Fast Nonradiative Relaxation Research Articles

A molecular approach to the intermediate band solar cell: The symmetric case

Molecular materials may overcome some of the difficulties for making an intermediate band solar cell by storing energy in long-lived triplet states. Both the problems of fast nonradiative interband relaxation and selective photon absorption can be solved by this approach. A practical implementation of the molecular intermediate band solar cell is considered with a symmetric band alignment, resting on a proven triplet-triplet annihilation process. The limiting power conversion efficiency for this system exceeds that of a single bandgap device over a broad range, peaking at 40.6% for 1.9eV under 1 sun illumination.

Exciton Energy Transfer in Pairs of Single-Walled Carbon Nanotubes

We studied the exciton energy transfer in pairs of semiconducting nanotubes using high-resolution optical microscopy and spectroscopy on the nanoscale. Photoluminescence from large band gap nanotubes within bundles is observed with spatially varying intensities due to distance-dependent internanotube transfer. The range of efficient energy transfer is found to be limited to a few nanometers because of competing fast nonradiative relaxation responsible for low photoluminescence quantum yield.

Identification of the I− → Pr3+ charge transfer transition in the excitation spectrum of LuI3:Pr3+

The excitation spectrum of LuI3:Pr3+ is dominated by a strong ultraviolet band centered at 320nm. This band is identified as the I−→Pr3+ charge transfer transition. The Pr3+ 4f2→4f15d1 interconfiguration optical transitions are located within the host lattice absorption band. The luminescence of LuI3:Pr3+ under charge transfer excitation is dominated by the Pr3+ 4f2→4f2 intraconfiguration optical transitions. The absence of luminescence from the charge transfer state is due fast (nonradiative) relaxation of the charge transfer state into the Pr3+ 4f2 states.

Gas sensitive light emission properties of tin oxide and zinc oxide nanobelts

Light emission properties of tin oxide and zinc oxide nanobelts and nanowires have been investigated by means of continuous wave and time resolved photoluminescence. In both kind of nanostructures, photoluminescence quenching was observed at increasing temperature and at interaction with 5ppm concentration NO2. Photoluminescence time decay measurements in the visible range suggest that adsorbed NO2 induces fast non-radiative relaxation of surface electrons, competing with radiative surface-localized recombination. Such fast interaction between electrons and molecular adlayer may reduce the number of possible radiative recombination events at surface without significantly changing the subsequent slower radiative processes. Our measurements also show that adsorbate-related non-radiative recombination time should be lower than 1ns.

Fast electronic relaxation in metal nanoclusters via excitation of coherent shape deformations

Electron-phonon relaxation in size-quantized systems may become inhibited when the spacing of discrete electron energy levels exceeds the magnitude of the phonon frequency. We show, however, that nanoclusters can support a fast nonradiative relaxation channel which derives from their distinctive ability to undergo Jahn-Teller shape deformations. Such a deformation represents a collective and coherent vibrational excitation and enables electronic transitions to occur without a multiphonon bottleneck. We analyze this mechanism for a metal cluster within the analytical framework of a three-dimensional potential well undergoing a spheroidal distortion. An expression for the time evolution of the distortion parameter is derived, the electronic level crossing condition formulated, and the probability of electronic transition at a level crossing is evaluated. An application to electron-hole recombination in a closed-shell aluminum cluster with 40 electrons shows that the short $(\ensuremath{\sim}250\phantom{\rule{0.3em}{0ex}}\mathrm{fs})$ excitation lifetime observed in recent pump-probe experiments can be explained by the proposed mechanism.

Intersublevel polaron laser with InAs∕GaAs self-assembled quantum dots

We propose a three-level scheme to achieve intersublevel population inversion, optical gain, and intersublevel lasing effect in n-doped InAs∕GaAs self-assembled quantum dots under optical pumping. The proposed Ruby-type laser scheme uses the natural splitting of the s-p polaron intersublevel transitions around 25μm wavelength. The relaxation time engineering, which leads to optical gain in the system, relies (i) on the slow polaron relaxation from the P− state down to the S ground state, governed by the specific strong coupling regime for the electron-phonon Fröhlich interaction and (ii) on the fast nonradiative relaxation of the polaron between the P+ and P− levels through the irreversible emission of acoustic phonons. TE-polarized optical gain up to 330cm−1 is calculated for 80 quantum dot planes in an in-plane monomode waveguide geometry and a laser pump intensity threshold as low as 930W∕cm2, two orders of magnitude smaller than for quantum wells, is predicted.

Dimerization by Hydrogen Bonding and Photochemical Properties of Dipyridone

The ground state and the excited state of dipyridones are studied by various spectroscopic methods: absorption spectra, fluorescence spectra, time resolved fluorescence measurements, and nanosecond transient absorption spectra. In the system of 3-[(1,6-dihydro-6-oxo-2-pyridinyl)ethynyl]-2(1H)-pyridinone, the absorption and fluorescence spectra are assigned to the keto form in comparison to model compounds. The absorption spectra and the emission decay depend on the concentration in chloroform solution whereas they do not in methanol. This difference can be explained by the formation of a hydrogen-bonded dimer only in chloroform solution. The excited singlet states of the monomer and the dimer have different lifetimes even though they have same keto form. The origin of the fast nonradiative relaxation processes in the dimer system is discussed with the results of transient absorption spectroscopy. As an application of the formation of dimers by hydrogen bonding, the effect of the dimerization on photochem...

Excitation relaxation in films of dipolar N, N-dimethylaminobenzylidene 1,3-indandione molecules

Ultrafast relaxation of self-trapped (ST) excitons in polycrystalline film of N, N-dimethylaminobenzylidene 1,3-indandione (DMABI) molecules, which possess pronounced donor–acceptor properties, is studied by using femtosecond luminescence up-conversion spectroscopy. Exciton relaxation dynamics is compared to excitation decay obtained for DMABI molecules embedded in a rigid transparent matrix. Excitons in the film undergo rapid self-trapping on the time scale below 100 fs. A strong nonradiative recombination of ST excitons is observed on the subpicosecond time scale, which is shown to occur concurrently with their final localization. This final localization is attributed to the cooling of locally heated (up to 600 K) ST excitons, that occurs with the time constant τ c=800 fs. The fast nonradiative relaxation is assigned to the thermally activated transition of the hot excitons to the ground state during their thermalization. After the fast relaxation stage is completed, the second, slow exciton relaxation stage emerges, which is attributed to one-dimensional migration of excitons and their nonradiative decay on structural defects.

Fast decay of high vibronic S1 states in gas-phase benzene

Lifetimes of different vibronic levels at high excess energies in the S1 state of isolated benzene molecules are measured using for the first time a two-photon ionization pump–probe technique with UV femtosecond pulses. For the 6113 state (3290 cm-1 excess energy) at the onset of the ‘channel three’ a biexponential decay is found with a fast (τf=20 ps) and a slow (τs>500 ps) component. The values are in line with previous sub-Doppler high-resolution measurements of this band in our laboratory. We explain the measured faster decay (τf=900 fs, τs=100 ps) of the 71 state at a lower excess energy of 3077 cm-1 by a simultaneous excitation of the adjacent 61 013 0161 1 hot band leading to a vibronic state of higher excess energy and faster decay. The vibronic levels 6114 and 7111 at a higher excess energy of ≳4000 cm-1 show a biexponential decay of τf=550 fs, τs=30 ps and τf≲300 fs, τs=20 ps, respectively. The experimental results point to dynamic processes within the vibronic level manifold of the S1 state and a fast nonradiative electronic relaxation process.

Spectroscopy and energy relaxation processes of Hg-doped solid neon, argon, and xenon

Emission, absorption, and excitation spectroscopy has been used for a detailed analysis of the optical transitions of Hg2 trapped in cryogenic matrices. Upon excitation of electronic states correlating to the 3P1 or the 1P1 asymptote, fast nonradiative relaxation leads to emission from the lowest excited A0g+ state in all matrices, which decays monoexponentially in 1 ms in Ne, 280 μs in Ar, and 12 μs in Xe. In addition, electronically unrelaxed emission of Hg2 is reported in neon and in xenon matrices and attributed to the B1g state in neon and to the B1g state and the C0u− or A0g− states in xenon. The results are rationalized by assuming: (a) that population of the excited states occurs mainly close to the asymptotic limit, where branching is determined by nonadiabatic coupling and energetics, that are strongly environment dependent, and (b) that in Xe matrices the Hg2 states correlating to the 3P1 and 3P0 asymptotic limits are stabilized in different configurations, as a result of the very different solvation properties of the atomic 3P1 and the 3P0 state. Further emission bands are found in the vicinity of the dimer transitions, which we attribute to Hg3 and to site effects on Hg2. In particular, electronically unrelaxed emission from excited states of Hg3 is reported.

Spectroscopic and photochemical properties of Malachite Green noncovalently bound to bovine serum albumin

The noncovalent binding of the cationic triarylmethane dye Malachite Green (MG+) to bovine serum albumin (BSA) was investigated using fluorescence and UV-vis spectroscopy. The results indicate that the BSA binding sites are very effective in hindering fast nonradiative relaxation processes that occur via rotational motion of the aromatic rings of this triarylmethane. As a result, pronounced increases in both fluorescence yield and dye photoreactivity were observed upon protein binding. The 532 nm laser-induced photobleaching of protein-bound MG+ yields leuco malachite green and 4-dimethylaminobenzophenone as major reaction photoproducts. Based on the nature of these products, the first step of the bleaching process is postulated to be an electron or hydrogen atom transfer from the protein to the dye moiety.

Laser Flash Photolysis Studies on the Monohydroxy Derivatives of Benzophenone

Time-resolved studies in the pico- and nanosecond time domain have been performed to characterize the triplet states of monohydroxy-substituted benzophenones, namely, para- (p-), meta- (m-), and ortho- (o-) hydroxybenzophenones (HOBP). Due to a very fast intersystem crossing (ISC) process, only the triplet states have been detected in the subnanosecond time domain. Spectral characteristics and lifetimes of the triplet states of HOBP have been seen to be extremely sensitive to the position of the OH group in the phenyl ring as well as the solvent characteristics. In case of m-HOBP and p-HOBP, the excited triplet state in non-hydrogen-bond-forming solvents has an nπ* configuration and is capable of abstracting a hydrogen atom from another unexcited molecule to form ketyl and phenoxy type radicals. But in hydrogen-bond-forming solvents, the triplet state, which is strongly associated with the solvent molecules as a hydrogen-bonded complex, is very short-lived due to fast nonradiative relaxation via hydrogen-...

Fluorescence quenching of the Nd 3+ ions in different optical centers in fluorite-type crystals

The results of measurements of fast nonradiative relaxation rates from the strongly quenched 4G 5 2 ; 2G 7 2 multiplet of the Nd 3+ ion in the LiYF 4 crystal and in the fluorite-type crystals (CaF 2 : NdF 3(0.6%), CaF 2: NdF 3(0.25%): LaF 3(0.25%) and CaF 2: NdF 3(0.25%): CeF 3(0.25%)) with different Nd 3+ optical centers by direct laser-selective-excitation and selective fluorescence kinetics decay measurements are presented. The multiphonon relaxation time of 4G 5 2 + 2G 7 2 multiplet of the Nd 3+ ion in the single-site LiYF 4 crystal ( τ = 4.5 ns); and in a multi-site CaF 2 crystal for a tetragonal symmetry the L-center ( τ L < 2.0 ns), for a rhombic symmetry pair (the M- and M′-centers) ( τ M = 5.2 ns and τ M′ = 7.1 ns, respectively), and the cross-relaxational energy transfer rate inside the Nd ∗ Nd pair in CaF 2 ( τ tr −1 = ( τ M −1 − τ M′ −1) −1 = 5.2 × 10 7 s −1) were measured. Comparison of the experimental multiphonon relaxation rates with the calculated ones by the nonlinear theory of multiphonon relaxation was done. The dependence of multiphonon relaxation rates on crystal matrix parameters is discussed. Comparison of the quenched energy transfer rate between two Nd 3+ ions at minimal distance for the 4G 5 2 ; 2G 7 2 multiplet in fluoride crystals was done. The fastest rate in CaF 2 ( W = 5.2 × 10 7 s −1) with R min = 0.386 nm and the slowest in LaF 3 ( W = 4.35 × 10 7 s −1) with R min = 0.410 nm were found. The analysis of the dominant mechanisms and possible channels of cross relaxation was made.

Luminescence properties of chromium(V) doped into various host lattices

The luminescence properties of the CrO 3− 4 ion (3d 1) in various host lattices are reported and discussed. The luminescence is due to ligand field transitions and consists of two overlapping emissions due to the doublet splitting of the 2E ground state in lower symmetries than T d. Already at 15 K the luminescence decay times are dominated by fast nonradiative relaxation processes and the luminescence quantum yields at room temperature are 1% or less for all systems. The distortion of the host tetrahedron largely determines the variation of the nonradiative rates for the present materials.

MEASUREMENTS OF FAST RELAXATION PROCESSES USING PUMP-PROBE PHOTOTHERMAL SPECTROSCOPY

A novel technique by adopting pump-probe detection to thermal lens spectroscopy has been proposed for investigating fast nonradiative relaxation processes. It was applied to measurements of the relaxation of Rhodamine 6G in various solutions. The results show that nonradiative relaxation processes can be observed with time resolution of sub-nanosecond.

Electronic Spectra of 1‐Phenylethylamine and its Derivatives in Supersonic Jets

The S1 ← S0 fluorescence excitation spectra and dispersed fluorescence spectra of jet‐cooled (+)‐, (‐)‐ and (±)‐1‐phenylethylamine and their derivatives (amides) have been observed. The 0‐0 band of the amine locates at 37,641 cm‐1. The amides which were synthesized from (+)‐amine or (‐)‐amine with (+)‐tartaric acid are diastereomers. It was found that the two diastereomers give the identical spectra with the 0‐0 band at 34,757 cm-1. No difference in the spectrum indicates that the excitation is localized in the phenyl group which is far from the asymmetric carbon causing diastereoism. It was also found that 1‐phenylethylamine has a fast nonradiative relaxation process in the Sstate, but such a process is removed by the formation of the amide.

Carotenoid fluorescence

Fluorescence excitation and emission spectra are reported from the carotenoids β-carotene, rhodopin and spheroidenone in carbon disulfide. From the small Stokes shift and the high emission anisotropy it is concluded that the fluorescence is emitted during the 1B u→1A g transition. From the small quantum yields, of about 3×10 −5 for spheroidenone and 6×10 −5 for β-carotene, and the 15 ps ground-state recovery lifetime for spheroidenone combined with a natural radiative lifetime of 1–10 ns, it is concluded that a fast (30–300 fs) non-radiative relaxation occurs. This probably represents the transition from the 1B u to a low-lying 2A g state. Anomalities in the excitation spectra also indicate that this process competes efficiently with vibrational relaxation within the 1B u state.

Resonant excitation of the GR centre in diamond

Photoluminescence in the GR1 zero-phonon region has been examined in detail, using a tunable dye laser resonant with the higher excited states GR2 to GR8. The authors conclude that the temperature-dependent structure observed when electron beam excitation is used is not due to perturbation of the centre resulting from the fast non-radiative relaxation from the higher levels to the first excited state.

Lifetimes of single rotational states in the ‘‘channel three’’ region of C6H6

Lifetimes of single rotational states in the 14112 vibrational state of benzene, C6H6, at 3412 cm−1 excess energy were measured. The lifetimes depend strongly on the rotational quantum number J and we find them to agree with our recent homogeneous linewidth measurements of the same states. We present a coupling scheme including the coupling of the excited narrow light zero order state with a broadened dark background state. The broadening is due to a fast electronic nonradiative relaxation process typical for a state containing quanta of out-of-plane modes. It is demonstrated that the observed J dependence is due to perpendicular Coriolis coupling in the weak coupling limit. The IVR process resulting from this coupling explains the onset of ‘‘channel three’’ in a natural way without any further mystery states in the model.

Measurements on gain and fast nonradiative relaxation of optically excited F-centers in KCI

The nonradiative relaxation times of F-centers in KCI representing the duration for the formation of relaxed configuration in the excited and ground state are experimentally determined by use of picosecond laser spectroscopy. They are found to be less than 10 ps. The measurements also allow to estimate the cross-section of stimulated emission of the F-centers to be σ e ≈ 1.2 × 10 -18 cm 2 and the nonradiative deactivation time of optically excited F-centers in NaBr.