Electronic excitation in phosphorus-containing molecules. II. Inner shell electron energy loss spectra of PF5, OPF3 AND OPCl3

Electron energy loss Spectroscopy has been used to obtain the inner shell excitation spectra of PF₅, OPF₃ and OPCl₃ in the P 2p,2s (L-shell) region as well as in the respective ligand K shell (F 1s, O 1s) and L shell (Cl 2p and 2s) regions. The spectra are compared and contrasted with earlier reported spectra obtained on the trivalent phosphorus compounds (PH₃, PCl₃, PF₃ and P(CH₃)₃). The spectra were obtained using an impact energy of 2.5keV and a scattering angle of about 1°. The spectra reported here are typical of molecules with electronegative ligands in that the discrete portions of the spectra show strong transitions to virtual molecular orbitais. In addition, intense features are observed at or just beyond the ionization edge attributable to transitions to trapped inner well states, while broad features further into the continuum can be ascribed to σ^*(P—L) shape-resonances (L = ligand). This resonance assignment was supported by a comparison with the corresponding spectra for PF₃ and PCl₃.     

Unusual "AB" Spectra in High-Resolution Magic-Angle-Spinning NMR-Studies of Solids

Phosphorus-31CP-MAS spectra of Cd(NO₃)₂(PPh₃)₂ have been obtained as a function of spinning frequency. Although the two ³¹P nuclei are crystallographically equivalent and have the same isotropic chemical shifts in the solid state, they exhibit spinning-rate-dependent MAS spectra which have been analyzed to obtain the value of ²J(P, P). At high spinning rates, the spectra are analogous to "A₂" spectra in isotropic solutions, while at slower spinning rates, the spectra are more characteristic of strongly coupled "AB" solution spectra. The AB spectra are unusual in that δ_A = δ_B and J(A, B) is given by the splitting between the alternate peaks in the four-peak multiplet as opposed to the splitting between the outer and adjacent inner lines. This assignment was confirmed by a 2D CP-MAS J-resolved experiment. The unusual spinning-rate-dependent MAS lineshapes result from recoupling of the J interaction between the two crystallographically equivalent nuclei via anisotropic interactions, i.e., weak homonuclear dipolar coupling and differences in the orientation dependence of the chemical-shift tensors. Such spinning-rate-dependent MAS lineshapes are predicted to be a more frequent observation at higher applied magnetic fields.     

Spectral behavior study of 3-(4-dimethylamino-phenyl)-1-{6-[3-(4-dimethylamino-phenyl)-acryloyl]-pyridin-2-yl}-propanone

The photophysical properties such as singlet absorption, molar absorptivity, fluorescence spectra, fluorescence quantum yield (?_f) and transition dipole moment (μ₁₂) of 3-(4-dimethylamino-phenyl)-1-{6-[3-(4-dimethylamino-phenyl)-acryloyl]-pyridin-2-yl}-propanone (DMAPAPP) have been studied in different media. DMAPAPP exhibits a large red-shift in both absorption and emission spectra as the solvent polarity increases, indicating a large change in dipole moment of molecule upon excitation. The fluorescence quantum yield depends on the nature of the solvent. The absorption and emission spectra of DMAPAPP in dioxane–water mixture are also studied. The effect of different type of surfactants to determine their critical micellar concentration (CMC) and the microemulsion effect on the electronic absorption and emission spectra of DMAPAPP are recorded. The effect of acidity on the electronic absorption and emission spectra of DMAPAPP is studied to determine the pK_a and pK_a^? values.     

Reinvestigation of microwave spectrum of dimethyl ether and rs structures of analogous molecules

Microwave spectra of dimethyl ether and its sixteen isotopic species have been measured. For species with singlet spectra, a least-squares analysis of observed transition frequencies gave rotational and five quartic centrifugal distortion constants. For species with multiplet spectra due to the methyl internal rotation, a least-squares analysis of observed multiplet frequencies gave not only unperturbed rotational and five quartic centrifugal distortion constants but also the quantities related to the methyl internal rotation. The r_s structures from (CH₃)₂O, CH₃OCD₃, and (CD₃)₂O species as the parent species, respectively, were compared with one another. The proposed r_s structure has been established from all the species measured and was compared with the r_s-like structure obtained by a diagnostic least-squares method and with the reported structure. The r_s structure of the present molecule was compared with the reported structures of dimethyl sulfide and dimethyl silane in relation to the tilt phenomenon. The r_s structure of dimethyl sulfide was revised based on the present comparison.     

Dynamical structure of peptide molecules: Fourier transform microwave spectroscopy of N-methylpropionamide

In order to clarify the dynamical aspects of the peptide structure, N-methylpropionamide (NMPA) was investigated as an example of peptide molecules: XCONHY (X=CH₃CH₂ and Y=CH₃ for NMPA), paying special attention to the internal rotation of the two methyl groups. NMPA was found to have an almost planar skeleton with an extended syn/trans conformation, as indicated by the observed value of I_aa+I_bb−I_cc, and its rotational spectra were interpreted in terms of group G₁₈ consisting of six symmetry species: A₁, A₂, E₁, E₂, E₃, and E₄. The A₁ and E₂ spectra were observed split in most of b-type transitions, yielding the internal-rotation potential barrier V₃ of 796 (21) cm⁻¹ for CH₃ in the ethyl group referred to as C-CH₃. The spectra of the three E species: E₁, E₃, and E₄ appeared several tens to thousands MHz apart from the corresponding A₁ spectra, suggesting the internal-rotation potential barrier of CH₃ bonded to the nitrogen, called N-CH₃, to be quite low. In sharp contrast with the A₁ spectra, which were well fitted to the ordinary asymmetric-rotor spectral pattern, a few higher-order terms were required to reproduce the E₁ spectra, presumably because of the low N-CH₃ barrier. The spectral analysis thus performed, in fact, led to the V₃ of 80.06487 (14) cm⁻¹, an order of magnitude lower than that of C-CH₃. The E₃ and E₄ spectra were found to form triplets with the corresponding E₁ lines at the center, and the E₃-E₁ and E₄-E₁ splittings were explained essentially by the contributions of the C-CH₃ internal rotation combined with the kinetic-energy coupling between the two methyl groups. The torsion around the C-C bond between the ethyl and carbonyl groups was suggested by an ab initio calculation to be of double minimum nature, but the observed A₁ spectra did not show any indication of such a double-minimum potential for the C-C torsion, although the possibility of a small hump being present at a planar conformation could not be entirely eliminated. The present results on NMPA along with those obtained on other peptide molecules will be of some significance in clarifying important problems of structural biology such as protein folding and signal transfer through biological systems.     

Electronic spectrum of acetaldehyde: Vibrational analysis of the 182-nm system

The electronic absorption spectra of CH₃CHO, CH₃CDO, CD₃CHO, and CD₃CDO have been obtained in the vacuum ultraviolet region. Vibrational analysis of the 182-nm system was made with the help of wavenumbers from their infrared spectra and also geometries and wavenumbers obtained from ab initio calculations. This system is assigned as a single Rydberg (3s ? n) electronic transition.     

Microwave spectra of partially deuterated acetic acid: Internal rotation,potential energy function,and methyl group geometry

The microwave spectra of CH₂DCOOH and CHD₂COOH have been studied by means of microwave-microwave double resonance. For the asy rotamers torsional splittings (5898 and 530 MHz, respectively) and effective rotational constants were determined in the ground state. Effective barrier parameters were provisionally estimated and used to predict excited-state spectra. Here significant interaction between sy and asy rotamers occurred, and a Hamiltonian based on an extension of the IAM method to the case of an asymmetric internal rotor was used to account for the spectra. A few direct sy-asy transitions were observed as well as spectra originating from the second excited torsional state. Effective potential energy coefficients, V₁ through V₆, were determined accurately; apart from V₃ and V₆, which are comparable to values in CH₃COOH and CD₃COOH, large V₂ terms occur (28.5 cm^?1 in CH₂DCOOH and ?25.4 cm^?1 in CHD₂COOH). These terms provide localization in the ground state wave functions, and can be rationalized as arising from the zero-point energies of the other normal vibrations. Also determined were Fourier components of the rotational constants, which were in fair agreement with results from model calculations when geometry relaxation was included. After correction of the ground state inertial moments for effects of the torsion a consistent set of inertial moments was obtained for the various isotopic species, and a complete substitution structure could be determined. The HCH angles in the methyl group were found to differ by 2.7°.     

The oscillator strengths for C1s and O1s excitation of some saturated and unsaturated organic alcohols,acids and esters

Electron energy loss Spectroscopy (ISEELS) under dipole scattering conditions is used to obtain the carbon and oxygen K-shell oscillator strength spectra of methanol (CH₃OH), propanol (CH₃CH₂CH₂OH), propenol (CH₂=CHCH₂OH), propargyl alcohol (HC≡CCH₂OH), propanoic acid (CH₃CH₂COOH), acrylic acid (CH₂=CHCOOH) and propiolic acid (HC≡CCOOH). A detailed interpretation of these spectra is presented, along with a comparison with the NEXAFS spectra of multilayers of these molecules adsorbed on a Si(111) surface, as recently reported by Outka et al. (Surf. Sci., 185 (1987) 53). Good agreement is found between the multilayer NEXAFS and the gas phase ISEEL spectra, except for the carboxylic acids which differ dramatically in the discrete portion of the O1s spectrum. Possible origins for this difference are discussed. The C1s and O1s spectra of methyl formate (HCOOCH₃) are also reported and interpreted in comparison with the spectra of formic acid and methanol.     

Comparison of the sensitivities of IR absorption and raman scattering spectra to hydrogen bonding in methanol

The sensitivity of infrared (IR) absorption and Raman scattering (RS) spectra to hydrogen bonding in methanol is quantified by a comparative sensitivity factor, which can be expressed as the K = (I _b /I _ub )_IR/(I _b /I _ub )_RS, where the indices b and ub refer to H-bonded and unbound (free) OH groups, respectively. The resulting value for methanol, K ～ 20, is identical to that found previously for n-butanol, although the methods of measuring the degree of hydrogen bonding for these two alcohols were quite different. It is shown that, in some cases, Raman spectra can be converted to IR absorption spectra and vice versa. It is important for understanding differences in the shapes of OH stretching vibration bands in IR absorption and Raman spectra.     

Calculations of the energy spectra of Zn,Ga, and Ge isotopes by the shell model

The effective Hamiltonian which was determined empirically by Koops and Glaudemans is tested in shell model calculations for the^65–68Zn,^67–69Ga, and^68–70Ge nuclei in the full (1p _3/2,0f _5/2,1p _1/2) ⁿ space. The resulting energy spectra are compared with the experimental spectra and results of previous calculations. The overall agreement with experiment is as satisfactory for these nuclei as for the Ni and Cu isotopes, by which the Hamiltonian was determined. It is noticed that the spectra of⁶⁷Zn and^67,69Ga calculated in this work are similar to those provided by the Alaga model.     

Microwave spectra of several molecular isotopes of toluene

The microwave rotational spectra of five further isotopic species of toluene have been measured. Spectra characteristic for the presence of a very small sixfold barrier potential V₆ hindering the internal methyl rotation were observed. It is demonstrated that the sets of rotational constants obtained from the ground state spectra, A′ (frame alone), B, C, can be used for the determination of a (partial) substitution structure also in the case of off-axis substituted isotopes. Attempts to obtain a more complete r₀-structure were less successful. Values of V₆ were derived for all isotopes from spectra in excited states of internal rotation. A 15 percent reduction of V₆ upon methyl deuteration was observed, just as in earlier, similar cases.     

Optical investigation of the vertical diffusion of manganese in planar structures based on CdTe and Cd1 − x Mg x Te with ultrathin MnTe layers

The emission spectra of planar structures based on CdTe and Cd_{1 ? x} Mg _x Te containing periodically built-in MnTe layers with a nominal thickness of one monolayer have been investigated. The luminescence spectra and luminescence excitation spectra of manganese ions and excitons, as well as the dependences of the spectra on the temperature and magnetic field strength, are used to determine the actual distribution of manganese ions. The full width at half-maximum of the profile describing the change in the concentration of manganese in the growth direction of the structures is estimated to be 7–8 monolayers.     

The near-ultraviolet spectrum of diacetylene trapped in solid argon at 9 K

The absorption spectra of normal and deuterated diacetylene trapped in solid argon at 9 K were investigated in the near-ultraviolet region between 2000 and 3000 Å. The vibrational structure observed at low temperature for the band system at 2448 Å, which was previously reported by Haink and Jungen in the gas phase, and identified as a ¹Δ_u ← X¹Σ_g⁺ transition, was analyzed. A comparison between the spectra for C₄H₂ and C₄D₂ suggests some revisions in the upper-state vibrational assignments. The possibility that the upper state assumes a C_2htrans configuration of ¹A_u symmetry is examined. The matrix spectra also appear to indicate that the absorption spectrum observed at 2576 Å in the gas phase and which has been assigned to a ¹Σ_u^? ← X¹Σ_g⁺ transition may not originate in the ground state of C₄H₂.     

Plasmaresonanzemission und Plasmaresonanzabsorption an dünnen Kaliumschichten

Optical transmission and emission spectra in the region of the plasma frequencyω _p were measured on thin potassium films evaporated on sapphire substrates of various temperatures. For the foils condensed at a temperatures of the substrateT _a T _a >?165 °C transmission spectra were observed which transmission minima are displaced to frequencies less thanω _p . Plasma resonance emission was observed from these foils. Comparison with calculated emission spectra gives an approximate agreement.     

F-center induced Raman scattering in RbCl

Experimental Raman spectra of RbCl containing F-centers are reported. Lines of A_1u, E_u and F_2u symmetry were observed. Calculations of the first-order Raman spectra are presented; they have been based on the assumption that the electron-phonon interaction is linear with displacements of F-center nearest neighbours. A shell-model fitted to neutron data was used to evaluate the Green-function matrix elements and the projected densities of states. The best agreement between theory and experiment was found to occur when using a zero-force-constant change for the F_2u spectrum and a 0·35 fractional decrease in the longitudinal force-constant A₁₂ for the A_3u and E_u spectra.     

Energy fluctuations in optical transitions in CuHal crystals

This paper reports on a comprehensive study of fundamental-absorption spectra of CuCl and CuBr nanocrystals measured in the temperature range extending from 80 K up to the melting point. The temperature dependences of the energy E, oscillator strength f, and temperature-broadening coefficient σ of optical-transition spectral to excitonic states were determined. A new approach to the calculation of the shape and temperature-broadening of absorption spectra, based on quantum statistics, was employed. The crystal is treated as a quantum statistical ensemble. The set of atoms involved in the electronic transition initiated by a photon absorption is taken as an element of the ensemble. Energy fluctuations of electronic states occurring in the elements of the ensemble give rise to temperature broadening of the optical-transition spectra. The temperature dependence of the χ coefficient of thermodynamic fluctuations of the Z _{1, 2} and Z ₃ exciton states was found. A theoretical substantiation of the Urbach rule was obtained in terms of the model proposed.     

Lowering of magnetic field intensity at the surfaces of α-Fe2O3 and FeBO3 single crystals

Depth-selective conversion electron Mössbauer spectroscopy was used to study magnetic properties of the thin surface layers of the α-Fe₂O₃ and FeBO₃ single crystals. An analysis of the experimental spectra indicates that the magnetic properties of the layers at a depth of more than ～100 nm from the surface are similar to the properties of crystal bulk, and the corresponding spectra consist of narrow lines. The lines gradually broaden as the crystal surface is approached. The spectra of the ～10-nm-thick surface layers consist of broad lines, indicating a wide distribution δ=2.1 T of the effective magnetic field about its mean value of 32.2(4) T. The experimental spectra were used to determine the effective magnetic fields (H _eff) for the iron ions situated in the surface layers of thickness ～100 nm. The effective fields in these layers were found to gradually decrease at room temperature (291 K) as the crystal surface was approached. The H _eff values in the 2.4(9)-nm-thick surface layer of the α-Fe₂O₃ crystal and 4.9(9)-nm layer of FeBO₃ are 0.7(2) and 1.2(3)%, respectively, smaller than for the nuclei of the ions in the bulk of these crystals.     

Electronic photodissociation spectra of the Arn-C4H2 (n=1-4) weakly bound cationic complexes

Electronic spectra of a series of weakly bound clusters consisting of argon (Ar_n, n=1-4) bound to the butadiyne cation, C₄H₂⁺, have been recorded in the visible range from 440 to 520 nm by photodissociation. The C₄H₂⁺ fragment signal was recorded as a function of the laser wavelength during excitation of the A←X electronic transition. The observed transitions were assigned to the band origin of the cationic complexes and to vibronic bands involving excitation of the ν₃ and ν₇ vibrational modes of the C₄H₂⁺ moiety, as well as combination bands of these modes. Comparison of the photodissociation spectra of the various clusters reveals a small blue shift, 25 cm⁻¹ of the band maxima relative to the corresponding transitions reported from gas phase spectra of the bare C₄H₂⁺ cation. The magnitude of the blue shift of each band increases with successive Ar solvation up to n=3. Furthermore, each band becomes increasingly broadened towards the red with the addition of Ar atoms due to an increasing number of unresolved transitions involving excited intermolecular modes.     

Broadening of the lines due to the 4f n -4f n − 15d transitions of Ce3+, Pr3+, and Tb3+ ions in the absorption spectra of CdF2 crystals

The structure of the absorption spectra of Ce³⁺, Pr³⁺, and Tb³⁺ ions in the vicinity of 4f-5d transitions has been investigated. At low temperatures the absorption spectra exhibit a weakly pronounced fine structure, in contrast to narrow-line spectra in crystals of Ca, Sr, and Ba fluorides. The spectra of Ce³⁺, Pr³⁺, and Tb³⁺ ions in CdF₂ can be considered as the absorption spectra of these ions in alkali-earth fluorides, broadened by 60–75 cm^?1. The broadening is related to the autoionization of electron from the local 5d(e _g ) level to the energy-degenerate states of the conduction band of CdF₂ crystal.     

Raman spectroscopy of SrB<Subscript>4</Subscript>O<Subscript>7</Subscript> single crystals in the temperature range 300–1273 K

The polarized Raman spectra of SrB₄O₇ (SBO) single crystals are studied in detail in the temperature range of 300–1273 K. The TO, LO, and IO phonon lines of A₁, A₂, B₁, and B₂ symmetries of rhombic SBO at 300 K are identified. The behavior of the Raman spectra of SBO crystals is studied upon heating up to their melting. The relation of Raman spectra with the structure of boron–oxygen fragments, as well as the transformation of spectra in the process of melting of SBO crystals, is discussed.