Nonradiative Decay Mechanism Research Articles

Multiphonon energy gap law in rare-earth doped chalcogenide glass

The parameters for multiphonon relaxation of rare earth (RE) ions in the sulfide glass Ge 25As 8.33Ga 1.67S 65 have been re-evaluated using the temperature dependence of the fluorescence lifetime to separate out true multiphonon decay from other non-radiative processes. It is found that for energy gaps to the next lowest level greater than 2500 cm −1, other non-radiative processes become dominant over multiphonon decay. The most likely process for this additional non-radiative decay is energy transfer to vibrational impurities such as OH and SH. The newly derived parameters lead to an electron–phonon coupling parameter ϵ=0.058, which is more in line with other glass types than the previously accepted value of ϵ=0.36. These new multiphonon relaxation parameters and the existence of additional non-radiative decay mechanisms has implications for the modeling of chalcogenide-based active devices.

Singlet excited-state lifetimes of cytosine derivatives measured by femtosecond transient absorption.

Lifetimes of the lowest excited singlet (S1) electronic states of various derivatives of the pyrimidine nucleobase cytosine (Cyt) were measured by the femtosecond transient absorption technique. The bases were excited in room-temperature aqueous solution at 265 nm using approximately 200 fs pump pulses from a titanium-sapphire laser system. The decay of excited-state absorption (ESA) at visible probe wavelengths was used to determine the S1 lifetimes of a variety of modified Cyt compounds at different pH values by global fitting. Identical lifetimes were observed for Cyt and cytidine (Cyd) within experimental uncertainty, but ESA by the ribonucleoside was considerably stronger, suggesting that the ribose group increases the oscillator strength of the S1 --> SN transition. The S1 lifetime of the important minor base 5-methylcytosine (m5Cyt) is 7.2 +/- 0.4 ps at pH 6.8. The same lifetime was measured for the ribonucleoside 5-methylcytidine, but sugar substitution again increased the strength of the ESA signal. Protonation of Cyd and m5Cyt at low pH led to a modest decrease in their S1 lifetimes. On the other hand, deprotonation of Cyt and m5Cyt significantly increased the lifetime of their respective S1 states. These trends support the intermediacy of the n,pi* state localized on the carbonyl oxygen in the nonradiative decay mechanism of Cyt. Longer S1 lifetimes were observed for 5-fluorocytosine and N4-acetylcytosine. Collectively, these results illustrate the great potential of femtosecond laser spectroscopy for investigating excited-state dynamics in DNA and DNA components.

On the mechanism of nonradiative decay of DNA bases: ab initio and TDDFT results for the excited states of 9H-adenine

Minimum-energy reaction paths and corresponding potential-energy proles have been com- puted for the lowest excited states of the amino form of 9H-adenine. Complete-active- space self-consistent- eld (CASSCF) and density functional theory (DFT) methods have been employed. The potential-energy function of the lowest 1 state, nominally a 3s Rydberg state, is found to be dissociative with respect to the stretching of the NH bond length of the azine group. The 1 potential-energy function inter- sects not only those of the 1 and 1 n excited states, but also that of the electronic ground state. The 1 { 1 and 1 {S0 intersections are converted into conical intersections when the out-of-plane motion of the active hydrogen atom is taken into account. It is argued that the predissociation of the 1 and 1 n states by the 1 state and the conical intersection of the 1 state with the S0 state provide the mechanism for the ultrafast radiationless deactivation of the excited singlet states of adenine.

Impurity local phonon nonradiative quenching of Yb/sup 3+/ fluorescence in ytterbium-doped silicate glasses

We have studied the concentration quenching of Yb/sup 3+/ ion fluorescence in Yb-doped silicate glasses containing up to 3.4/spl times/10/sup 21/ cm/sup -3/ Yb/sup 3+/ ions. The absorption and fluorescence spectra are similar to those obtained for the Yb/sup 3+/ ion in many different matrices, with a radiative lifetime of approximately 1400 /spl mu/s. The fluorescence decay curves were different among samples, with strong dependence on the Yb concentration. The decay curves could always be resolved into two exponential components, indicating that the ions reside in two different sites, each of a different characteristic nonradiative decay mechanism. The fast decay times ranged between 6 and 300 /spl mu/s, and the slow ones ranged between 190 and 1250 /spl mu/s in different samples. The sites where ions exhibit the fast decay most probably consist of pairs of Yb/sup 3+/ ions. The nonradiative decay probabilities for each site mere directly proportional to the Yb/sup 3+/ concentration in the same site. We propose that the fluorescence quenching occurs by multiphonon nonradiative transitions involving polar local phonon bands created by the presence of the Yb/sup 3+/ ion.

The predissociation mechanism of the B̃ 2A′ state of HCO via the conical intersection with the X̃ 2A′ state

We use large-scale multiconfiguration self-consistent field/configuration interaction calculations to characterize the predissociation mechanism of the B̃ 2A′ state of HCO through conical intersection with the X̃ 2A′ ground state. We locate two regions of intersection: the first represents a highly bent HCO that is 8 kcal/mol energetically lower than the B̃ state minimum, with a barrier height of 26 kcal/mol. Energy points on the B̃ 2A′ potential surface connecting these extrema were also calculated. This region emphatically illustrates the feasibility of a nonradiative decay mechanism consistent with latest experimental findings of purely vibronic coupling mechanism. The second region of intersection represents a confluence of three linear Σ+2–2Π states crossings, 53 kcal/mol below the B̃ state minimum. A barrier about 21 kcal/mol above the B̃ state equilibrium structure is located and assigned to the entrance channel of H–CO(3Π).

On the mechanism of rapid non-radiative decay in intramolecularly hydrogen-bonded π systems

Reaction paths and potential-energy profiles of excited singlet states of malonaldehyde have been investigated using the ab initio CASSCF method. The results confirm the barrierless nature of the hydrogen-transfer process in the π π ∗ excited singlet state. A hitherto unknown 1 π σ ∗ state has been identified which is strongly repulsive with respect to in-plane detachment of the mobile hydrogen atom, leading to a low-lying conical intersection with the electronic ground state. Vibronic coupling of the 1 π π ∗ with the 1 π σ ∗ state by out-of-plane modes connects the former, via a low barrier, to the conical intersection, providing a mechanism for efficient internal conversion to the ground state. It is argued that these findings reflect general properties of the H-chelate ring in intramolecularly hydrogen-bonded molecules and can explain the photostability of these systems.

Photoluminescence of quasi-direct transitions in disordered In/sub 1-x/Ga/sub x/P/graded GaP alloys

We have examined the photoluminescence and photoluminescence kinetics of a series of In/sub 1-x/Ga/sub x/P alloys in an effort: 1) to elucidate the electronic structure of the conduction band versus alloy composition, especially near the direct-indirect crossover; 2) to determine precisely the composition of the direct-indirect crossover, and its temperature dependence; and 3) to understand the nonradiative decay mechanism and its temperature dependence. We find that the fundamental bandgap is only determined by the /spl Gamma//sub 1c/ and X/sub 1c/ states in samples with Ga-compositions ranging from 0.58 to 0.75, and that the 2-K direct-indirect crossover from /spl Gamma//sub 1c/, to X/sub 1c/ occurs at x=0.69 and is not strongly temperature-dependent. Further, we find, in agreement with our spectroscopic ellipsometry measurements at room temperature, that the mixing near crossover is rather complicated and leads to the previous observation of quasi-direct transitions. Our combined photoluminescence and spectroscopic ellipsometry measurements have therefore clearly resolved the controversy regarding the bandgap crossover. This has strong implications for the realization of InGaP-based efficient light-emitting devices with emission at higher energies.

Nonradiative decay processes and mechanisms of frequency upconversion of Er3+ in ZrF4–BaF2–LaF3 glass

We have investigated the decay processes of the Er:4S3/2 and F39/2 states and the mechanisms of frequency upconversion under 800 nm excitation of Er3+ in ternary ZrF4–BaF2–LaF3 glasses. The decay processes of the Er:4S3/2 and F49/2 states are discussed on the basis of the Er concentration dependence of the fluorescence lifetime. The quantum efficiencies of emission of the S43/2 and F49/2 states are 73% and 25%, respectively. The difference in the quantum efficiencies between these states can be explained from the difference in the multiphonon decay rates: the multiphonon decay rate of the S43/2 state is much smaller than that of the F49/2 state. The multiphonon relaxation is a dominant nonradiative decay process of the F49/2 state. A dominant nonradiative decay process of the S43/2 state is self-quenching at high Er concentrations. The upconversion mechanisms under 800 nm excitation are also investigated from the dependence of upconversion luminescence on excitation wavelengths (800 and 980 nm), excitation laser power, and Er concentration. The mechanism of upconversion from 800 to 660 nm is mainly due to the energy transfer between the I49/2→4I13/2 and F49/2←4I11/2 transitions. We also show that the upconversion from 800 to 550 nm is due to excited-state absorption.

Photoluminescence studies of broadband excitation mechanisms for Dy3+ emission in Dy:As12Ge33Se55 glass

Photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy of Dy2S3-doped As12Ge33Se55 glasses demonstrate that the broad, below-gap PLE mechanism observed previously for rare-earth emissions in Er- and Pr-doped chalcogenide glasses is a general or universal feature of rare-earth-doped chalcogenide glasses, provided the transition energies of the rare earths are in the correct energy range. The PL spectrum excited in the 815 nm Dy3+ absorption band shows the 1150, 1340, and 1700 nm sequence of Dy3+ transitions expected for Dy-doped chalcogenide glasses. The PLE spectra of the 1340 (6F11/2,6H9/2→6H15/2) and 1700 nm (6H11/2→6H15/2) Dy3+ emissions exhibit broad excitation bands from ∼500 to 1000 nm, upon which the sharp intra F-band absorptions of Dy3+ are superimposed. These broad PLE bands are characterized by an exponentially decreasing slope with decreasing energy in the spectral range below the Urbach edge which is associated with the below-gap, defect- and impurity-induced exponential tails observed in the below-gap absorption spectra of chalcogenide glasses. At high energy, the exponentially rising Urbach absorption edge of the host glass, which leads to competing nonradiative decay mechanisms, eventually dominates the absorption spectrum and imposes an exponentially decreasing slope on the PLE spectra. These features of the broadband PLE have been observed in the PLE spectra of the Er-, Pr- and Dy-doped chalcogenide glasses we have studied. There is a difference in the relative strengths of the broad PLE bands, with the broadband for the 1340 nm PL band being a factor of 3 stronger than that for the 1700 nm PL emission. In addition, there is a shift in the peak energy of the different PLE spectra. Qualitatively speaking, the higher the energy of the luminescence transition, the higher the energy of the corresponding broad PLE peak. Proposed mechanisms for the broadband PLE of rare-earth emissions in chalcogenide glasses are discussed in the context of models for the below-gap, defect- and impurity-induced exponential absorption tails, including the possible role of lattice relaxation associated with charge transfer transitions and the involvement of transition metal impurities.

Time-delayed two-color photoelectron spectra of aniline, 2-aminopyridine, and 3-aminopyridine: Snapshots of the nonadiabatic curve crossings

We present time-delayed two-color photoionization photoelectron spectra of aniline, 2-aminopyridine, and 3-aminopyridine seeded in a cold molecular beam. The molecules are prepared in their S1 electronic states by a picosecond UV laser pulse and ionized by a time-delayed 200 nm probe pulse. The photoelectron spectrum is observed with a time-of-flight spectrometer. All time-delayed spectra reveal only one product of the nonradiative relaxation process. Careful considerations of electronic and vibrational overlap propensity rules for the ionization step lead to the conclusion that the dominant nonradiative decay mechanism in these molecules is the intersystem crossing to a bath of vibrationally excited levels of the T1 electronic state. Our observations reveal no admixtures of T2 or higher triplet levels. The pathway of the nonradiative electronic relaxation in 2-aminopyridine is found to be independent of the initially prepared vibrational states up to 1000 cm−1 of vibrational energy. We find no evidence of intramolecular vibrational relaxation preceding the electronic curve crossing.

Suppression of atomic radiation in a cylindrical nanocavity

An excited atom placed inside a cylindrical nanocavity cannot radiate if the frequency of emission is below the cutoff frequency for electromagnetic wave propagation in the cavity. However, we demonstrate that it can be de-excited through the emission of surface plasmons. The observable effect will be a substantial enhancement in the lifetime of dipole-allowed transitions. The recently discovered carbon nanotubules will be explored as potential nanometer scale cylindrical cavities and the lifetimes of excited states of Helium (2p → 1s) in the earbon nanocavities will be calculated as an example of this nonradiative decay mechanism. The dependence of the transition rate as a function of the radial distance from the tube surface is studied by varying the initial radial quantum number of the center-of-mass wavefunction for an atom confined within the nanotube. The results of the calculation are used to explore the possible application of concentric carbon nanotube structures as TEM waveguides.

Cavity quantum electrodynamics in a carbon nanotube

Abstract An excited atom placed inside a metallic carbon nanotube cannot radiate if the frequency of its emission is below the cutoff frequency for electromagnetic wave propagation in the cavity. However, we demonstrate that it can be de-excited through the emission of surface plasmons on the nanotube. The observable effect will be a substantial enhancement in the lifetime of dipole-allowed transitions. The lifetimes of excited states of Helium (2p → 1s) in the carbon nanocavities are calculated for this nonradiative decay mechanism and the radial dependence of the lifetimes discussed.

Auger and photoelectron spectra of K + in solids at resonant 2 p6 to 2 p53 d excitation

We present here the 2 p-3 d-resonant photoelectron and Auger spectra of K + in KMnF 3 and compare them with analogous spectra of KCl. The spectra are very similar for both compounds. The most conspicious structure which arises during the LMM Auger spectator decay of the 2 p 53 d configuration is due to the 3 p −23 d final configuration. Its remarkable property, the linear dispersion in the whole resonance region, demonstrates that i) the Auger transition is so fast that at resonant excitation it is not possible separate it from the photon absorption process, ii) the L 2 L 3 M 45 Coster-Kronig transition is an important link in the nonradiative decay of the resonantly-created L 2 hole, and iii) the crystal-field splitting of the intermediate 2 p 53 d configuration does not determine the general mechanism of nonradiative decay of this configuration.

Novel broad-band excitation of Er3+ luminescence in chalcogenide glasses

Photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy carried out on bulk samples of Er2S3-doped Ge33As12Se55 glasses demonstrate that Er3+ is incorporated in optically active sites in the glass and gives rise to a broad ∼1500–1600 nm 4I13/2→4I15/2 PL spectrum similar to those observed in Er-doped oxide glasses. The novel PLE spectrum of the 1550 nm Er3+PL band comprises a superposition of relatively sharp peaks which are attributable to the characteristic 4I15/2→4I11/2 and 4I15/2→4I9/2 Er3+ absorption transitions at 810 and 980 nm, respectively, and a broad, below-gap PLE band characteristic of the weak defect or impurity absorption tails in chalcogenide glasses which decrease exponentially with decreasing photon energy. At high energy the exponentially rising Urbach absorption edge, which leads to competing nonradiative decay mechanisms, imposes an exponentially decreasing slope on the PLE spectrum.

Time-resolved spectroscopy of visibly emitting porous silicon

Time-resolved emission spectra and photoluminescence decay kinetics are analyzed in porous Si samples exhibiting a noticeable inhomogeneous broadening under steady-state selective excitation. The gated emission spectra are found to be substantially narrower compared to a steady-state one, with a pronounced redshift with time. Considerable dependence of the decay parameters upon the emission wavelength is found. The decay time distribution analysis applied leads to a model where the nonradiative decay mechanism is attributed to traps of the same type, the number of traps per Si grain obeying the Poissonian statistics.

New ultrafast nonradiative decay mechanism in the benzene radical cation

The simultaneous vibronic interactions between the X̃ 2E 1g, B̃ 2E 2g and C̃ 2A 2u states of the benzene radical cation are investigated theoretically. A new type of conical intersection between the X̃ and B̃ state potential energy surfaces is found which is triggered by the Jahn—Teller effects in the two degenerate states, the coupling being provided by the out-of-plane mode ν 8. Extensive wavepacket dynamical calculations have been performed involving all three coupled electronic states and up to five nonseparable vibrational modes (≈2 million basis states). They reveal a decay to the X̃ 2E 1g state upon broadband excitation to the C̃ 2A 2u state within ≈250 fs. This can explain the absence of C̃ å X̃ radiative emission as well as the fact that the benzene cation fragments via the X̃ state even if higher electronic states are populated initially. A comparison with (partly) halogenated derivatives indicates that the absence of a low-lying 2E 2g ( 2E′) state is responsible for the observation of Ã→X̃ radiative emission in these species.

Temperature dependence of the lowest excited singlet-state lifetime of all-t r a n s-β-carotene and fully deuterated all-t r a n s-β-carotene

A 4 ps, 450 nm laser pulse was used to electronically excite all-trans-β-carotene and all-trans-β-carotene-d56 in 3-methylpentane. The transient absorption spectra of these molecules were measured as a function of temperature down to 20 K. In all cases the 400–500 nm electronic absorption band of each carotene bleaches and a new absorption band near 560 nm appears immediately upon excitation. These bands recover with single exponential kinetics: τ=8.1±0.5 ps for all-trans-β-carotene, and τ=10.5±0.6 ps for all-trans-β-carotene-d56 at 294 K. These recovery times increase by about a factor of 2 in glassy 3-methylpentane, and are nearly independent of temperature from 100 to 20 K. The weak dependencies of the lowest excited single-state lifetime of all-trans-β-carotene on deuteration and temperature are discussed in terms of nonradiative decay mechanisms within carotenoids.

Analytical applications of enhanced drug luminescence

Luminescence emission from drugs is strongly dependent on their physicochemical environment. Several biomedically and environmentally important compounds and pharmaceuticals exhibit sufficient intrinsic luminescence properties to allow their determination by high-performance liquid chromatography (HPLC) with fluorimetric, chemiluminescence or room temperature phosphorimetric detection. In the case of weakly fluorescing compounds it is possible to use the dependence of the emitted radiation on the molecular environment at the moment of measurement. The composition of the eluent, i.e. solvents, added salts and buffers, pH and ionic strength, oxygen content and temperature, are of the highest importance for the luminescence detection of drugs in solution (e.g. in liquid chromatography) or adsorbed onto solid surfaces (e.g. in thin-layer chromatography). Post-column or post-plate acid—base manipulation and the use of specific reagents may remarkably enhance the observed luminescence of several molecules. The term “enhancement” of luminescence comprises various sample treatments leading to an increase of the emitted radiation. These treatments include the addition of non-fluorescent compounds to, or the creation of organized media (surfactants, cyclodextrins, heavy atoms) in, the sample to be measured. They may also involve changes in molecular environment, pH, the application of excessive drying conditions, the removal of oxygen, the protection of adsorbed compounds against non-radiative decay mechanisms by means of specific spraying or dipping conditions, amongst others. The use of organized media in luminescence spectroscopy is growing. Many of the recent studies have involved micelles for enhancing the fluorescence, room temperature phosphorescence and chemiluminescence of several chemicals. Cyclodextrins are increasingly used for various analytical applications. Liquid paraffin, triethanolamine, dodecane, Triton X-100 and Fomblin Y-Vac are commonly used fluorescence enhancers in chromatographic assays. Examples of these systems in drug analysis are presented.

Photoluminescence characterization of rare-earth (Er, Yb)-doped InP grown by metalorganic chemical vapor deposition

Er-doped and Yb-doped InP epitaxial crystals were grown by MOCVD and photoluminescence (PL) of intra-4f-shell transitions of the rare earth ions was observed. Rare-earth-ion-related PL intensities are observed to increase only sublinearly as a function of excitation photon density. Their decay time constants are about one order of magnitude smaller than those of corresponding rare earth ions in CdS. These results indicate that a strong nonradiative bimolecular decay mechanism is in effect in state-of-the-art crystals and that intra-4f-shell transitions of rare earth ions depend strongly on the host crystals in which the ions are incorporated.

Photoinduced reactions. 158. Photochemical hydrogen-deuterium exchange reaction of tryptophan. The role of nonradiative decay of singlet tryptophan

The mechanism of nonradiative decay of singlet excited tryptophan (Trp) in aqueous solution was investigated by a highly selective photosubstitution of the C-4 hydrogen of Trp with deuterium of solvent D/sub 2/O. It was concluded that intramolecular proton transfer from the ..cap alpha..-ammonia group giving rise to formation of a protonated species plays an important role in the nonradiative decay of singlet Trp at neutral pH. 11 references, 1 figure.