Spectroscopy Of Excited States Research Articles

Conformational Effects on Excitonic Interactions in a Prototypical H-Bonded Bichromophore: Bis(2-hydroxyphenyl)methane

Laser-induced fluorescence, single-vibronic level fluorescence (SVLF), UV hole burning, and fluorescence dip infrared (FDIR) spectroscopy have been carried out on bis-(2-hydroxyphenyl)methane in order to characterize the ground-state and first excited-state vibronic spectroscopy of this model flexible bichromophore. These studies identified the presence of two conformational isomers. The FDIR spectra in the OH-stretch region determine that conformer A is an OH...O H-bonded conformer, while conformer B is a doubly OH...pi H-bonded conformer with C(2) symmetry. High-resolution ultraviolet spectra ( approximately 50 MHz resolution) of a series of vibronic bands of both conformers confirm and refine these assignments. The transition dipole moment (TDM) direction in conformer A is consistent with electronic excitation that is primarily localized on the donor phenol ring. A tentative assignment of the S(2) origin is made to a set of transitions approximately 400 cm(-1) above S(1). In conformer B, the TDM direction firmly establishes C(2) symmetry for the conformer in its S(1) state and establishes the electronic excitation as delocalized over the two rings, as the lower member of an excitonic pair. The S(2) state has not been clearly identified in the spectrum. Based on CIS calculations, the S(2) state is postulated to be several times weaker than S(1), making it difficult to identify, especially in the midst of overlap from vibronic bands due to conformer A. SVLF spectra show highly unusual vibronic intensity patterns, particularly in conformer B, which cannot be understood by simple harmonic Franck-Condon models, even in the presence of Duschinsky mixing. We postulate that these model flexible bichromophores have TDMs that are extraordinarily sensitive to the distance and orientation of the two aromatic rings, highlighting the need to map out the TDM surface and its dependence on the (up to) five torsional and bending coordinates in order to understand the observations.

Excited-state spectroscopy of single NV defects in diamond using optically detected magnetic resonance

Using pulsed optically detected magnetic resonance techniques, we directly probe electron-spin resonance transitions in the excited-state of single nitrogen-vacancy (NV) color centers in diamond. Unambiguous assignment of excited state fine structure is made, based on changes of NV defect photoluminescence lifetime. This study provides significant insight into the structure of the emitting 3E excited state, which is invaluable for the development of diamond-based quantum information processing.

Excited-state spectroscopy of single Pt atoms in Si

Resonant peaks superimposed on the direct tunneling current in the low-temperature transport of Schottky barrier metal-oxide-semiconductor field-effect transistor inversion layers are investigated. The resonances are attributed to single Pt atoms that have diffused from the metallic contacts into the depletion width near the metal/semiconductor interface. Excited-state spectroscopy and magnetic fields are used to identify different levels. A double donor level in a singlet state and another level that is attributed to a triple donor state are observed.

Comment on “Level structure ofRh92: Implications for the two-proton decay of94Agm”

A recent publication describes results obtained by in-beam $\ensuremath{\gamma}$-ray spectroscopy of excited states of $^{92}\mathrm{Rh}$ and concludes, by mainly using extrapolated $Q$ values, that the previously reported direct two-proton decay of $^{94}\mathrm{Ag}$ (${21}^{+}$) to $^{92}\mathrm{Rh}$ is very difficult to reconcile with the $\ensuremath{\gamma}$-ray data. We present evidence that this conclusion may be misleading.

Excited-state spectroscopy on a quantum dot side coupled to a quantum wire

We report excited-state spectroscopy on a quantum dot side coupled to a quantum wire with accurate energy estimation. Our method utilizes periodic voltage pulses on the dot, and the energy calibration is performed with reference to the bias voltage across the wire. We demonstrate the observation on the orbital excited state and the Zeeman splitting in a single dot.

A Spectroscopic Study of the Interaction of the Fluorescent β-Carboline-3-carboxylic Acid N-methylamide with DNA Constituents: Nucleobases, Nucleosides and Nucleotides

Interaction between beta-carboline-3-carboxylic acid N-methylamide, betaCMAM, and nucleobases, nucleosides and nucleotides is studied in the ground state with UV-visible, (1)H NMR and (31)P NMR spectroscopies and in the first excited state, with steady-state and time-resolved fluorescence spectroscopy. Job plots show a predominant 1:1 interaction in both electronic states. Association constants are estimated from changes in the absorption spectra, and show that the strongest interaction is produced with the nucleosides: 2'-deoxyadenosine (dAdo) and thymidine (Thd), and with the mononucleotides: 2'-deoxycytidine 5'- monophosphate (5'-dCMP) and uridine 5'- monophosphate (5'-UMP). These results are corroborated by the upfield shifts of two (1)H NMR resonances of the betaCMAM indole group. The (31)P NMR resonance of nucleotides is shifted downfield, suggesting the presence of electrostatic or hydrogen bond interaction with betaCMAM. In the first electronic singlet excited state, static and dynamic quenching of betaCMAM emission is achieved upon addition of nucleobases, nucleosides and nucleotides. This has been analysed using Stern-Volmer kinetics.

Direct observation of intersystem crossing in charge-pair states in polyfluorene polymer blends

In conjugated polymers, the process of interconversion between singlet and triplet bound charge-pairs is important in determining the efficiency limits both for light-emitting diodes (LEDs) and for photovoltaics. We probe the kinetics of spin-mixing in charge-pairs by studying the formation of triplet excitons after photoexcitation of blends of poly(9,9-dioctylfluorene- co-benzothiadiazole) (F8BT) with poly(9,9-dioctylfluorene- co-bis- N, N′-(4-butylphenyl)-bis- N, N′-phenyl-1,4-phenylene-diamine) (PFB) using excited-state transient absorption spectroscopy. We see clear evidence of triplet formation from long-lived charge-pairs during a period up to 100 ns after the excitation pulse, and we find that the intersystem crossing rate in these charge-pairs exceeds 2.0 × 10 6 s −1.

Effect of thermal treatment on excited state spectroscopy of oxyfluoride borosilicate glass activated by Pr 3+ ions

In order to obtain new kind of glass ceramics (with improved mechanical strength, thermal and chemical resistance), oxyfluoride borosilicate (SiO 2–B 2O 3–Na 2O–LaF 3) glass, containing Pr 3+ ions (0.82–0.41 wt% Pr 2O 3), has been melted and then exposed to thermal treatment at 555 °C and 620 °C. Changes in the absorption and emission spectra and in the fluorescence kinetics are observed and analyzed. In this work we report in detail on the results of measurements along with interpretation which indicate an existence of a nano-crystalline phase in the glass as a result of thermal treatment.

Scientific opportunities in nuclear resonance spectroscopy from source-driven revolution

From the beginning of its discovery the Mossbauer effect has continued to be one of the most powerful tools with broad applications in diverse areas of science and technology. With the advent of synchrotron radiation sources such as the Advanced Photon Source (APS), the European Synchrotron Radiation Facility (ESRF) and the Super Photon Ring-8 (SPring-8), the tool has enlarged its scope and delivered new capabilities. The popular techniques most generally used in the field of materials physics, chemical physics, geoscience, and biology are hyperfine spectroscopy via elastic nuclear forward scattering (NFS), vibrational spectroscopy via nuclear inelastic scattering (NRIXS), and, to a lesser extent, diffusional dynamics from quasielastic nuclear forward scattering (QNFS). As we look ahead, new storage rings with enhanced brilliance such as PETRA-III under construction at DESY, Hamburg, and PEP-III in its early design stage at SLAC, Stanford, will provide new and unique science opportunities. In the next two decades, x-ray free-electron lasers (XFELs), based both on self-amplified spontaneous emission (SASE-XFELs) and a seed (SXFELs), with unique time structure, coherence and a five to six orders higher average brilliance will truly revolutionize nuclear resonance applications in a major way. This overview is intended to briefly address the unique radiation characteristics of new sources on the horizon and to provide a glimpse of scientific prospects and dreams in the nuclear resonance field from the new radiation sources. We anticipate an expanded nuclear resonance research activity with applications such as spin and phonon mapping of a single nanostructure and their assemblies, interfaces, and surfaces; spin dynamics; nonequilibrium dynamics; photochemical reactions; excited-state spectroscopy; and nonlinear phenomena.

Fate of Excited States in Jet-Cooled Aromatic Molecules: Bifurcating Pathways and Very Long Lived Species from the S1 Excitation of Phenylacetylene and Benzonitrile

Pump-probe delayed ionization studies on phenylacetylene and benzonitrile in a supersonic beam reveal the production of a low-ionization-potential (approximately 5.7 eV) species lasting more than hundreds of microseconds after excitation to the S1 state. Excitation of the molecules was done with a frequency-doubled, Fourier transform-limited, pulse-amplified cw laser, and the rotationally resolved structure of the S1-S0 transition ensures that the excited molecules are monomers. Excited-state photoelectron spectroscopy shows that the long-lived species are formed during the light pulse but not by transfer from the fluorescing S1 population after the pulse, even though the S1 spectral signature is present in the long-lived action spectrum. This behavior differs greatly from that found in benzene and with most commonly held pictures of radiationless transitions in large molecules.

Nonlinear optical spectroscopy of excited states in polyfluorene

We used a variety of nonlinear optical (NLO) spectroscopies to study the singlet excited-state order and primary photoexcitations in polyfluorene, an important blue-emitting $\ensuremath{\pi}$-conjugated polymer. The polarized NLO spectroscopies include ultrafast pump-probe photomodulation in a broad spectral range of $0.2--2.6\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, two-photon absorption in the range of $3.2--4.2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, and electroabsorption covering the spectral range of $2.8--5.0\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. For completeness, we also measured the linear absorption and photoluminescence spectra. We found that the primary photoexcitations in polyfluorene are singlet excitons with $\ensuremath{\sim}100\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$ lifetime that have a characteristic photomodulation spectrum comprising of two photoinduced absorption (PA) bands, ${\mathrm{PA}}_{1}$ at $0.55\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and ${\mathrm{PA}}_{2}$ at $1.65\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, and a strong stimulated emission band that peaked at $\ensuremath{\sim}2.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The two-photon absorption and electroabsorption spectra identify the exciton PA bands with optical transitions between the lowest-lying odd symmetry $1{B}_{u}$ exciton at $3.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and two strongly coupled even symmetry states, namely, $m{A}_{g}$ at $3.7\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and $k{A}_{g}$ at $4.7\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The excited state manifold also contains a strongly coupled odd symmetry exciton $n{B}_{u}$ at $4.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, which identifies the continuum band threshold. A polarization memory of $\ensuremath{\sim}2.1$, typical to films of $\ensuremath{\pi}$-conjugated polymers, characterizes all three NLO spectra, reflecting the highly anisotropic third-order NLO coefficient, ${\ensuremath{\chi}}^{(3)}$ of the polymer chains. The four essential states, namely, $1{A}_{g}$, $1{B}_{u}$, $m{A}_{g}$, and $n{B}_{u}$, were used to satisfactorily fit the ${\ensuremath{\chi}}^{(3)}$ optical spectra using the summation over states model. The combination of the three NLO spectra and the model fit conclusively show that the band model, typical in inorganic semiconductors, cannot properly describe the poly(9,9-dioctylfluorene) polymer. On the contrary, a strongly bound exciton with intrachain binding energy of $\ensuremath{\sim}1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ dominates the linear and NLO spectra of this polymer.

Photon-Assisted Tunneling in a Carbon Nanotube Quantum Dot

We report on photon-assisted tunneling (PAT) experiments in a carbon nanotube quantum dot using microwave frequencies between 20 and 60 GHz. In addition to the basic PAT effect, revealed by the appearance of two extra resonances in the current through the dot, we use PAT for spectroscopy of excited states. The experimental data are in good agreement with simulations.

γ-ray spectroscopy of excited states in 61 30Zn31

γ-ray spectroscopy of excited states in 61 30Zn31

Excited State Spectroscopy in Carbon Nanotube Double Quantum Dots

We report on low-temperature measurements in a fully tunable carbon nanotube double quantum dot. A new fabrication technique has been used for the top-gates in order to avoid covering the whole nanotube with an oxide layer as in previous experiments. The top-gates allow us to form single dots and control the coupling between them, and we observe 4-fold shell filling. We perform inelastic transport spectroscopy via the excited states in the double quantum dot, a necessary step toward the implementation of new microwave-based experiments.

Photoelectron spectroscopy of E′ centers in crystalline and glassy silicon dioxide

Radiation-induced E′ centers in SiO2 were studied to test the possibility of applying optically stimulated electron emission (OSEE) to the spectroscopy of excited states of point defects in dielectrics. The spectral responses of the OSEE of crystalline α quartz and silica glass irradiated by 10-MeV electrons were measured and studied. It was established that volume E′ centers in the crystalline and glassy SiO2 modifications are dominant emission-active defects. Surface E’s (1) centers were also detected in glassy SiO2. A model of the energy structure of E′ centers accounting for the absence of luminescence and taking into account the presence of two nonradiative (intracenter and ionization) relaxation channels is proposed. This model was used to explain the mechanism of photothermal decay of the E′ centers and to determine the ionization activation barriers and quantum yields of these centers. The emission, spectral, and kinetic parameters of the volume and surface E′ centers in glassy SiO2 were obtained, showing the excited states of these defects to have identical atomic configurations.

Spin filling of a quantum dot derived from excited-state spectroscopy

We study the spin filling of a semiconductor quantum dot using excited-state spectroscopy in a strong magnetic field. The field is oriented in the plane of the two-dimensional electron gas in which the dot is electrostatically defined. By combining the observation of Zeeman splitting with our knowledge of the absolute number of electrons, we are able to determine the ground state spin configuration for one to five electrons occupying the dot. For four electrons, we find a ground state spin configuration with total spin S = 1, in agreement with Hund's first rule. The electron g-factor is observed to be independent of magnetic field and electron number.

Comparison of the chain length dependence of the singlet- and triplet-excited states of oligofluorenes

Using time-resolved photoluminescence and excited-state absorption spectroscopy, we investigate the chain length dependence of the energies of the singlet (S 1) and triplet (T 1 and T n ) excited states of a homologous series of conjugated 9,9-dihexylfluorene-2,7-diyl oligomers ( nF, n = 1, 3, 5, 7, ∼50) in frozen solution. The energies of the optical transitions in absorption, fluorescence, phosphorescence, and triplet–triplet absorption initially decrease linearly with the reciprocal number of repeat units, but saturate at n⩾5. The saturation and dispersion of the optical transition energies indicate that the T 1 state is only slightly more confined than the S 1 state.

Challenges for lasers in liquid oxygen

We review some aspects of the spectroscopy and kinetics of excited electronic states and vibrational levels and identify challenges that may impede development of high-energy lasers based on optical pumping of liquid oxygen.

Cathodoluminescence imaging and spectroscopy of excited states in InAs self-assembled quantum dots

We have examined state filling and thermal activation of carriers in buried InAs self-assembled quantum dots (SAQDs) with excitation-dependent cathodoluminescence (CL) imaging and spectroscopy. The InAs SAQDs were formed during molecular-beam epitaxial growth of InAs on undoped planar GaAs (001). The intensities of the ground- and excited-state transitions were analyzed as a function of temperature and excitation density to study the thermal activation and reemission of carriers. The thermal activation energies associated with the thermal quenching of the luminescence were measured for ground- and excited-state transitions of the SAQDs, as a function of excitation density. By comparing these activation energies with the ground- and excited-state transition energies, we have considered various processes that describe the reemission of carriers. Thermal quenching of the intensity of the QD ground- and first excited-state transitions at low excitations in the ∼230–300-K temperature range is attributed to dissociation of excitons from the QD states into the InAs wetting layer. At high excitations, much lower activation energies of the ground and excited states are obtained, suggesting that thermal reemission of single holes from QD states into the GaAs matrix is responsible for the observed temperature dependence of the QD luminescence in the ∼230–300-K temperature range. The dependence of the CL intensity of the ground-and first excited-state transition on excitation density was shown to be linear at all temperatures at low-excitation density. This result can be understood by considering that carriers escape and are recaptured as excitons or correlated electron–hole pairs. At sufficiently high excitations, state-filling and spatial smearing effects are observed together with a sublinear dependence of the CL intensity on excitation. Successive filling of the ground and excited states in adjacent groups of QDs that possess different size distributions is assumed to be the cause of the spatial smearing.

Charge sensing of excited states in an isolated double quantum dot

Pulsed electrostatic gating combined with capacitive charge sensing is used to perform excited state spectroscopy of an electrically isolated double-quantum-dot system. The tunneling rate of a single charge moving between the two dots is affected by the alignment of quantized energy levels; measured tunneling probabilities thereby reveal spectral features. Two pulse sequences are investigated, one of which, termed latched detection, allows measurement of a single tunneling event without repetition. Both provide excited-state spectroscopy without electrical contact to the double-dot system.