vibrationally excited. To account for this effect we propose a mechanism, in which the upper potential surface has a saddle point at a greater Sn-Cl₂ distance than the ground state. The angular distribution exhibits an anisotropy parameter β = 0.21. A model was developed, based on a charge transfer from the p nonbonding orbitals on the Cl atoms to the p_x orbital on the Sn atom. The model predicts that A₁ → B₂ transitions dominate over A₁ → B₁ transitions and that A₁ → B₁ transitions are negligible in good agreement with the experimental observations.     

Water photoelectrolysis using nickel titanate and niobate as photoanodes

Nickel titanate (NiTiO₃) and nickel niobate (NiNb₂O₆), both with a cationic valence and conduction band, were examined for their photoelectrochemical properties. Applied as photoanode in a photoelectrochemical cell for water electrolysis, reduced pellets of these oxides show a photoresponse when irradiated in the optical band gap. The corresponding absorption is due to Ni²⁺ → Ti⁴⁺ and Ni²⁺ → Nb⁵⁺ charge-transfer transitions, respectively. These are situated at lower energy than the O^2? → Ti⁴⁺ and O^2? → Nb⁵⁺ charge-transfer transitions. The flatband potentials of both compounds were determined from photocurrent versus applied potential measurements. During reduction both compounds showed superficial decomposition. The effect of this decomposition on the photocurrents is discussed.     

Photoelectron spectrum and ground state electronic structure of chromyl chloride vapor

The electronic structure of chromyl chloride CrO₂Cl₂ has been investigated by ultraviolet (HeI) photoelectron spectroscopy. Mulliken-Wolfsberg-Helmholtz molecular orbital calculations have been performed in order to provide a model for interpretation of the photoelectron spectra and to assist in assigning the low-energy optical absorption and emission transitions. The first ionization potential of CrO₂Cl₂ at 11.8 eV is due to ionization of the near-degenerate oxygen and chlorine nonbonding 2a₂, 4b₁, and 4b₂ MO's. The first unoccupied orbital is basically a chromium dπ^* orbital. The excitations (2a₂, 4b₁, 4b₂)→ 7a₁^* correlate well with the three low energy absorption transitions observed.     

Two-photon induced fluorescence and resonance-enhanced ionization of sulfur atoms

Two-photon induced fluorescence and resonance-enhanced photoionization have been observed in atomic sulfur originating from both the ³P_2,1,0 and the ¹D₂ states. Sulfur atoms are generated by the sequential multiphoton dissociation of CS₂ at probing wavelengths. The two-photon absorption process involves the 3 ³P_2,1,0 → 4 ³P_2,0,1 or the 3 ¹D₂ → 4 ¹F₃ transitions with resolution of the individual J″ → J′ transitions in most cases. Intensities of the 3³P_J″ → 4 ³P_J′ transitions have been compared with Hartree-Fock calculated transition probabilities from the analogous transitions in atomic oxygen. Photoionization is observed in a three-photon (two to resonance) ionization originating from the ³P_2,1,0 and the ¹D₂ states. Induced fluorescence is observed at 167 and 180 nm which is dipole-allowed radiation from the intermediate ³S⁰₁ and ¹D⁰₂ states, respectively.