Higher Electronic States Research Articles

Modeling deoxyribose radicals by neutralization-reionization mass spectrometry. Part 2. Preparation, dissociations, and energetics of 3-hydroxyoxolan-3-yl radical and cation

The title radical ( 1) is generated in the gas-phase by collisional neutralization of carbonyl-protonated oxolan-3-one. A 1.5% fraction of 1 does not dissociate and is detected following reionization as survivor ions. The major dissociation of 1 (∼56%) occurs as loss of the hydroxyl H atom forming oxolan-3-one ( 2). The competing ring cleavages by OC-2 and C-4C-5 bond dissociations combined account for ∼42% of dissociation and result in the formation of formaldehyde and 2-hydroxyallyl radical. Additional ring-cleavage dissociations of 1 resulting in the formation of C 2H 3O and C 2H 4O cannot be explained as occurring competitively on the doublet ground ( X ) electronic state of 1, but are energetically accessible from the A and higher electronic states accessed by vertical electron transfer. Exothermic protonation of 2 also produces 3-oxo-( 1H)-oxolanium cation ( 3 + ) which upon collisional neutralization gives hypervalent 3-oxo-( 1H)-oxolanium radical ( 3). The latter dissociates spontaneously by ring opening and expulsion of hydroxy radical. Experiment and calculations suggest that carbohydrate radicals incorporating the 3-hydroxyoxolan-3-yl motif will prefer ring-cleavage dissociations at low internal energies or upon photoexcitation by absorbing light at ∼590 and ∼400 nm.

Strong evidence of field-induced superconductivity and Shubnikov-de Haas oscillation in κ -(BETS)2FeBr4

High field electronic state of the quasi-two dimensional organic superconductor, κ-(BETS) 2 FeBr 4 was investigated. Under the magnetic field exactly parallel to the conducting layers, zero resistance was observed in the region of about 11 ∼ 14 T at 27 mK. This is the strong evidence of the field induced superconductivity in this salt. In the antiferromagnetic state below 5 T, Shubnikov-de Haas oscillation with a low frequency of 62 T was observed. This new frequency can be ascribed to a small pocket formed by the reconstruction of the Fermi surface due to the antiferromagnetic order.

Phase diagram of magnetic-field-induced superconductor λ-(BETS)2FeCl4-xBrx

Resistance measurements of λ-(BETS) 2 FeCl 4-x Br x (x=0.7) have been performed to investigate the high magnetic field electronic state. When the magnetic field is applied exactly parallel to the c axis (in the conducting layers), superconducting phase is stabilized above 27 T for T< 2K after the antiferromagnetic insulating (AFI) phase is removed. The critical temperature T has a maximum at about 32 T, which is in good agreement with the case for x=0. However, the AFI phase is more stabilized for x=0.7 than for x=0.

The time-resolved spectroscopy of InGaAs/AlGaAs heterostructures with asymmetric funnel-shape quantum wells for near- and mid-IR lasing

The dynamics of interband transitions in InGaAs/AlGaAs heterostructures with funnel-shaped quantum wells, when a narrow and deep well is positioned asymmetrically in wide and shallow ones, has been studied experimentally. These structures have been proposed as a dual-colour laser in mid- and near-IR range simultaneously. The lifetime of electrons in the ground and excited states in the quantum wells has been determined by the time of the photoluminescence intensity decline. The obtained lifetime values allow precise identification of peaks in the photoluminescence spectra and a deep insight into the process of high electron states populating. The experimentally found electron lifetime for the interband transitions corresponds on the order of magnitude with the interband transitions lifetime in GaAs. Analysis of the photoluminescence spectra as well as the time dependence of the photoluminescence intensity decline indicates a possibility of population inversion achievement in these structures under high excitation.

X-ray crystallographic, spectroscopic and molecular sensing properties of fac-tricarbonylchloro(di-2-pyridylketonebenzoylhydrazone)rhenium(I) dimethylformamide solvate

The molecular sensor fac-[Re(CO) 3(dpkbh)Cl]·DMF ( 1) where DMF= N, N-dimethyl formamide and dpkbh=di-2-pyridylketonebenzoylhydrazone crystallized in the acentric triclinic space group P1. Structural analysis shows two units of ( 1) with rhenium in distorted octahedral geometry, the carbonyl groups are in facial positions with an average C–Re–C angle of 90° and the N, N-bidentate dpkbh forms six-membered metallocyclic ring with rhenium in a boat conformation. The molecular packing shows the binding of DMF to fac-[Re(CO) 3(dpkbh)Cl] and helical strands of ( 1) interlocked via a network of hydrogen bonds. Optical measurements on fac-[Re(CO) 3(dpkbh)Cl] in DMF show the presence of two interlocked charge transfer bands at 440 and 318 nm that are highly sensitive to slight variations in their surroundings. As the concentration decreases, the intensity of the low energy absorption band at 440 nm increases and the reverse is observed when the concentration increases. Thermo-optical measurements on fac-[Re(CO) 3(dpkbh)Cl] in DMF established the reversible interconversion between the high and low energy electronic states of fac-Re(CO) 3(dpkbh)Cl and changes in enthalpy (Δ H ∅) of+17.44±0.40 kJmol −1, entropy (Δ S ∅) of +37.45±0.50 J mol −1 and free energy (Δ G ∅) of +6.27±0.30 kJ mol −1 in DMF were calculated. Acids or bases in concentration as low as 1.0×10 −9 M can be detected and determined using fac-[Re(CO) 3(dpkbh)Cl] in DMF. 1H-NMR measurements on fac-[Re(CO) 3(dpkbh)Cl] in DMF in non-aqueous media reveal high sensitivity to solvent and temperature variations and points to strong solvent-complex interactions. The amide protons and solvate protons show high sensitivity to temperature variations than the aromatic protons pointing to the participation of the amide proton in solvate-complex hydrogen bonds in non-aqueous solutions of fac-[Re(CO) 3(dpkbh)Cl].

Single-electron-phonon interaction in a suspended quantum dot phonon cavity.

An electron-phonon cavity consisting of a quantum dot embedded in a freestanding GaAs/AlGaAs membrane is characterized using Coulomb blockade measurements at low temperatures. We find a complete suppression of single electron tunneling around zero bias leading to the formation of an energy gap in the transport spectrum. The observed effect is induced by the excitation of a localized phonon mode confined in the cavity. This phonon blockade of transport is lifted at discrete magnetic fields where higher electronic states with nonzero angular momentum are brought into resonance with the phonon energy.

Nuclear magnetic resonance and optosensing properties of di-2-thienyl ketone p-nitrophenylhydrazone (DSKNPH) in non-aqueous media

The compound di-2-thienyl ketone p-nitrophenylhydrazone (DSKNPH) melting point 168–170 °C was isolated in good yield from the reaction between di-2-thienyl ketone (DSK) and p-nitrophenylhydrazine in refluxing ethanol containing a few drop of concentrated HCl. Nuclear magnetic resonance studies on DSKNPH in non-aqueous solvents revealed strong solvent and temperature dependence due to solvent–solute interactions. Optical measurements on DSKNPH in DMSO in the presence and absence of KPF 6 gave extinction coefficients of 83,300±2000 and 25,600±2000 M −1 cm −1 at 612 and 427 nm at 295 K. In CH 2Cl 2, extinction coefficient of 34,000±2000 M −1 cm −1 was calculated at 422 nm. When DMSO solutions of DSKNPH were allowed to interact with DMSO solutions of NaBH 4 the low energy electronic state becomes favorable and when DMSO solutions of DSPKNPH where allowed to interact with DMSO solutions of KPF 6 or NaBF 4, the high energy electronic state becomes favorable. The reversible BH 4 −/BF 4 − interconversion points to physical interactions between these species and DSKNPH and hints to the possible use of DSKNPH as a spectrophotometric sensor for a variety of physical and chemical stimuli. Thermo-optical measurements on DSKNPH in DMSO confirmed the reversible interconversion between the high and low energy electronic states of DSKNPH and allowed for the calculations of the thermodynamic activation parameters of DSKNPH. Changes in enthalpy (Δ H ∅) of +57.67±4.20; 27.15±0.90 kJ mol −1, entropy (Δ S ∅) of +160±12.88; 83±2.91 J mol −1 and free energy (Δ G ∅) of −8.52±0.40; 2.66±0.25 kJ mol −1 were calculated at 295 K in the absence and presence of NaBH 4, respectively. Manipulation of the equilibrium distribution of the high and low energy electronic states of DSKNPH allowed for the use of these systems (DSKNPH and surrounding solvent molecules) as molecular sensors for group I and II metal ions. Group I and II metal ions in concentrations as low as 1.00×10 −5 M can be detected and determined using DSKNPH in DMSO.

Features of Emission and Absorption of Indotricarbocyanine Dyes on Excitation to Higher Electronic States

The manifestation of ion pairs in the absorption and emission of fluorescence by symmetric indotricarbocyanine dyes with the same cation and anions Br, I, and BF4 in ethanol, methylene chloride, and dichlorobenzene has been investigated. It is shown that the formation of ion pairs on passing to low-polarity solvents, along with the changes in the electronic absorption and emission spectra, manifests itself in the polarization and fluorescence excitation spectra. We were the first to establish that the presence of an equilibrium mixture of contact ion pairs and free ions in solutions has an effect on the position and probabilities of transitions not only for the first but also for higher excited singlet states of the molecules of polymethine dyes. The formation of ion pairs manifests itself more clearly when the spectral-luminescence properties of compounds in the shortwave spectral region are analyzed. It has been established that marked changes arise in the absorption, excitation, emission, and polarization fluorescence spectra, as well as in the lifetime and quantum yield of fluorescence in this spectral region when the solvent is changed.

Energy transfer involving higher electronic states: a new direction for molecular logic gates

The design of a molecular logic circuit based on sequential forward S 2–S 2 energy transfer and back S 1–S 1 energy transfer (cyclic energy transfer) for a system comprising of an azulene and a zinc porphyrin is discussed. Using a computational dynamics simulation, the effect of varying the orientation factor via the viscosity of the medium on the cyclic energy transfer dynamics is investigated and is incorporated into the logic circuit.

Magnetic and optical properties of single 4f n and mixed 4f n−1 5d electronic configurations of trivalent rare earth ions in wide band gap dielectric crystals

The investigation of the optical and magnetic properties of the rare earth ions in wide band gap fluoride dielectric crystals could trigger a new variety of applications using high excited electronic states. By applying laser-induced fluorescence spectroscopy in the vacuum ultraviolet (VUV) region of the spectrum (100–200 nm), it was found strong emission in the VUV and UV spectral regions due to the strong dipole 4f n−1 5d→4f n interconfigurational transitions of rare earth (RE) ions in wide band gap fluoride crystals. The emitted radiation is highly directional suggesting a strong paramagnetic response of the 4f n−1 5d electronic configuration following VUV laser excitation at 157 nm. In addition, the concentration of the Pr 3+ and Tm 3+ ions in KY 3F 10 and CaF 2 crystal hosts can be accurately evaluated by measuring the magnetic moment of the Pr 3+ and Tm 3+ ions, using the vibrating sample magnetometer method, in a non-destructive way for the first time to our knowledge. Since the efficiency of the laser crystals doped with trivalent RE ions depends on the degree of the homogeneous distribution of the concentration of the trivalent RE ions over the crystal volume, the method can be used to evaluate the quality of the laser crystals.

Ultrafast Dynamics of Porphyrins in the Condensed Phase: II. Zinc Tetraphenylporphyrin

Femtosecond spectroscopic studies of zinc tetraphenylporphyrin (ZnTPP) in benzene and dichloromethane are reported, combining both fluorescence up-conversion and transient absorption measurements. The purpose is to investigate the initial electronic and vibrational relaxation of the S_1 and S_2 excited states, in a system in which interference from solvent rearrangement is insignificant as evidenced by the small Stokes shift in the fluorescence. Excitation of the low-lying singlet excited state (S_1) results in nanosecond relaxation, while excitation to S_2, the Soret band, leads to multiple electronic and vibrational relaxation time scales of S_2 and S_1 populations, from hundreds of femtoseconds to tens of picoseconds. The systematic and detailed studies reported here reveal that the Soret fluorescence band decays with a lifetime in benzene of 1.45 ps for excitation at 397 nm, while emission monitored at the same wavelength, but for two-photon 550 nm excitation, decays biexponentially with 200 fs and 1.0 ps time constants. In addition, the Soret fluorescence decay lifetime for 397 nm excitation is distinctly longer than the rise time of S_1 fluorescence for the same excitation, which varies with wavelength. These observations are consistent with the model presented here in which the Soret band structure consists of absorption from S_0 to two manifolds of states with distinct electronic and vibrational couplings to S_1 and higher electronic states. To compare with literature, we also measured the S_2 lifetime in dichloromethane and found it to be 1.9 ps, a lengthening from its value in benzene. However, the transient fluorescence intensity is greatly reduced. These observations in dichloromethane provide evidence of an ultrafast (<100 fs) channel for electron transfer from ZnTPP to dichloromethane for a subset of excited molecules in favorably oriented contact with the solvent, that is, a bifurcation of population. Finally, solvent-induced vibrational relaxation of the S_1 population following internal conversion from S_2 occurs over a range of time scales (picoseconds to tens of picoseconds) depending on the wavelength (fluorescence or transient absorption), and the observed rate indeed changes with solvent.

Photodissociation of NO2 in the Region 217−237 nm: Nascent NO Energy Distribution and Mechanism

Photodissociation of NO2 in the region of 217−237 nm is investigated by probing the nascent NO generated using a one-laser photofragmentation/fragment-detection technique. By mixing O2 in the sample (20%−100%) and by setting proper detection timing, only mass-resolved excitation spectra (MRES) of photofragmented NO are obtained and examined. The nascent NO spectrum changes depending on the intensity (W/cm2) of the laser beam. For low laser intensity, the dissociation of NO2 produces rotationally and vibrationally cold NO through the NO2 22B2 (B̃) excited state. Excess energy for this photofragmentation is ∼0.4 eV at 226 nm. This excess energy is distributed almost entirely to the kinetic energy of NO(X 2A2) and O(1D) products. The spectrum of photofragmented NO becomes very crowded as the laser intensity is increased because higher rotational and vibrational levels of NO become populated through multiphoton excitation and eventual photofragmentation of NO2 from higher-energy electronic states. The rotational temperature of NO is ca. 200 K for high laser intensity and less than 30 K for low laser intensity. Near 230 nm, a rovibronic spectrum is observed that cannot be attributed to NO even though it is detected in the NO mass channel. The time-of-flight mass spectrum line width for photofragmented NO in this photolysis region increases by a factor of 2 to about 40 ns (estimated NO kinetic energy is 0.40 ± 0.04 eV) for high laser intensity. These observations lead to the conclusion that a higher electronic state of NO2 has been accessed to generate highly excited NO through multiphoton absorption processes for the high laser intensity experiments. Such intermediate states can be Rydberg, ion pair, or other high-energy states of NO2 accessed by multiphoton absorption.

Ab Initio Study of Structure and Spectra of MnH2, MnH2-, and MnH3

Molecules of manganese hydrides MnH 2 , MnH 2 -, and MnH 3 were studied using large basis sets up to spdfg quality and the coupled cluster technique for the treatment of electron correlation. Both MnH 2 and MnH 2 - have linear equilibrium structures in their high spin ground electronic state, the MnH 2 - molecule having a rather flat bending potential. The MnH 3 molecule is Y-shaped with valence angle α(H-Mn-H) = 44.6° in the 5 B 2 ground electronic state due to the strong Jahn-Teller distortion of the trigonal planar structure. The harmonic vibrational frequencies, IR intensities, and the relative energies of excited states were calculated. Fundamental vibrational wavenumbers and electron affinity were also found for MnH 2 . The results are compared with available spectroscopic data.

Alkyl-substituted magnesium phthalocyanine: phototoxicity after excitation of higher electronic states in cellsin vitro

Photodynamic tumour therapy (PDT) on the basis of a sequential two-photon excitation of suitable sensitizers is expected 1) to prevent skin phototoxicity caused by day-light and 2) may occur via an oxygen-independent mechanism of photosensitization. Here we investigated cellular uptake, localization and phototoxicity of ( t-butyl )4- PcMg , a promising dye for a sequential two-step activation, in Jurkat cells (human T cell line) and murine hybridoma cells (B cells) in vitro. Pheophorbide a (pheo) was used as a classical Type II reference sensitizer. Excitation of higher singlet states was performed both by direct UV-excitation and by sequential two-step laser excitation in the Q-band with maximum photon flux densities of about 1026phot.cm−2.s−1. Irradiation of ( t-butyl )4- PcMg in cells in vitro is accompanied by a rapid destruction of the dye, as was proved by confocal fluorescence microscopy. Irradiated cells exhibit a less pronounced decrease of viability and a transient cell cycle arrest of about 24 h. Pheo incubated cells are characterized by a complete stop of proliferation. In contrast, no long-lasting phototoxicity was observed with ( t-butyl )4- PcMg . Low fluorescence levels and rapid photobleaching prevented the identification of intracellular dye-localization. Our data indicate the presence of a radical mechanism as origin of an irreversible dye destruction that accounts for the low phototoxicity of ( t-butyl )4- PcMg .

Two-Photon Excited Fluorescence from Higher Electronic States of Chlorophylls in Photosynthetic Antenna Complexes: A New Approach to Detect Strong Excitonic Chlorophyll a/ b Coupling

Stepwise two-photon excitation of chlorophyll a and b in the higher plant main light-harvesting complex (LHC II) and the minor complex CP29 (as well as in organic solution) with 100-fs pulses in the Q y region results in a weak blue fluorescence. The dependence of the spectral shape of the blue fluorescence on excitation wavelength offers a new approach to elucidate the long-standing problem of the origin of spectral “chlorophyll forms” in pigment-protein complexes, in particular the characterization of chlorophyll a/ b-heterodimers. As a first result we present evidence for the existence of strong chlorophyll a/ b-interactions (excitonically coupled transitions at 650 and 680 nm) in LHC II at ambient temperature. In comparison with LHC II, the experiments with CP29 provide further evidence that the lowest energy chlorophyll a transition (at ∼680 nm) is not excitonically coupled to chlorophyll b.

Optosensing properties of fac-Re(CO) 3(dpknph)Cl (dpknph=di-2-pyridyl ketone p-nitrophenyl hydrazone) toward chemotactic N-formylamino acids

Optical measurements on fac-Re(CO) 3(dpknph)Cl in polar non-aqueous solvents in the presence and absence of NaBH 4/KPF 6 revealed reversible interconversion between the high (β-) and low (α-) energy electronic states of fac-Re(CO) 3(dpknph)Cl. The reversibility of these interactions and the disturbance of the equilibrium distribution of the low (α-) and high (β-) energy electronic state upon addition of NaBH 4/KPF 6 mark improvement in the optosensing properties of fac-Re(CO) 3(dpknph)Cl. The optical behavior of fac-Re(CO) 3(dpknph)Cl in the presence and absence of the l-methionine and chemotactic N-formylamino acids: N-formyl- l-methionine (NFM), N-formyl- l-glycine (NFG) and N-formyl- l-phenylalanine (NFP) shows the α- and β-electronic states of fac-Re(CO) 3(dpknph)Cl to be insensitive to l-methinonine and highly sensitive to N-formylamino acids. N-formylamino acids in concentrations <1.0×10 −5 M can be determined using the optical sensor fac-Re(CO) 3(dpknph)Cl in non-aqueous polar solvents. The optosensing power of fac-Re(CO) 3(dpknph)Cl towards N-formylamino acids depends on the concentration and polarity of the side chain of the amino acids and increases in the following order: NFM>NFG>NFP.

Control of the fragmentation of excited ammonia clusters by femtosecond infrared laser pulses

Applying photoion and coincident photoelectron detection in femtosecond pump–probe experiments, we have studied the change of the fragmentation behavior of ammonia clusters excited by femtosecond (fs) laser pulses at 200 nm to the electronic à state which absorb an additional fs control photon 1–2 ps after the pump photon. Only a few 100 fs after the primary excitation, the (NH3)n clusters are partially transferred to the vibrationally highly excited H-transfer state (NH3)n−2NH4NH2 with a lifetime of a few ps. By irradiating the clusters in this state with control photons of a wavelength in the range of 1200–1400 nm, we were able to excite the clusters resonantly to the next higher electronic state in the H-transfer configuration with a strongly reduced vibrational energy. The excited H-transfer state corresponds to the 3s→3p transition in the NH4 component of the internally hydrogenated clusters. Due to the strong reduction of the vibrational energy after the control photon absorption, the fragmentation probability in the excited H-transfer state and correspondingly in the ionic proton transfer state is drastically reduced. For example, for the ammonia dimer the signal ratio of [(NH3)2+] to [NH4+] has been enlarged by nearly one order of magnitude by the resonant control photon absorption. Whereas the lifetime of the ammonia clusters in the nonexcited H-transfer state is nearly identical for all cluster sizes (2–4 ps) we found distinct lifetimes τ6 for the excited H-transfer state of the dimer and the trimer. For the dimer a lifetime τ6=130±50 fs has been obtained for undeuterated as well as for deuterated ammonia molecules. In contrast, for the trimer the lifetime τ6 is significantly larger and depends on the control wavelength as well as on the isotope composition.

Field induced ion-pair formation from ICl studied by optical triple resonance

Multiphoton pathways to the ion-pair states of ICl, at energies close to dissociation, are presented. These very high vibrational levels (v > or = 10(4)) are detected in the I+ and Cl- channels by pulsed field ionisation. Using a variable time delay before field ionisation, ion-pair states up to 50 cm-1 below the dissociation limit are shown to survive for at least 2 microseconds, indicating a stabilisation process analogous to that operating in high Rydberg electronic states. The atomic ion production signal is highly structured both above and below the free ion threshold, indicating the role of doorway states which are coupled to the dense ion-pair manifold near dissociation. This initial coupling appears to be very efficient and competes successfully with radiative decay and further up-pumping.

Mapping of wave packets in direct fragmentation via pump–probe frequency integrated fluorescence spectroscopy

We consider femtosecond excitation of a molecule to a dissociative electronic state. The quantum dynamics is recorded via delayed excitation to a higher electronic state and measurement of the total fluorescence from this state detected as a function of delay time. It is shown that the signal can be used to determine the probability density distribution of the outgoing wave packet describing the fragmentation. This, in particular, applies to the case of fragment detection since then the time-dependent signal directly measures the probability flux at a fixed value of the dissociation coordinate. Numerical examples illustrate the procedure.

Atomic fluorine emission cross sections observed in the 600-900 nm region following 0-500 eV electron impact on fluoromethanes (CF4, CHF3, CH2F2 and CH3F)

Absolute emission cross sections (em) of F I atom electronic transitions in the 600-900 nm region, following the electron impact on CHxF4-x (x = 0-3) molecules and determined by the optical method, are reported. The range of electron energies studied varies from threshold to 500 eV. In the electron impact on CF4 at pressures higher than 2 mTorr, a continuum background band in the 500-700 nm region is observed and has been assigned to the CF3* (2A2´´1A1´ and/or 1E´1A1´) transitions. Continuum bands of the same features were also found in collisional processes on CHF3 and CH2F2, although their maxima are shifted to the blue with respect to that of CF4 and have been tentatively assigned to the F2* ion pairs. For low F content fluoromethanes high electronic excited states (3d) of atomic F are observed and may be due to redistribution of the excess electron energy over that for C-F bond dissociation.