Well-resolved Vibrational Structure Research Articles

Bright yellow and green Eu(ii) luminescence and vibronic fine structures in LiSrH3, LiBaH3 and their corresponding deuterides

The luminescence of Eu(2+) in hydride and deuteride perovskite hosts LiMH3 and LiMD3 (M = Sr, Ba) is reported. Bright yellow (M = Sr) and green (M = Ba) emission is observed and assigned to 4f(6)5d-4f(7) emission from Eu(2+) in the highly symmetric 12-coordinated M(2+) site (m3[combining macron]m). The long wavelength of the emission is explained by the strong covalence and crystal field splitting in europium's coordination by hydride anions. A well-resolved vibrational structure in the emission and excitation spectra of Eu(2+) in the Sr-compounds allows for an accurate determination of the energy of the lowest 4f(6)5d state and vibrational frequencies, for both the hydride and deuteride. The isotope effect on the energy of the fd states is small (∼70 cm(-1)), as expected. Surprisingly, also the vibrational energies observed in the vibronic progression are similar for the d-f emission spectra in LiSrH3 and LiSrD3. This is explained by strong coupling of the d-f emission with low energy acoustic phonons which, contrary to optical phonons, are not strongly affected by replacing H by D. The present results provide insight into the long wavelength Eu(2+) emission in hydride coordination and the influence of isotope replacement on the luminescence.

Electronic structures of WAlOy and WAlOy− (y = 2–4) determined by anion photoelectron spectroscopy and density functional theory calculations

The anion photoelectron spectra of WAlO(y)(-) (y = 2-4) are presented and assigned based on results of density functional theory calculations. The WAlO(2)(-) and WAlO(3)(-) spectra are both broad, with partially resolved vibrational structure. In contrast, the WAlO(4)(-) spectrum features well-resolved vibrational structure with contributions from three modes. There is reasonable agreement between experiment and theory for all oxides, and calculations are in particular validated by the near perfect agreement between the WAlO(4)(-) photoelectron spectrum and a Franck-Condon simulation based on computationally determined spectroscopic parameters. The structures determined from this study suggest strong preferential W-O bond formation, and ionic bonding between Al(+) and WO(y)(-2) for all anions. Neutral species are similarly ionic, with WAlO(2) and WAlO(3) having electronic structure that suggests Al(+) ionically bound to WO(y)(-) and WAlO(4) being described as Al(+2) ionically bound to WO(4)(-2). The doubly-occupied 3sp hybrid orbital localized on the Al center is energetically situated between the bonding O-local molecular orbitals and the anti- or non-bonding W-local molecular orbitals. The structures determined in this study are very similar to structures recently determined for the analogous MoAlO(y)(-)/MoAlO(y) cluster series, with subtle differences found in the electronic structures [S. E. Waller, J. E. Mann, E. Hossain, M. Troyer, and C. C. Jarrold, J. Chem. Phys. 137, 024302 (2012)].

Ultraviolet absorption and luminescence of matrix-isolated adenine

We have investigated the absorption, the fluorescence and phosphorescence emission and the fluorescence lifetimes of adenine in low-temperature argon and nitrogen matrices at 15 K. Compared to other environments the absorption spectrum shows higher intensity at the shortest wavelengths, and a weak apparent absorption peak is observed at 280 nm. The resolved fluorescence excitation spectrum has five peaks at positions corresponding to those observed in the absorption spectrum. The position of the fluorescence maximum depends on the excitation wavelength. Excitation below 220 nm displays a fluorescence maximum at 305 nm, while for excitations at higher wavelengths the maximum occurs at 335 nm. The results suggest that multiple-emission excited electronic states are populated in low-temperature gas matrices. Excitation at 265 nm produces a phosphorescence spectrum with a well-resolved vibrational structure and a maximum at 415 nm. The fluorescence decays corresponding to excitation at increasing energy of each resolved band could be fit with a double exponential, with the shorter and longer lifetimes ranging from 1.7 to 3.3 ns and from 12 to 23 ns, respectively. Only for the excitation at 180 nm one exponential is required, with the calculated lifetimes of 3.3 ns. The presented results provide an experimental evidence of the existence of multiple site-selected excited electronic states, and may help elucidate the possible deexcitation pathways of adenine. The additional application of synchrotron radiation proved to result in a significant enhancement of the resolution and spectral range of the phenomena under investigation.

Poly(thiophenes) derivatized with oligo(oxyethylene) chains as donor materials for organic solar cells

Regioregular poly[3-(3,6-dioxaheptyl)thiophene] (PDHT) has been synthesized by Grignard metathesis (GRIM). The UV–vis absorption spectrum of spin-cast films of PDHT presents a well-resolved vibrational fine structure typical of well-ordered polymers. The characterization of bulk heterojunction solar cells fabricated using PCBM as acceptor shows that PDHT leads to power conversion efficiencies in ca 4–5 times lower than that obtained with regioregular poly(3-hexylthiophene) (P3HT). In contrast, the characterization of bilayer solar cells realized by thermal evaporation of C 60 on spin-cast polymer films shows that PDHT leads to performances similar or even better than P3HT both before and after thermal treatment.

Resonance-enhanced multiphoton ionization spectra of jet-cooled methanol and ethanol

The first spectroscopic study of the 3p ← n O multiphoton excitation of jet-cooled methanol and ethanol is presented. The mass-resolved resonance-enhanced multiphoton ionization (REMPI) spectra of CH 3OH and CD 3OD are compared to the VUV absorption spectra. The 2 + 1 REMPI spectra of CH 3CH 2OH and CH 3CH 2OD have a well-resolved vibrational structure and are attribute to trans ethanol. The production of the methyl radical (from methanol and ethanol) is revealed by its 3 p z 2 A 2 ″ ← X ∼ 2 A 2 ″ 2 + 1 REMPI spectrum.

Chemical and electronic characterization of methyl-terminated Si(111) surfaces by high-resolution synchrotron photoelectron spectroscopy

The chemical state, electronic properties, and geometric structure of methyl-terminated Si(111) surfaces prepared using a two-step chlorination/alkylation process were investigated using high-resolution synchrotron photoelectron spectroscopy and low-energy electron diffraction methods. The electron diffraction data indicated that the methylated Si surfaces maintained a (1×1) structure, where the dangling bonds of the silicon surface atoms were terminated by methyl groups. The surfaces were stable to annealing at 720 K. The high degree of ordering was reflected in a well-resolved vibrational fine structure of the carbon 1s photoelectron emission, with the fine structure arising from the excitation of C-H stretching vibrations having hnu=0.38±0.01 eV. The carbon-bonded surface Si atoms exhibited a well-defined x-ray photoelectron signal having a core level shift of 0.30±0.01 eV relative to bulk Si. Electronically, the Si surface was close to the flat-band condition. The methyl termination produced a surface dipole of –0.4 eV. Surface states related to piCH3 and sigmaSi-C bonding orbitals were identified at binding energies of 7.7 and 5.4 eV, respectively. Nearly ideal passivation of Si(111) surfaces can thus be achieved by methyl termination using the two-step chlorination/alkylation process.

Fine-structure vibronic spectra and NH-phototautomerism in free-base unsubstituted 2,3-naphthalocyanine in naphthalene at 6 K

Abstract Spectral-luminescent investigations of free-base unsubstituted 2,3-naphthalocyanine (2,3-NcH2) in naphthalene and tetrakis(tert-butyl)-derivative of 2,3-NcH2 in various solvents at room temperature, 77 and 6 K have been carried out. An essential dependence of the S2–S1 energy interval of the compounds investigated, Δ E S 2 S 1 , on the nature of the matrix environment has been found (the values of Δ E S 2 S 1 lie within the limits 100 and 300 cm−1). At 6 K the fluorescence spectrum of 2,3-NcH2 has a well-resolved vibrational structure under broadband excitation, resulting practically from the emission arising from two main sites and therefore consisting of “doublets” of narrow lines. The qualitative analysis of the vibrational structure of the fine-structure fluorescence spectra of 2,3-NcH2 enabled assessment of the form of active modes. For the two main sites efficient NH photoisomerization of the 2,3-NcH2 molecules under selective laser excitation has been established.

Structure of excitation and fluorescence spectra recorded at the10u+(51P1)–X10g+transition ofCd2

Excitation and fluorescence ultraviolet spectra of ${\mathrm{Cd}}_{2}$ recorded at the ${}^{1}{0}_{u}^{+}(5{}^{1}{P}_{1})--X{}^{1}{0}_{g}^{+}$ transition are reported. The ${\mathrm{Cd}}_{2}$ molecules (seeded in Ar) produced in a continuous free-jet supersonic beam were excited in a vacuum chamber with a pulsed dye-laser beam. A well-resolved vibrational structure of the ${}^{1}{0}_{u}^{+}\ensuremath{\leftarrow}X{}^{1}{0}_{g}^{+}$ excitation spectrum as well as the isotopic structure of the vibrational components were recorded. Analysis of the spectrum yielded vibrational constants for the ${}^{1}{0}_{u}^{+}$ state: ${\ensuremath{\omega}}_{e}^{\ensuremath{'}}=100.50\ifmmode\pm\else\textpm\fi{}0.25{\mathrm{cm}}^{\ensuremath{-}1},$ ${\ensuremath{\omega}}_{e}^{\ensuremath{'}}{x}_{e}^{\ensuremath{'}}=0.325\ifmmode\pm\else\textpm\fi{}0.003{\mathrm{cm}}^{\ensuremath{-}1},$ ${D}_{0}^{\ensuremath{'}}=8638\ifmmode\pm\else\textpm\fi{}15{\mathrm{cm}}^{\ensuremath{-}1},$ ${D}_{e}^{\ensuremath{'}}=8688\ifmmode\pm\else\textpm\fi{}15{\mathrm{cm}}^{\ensuremath{-}1},$ and $\ensuremath{\Delta}{R}_{e}{=R}_{e}^{\ensuremath{''}}\ensuremath{-}{R}_{e}^{\ensuremath{'}}=1.04\ifmmode\pm\else\textpm\fi{}0.01\AA{}$ derived for the ${}^{226}{\mathrm{Cd}}_{2}$ isotopomer. The ${}^{1}{0}_{u}^{+}$ state potential-energy (PE) curve was obtained numerically using an inverse perturbation approach (IPA) procedure. Condon internal diffraction (CID) patterns in the ${}^{1}{0}_{u}^{+}\ensuremath{\rightarrow}X{}^{1}{0}_{g}^{+}$ fluorescence band, emitted upon the selective excitation of the ${v}^{\ensuremath{'}}=38$ and ${v}^{\ensuremath{'}}=39$ vibrational components of the ${}^{226}{\mathrm{Cd}}_{2}$ isotopomer, were observed and improved the ${v}^{\ensuremath{'}}$ assignments derived from the analysis of the isotopic structure. Analysis of the fluorescence spectrum yielded information on the repulsive part of the ground-state interatomic potential. The result confirms a relatively soft repulsion between two Cd atoms in the short-range (2.53--4.05 \AA{}) region and makes allowance for a covalent admixture to the ground-state van der Waals bonding. Quasirelativistic valence ab initio calculations on the PE curves for the investigated states have been performed at the complete-active-space multiconfiguration self-consistent-field (CASSCF/CAS) multireference second-order perturbation theory (CASPT2) level with the total of 40 correlated electrons. In the calculations, the Cd atom is considered as a 20-valence electron system whereas the ${\mathrm{Cd}}^{20+}$ core is replaced by an energy-consistent pseudopotential which also accounts for scalar-relativistic effects and spin-orbit interaction within the valence shell. A comparison with results obtained from other experiments and ab initio calculations is presented.

Long-lived room temperature phosphorescence of a naphthalene-β-cyclodextrin-adamantane complex in the presence of oxygen

Naphthalene-d8—β-cyclodextrin—adamantane triple complexes were prepared in an aqueous solution at room temperature. Irradiation (λ = 285 nm) of the solution in the presence of molecular oxygen results in the long-lived (τ = 10.3 s) room temperature phosphorescence (RTP). The removal of oxygen from the solution increases the RTP intensity and phosphorescence lifetime by 1.5 times. The RTP spectrum contains a well-resolved vibrational structure, whose bands are assigned to full symmetric vibrations of naphthalene, their overtones, and the combination tones of full symmetric vibrations. The quantum-chemical calculation of the triple complex structure confirms that both naphthalene and adamantane can simultaneously be included into the β-cyclodextrin cavity and suggests that the role of the latter as the third component is the more efficient shielding of naphthalene from the oxygen effect due to both the formation of three-component complexes and their aggregation to form submicronic particles.

An anionic organic mixed-valence system with a remarkably well-resolved vibrational structure in its intervalence band.

The reduction of a phenylene-bridged bis(dioxaborine) affords a strongly delocalised organic mixed-valence system; for the first time details of the vibrations coupled to the electron transfer have been extracted from the intervalence band.

Absorption and luminescence spectroscopy of the MnO4 centers in alkali halide crystals

The low-temperature (8 K) optical absorption, near-infrared photoluminescence (NIRPL) and photoluminescence excitation spectra (PLE) of MnO4− and MnO42− in the KI and RbI crystals are reported. The most intense low-energy absorption bands in visible region are assigned to ligand-to-metal charge transfer (LMCT) bands due to 1A1→1T2 (1t1–2e) (MnO4−) and 2E→2T2 (1t1–2e) (MnO42−) intramolecular transitions, respectively. In both crystals, LMCT bands exhibit a fairly well-resolved vibrational structure due to the Mn–O stretching mode (∼750 cm−1). The broad NIRPL bands, observed for both systems in the similar (850–1200 nm) spectral region, display pronounced vibrational structure caused by electron coupling mostly with the bending O–Mn–O mode (∼340 cm−1). The model is proposed that explains the NIRPL spectra in MnO4−- and MnO42−-doped crystals as activated by two types of the MnO42− centers with Td and Cs symmetries. The NIRPL bands are attributed to d–d (2T2→2E) ligand–field transition of the single 3d-electron of manganese in MnO42−.

Butterfly vibration of the tetrafluorobenzene cation studied by pulsed field ionization-zero kinetic energy photoelectron spectroscopy

Abstract Pulsed field ionization-zero kinetic energy photoelectron spectroscopy has been applied to 1,2,4,5-tetrafluorobenzene in a supersonic jet. The spectrum measured by selecting a specific vibronic level of the butterfly vibrational mode in S 1 by the first laser shows a well-resolved vibrational structure of the cation. A long progression has been assigned to the out-of-plane butterfly vibrational mode 11 with an even quantum number in the cation. From the harmonisity and Franck–Condon factor, it has been concluded that the molecular structure of the tetrafluorobenzene cation is flat, though that in S 1 is non-planer along the butterfly vibrational mode. The geometrical change upon ionization is discussed in terms of the electronic structure.

Negative-ion photoelectron spectroscopy of CH3S−

A 364-nm negative-ion photoelectron spectrum of CH 3 S − has been recorded. The electron affinity is 1.867±0.004 eV. Well-resolved vibrational structure has been observed such that the C–S symmetric stretch and the C–H symmetric stretch have been measured at 725±15 cm −1 and 1260±30 cm −1 , respectively. The spin–orbit splitting is reported at 265±15 cm −1 . The large spin–orbit splitting quenches the Jahn–Teller modes seen in the analogous radicals, CH 3 O and CF 3 S.

Initial state resolved electronic spectroscopy of HNCO: Stimulated Raman preparation of initial states and laser induced fluorescence detection of photofragments

Stimulated Raman excitation (SRE) efficiently prepares excited vibrational levels in the ground electronic state of isocyanic acid, HNCO. Photofragment yield spectroscopy measures the electronic absorption spectrum out of initially selected states by monitoring laser induced fluorescence (LIF) of either NCO (X 2Π) or NH (a 1Δ) photofragments. Near threshold, the N–H bond fission is predissociative, and there is well-resolved rotational and vibrational structure in the NCO yield spectra that allows assignment of Ka rotational quantum numbers to previously unidentified vibrational and rotational levels in the ν1 N–H stretch and ν3 N–C–O symmetric stretch fundamentals in the ground electronic state of HNCO. The widths of NCO yield resonances depend on the initial vibrational state, illustrating one way in which initial vibrational state selection influences dissociation dynamics. Initial excitation of unperturbed ν1 (N–H stretch) states leads to diffuse NCO yield spectra compared to excitation of mixed vibrational levels. The higher energy dissociation channel that produces NH (a 1Δ) has coarser structure near its threshold, consistent with a more rapid dissociation, but the resonance widths still depend on the initially selected vibrational state.

TheG0u+(61P1)–X0G+Excitation and Fluorescence Spectra of Hg2Excited in a Supersonic Jet

TheG0u+(61P1)–X0G+excitation and fluorescence spectra of Hg2van der Waals molecules, produced in a pulsed free-jet supersonic expansion beam crossed with a pulsed dye-laser beam, were studied using Ar as the carrier gas. A well-resolved vibrational structure of theG0u+←X0g+excitation spectrum was recorded, as well as the isotopic structures of the vibrational components. A Condon internal diffraction pattern in theG0u+→X0g+fluorescence band, emitted upon the selective excitation of the v′ = 39 vibrational component, was also observed and confirmed the assignments derived from the analysis of the isotopic structure. Analyses of the spectra yielded molecular constants for both states, as well as information on the ground state potential energy curve. The results are compared with those obtained from other experiments and from theoretical calculations.

Electron transfer in the reactions of titanocene-bis(trimethylsilyl) acetylene complexes with 2,2'-bipyridine and 4,5-diazafluorene. The crystal structure of (4,5-diazafluorenyl) bis(pentamethylcyclopentadienyl) titanium(III)

The titanocene-bis(trimethylsilyl)acetylene complexes Cp' 2Ti[η 2-C 2(SiMe 3) 2] (Cp' = C 5H 5, C 5HMe 4, and C 5Me 5) react with 2,2'-bi-pyridine (bpy) to give the corresponding Cp' 2Ti(bpy) complexes 1–3. EPR evidence of the electronic triplet state in 1–3 implies the transfer of one of the Ti d 2 electrons to the bpy ligand. An analogous electron transfer to 4,5-diazafluorene (dafH) affords (4,5-diazafluorenyl)titanocene (Ti III) complexes Cp' 2Ti(daf) (Cp' = C 5H 5 ( 4), C 5HMe 4 ( 5), and C 5Me 5 ( 6)) with a release of one hydrogen atom from the sp 3 carbon atom of the ligand. EPR spectra of 4–6 in toluene solutions are characterized by g = 1.9798–1.9820, coupling to 47Ti and 49Ti isotopes a Ti = 11.9–12.6 G, and for 4 and 6 also by the coupling to two equivalent nitrogen atoms a n = 2.0 G. Rhombic g-tensors with g av in the range 1.981–1.982 were obtained in frozen toluene glass. The X-ray crystal structure of 6 (monoclinic, P2 1/ c (No. 14), a = 11.7309(9) Å, b = 14.7075(10) Å, c = 14.7893(11) Å, β = 94.68(6)°, V = 2543.1(4) Å 3, Z = 4) proved that both nitrogen atoms of an approximately planar 4,5-diazafluorenyl ligand coordinate to a pseudotetrahedrally coordinated titanium atom. The electronic absorption spectra of 4–6 in solution showed a well-resolved vibrational structure of the (daf) − anion with positions of all vibronic bands shifted to longer wavelength by ca. 20–30 nm, and a low intensity band probably of the d-d transition in the range 820–940 nm.

Optical and Structural Properties of Electrodeposited Polybithiophene Films During the Film Growth as a Function of the Redox Potential

Abstract The optical properties of p-doped polybithiophene films were investigated during the polymerization and discharging process using in situ spectroscopy, while the fluorescence and absorption spectra of the undoped films were ex situ monitored as functions of the film thickness and temperature. The in situ absorption spectra of polybithiophene were observed to depend significantly on the film thickness showing that both the mean effective conjugation length of the polymer chains and the morphology of the films changes during the polymerization. The absorption maximum at 692 nm is assigned to bicationic segments. The similarly structureless absorption spectrum of undoped polybithiophene exhibits a maximum at 480 nm which corresponds to a conjugation length of 7 thiophene units, whereas the red-shifted fluorescence spectrum shows a well-resolved vibrational structure and is attributed to the longest-conjugation length segments (ca. 11 thiophene units) in the film.

STUDY OF SMALL SEMICONDUCTOR CLUSTERS USING ANION PHOTOELECTRON SPECTROSCOPY: GERMANIUM CLUSTERS (Gen, n=2−15)

The technique of anion photoelectron spectroscopy has been used at 266, 355, and 416 nm to study small germanium clusters, [Formula: see text], n=2–15. The electron affinities for these cluster anions have been determined from the photoelectron spectra. The spectra for [Formula: see text] and [Formula: see text] show well-resolved vibrational structure.

Vapor Phase Electronic Absorption and MCD Spectra for Dimethyl Sulfide, Dimethyl Selenide, and Dimethyl Telluride in the UV Region

Electronic absorption and magnetic circular dichroism spectra for (CH3)2X, X = S, Se, and Te, in the vapor phase were measured in the range 2.5−5.3 μm-1 with a spectral bandwidth of ≤0.003 μm-1. The spectra show well-resolved vibrational structure for the nX → (n + 1)p Rydberg system with positive MCD B terms for (CH3)2Se and (CH3)2Te, but a negative B term for (CH3)2S. The difference in the MCD spectra is interpreted in terms of the influence of magnetic coupling with the broad nX → σ* valence transition, which lies rather close in energy to the Rydberg system for X = S, but is separated to lower energy for X = Se or Te. The interpretation requires a nX → σ* transition assignment of 1A1 → 1B1 and provides support for the most recent 1A1 symmetry assignment of the nX → (n + 1)p Rydberg excited state. The assignment of several weak, previously unreported, bands for X = Se and Te is also given.

First ion-pair charge-transfer complex showing a specific CT absorption spectrum with well-resolved vibrational structure in solution

An ion-pair charge-transfer complex showing a charge-transfer absorption spectrum with a well-resolved vibrational structure of about 1100–1400 cm −1 was first observed for the diquat (2,2′-bipyridinium) salt with tetrakis[3,5-bis(trifluoromethyl)phenyl]borate in 1,2-dimethoxyethane solution at room temperature. It was ascribed to the highly ordered ternary complex of this ion pair in the less polar 1,2-dimethoxyethane solution.