Interpretation of the optical dephasing time and lineshape function in the S0 → T1 exciton transitions of 1,4-dibromonaphthalene

The lineshape function for the S₀ → T₁ absorption in 1,4-dibromonaphthalene (DBN) is analyzed in terms of exchange theory. It is shown that the dominant optical dephasing mechanism for the electric dipole transition to the k = 0 state in the band results from the absorption and emission of a low energy optic phonon. This process dephases the optical absorption because of frequency differences of the phonon in the ground and excited state. In addition, it is shown how to extract the energy of the phonon responsible for dephasing, the phonon absorption rate, and the lifetime in the phonon promoted state from the data. The analysis of the data for DBN shows that very little dephasing of the optical transition occurs before ≈ 15 K but from 15 K to ≈ 40 K the singlet-triplet transitions to site I (20192 cm^?1) and site II (20245 cm^?1) are dephased by absorption and emission of an ≈ 38 cm^?1 and 45 cm^?1 phonon respectively. The phonon absorption rates by the k = 0 state in the exciton band are similar for both sites being 5 × 10⁶ s^?1 and 3 × 10⁵ s^?1 at 4 K and 7 × 10¹¹ s^?1 and 4 × 10¹¹ s^?1 at 30 K for site I and II respectively. Finally, the lifetimes in the phonon promoted state for sites I and II are 0.23 and 0.28 ps over the range 15–40 K.     

Optical studies of the electronphonon interaction in the pyrene-PMDA complex

Low temperature phosphorescence spectra of pyrene-PMDA (pyromellitic acid dianhydride) imbedded in a naphthalene-PMDA host crystal are reported. The spectra exhibit resolved zero-phonon and multi-phonon structure which is significant since the phosphorescent state is characterized by ≈36% charge-transfer character. Several different phonons contribute to the structure with the dominant phonon having ground and excited state (from hot band analysis) frequencies of 25 and 15 cm^?1. The linear electronphonon coupling strength for the 25cm^?1 phonon is computed. This phonon is tentatively assigned to rotational motion of the rigid complex. Linewidth data yield a relaxation time of 0.4 ps at 2 K for the 25 cm^?1 phonon which is believed to be a pseudolocalized or resonant mode.     

Mixed-symmetry states in weakly deformed nuclei from (n,n’γ) reaction studies

Summary The current status of 2⁺ mixed-symmetry states in weakly deformed nuclei is described, and recent contributions from the (n,n’γ) reaction to our understanding of these states in nuclei around ⁹⁴Mo are reviewed. The recent identification and characterization of two-phonon states, which are composed of the 2⁺ mixed-symmetry state coupled with the 2⁺ symmetric quadrupole phonon, are particularly significant and help clarify the picture of mixed-symmetry states as being generally occurring collective excitations.     

Phonon mediation of solid state photoreaction: Photodimerization of o -methoxy cinnamic acid

The solid state photodimerization reaction ofo-methoxy cinnamic acid is shown to be mediated by a lattice phonon. The phonon participation, in this case, is through a mode softening and not through strong exciton-phonon coupling as is generally observed. Raman phonon spectroscopy suggests that the reaction is heterogeneous. Infrared spectroscopy has been used to study the internal vibrations of the reactant and the product. Partly presented at the International Laser Science Conference II, 1986 held at Seattle, USA.     

Density functional theoretical study of lattice-specific heat and thermal properties of magnesium nitride

A comprehensive first principle study of thermodynamic properties of MgN is reported within the density functional theory scheme. The ground state properties such as lattice constant, Bulk modulus etc. of MgN in rock-salt (RS) phase have been determined. The thermodynamical properties have been analyzed in the light of phonon density of states of MgN and its constituent atoms. The variation of lattice-specific heat with temperature obeys the classical Dulong–Petit’s law at high temperature while at low temperature it obeys Debye T ³ law. The phonon spectrum shows the presence of all positive phonons and zero phonon density of states at zero energy confirming a dynamically stabilized structure of MgN in RS phase.     

Lattice phonon mediation of solid state photoreaction in 7-methoxycoumarin crystal: laser Raman spectroscopic study

The photodimerization reaction in 7-methoxycoumarin (7MC) in the crystalline state is shown to be mediated by a lattice phonon. The phonon participation is through a mode softening in the excited state of the crystal on photoirradiation and not via strong exciton—phonon coupling as in generally observed. IR and Raman spectroscopy were used to characterize the reactant and product, and it is confirmed that the reaction occurs by cyclobutane ring formation through the bond of the pyrone ring. Electronic spectroscopy suggests that the exciton—phonon coupling is weak in the monomer crystal.     

Ultrafast Spectroscopy Studies on Thickness Dependence of Acoustic Phonon Modes in Silver Nanoprisms

Time-resolved transient absorption technique is used to investigate the thickness dependence of acoustic phonon modes of silver nanoprisms with two thicknesses, 7.8 ± 1.2 and 8.5 ± 0.69 nm, and a similar bisector length of 31.4 ∼ 31.6 nm. Coherent acoustic phonon signals are observed. A new acoustic phonon frequency within 7.81 cm⁻¹ ∼ 11.7 cm⁻¹ is found and this phonon mode is associated with the thickness of the nanoprism. Another phonon frequency between 1.95 cm⁻¹ and 1.71 cm⁻¹ is also observed, and its origin can be associated with the bisector length of the nanoprism.     

Exciton dynamics in molecular crystals from line shape analysis. Spectral moment analysis of the origin region of the 4000 Å transition of crystal anthracene

A spectral moment analysis of the line shape function ω?₂(ω) in the region of the (0—0) band of the 4000 ŹB_2u ← ¹A_g transition in crystal anthracene at various temperatures is performed. The data are compared with the predictions of three coupling models, viz., weak exciton—photon with weak exciton—phonon coupling, strong exciton—photon with weak exciton—phonon coupling, weak exciton—photon with strong exciton—phonon coupling. The terms contributing to each spectral moment for each model are rendered explicit. The experimental data indicate that the exciton—intermolecular phonon coupling is primarily weak. The exciton interacts with optical phonons of about 90 cm^-1 frequency with a coupling strength of about 140 cm^-1 , a value near that predicted by a weak coupling model. The coupling strength is nearly the same irrespective of whether the exciton is created by b- or a-polarized light probably indicative of the importance of higher multipole contributions to the coupling although the existence of strong interband scattering could affect that suggestion. The coupling parameters g_op and g_ac are about 10^-1 and 10^-4 respectively.     

Laser studies of vibrational energy transfer and relaxation of CO trapped in solid neon and argon

The infrared emission of CO trapped in solid Ne and Ar is observed at low temperature. The first vibrational level of ¹²C¹⁶O is excited by a Q-switched frequency doubled CO₂ laser. The emission spectrum consists of several lines arising from upper vibrational levels of ¹²C¹⁶O and also of ¹³C¹⁶O and ¹²C¹⁸O which are present in natural abundance. An interpretation is proposed which is based on the assumption that long range dipole—dipole interaction is the main physical process involved in these experiments. Resonance energy transfer produces an energy migration among ¹²C¹⁶O molecules without any change in vibrational populations. Phonon assisted energy transfer takes place between vibrational levels of the various isotopic species present in the solution. In order to satisfy the resonance condition a phonon is emitted or absorbed whose energy compensates for the energy mismatch between the transitions in each interacting molecule due to isotopic effect and or vibrational anharmonicity. The range of this process is greatly extended by energy migration. At the low phonon bath temperature phonon emission is much more probable than phonon absorption. So a strong excitation of upper vibrational levels with in some cases population inversions is observed.Molecular impurities act as efficient quenching centers even at very low concentration. When highly purified samples are used, the fluorescence decay time is found to be 20.6 ms in Ne and 14.5 ms in Ar and does not significantly depend upon concentration and temperature. It is concluded that radiationless relaxation is unimportant.     

Temperature dependence of the exciton—phonon coupling in the strong coupling limit: Results for solid nitrogen

High resolution (ΔE = 0.75 meV) absorption profiles of the vibronic bands in the range of the w¹Δ_u ← X¹Σ⁺_g and a¹II_g ← X¹Σ⁺_g exciton progressions at hv ≈ 8.9 eV in solid N₂ have been measured in the temperature range between 6 K and 30 K. These excitations are strongly localized so that the observed temperature dependence of the fine structure, consisting of a zero phonon line and a phonon side band, can be described very well in the model of strong exciton—phonon coupling at point defects. The experimental results for the w¹Δ_u transition are found to be consistent with the assumption of a Debye spectrum for the phonon density of states and we derive a value for the Debye temperature of θ = 78 K, which is in very good agreement with that derived from other measurements.     

Dynamics of radiationless transitions in large molecules: 3. Decay of vibration-phonon states

The statistical operator of the ensemble of high-frequency intramolecular vibrations associated with the phonon reservoir depends on the phonon occupation numbers under thermal equilibrium conditions. The eigenvalues of energy of statistically averaged vibration-phonon (VP) states are complex quantities. In the case of weak VP coupling, only one- and two-phonon transitions are taken into consideration for calculating the decay rate constant, in which the difference of phonon energies compensates for the difference in energy between the initial and final intramolecular states. Although the fast evolution of amplitudes of VP states is due to intramolecular redistribution of energy and is not reduced to exponential decay of the initial state, the imaginary components of the eigenvalues coincide with those predicted by Fermi’s golden rule. The relative contribution of two-phonon (combination) transitions compared with one-phonon transitions increases with an increase in the density of intramolecular states and temperature, becoming prevalent for large molecules at T ≥ D ? Δ₀ (D = 100–200 K (70–140 cm^?1) is the Debye temperature and Δ₀～10 cm^?1 is the spacing between neighboring intramolecular vibration levels). When T ≥ D, the decay rate constant is K ∝ T ².     

Theoretical study on the reaction mechanism of (CH<Subscript>3</Subscript>)<Subscript>3</Subscript>CO<Superscript>.</Superscript> radical with NO

The reaction mechanism of (CH₃)₃CO^. radical with NO is theoretically investigated at the B3LYP/6-31G* level. The results show that the reaction is multi-channel in the single state and triplet state. The potential energy surfaces of reaction paths in the single state are lower than that in the triple state. The balance reaction: (CH₃)₃CONO⇔(CH₃)₃CO^.+NO, whose potential energy surface is the lowest in all the reaction paths, makes the probability of measuring (CH₃)₃CO^. radical increase. So NO may be considered as a stabilizing reagent for the (CH₃)₃CO^. radical.     

The use of single crystal rare gas lattices to study homogeneous lineshapes of impurities: SF6 in xenon

The IR absorption spectrum of SF₆ in single crystal xenon has been studied as a function of temperature. Only one trapping site is stable in single crystal xenon and the v₃ linewidth for this site is dominated by homogeneous broadening throughout the entire temperature range studied (14–160 K). The v₃ linewidth shows an approximate e^αT dependence which is inconsistent with either elastic phonon scattering or local phonon modes independently as the dominant dephasing mechanism. The results suggest that dephasing involves both site local and bulk phonon modes with the former dominating at low temperatures and the latter at higher temperatures.     

Progress of estuarine research in China over last 50 years

Under 216 nm UV-excitation, cascade emission of Pr³⁺ occurs from¹S₀ state in LaBaB₉O₁₆:Pr³⁺ that converts a single UV photon of high energy into two visible photons. The observation of the cascade emission in this oxide matrix is largely due to the weak crystal field on the lanthanum sites. The analysis of the vibrational coupling indicates that the radiative transition from the³ P ₀ state is related to the low phonon frequency of the BO₄ borate groups bonded to the lanthanum atoms(hω _max∼850 cm^-1). On the other hand, the cascade emission does not take place in a closely related material, YBaB₉O₁₆:Pr³⁺, which can be interpreted by the fact that the 4f5d levels are located below the¹S₀ level in this material.     

Photon-echo decay of naphthalene in durene and perdeutero-naphthalene

Using two frequency-doubled, nitrogen laser pumped, dye-lasers we have measured the decay of the photon-echo in the lowest ¹B_1u ← ¹A_1g singlet state of naphthalene in durene and in perdeutero-naphthalene. Optical phase relaxation in the durene mixed crystal is believed to be caused by resonant phonon scattering in the ground and excited state, and in the isotopically mixed crystal by scattering of the excited state into the singlet exciton band. At the lowest achievable temperature (1.4 K), the photon-echo decay time (T₂) in both systems is still found to be much shorter than expected from the fluorescence decay times. Energy transfer is held responsible for this discrepancy.     

Progress of estuarine research in China over last 50 years

Under 216 nm UV-excitation, cascade emission of Pr³⁺ occurs from¹S₀ state in LaBaB₉O₁₆:Pr³⁺ that converts a single UV photon of high energy into two visible photons. The observation of the cascade emission in this oxide matrix is largely due to the weak crystal field on the lanthanum sites. The analysis of the vibrational coupling indicates that the radiative transition from the³ P ₀ state is related to the low phonon frequency of the BO₄ borate groups bonded to the lanthanum atoms(hω _max～850 cm^-1). On the other hand, the cascade emission does not take place in a closely related material, YBaB₉O₁₆:Pr³⁺, which can be interpreted by the fact that the 4f5d levels are located below the¹S₀ level in this material.     

Raman scattering study of the geometrically frustrated pyrochlore Y2Ru2O7

Geometrically frustrated pyrochlore Y₂Ru₂O₇, which shows a spin-glass-like transition at T_G ∼ 80 K, were investigated by temperature-dependent Raman scattering. Three discernable phonons appear around 315, 410, and 510 cm⁻¹ without any abrupt change in the number of Raman active modes within the temperature range of 10–300 K. Fitting each phonon with Lorentz oscillators, we analyzed the effects of temperature on the phonon frequencies and the linewidths. The temperature-dependence of the mode near 510 cm⁻¹ shows abnormal behavior below T_G, while the other two phonons follow the usual thermal effect of lattice vibration. This behavior can be understood in terms of spin–phonon coupling. Considering the atomic modulations of each phonon mode, it is conjectured that the 510 cm⁻¹ phonon mode is isotropically coupled to the spin degree of freedom, while the other modes are not.     

Control of Luminescence by Tuning of Crystal Symmetry and Local Structure in Mn4+‐Activated Narrow Band Fluoride Phosphors

Mn⁴⁺‐doped fluoride phosphors have been widely used in wide‐gamut backlighting devices because of their extremely narrow emission band. Solid solutions of Na₂(Si_xGe_1?x)F₆:Mn⁴⁺ and Na₂(Ge_yTi_1?y)F₆:Mn⁴⁺ were successfully synthesized to elucidate the behavior of the zero‐phonon line (ZPL) in different structures. The ratio between ZPL and the highest emission intensity υ₆ phonon sideband exhibits a strong relationship with luminescent decay rate. First‐principles calculations are conducted to model the variation in the structural and electronic properties of the prepared solid solutions as a function of the composition. To compensate for the limitations of the Rietveld refinement, electron paramagnetic resonance and high‐resolution steady‐state emission spectra are used to confirm the diverse local environment for Mn⁴⁺ in the structure. Finally, the spectral luminous efficacy of radiation (LER) is used to reveal the important role of ZPL in practical applications.     

Genesis of the phonon spectra of A2B4C 2 5 and A1B3C 2 6 crystals with a chalcopyrite lattice from the vibrational states of their sublattices

The phonon spectra of a series of A²B⁴C ₂ ⁵ and A¹B³C ₂ ⁶ crystals with the chalcopyrite structure are calculated in the Keating model in the basis set of polarization vectors of their sublattices. The dependences of frequency values and partial contributions of sublattices to the polarization vectors on the chemical composition are studied. The effect of the mass ratio and the strengths of covalent bonds between the compound components on the phonon spectrum formation is found.     

Improvement of the Water Resistance of a Narrow‐Band Red‐Emitting SrLiAl3N4:Eu2+ Phosphor Synthesized under High Isostatic Pressure through Coating with an Organosilica Layer

A SrLiAl₃N₄:Eu²⁺ (SLA) red phosphor prepared through a high‐pressure solid‐state reaction was coated with an organosilica layer with a thickness of 400–600 nm to improve its water resistance. The observed 4f⁶5d→4f⁷ transition bands are thought to result from the existence of Eu²⁺ at two different Sr²⁺ sites. Luminescence spectra at 10 K revealed two zero‐phonon lines at 15377 (for Eu(Sr1)) and 15780 cm^?1 (for Eu(Sr2)). The phosphor exhibited stable red emission under high pressure up to 312 kbar. The configurational coordinate diagram gave a theoretical explanation for the Eu^2+/3+ result. The coated samples showed excellent moisture resistance while retaining an external quantum efficiency (EQE) of 70 % of their initial EQE after aging for 5 days under harsh conditions. White‐light‐emitting diodes of the SLA red phosphor and a commercial Y₃Al₅O₁₂:Ce³⁺ yellow phosphor on a blue InGaN chip showed high color rendition (CRI=89, R9=69) and a low correlated color temperature of 2406 K.