Vibronic Progression Research Articles

Vibronic coupling to nearly localized modes in diamond

Abstract The presence of sharp low energy vibronic progressions in several luminescing bands of brown diamonds is interpreted as due to a nearly localized vibrational mode, showing a weak coupling to resonant lattice phonons. The effect of this mode on the vibronic coupling is discussed and compared with experimental data.

Intraconfigurational absorption spectroscopy of IrCl 2−6 and IrBr 2−6 in A 2MX 6-type host crystals

Near-IR absorption spectra for Ir 4+ in A 2MX 6 (A  K, Rb, NH 4, M  Sn, Pt, Re, X  Cl, Br)-type hosts are discussed and compared to earlier work. Our data indicate that the effect of reduced host symmetry is quite small in this d 5 system and that the IrCl 2− 6 spectra can be interpreted in a manner paralleling previous studies on Ir 4+ in cubic hosts. In some hosts, e.g., K 2PtCl 6, the IrBr 2− 6 appears to have a vibronic pattern outside the experience with previous MX 2− 6 results. Origin shifts and the nature of a vibronic progression were seen to correlate with host lattice counterion. By comparison to luminescence results of Flint and Paulusz, the latter data are proposed to be indicative of a strongly Jahn-Teller active e g mode.

Ultraviolet absorption spectra of all- trans-octatetraene and -decapentaene in solid argon

Ultraviolet absorption spectra for all- trans-octatetraene and -decapentaene in solid argon give well-resolved vibronic progressions in single and double carbon-carbon bond stretching modes. The vibronic bands in argon matrix spectra are sharper than bands in glassy matrix spectra.

Analysis of the visible-near-ultraviolet spectrum of 9-aminoacridine using dichroic spectra in stretched polymer films

We have achieved a deconvolution of the absorption spectrum of 9-aminoacridine dispersed in polyvinyl alcohol films into 11 separate harmonic vibronic progressions of which 5 are polarised along the long axis and 6 along the short axis of the dye. The correlation between the experimental electronic transitions in this range and the calculated (π *, π) transitions is very satisfactory.

The red luminescence spectrum of brown diamonds: uniaxial-stress measurements

Natural brown diamonds that emit yellow luminescence under 3.40 eV (365 nm) excitation are known to exhibit also a red luminescence band when excited with 2.69 eV (460 nm) radiation. Four zero-phonon red-luminescent lines have been studied under uniaxial stress. One of them, at 2.145 eV, believed to be the origin of the major vibronic progression known as the red band, is found to be a transition between A states at a monoclinic I centre. The other lines at 2.133, 2.156 and 2.166 eV are shown to be transitions between A and B states at monoclinic II centres, with the C2 point group. It is the first time that centres with this point group have been reported in diamond. The authors suggest that a relationship between the centres giving rise to the yellow and to the red luminescent bands, by comparison of their phonon sidebands, symmetries, responses to stress and zero-phonon energies.

Thermostable sites of palladium phthalocyanine in Shpol'skii matrices of mixed solvents

A study was made of the effects of three different mixed solvent Shpol'skii matrices at 4.2 K on site formation and on vibronic progressions of palladium phthalocyanine. It is found that only a limited number of thermostable sites are formed and these sites are common to all three matrices, but vary in their statistical weights. However, in all solvents the statistical weights of the first few most important sites are similar, with the predominant site at least eight or nine times stronger than the next. As well, the vibrational features of both the ground state and the first excited states of a few of the most populated sites examined are practically the same for the different mixed solvents. The absence of any significant differences in site splitting implies the absence of any dipolar interaction effects on these splittings.The above findings provide a better insight into the interaction between phthalocyanine, intermediate solvent, and basic solvent (n-octane) in the three-component solutions and give a clear guideline for choosing convenient solvents and for using Shpol'skii matrices as a simple effective mechanism for producing quasi-line vibronic spectra of phthalocyanines and other related large molecules insoluble in the alkanes.

The coexistence of zone and self-localized exciton states in vibronic spectra of nitrogen cryocrystals

The shape of bands and their evolution along the vibronic progression of the exciton resonances in the VUV spectra are correlated for nitrogen in its own lattice and in argon matrix. The fact that in solid nitrogen the zone states make a contribution to the formation of a 1 II g ← X 1 Σ + g and w 1 Δ u ← X 1 Δ + g progression vibrons is supported by the experimental results. The exciton-phonon interaction is treated in terms of the model of coexistence of zone and self-localized states. The role of optical and acoustic modes is discussed and the main parameters of exciton-phonon coupling are estimated.

The relationship of εL and εR from consideration of bisignate C.D. curves of α,β-unsaturated ketones

From reconsideration of experimental evidence of ∆ε and ε curves and extrapolation of comments by Beecham and Collins1 it is concluded that the occurrence of bisignate n- π* c.d. curves for ketones and α,β-unsaturated ketones does not require hypothesizing the existence of two spectral bands of opposite sign and therefore indicates that the relationship between εL and εR for a single transition does not necessarily involve a simple constant amplitude factor as the conventional interpretation holds. An alternative interpretation of bisignate curves is proposed, based on one spectral band, the relationship between εL and εR involving differential half-widths of the vibronic components of the L series relative to the R series and a variable relative-amplitude scale along the vibronic progression. If appropriate scale and differential half-width parameters are used, the characteristics of monosignate and bisignate curves can be demonstrated by means of synthetic curves.

Selective vibronic excitation of singlet oxygen–furan reactions in cryogenic matrices

The reactions of 2,5-dimethylfuran (DMF), 2-methylfuran (MF), and furan with molecular oxygen to form endoperoxides have been induced in Ar and O2 matrices at 12 K with selective vibronic excitation of O2 using near infrared light. Reaction was induced through excitation of the 1Δg(v′=0, v′=1) and 1Σg+(v′=0, v′=1) O2 states near 8000 and 13 100 cm−1, respectively, as well as by the (1Δg, 1Δg) simultaneous transitions of (O2)2 at 15 900 and 17 300 cm−1. The 1Δg ← 3Σg− vibronic progression in solid O2 was recorded by FTIR spectroscopy, whereas members of the 1Σg+ ← 3Σg− and (1Δg, 1Δg) ← (3Σg−, 3Σg−) progressions in Ar matrices were located by ‘‘reaction excitation’’ spectroscopy. The DMF+O2 reaction is a single photon process, apparently with unit quantum yield, for all vibronic levels excited. For MF+O2(1Δg, v′=0) and furan +O2(1Δg, v′=0), quantum yields were high, ∼0.6 and 0.4, respectively, but the reservoirs of reactive pairs were 10–20 times smaller than for DMF+O2. The furan+O2 reaction rate showed an 18O2 isotope effect 0.78±0.15, which can be interpreted in terms of quantum mechanical tunneling on the lowest singlet hypersurface. The IR matrix spectra of DMF endoperoxide and its peroxidic 18O counterpart are reported, and a partial vibrational assignment for this intermediate is given. The endoperoxide vibration that absorbs at 815.7 cm−1 is dominated by the bridge O–O stretching motion.

Nonadiabatic representations of the 1Σ+u and 1Πu states of the N2 molecule

The vibronic bands in the dipole-allowed absorption spectrum of N2 associated with the lowest three electronic 1Σ+u and the lowest three electronic 1Πu states are represented in a basis of electronically coupled diabatic states as well as in the basis of nuclear-momentum coupled adiabatic states. Parameters defining the diabatic states and their electronic coupling energies are first evaluated by fitting the eigenvalues of a vibronic interaction matrix to the observations. The coupled-oscillator equations are then solved directly by Johnson’s numerical integration method and the diabatic representation is redetermined via the matrix method and coupled equations iteratively. The fit of the experimental vibronic terms, B values, and absorption intensities achieved with R-independent electronic coupling energies in a diabatic basis of valence and Rydberg-type states (b′+c′+e′)1Σ+u and (b+c+o)1Πu is satisfactory. Comparison with the corresponding adiabatic representation shows that the nonadiabatic perturbations are larger in that basis than in the diabatic one. The vibronic intensity distributions observed in the absorption spectrum show numerous interesting examples of intensity envelopes over discrete vibronic progressions with Fano-type resonance profiles as well as with distinctly non-Fano-type profiles which can be attributed to variations of the relevant coupling terms over the widths of the vibronic resonances.

Novel ultraviolet electroluminescence from Al/octaethylporphyrin/Ag film sandwich cells

The electroluminescence of a [glass/Al/H 2OEP/Ag] sandwich cell under pulsed voltage is presented. The luminescence obtained shows three types of light. (i) There is a red fluorescence characteristics of the octaethylporphyrin material, with the origin at ≈ 630 nm. (ii) There is a UV emission consisting of two origin bands at 315.0 nm and 336.5 nm. An extensive vibronic progression is built on the latter which contains two vibrational energies, 1350 and 1680 cm -1, which are known to be porphyrin ring vibrations. The progression is explained as electron-hole recombination, i.e. P + +P - →2P(v), Where v is a vibrational level. Two possible explanations are offered for the 315.0 nm origin band, on which no progression is built. (iii) The third type of light comes in the form of burst attributed to dielectric breakdown. The burst last for about 10–20 ns.

Vibronic structures in the excitation spectra associated to the Suzuki phase in NaCl:Mn

Vibronic structures in the excitation spectra of NaCl:Mn, forming Suzuki-phase precipitates, have been investigated. The occurence of such a phase has been ascertained by means of Raman spectroscopy. Vibronic progressions have been interpreted along the lines previously developed for manganese dihalide crystals. The frequencies of the active phonons inferred from the vibronic progressions compare very well with the values determined through the Raman spectra. It is suggested that vibronic spectroscopy may be an interesting tool to study ordered phases of optically active impurities in dielectric crystals.

Electronic transitions of vanadyl phthalocyanine in solution and in the solid state

The electronic absorption and luminescence of vanadyl phthalocyanine (Pc) in dilute solution. and solids at 300 K were measured and analyzed qualitatively with the Ake and Gouterman(AG) mode [1] and the exciton in molecular crystal model [2]. Good agreement between the group theoretical predictions and the observed spectra was found. The presence of trace aggregates in VOPc dilute solutions was clearly illustrated by their characteristic O-O fluoreseences. The dimer fluoresces on the high energy side of the monomer, whereas the signal from the heavier aggregates appears on the low energy side. The presence of an unpaired electron in the monomer results in a much smaller fluorescence yield as compared to the dimeric form in which the odd electrons are paired. The absorption and fluorescence vibronic progressions suggest that the monomer is quite stable upon optical excitation to the first excited singlet. Our spectra show that there are at least two crystal forms in the phase I solid in agreement with Griffiths et al.`s calorimetric observation [3]. One of the two forms is dimeric as proposed by Griffiths et al. and confirmed by us. The extremely low radiative yield of the dimeric form is attributed to ultrafast exciton migration and to the intrinsic low yield of the molecules. In the phase II crystal, two prominent electronic absorptions were predicted and assigned. The low luminescence yield of this phase is ascribed to the rapid 2Q ⇝ 2T 1 relaxation which also leads to the greatly red shifted luminescence (i.e. 2T 1 → 2T) relative to the absorption, 2T → 2Q. Molecules in this phase are treated as monomeric rather than dimeric as in one of the phase I forms. Thus, the characteristics of the unpaired electron of the single molecule are preserved in the phase II crystal.

Analysis of the near-ultraviolet circular dichroic spectra of staphylococcal enterotoxins A, B and C

The near-ultraviolet CD spectra from 260 to 300 nm of staphylococcal enterotoxins A, B and C were resolved empirically into Gaussian curves. Each spectrum contained the same six components with maximum ellipticities at virtually identical wavelengths. The rotatory strength of the bands of enterotoxin A, however, differed significantly from that of enterotoxins B and C. Each Gaussian curve was identified as corresponding to a prominent CD transition of an aromatic chromophore: two phenylalanine bands from its 1L b transition ( 0-0 and 0 + 930 cm −1 ); four tyrosine bands from its 1L b transition (the 800 cm −1 primary vibronic progression) with the weakest of these overlapping a phenylalanine band; and one 1L b tryptophan band. At alkaline pH, tyrosylate CD bands arose, centered at 247–249 nm and at 295–298 nm. The low intensity of the 295–298-nm bands indicated that most of the tyrosine CD in the neutral enterotoxins was contributed by buried residues. The tryptophan contribution to the CD of enterotoxin A was positive, while that of B and C was negative. Also indicative of a different milieu was the ready oxidation of the A toxin by N- bromosuccinimide and the unavailability of the single tryptophan residue in the other two toxins to this reagent. The environment of the disulfide bond was markedly diverse in the three enterotoxins. Enterotoxin C was refractory to reduction by mercaptoethanol under conditions where enterotoxins B and A were readily reduced. The contribution of the disulfide of enterotoxin B to its CD spectrum was positive with an intensity of about 9000 deg · cm 2 · dmol −1 and was centered near 273 nm. The difference spectrum between native and reduced enterotoxin A was much smaller and appeared to be conformational in origin.

Erratum: Evidence of two-phonon vibronic progressions in layered3dmetal dihalides

Experimental evidence of two-phonon vibronic progressions due to second-order linear electron-phonon interactions is obtained in Ni${\mathrm{Cl}}_{2}$ and Ni${\mathrm{Br}}_{2}$ crystals, by comparing the phonon structure observed in $^{3}A_{2}(F)\ensuremath{\rightarrow}^{1}E(D)$ transition with Raman data. The vanishing of the spin-dependent first-order electron-phonon interaction is shown to be a necessary condition for observing prominent second-order phonon structures in isoconfigurational crystal-field transitions. According to this rule we predict and discuss the appearance of second-order progressions in other $d$-metal layered compounds.

Argon matrix absorption spectra of ClO, BrO, and IO and emission spectra of IO

Absorption spectra of ClO, BrO, and IO and the emission spectrum of IO have been observed from argon matrix samples prepared by microwave discharging the reagent mixture before condensation. Vibronic progressions were observed for each system. The spectroscopic scopic constants T e, ω′ e, ω e x′ e, ω″ e, and ω e x″ e were evaluated from the absorption and emission data for comparison with gas-phase constants. Very good agreement is found for ClO. The argon matrix observations dictate a revision of the gas-phase vibronic assignments for BrO. The ground-state vibrational fundamental and T e for argon matrix isolated IO are similar to the gas-phase values, but a lower excited-state spacing is found in the matrix.

Optical vibronic absorption spectra in 14.8 MeV neutron damaged sapphire

Preliminary observations of several anisotropic optical absorption bands are reported for high-purity sapphire following exposure to a pure 14.8 MeV neutron beam. These bands are compared with and found to be similar to those reported following fission-spectrum exposure, and are associated with lattice displacement damage. Two of these bands at 3.47 and 4.09 eV show similar sharpline features at low temperature; this ≈200 cm −1 vibronic progression it tentatively associated with a local mode.

Raman spectrum of 1,1′-diethyl-2,2′-cyanine iodide near the π-π* electronic transition

The Raman spectrum of 1,1′-diethyl-2,2′-cyanine iodide in a methyl alcohol solution (4 × 10 −5 m) in which the lines of the argon ion laser at 457.9, 488.0, 496.5, 501.7 and 514.5 nm were employed as excitation sources has been investigated. The strongest lines in the Raman spectrum appear in the region from 1200–1700 cm −1 and are strongly polarized (ϱ= 0.2). Two vibrational modes have been identified with reasonable certainty: one a doublet at 1361 and 1380 cm −1 which involves the stretching of the conjugated chain in the ring of the quinolinic end groups, and the other at 1639 cm −1 which characterizes the stretching of the conjugated chain which bridges the quinolinic end groups. The splitting of the line associated with the ring stretching mode probably arises from the resonance interaction between the two quinolinic end groups. The intensity and polarization of the lines in the spectrum indicate that the vibrational modes of the strongest lines are totally symmetric. The visible electronic spectrum shows maxima at 490.6 and 522.9nm; an analog resolution of the spectrum shows that the contour of the vibronic band can be accounted for by a vibronic progression with a spacing of approximately 1200 cm −1. Both the observed variation of intensity and an approximate calculation based on A-term scattering of totally symmetric vibrational modes support the conclusion that a maximum enhancement of the intensity will be attained when the frequency of exciting radiation is equal to that of the pure electronic transition at 522.9 nm.

Evidence of two-phonon vibronic progressions in layered3d-metal dihalides

Experimental evidence of two-phonon vibronic progressions due to second-order linear electron-phonon interactions is obtained in Ni${\mathrm{Cl}}_{2}$ and Ni${\mathrm{Br}}_{2}$ crystals, by comparing the phonon structure observed in $^{3}A_{2}(F)\ensuremath{\rightarrow}^{1}E(D)$ transition with Raman data. The vanishing of the spin-dependent first-order electron-phonon interaction is shown to be a necessary condition for observing prominent second-order phonon structures in isoconfigurational crystal-field transitions. According to this rule we predict and discuss the appearance of second-order progressions in other $d$-metal layered compounds.

The EF and GK1Σ g+ states of hydrogen : Calculation of nonadiabatic coupling

The mutual vibronic coupling between the EF and GK states of hydrogen has been computed in an adiabatic basis of electronic-vibrational states. The electronic matrix elements 〈 EF|▿ R| GK〉 and 〈 EF|Δ R| GK〉 are presented over the relevant range 2.0 ≤ R ≤ 4.0 a.u. and the vibronic coupling matrix is displayed for H 2. Nonadiabatic vibroic energies and B values are calculated for H 2, HD, and D 2 and compared with experimental data. The irregularities of the observed vibronic EF and GK progressions are well reproduced by the calculation. The remaining errors of the calculated energies grow systematically as the dissociation limit is approached, indicating the importance of additional nonadiabatic interactions with higher states and possibly of R-dependent errors of the Born-Oppenheimer energies of the EF and GK states at R values far from the potential minima.