Increasing Excitation Energy Research Articles

Hybrid Materials for Solid-State Dye Laser Applications

The quest for a solid-state tunable dye laser can be satisfied by sol-gel prepared organic-inorganic hybrids. A photostability study of porous silica-Rhodamine 6G hybrids prepared via a sol-gel method is presented. The dye molecules can be incorporated into the silica matrix by forming weak or covalent bonds (hybrids of classes I and II, respectively). New class II samples and traditional class I materials prepared by the pre-doping method were synthesized. Samples were characterized by photoluminescence measurements to compare the emission properties and the photostability of the samples. The decay of the fluorescence signal as the cumulative excitation energy increases is reported and interpreted by hypothesizing that the dye molecules can be hosted in different surroundings within the porous glass matrix. The reported photoluminescence and photobleaching features indicate the class II samples as good candidates for solid-state dye lasers.

Linear and Nonlinear Optical Properties of a Macrocyclic Trichromophore Bundle with Parallel-Aligned Dipole Moments

A macrocyclic trichromophore bundle 1 with parallel-aligned dipole moments has been synthesized to study the influence of aggregation and orientation of a nonlinear optical (NLO) chromophore on its optical properties. The linear and nonlinear optical properties of 1 and a single chromophore standard 2 have been studied by UV-vis absorption, fluorescence, solvatochromic spectrometry, and hyper-Rayleigh scattering (HRS). Reduced first-order hyperpolarizability beta, hypsochromic shift, enhanced solvatochromic shifts, and fluorescence quenching for individual chromophores were observed when 1 was compared with 2. Analysis of the data showed that the transition dipole moment changes only slightly when the chromophores are parallel aligned in the bundle architecture. However, the apparent hyperpolarizability of the individual chromophores decreased significantly by about 20%. The reduction in beta for the individual chromophores in 1 is largely due to the hypsochromic shift, i.e., excitation energy increase of the interband (charge-transfer) energy gap and the reduced difference between the ground-state and excited-state dipole moments. The hypsochromic shift and fluorescence quenching are consistent with exciton theory. Possible reasons for the enhanced solvatochromic shift are discussed.

Heavy-residue isoscaling as a probe of the symmetry energy of hot fragments

The isoscaling properties of isotopically resolved projectile residues from peripheral collisions of 86Kr (25 MeV/nucleon), 64Ni (25 MeV/nucleon) and 136Xe (20 MeV/nucleon) beams on various target pairs are employed to probe the symmetry energy coefficient of the nuclear binding energy. The present study focuses on heavy projectile fragments produced in peripheral and semiperipheral collisions near the onset of multifragment emission E*/A = 2-3 MeV). For these fragments, the measured average velocities are used to extract excitation energies. The excitation energies, in turn, are used to estimate the temperatures of the fragmenting quasiprojectiles in the framework the Fermi gas model. The isoscaling analysis of the fragment yields provided the isoscaling parameters "alpha" which, in combination with temperatures and isospin asymmetries provided the symmetry energy coefficient of the nuclear binding energy of the hot fragmenting quasiprojectiles. The extracted values of the symmetry energy coefficient at this excitation energy range (2-3 MeV/nucleon) are lower than the typical liquid-drop model value ~25 MeV corresponding to ground-state nuclei and show a monotonic decrease with increasing excitation energy. This result is of importance in the formation of hot nuclei in heavy-ion reactions and in hot stellar environments such as supernova.

Auger decay calculations with core-hole excited-state molecular-dynamics simulations of water

We report a new theoretical procedure for calculating Auger decay transition rates including effects of core-hole excited-state dynamics. Our procedure was applied to the normal and first resonant Auger processes of gas-phase water and compared to high-resolution experiments. In the normal Auger decay, calculated Auger spectra were found to be insensitive to the dynamics, while the repulsive character of the first resonant core-excited state makes the first resonantly excited Auger decay spectra depend strongly on the dynamics. The ultrafast dissociation of water upon O(1s)-->4a(1) excitation was analyzed and found to be very sensitive to initial vibrational distortions in the ground state which furthermore affect the excitation energy. Our calculated spectra reproduce the experimental Auger spectra except for the Franck-Condon vibrational structure which is not included in the procedure. We found that the Auger decay of OH and O fragments contributes to the total intensity, and that the contribution from these fragments increases with increasing excitation energy.

Photoinduced Isomerization Kinetics of Diiodomethane in Supercritical Fluid Solution: Local Density Effects

The density dependence of diiodomethane photoinduced isomerization in supercritical (sc) CO2, CHF3, and C2H6 was investigated by transient absorption spectroscopy, covering a fluid density range from 0.7 to 2.5 (in reduced units). The solvent-caged photoproduct iso-diiodomethane is formed even at the lowest density, and its yield increases about 4-fold over the whole range. At the same time, isomer formation rate constants increase by roughly an order of magnitude and show little variation between CO2, C2H6, and CHF3. Furthermore, the formation rate constant decreases significantly with increasing excitation energy. We propose an isomer formation mechanism involving a rapidly established preequilibrium between a solvent-caged iodine atom-methyliodide radical pair and a loosely bound iodine-methyliodide radical complex, from which the reaction subsequently proceeds to the isomer. The latter step seems to be controlled by collisional stabilization of the initially hot radical moiety, as the formation rate constant increases linearly with sc solvent viscosity. The model predicts a quadratic dependence of relative isomer yield on fluid density. A corresponding correlation is found with the local fluid density, calculated via solute-solvent radial distribution functions obtained from molecular dynamics (MD) simulations.

Ab initio and RRKM study of photodissociation of azulene cation

The ab initio/Rice-Ramsperger-Kassel-Marcus (RRKM) approach has been applied to investigate the photodissociation mechanism of the azulene cation at different values of the photon energy. Reaction pathways leading to various decomposition products have been mapped out at the G3(MP2,CC)//B3LYP level and then the RRKM and microcanonical variational transition state theories have been applied to compute rate constants for individual reaction steps. Relative product yields (branching ratios) for the dissociation products have been calculated using the steady-state approach. The results show that a photoexcited azulene cation can readily isomerize to a naphthalene cation. The major dissociation channels are elimination of atomic hydrogen, an H2 molecule, and acetylene. The branching ratio of the H elimination channel decreases with an increase of the photon energy. The branching ratio of the acetylene elimination as well as that of the H2 elimination rise as the photon energy increases. The main C8H6+ fragment at all photon energies considered is a pentalene cation, and its yield decreases slightly with increasing excitation energy, whereas the branching ratios of the other C8H6+ fragments, phenylacetylene and benzocyclobutadiene cations, grow.

Single nucleon transfer reactions near Coulomb barrier on 197Au

Single neutron transfer reactions on 197Au induced by 12C and 16O at near Coulomb barrier energies have been studied using radiochemical and off-line gamma-ray spectrometric techniques. High spin yield fraction (HSF = σm/(σm+σg)) for 196m,gAu and formation cross section ratio (σ198/σ196) of 198Au and 196Au have been determined for both 197Au+12C and 197Au+16O reactions at different excitation energies near the Coulomb barrier. The HSF of 196m,gAu and σ198/σ196 values in both reactions are seen to sharply increase with increasing excitation energy near the Coulomb barrier and then slowly increase reaching a constant value at high projectile energy. These observations have been analyzed in terms of nuclear structural and kinematic parameters. A comparison the effects of Coulomb excitation and nucleon transfer process during heavy-ion interactions indicates a predominant effect of the latter process in the background of the Coulomb excitation. Observed reaction systematics seem to follow the quasi-elastic transfer process.

Photodissociation of Azulene at 193 nm: Ab Initio and RRKM Study

The ab initio/Rice-Ramsperger-Kassel-Marcus (RRKM) approach has been applied to investigate the photodissociation mechanism of azulene at 6.4 eV (the laser wavelength of 193 nm) upon absorption of one UV photon followed by internal conversion into the ground electronic state. Reaction pathways leading to various decomposition products have been mapped out at the G3(MP2,CC)//B3LYP level and then the RRKM and microcanonical variational transition state theories have been applied to compute rate constants for individual reaction steps. Relative product yields (branching ratios) for the dissociation products have been calculated using the steady-state approach. The results show that photoexcited azulene can readily isomerize to naphthalene and the major dissociation channel is elimination of an H-atom from naphthalene. The branching ratio of this channel decreases with an increase of the photon energy. Acetylene elimination is the second probable reaction channel and its branching ratio rises as the photon energy increases. The main C8H6 fragments at 193 nm are phenylacetylene and pentalene and the yield of the latter grows fast with the increasing excitation energy.

Kinetic energy spectra for fragments and break-up density in multifragmentation

We investigate the possibility, in nuclear fragmentation, to extract information on nuclear density at break-up from fragment kinetic energy spectra using a simultaneous scenario for fragment emission. It is found that a decrease of peak centroids for kinetic energy spectra of fragments with increasing excitation energy can be observed at constant low density, which is different from recently published results of Viola et al. [V.E. Viola, K. Kwiatkowski, J.B. Natowitz, S.J. Yennello, Phys. Rev. Lett. 93 (2004) 132701].

Femtosecond time-resolved two-photon photoemission study of organic semiconductor copper phthalocyanine film

We have carried out a femtosecond (fs) two-photon photoemission study of organic semiconductor copper phthalocyanine (CuPc) film. Furthermore, we have carried out the photoemission (PES) and inverse photoemission (IPES) studies. From the simultaneous PES and IPES measurements, the lowest unoccupied molecular orbital, highest occupied molecular orbital, and ionization potential of CuPc film have been directly determined. From the detailed time-resolved two-photon photoemission measurements, it is found that the relaxation lifetime of excited state is extremely short (all below 50 fs) and monotonously became faster with increasing excitation energy. From these results, we discuss the electronic structure and excited-state dynamics in the organic semiconductor CuPc film.

Complex nuclear-structure phenomena in the cooling down of highly excited nuclear systems

Complex structural effects in the nuclide production from the projectile fragmentation of 1 A GeV 238 U nuclei in a titanium target, manifested as an even-odd effect, are reported. The structure seems to be insensitive to the excitation energy induced in the reaction. This is in contrast to the prominent structural features found in nuclear fission and in transfer reactions, which gradually disappear with increasing excitation energy. Most of the features of the results are reproduced using the statistical model of nuclear reactions, treating the pairing correlations in a consistent way both in the masses and in the level densities. The structures appear as the result of the condensation process of heated nuclear matter while cooling down in the evaporation process. As such, it can be considered a manifestation of the passage from the normal liquid phase of the nucleus to its superfluid phase.

Fragment yield distribution and the influence of neutron composition and excitation energy in multifragmentation reactions

The isotopic properties of the primary and secondary fragment yield distribution in the multifragmentation of {sup 58}Fe+{sup 58}Ni and {sup 58}Fe+{sup 58}Fe reactions are studied with respect to the {sup 58}Ni+{sup 58}Ni reaction at 30, 40, and 47 MeV/nucleon. The reduced neutron and proton densities from the observed fragment yield distribution show primary fragment yield distribution to undergo strongly secondary deexcitations. The effect is small at the lowest excitation energy and smallest neutron-to-proton ratio and becomes large at higher excitation energies and higher neutron-to-proton ratio. The symmetry energy of the primary fragments deduced from the reduced neutron density is significantly lower than that for the normal nuclei at saturation density, indicating that the fragments are highly excited and formed at a reduced density. Furthermore, the symmetry energy is also observed to decrease slowly with increasing excitation energy. The observed effect is explained using the statistical multifragmentation model.

Quasi-Classical Trajectory Simulations of Intramolecular Vibrational Energy Redistribution in HONO2 and DONO2

By use of an analytic potential energy surface developed in this work for nitric acid, the quasi-classical trajectory method was used to simulate intramolecular vibrational energy redistribution (IVR). A method was developed for monitoring the average vibrational energy in the OH (or OD) mode that uses the mean-square displacement of the bond length calculated during the trajectories. This method is effective for both rotating and nonrotating molecules. The calculated IVR time constant for HONO(2) decreases exponentially with increasing excitation energy, is almost independent of rotational temperature, and is in excellent agreement with the experimental determination (Bingemann, D.; Gorman, M. P.; King, A. M.; Crim, F. F. J. Chem.Phys. 1997, 107, 661). In DONO(2), the IVR time constants show more complicated behavior with increasing excitation energy, apparently due to 2:1 Fermi-resonance coupling with lower frequency modes. This effect should be measurable in experiments.

Resonant Raman scattering in GeSi/Si superlattices with GeSi quantum dots

The resonant Raman scattering in GeSi/Si structures with GeSi quantum dots has been analyzed. These structures were formed at various temperatures in the process of molecular-beam epitaxy. It has been shown that Raman scattering spectra recorded near resonances with the E 0 and E 1 electronic transitions exhibit the lines of Ge optical phonons whose frequencies differ significantly from the corresponding values in bulk germanium. In the structures grown at low temperatures (300–400°C), the phonon frequency decreases with increasing excitation energy. This behavior is attributed to Raman scattering, which is sensitive to the size of quantum dots, and shows that quantum dots are inhomogeneous in size. In the structures grown at a higher temperature (500°C), the opposite dependence of the frequency of Ge phonons on excitation energy is observed. This behavior is attributed to the competitive effect of internal mechanical stresses in quantum dots, the localization of optical photons, and the mixing of Ge and Si atoms in structures with a bimodal size distribution of quantum dots.

Continuum and discrete pulsed cavity ring down laser absorption spectra of Br 2 vapor

The absorption cross-sections at room temperature are reported for the first time, of Br 2 vapor in overlapping bound–free and bound–bound transition of A 3Π 1u ← XΣ g +, X 1Π 1u ← X 1Σ g + and B 3Π 0u ← X 1Σ g +, using cavity ring down spectroscopy (CRDS) technique. We reported here, the A 3Π 1u ← X 1Σ g +, transition is included along with the two stronger X 1Π 1u ← X 1Σ g + and B 3Π 0u ← X 1Σ g transitions of Br 2. We obtained discrete absorption cross-section in the rotational structure, the continuum absorption cross-sections, and were also able to measure the absorption cross-section in separate contribution of A 3Π 1u ← X 1Σ g +, 1Π 1u ← X 1Σ g +, and B 3Π 0u ← X 1Σ g + transitions using CRDS method to use quantum yield of Br *( 2P 1/2). We obtained absorption cross-section order 10 −19 cm 2 and detection 10 13 molecule cm −3 (1 mTorr) of Br 2. The absorption cross-sections are increasing with increasing excitation energy in the wavelength region 510–535 nm.

Excitation energy dependence of photoluminescence in polycrystalline GaN films by post-nitridation technique

Polycrystalline gallium nitride (GaN) films were prepared on (0001) sapphire substrates by post-nitridation technique. The photoluminescence (PL) spectra of the films show a blue emission (BL) and an ultraviolet emission (UV) at room temperature. We tentatively explain the blue emission in our nominally undoped GaN films with a model of donor–acceptor pair (DAP) transition. It is suggested that the donor is attributed to oxygen and carbon acts as an acceptor. Continuous blueshift of emission peaks with increasing excitation energy is observed.

Local structure and persistent spectral hole burning of the Eu3+ ion in SnO2–SiO2 glass containing SnO2 nanocrystals

The local structure and persistent spectral hole burning (PSHB) of the Eu3+ ion in SnO2–SiO2 glass containing SnO2 nanocrystals were investigated. The excitation and emission spectra of the Eu3+ ions indicate that the Eu3+ ions are doped into the SnO2 nanocrystals and glass phases. The variation in the site symmetry of Eu3+ ions in the nanocrystal from D2h or C2h symmetry may be due to oxygen vacancies. Fluorescence line-narrowing spectra reveal that the Eu3+ ions in the glass phase have two sites, one of which exhibits a unique feature. The calculated second crystal-field parameters show that the coordinating oxygen ions of site I are closer to the Eu3+ ions with increasing excitation energy, while those of site II are farther away from the Eu3+ ions. This result leads to the opposite behavior of the fluorescence lines of the Eu3+ ions at the two sites. PSHB was also observed and multiholes with a narrow width can be burned in the SnO2–SiO2 glass containing SnO2 nanocrystals.

Influence of Shell on Pre-scission Particle Emission of a Doubly Magic Nucleus 208Pb**The project supported in part by the Foundation of Teaching & Researching of the Best Teacher of Southeast University

Using Smoluchowski equation, we study the shell effects on the emission of light particles in the fission process of a doubly magic nucleus 208Pb. Calculated results show that shell has a considerable effect on the neutron emission and that shell effect gradually becomes weak with increasing excitation energy. In addition, a dependence of shell effects in the neutron emission on the angular momentum has been found.

Energy-selective charging of type-II GaSb/GaAs quantum dots

The hole confinement in type-II self-organized GaSb/GaAs quantum dots (QDs) was investigated by combining optical excitation and time-resolved capacitance spectroscopy. The experimental results indicate energy-selective charging even for type-II QDs. With increasing excitation energy the apparent hole activation energy decreases, which is attributed to light absorption in sub-ensembles of QDs with decreasing hole localization. The large localization energy of about 450 meV and the possibility of optical-multiplexing makes type-II GaSb/GaAs QDs a potential material system for QD memory concepts.

Complex nuclear-structure phenomena revealed from the nuclide production in fragmentation reactions

Complex structural effects in the nuclide production from the projectile fragmentation of 1 A GeV 238U nuclei in a titanium target are reported. The structure seems to be insensitive to the excitation energy induced in the reaction. This is in contrast to the prominent structural features found in nuclear fission and in transfer reactions, which gradually disappear with increasing excitation energy. Using the statistical model of nuclear reactions, relations to structural effects in nuclear binding and in the nuclear level density are demonstrated.