Linear Absorption Spectra Research Articles

Nonlinear optical absorption via two-photon process in asymmetrical Gaussian potential quantum wells

In this paper, linear and nonlinear optical absorption spectrum in asymmetrical Gaussian potential quantum wells under the applied magnetic and electric fields are studied via investigating the phonon-assisted cyclotron resonance (PACR) effect. The results are calculated for GaAs and Ga1−xAlxAs materials. Our results show that the optical absorption behaviors and the half-width are significantly dependent on the height of the Gaussian potential, the well width, the magnetic field, and the temperature. It is also found that there is a clear monotonic behavior of the resonant peaks and the half-width as functions of the factors mentioned above in both one and two-photon absorption processes.

Exact stochastic unraveling of an optical coherence dynamics by cumulant expansion.

A numerically exact Monte Carlo scheme for calculation of open quantum system dynamics is proposed and implemented. The method consists of a Monte Carlo summation of a perturbation expansion in terms of trajectories in Liouville phase-space with respect to the coupling between the excited states of the molecule. The trajectories are weighted by a complex decoherence factor based on the second-order cumulant expansion of the environmental evolution. The method can be used with an arbitrary environment characterized by a general correlation function and arbitrary coupling strength. It is formally exact for harmonic environments, and it can be used with arbitrary temperature. Time evolution of an optically excited Frenkel exciton dimer representing a molecular exciton interacting with a charge transfer state is calculated by the proposed method. We calculate the evolution of the optical coherence elements of the density matrix and linear absorption spectrum, and compare them with the predictions of standard simulation methods.

Self-trapping of excitons, violation of Condon approximation, and efficient fluorescence in conjugated cycloparaphenylenes.

Cycloparaphenylenes, the simplest structural unit of armchair carbon nanotubes, have unique optoelectronic properties counterintuitive in the class of conjugated organic materials. Our time-dependent density functional theory study and excited state dynamics simulations of cycloparaphenylene chromophores provide a simple and conceptually appealing physical picture explaining experimentally observed trends in optical properties in this family of molecules. Fully delocalized degenerate second and third excitonic states define linear absorption spectra. Self-trapping of the lowest excitonic state due to electron-phonon coupling leads to the formation of spatially localized excitation in large cycloparaphenylenes within 100 fs. This invalidates the commonly used Condon approximation and breaks optical selection rules, making these materials superior fluorophores. This process does not occur in the small molecules, which remain inefficient emitters. A complex interplay of symmetry, π-conjugation, conformational distortion and bending strain controls all photophysics of cycloparaphenylenes.

Vibrational spectra and structures of bare and Xe-tagged cationic Si(n)O(m)⁺ clusters.

Vibrational spectra of Xe-tagged cationic silicon oxide clusters Si(n)O(m)(+) with n = 3-5 and m = n, n ± 1 in the gas phase are obtained by resonant infrared multiple photon dissociation (IRMPD) spectroscopy and density functional theory calculations. The Si(n)O(m)(+) clusters are produced in a laser vaporization ion source and Xe complexes are formed after thermalization to 100 K. The clusters are subsequently irradiated with tunable light from an IR free electron laser and changes in the mass distribution yield size-specific IR spectra. The measured IRMPD spectra are compared to calculated linear IR absorption spectra leading to structural assignments. For several clusters, Xe complexation alters the energetic order of the Si(n)O(m)(+) isomers. Common structural motifs include the Si2O2 rhombus, the Si3O2 pentagon, and the Si3O3 hexagon.

States of Excitons and Linear Optical Spectra in Metallic Single-Walled Carbon Nanotubes

Considering the exciton effect, the excitation energy and its binding energy of the metallic single-walled carbon nanotubes (SWNTs) are theoretically studied by using the simple tight-binding model, based on which the linear absorption spectra are also calculated. It is found that due to the trigonal warping effect, the excitation energies of the linear optical spectra all are split into two corresponding ones. Additionally, the splitting depends on both the chirality and the transition energy: (1) the splitting is maximal for the zigzag tubes, the splitting decreases with the increasing chiral angle; (2) the higher the transition energy is, the larger the splitting is. It is very interesting to find that the obtained results are in good agreement with the experimental results.

How to calculate linear absorption spectra with lifetime broadening using fewest switches surface hopping trajectories: a simple generalization of ground-state Kubo theory.

In this paper, we develop a surface hopping approach for calculating linear absorption spectra using ensembles of classical trajectories propagated on both the ground and excited potential energy surfaces. We demonstrate that our method allows the dipole-dipole correlation function to be determined exactly for the model problem of two shifted, uncoupled harmonic potentials with the same harmonic frequency. For systems where nonadiabatic dynamics and electronic relaxation are present, preliminary results show that our method produces spectra in better agreement with the results of exact quantum dynamics calculations than spectra obtained using the standard ground-state Kubo formalism. As such, our proposed surface hopping approach should find immediate use for modeling condensed phase spectra, especially for expensive calculations using ab initio potential energy surfaces.

Quantum coherent plasmon in silver nanowires: a real-time TDDFT study.

A plasmon-like phenomenon, arising from coinciding resonant excitations of different electronic characteristics in 1D silver nanowires, has been proposed based on theoretical linear absorption spectra. Such a molecular plasmon holds the potential for anisotropic nanoplasmonic applications. However, its dynamical nature remains unexplored. In this work, quantum dynamics of longitudinal and transverse excitations in 1D silver nanowires are carried out within the real-time time-dependent density functional theory framework. The anisotropic electron dynamics confirm that the transverse transitions of different electronic characteristics are collective in nature and oscillate in-phase with respect to each other. Analysis of the time evolutions of participating one-electron wave functions suggests that the transverse transitions form a coherent wave packet that gives rise to a strong plasmon resonance at the molecular level.

Influence of Coulomb-induced band couplings on linear excitonic absorption spectra of semiconducting carbon nanotubes

Starting from the extended Su-Schrieffer-Heeger model, multiband semiconductor Bloch equations are formulated in momentum space and applied to the analysis of the linear optical response of semiconducting carbon nanotubes (SCNTs). This formalism includes the coupling of electron-hole pair excitations between different valence and conduction bands, originating from the electron-hole Coulomb attraction. The influence of these couplings, which are referred to as nondiagonal interband Coulomb interaction (NDI-CI), on the linear excitonic absorption spectra is investigated and discussed for light fields polarized parallel to the tube direction. The results show that the intervalley NDI-CI leads to a significant increase of the band gap and a decrease of the exciton binding energy that results in a blueshift of the lowest-frequency excitonic absorption peak. The strength of these effects depends on the symmetry of the SCNT. Furthermore, for zigzag SCNTs with higher symmetry other nonintervalley NDI-CI terms also affect the spectral positions of excitonic absorption peaks.

Connecting the Dots: The Kinetics and Thermodynamics of Hot, Cold, and Surface-Trapped Excitons in Semiconductor Nanocrystals

The excitonics of semiconductor nanocrystals (NC) depend upon temperature in a complex manner due to the interplay between the kinetics of hot exciton relaxation/trapping and the thermodynamics leading to cold exciton recombination. We apply a semiclassical electron transfer model of surface trapping to temperature-dependent absorption and emission data to elucidate a microscopic picture of the factors which govern the fate of hot and cold excitons. The linear absorption spectra reveal a unique temperature-dependence to the energies of higher excitonic states, while oscillator strength is shown to be temperature invariant. We identify the phonon based origin to the anomalous low temperature peak energy trend in photoluminescence (PL) spectra. PL intensities, PL lifetimes, and absorption spectra are used to demonstrate that variation of quantum yield with temperatures arises from the thermally controlled fraction of NC which emit, rather than from an activated nonradiative pathway common to all NCs. Experimental quantum yield spectra are shown for several NCs and we perform a much-needed analysis of the role of surface PL in quantum yield. Finally, we show that a semiclassical electron transfer model including hot excitonic effects can explain experimental quantum yield spectra and suggests how to probe kinetic trapping processes via simple steady-state spectroscopy.

Synthesis, Characterization and Two-Photon Absorption Properties of Novel Blue Up-Converted Fluorescent Molecules

Three new acceptor-donor-acceptor (A-D-A) up-converted fluorescent molecules were designed and synthesized. Their structures were characterized using Fourier transform infrared and hydrogen nuclear magnetic resonance spectroscopy, mass spectrometry, and elemental analysis. The linear absorption spectra, singlephoton excited fluorescence spectra, and fluorescence quantum yields were measured in different solvents. The two-photon absorption and up-converted fluorescent properties were studied using a femtosecond laser. The results showed that the fluorescence quantum yields were 0.200.68, the two-photon absorption cross-sections were 16×10-101×10 cm4∙s∙photon-1, and they exhibited strong blue up-converted fluorescence.

Large-scale first principles configuration interaction calculations of optical absorption in aluminum clusters.

We report the linear optical absorption spectra of aluminum clusters Aln (n = 2-5) involving valence transitions, computed using the large-scale all-electron configuration interaction (CI) methodology. Several low-lying isomers of each cluster were considered, and their geometries were optimized at the coupled-cluster singles-doubles (CCSD) level of theory. With these optimized ground-state geometries, excited states of different clusters were computed using the multi-reference singles-doubles configuration-interaction (MRSDCI) approach, which includes electron correlation effects at a sophisticated level. These CI wave functions were used to compute the transition dipole matrix elements connecting the ground and various excited states of different clusters, and thus their photoabsorption spectra. The convergence of our results with respect to the basis sets, and the size of the CI expansion, was carefully examined. Our results were found to be significantly different as compared to those obtained using time-dependent density functional theory (TDDFT) [Deshpande et al. Phys. Rev. B: Condens. Matter Mater. Phys., 2003, 68, 035428]. When compared to the available experimental data for the isomers of Al2 and Al3, our results are in very good agreement as far as important peak positions are concerned. The contribution of configurations to many body wave functions of various excited states suggests that in most cases optical excitations involved are collective, and plasmonic in nature.

One- and two-photon absorption and emission properties of an oligo(phenylenethienylene)s series.

The photophysical and nonlinear absorption properties of an oligo(phenylenethienylene)s series (nTBT) are investigated in this article. The length of the chromophore is gradually increased from one to four phenylenethienylene repeating units in order to evaluate the effects of the electronic delocalization on the two-photon absorption cross sections (δ). According to the excitation anisotropy measurements and quantum chemical calculations, two electronic transitions with distinctive symmetries, 1Ag → 1Bu and 1Ag → 2Ag, are present in the low energy region of the linear absorption spectrum. The lowest-energy transition 1Ag → 1Bu is one-photon allowed but two-photon forbidden and implies an electronic charge delocalization all along the oligomer segment whereas the weakly-allowed 1Ag → 2Ag transition exhibits a transition moment perpendicular to the average plane of the chromophore. The latter transition mainly contributes to the two-photon absorption ability of the oligomers. All derivatives are poorly solvatochromic and the breakdown of the mirror symmetry rule observed between absorption and fluorescence spectra at room temperature has been attributed to a photoinduced geometrical relaxation leading to a very efficient planarization process of the oligomer irrespective of its size. Increasing the oligomer length results in a slight shift of the two-photon absorption band (∼1300 cm(-1)) and in a drastic increase of δ from 2 ± 1 GM up to 802 ± 160 GM for 1TBT and 4TBT respectively. Based on a three-level model, it was found that main contributions to the strong increase of δ stem from the transition moments Mge and Mee' which are multiplied by a factor of 2.8 and 5 when going from 1TBT to 4TBT.

Metal-induced dimensionality tuning in a series of bipyrimidine-based ligands: a tool to enhance two-photon absorption

We report the synthesis, the photophysical and the two-photon absorption (2PA) properties of a series of octupolar bipyrimidine-based ligands incorporating N-substituted amines as terminal donor groups. The effect of replacing phenylvinylene π-conjugated linkers by fluorenylvinylene ones was also investigated. The linear absorption spectrum of these compounds is dominated by an intensive charge transfer band which is sensitive to N-substitution and the π-bridge nature. The excitation anisotropy spectrum indicates that this band encompasses multiple S0 → Sn transitions, whose occurrence is well rationalized on the basis of the Frenkel exciton model. The 2PA spectrum also corroborates the presence of several electronic transitions. In apolar or moderately polar medium, excited ligands mainly deactivate through a highly emissive intramolecular charge transfer (ICT) state localized within a single branch of the chromophore. In highly polar medium, the solvent-induced stabilization of the low emissive twisted intramolecular charge transfer (TICT) state leads to a severe quenching of the fluorescence. The same mechanism is observed upon complexation with Zn2+. According to single-crystal X-ray analyses, metal-induced planarization of the bipyrimidine chelating site was observed for the short length ligands. Such a dimensional change from D2d to D2h symmetry leads to a decrease of 2PA cross-sections with respect to the free ligands. A divergent effect is observed for the complex with the long length ligand since the three-dimensional structure is maintained which induces a sizeable increase of the 2PA cross-section with a maximum value of up to 2000 GM.

Modelling infrared action spectra of protonated water clusters

Infrared spectra in the gas phase are experimentally obtained from the depletion signal after exposure to a laser field. This work aims to model such a dissociation probability for the (H2O)20H3O+ cluster, and compare it to the more conventional linear absorption spectrum. Using a flexible, polarisable but non-reactive potential, the response of the system to a laser pulse is simulated as a function of frequency, and for different laser intensities and initial temperatures. For a cluster initially cold, the energetic heating efficiency is found to mimic rather well the absorption spectrum, but with a generally lower spectral resolution and increasingly large red shifts of the bands as the laser intensity increases. The dissociation probability, evaluated using a simple Rice–Ramsperger–Kassel model, also displays non-trivial dependences on the field and observation conditions. Repeating the numerical experiments for a warmer initial temperature, the action spectrum is blurred owing to the dominant contribution of thermal evaporation. The comparison between action and absorption spectra confirms the importance of dynamical anharmonicities on the spectral shifts, broadenings and intensity ratios.

Direct observation of the optical anisotropy of C70 fullerene molecules in the Shpol’skii spectra

Linear optical absorption and emission spectra of C70 fullerene molecules in single-crystal toluene are investigated. It is established that the lines of purely electronic S 0-S 1 transitions are significantly polarized. The degree of linear polarization of the spectral lines depends on the position of the fullerene molecule in the toluene matrix and can be as high as 100%. The polarization characteristics of the lines can be understood in the context of a model in which the S 0 → S 1 electronic transition is represented by the excitation of a planar oscillator whose axis is oriented along the principal axis of the C70 molecule. The relationship between the polarization of the spectral lines and the position of the fullerene molecule in the matrix is consistent with the conclusions drawn from a theoretical analysis of different configurations possible upon the embedding of C70 molecules into crystalline toluene.

Plasmon enhanced molecular absorption: A mixed quantum–classical description of supramolecular complexes attached to a metal nanoparticle

The application of a mixed quantum–classical methodology for an investigation of single pheophorbide-a molecules (Pheos) and respective supramolecular complexes is continued to effects caused by a nearby placed metal nanoparticle (MNP). Therefore, the classical simulation of the molecular nuclear degrees of freedom is combined with a uniform quantum description of the molecular electronic excitations coupled to those of the MNP. To account for the short MNP plasmon life time the quantum dynamics of the electronic degrees of freedom is formulated in the framework of a system-bath theory. Linear absorption spectra are calculated for a spherical 14nm diameter Au–MNP decorated with isolated Pheos or with P16 complexes formed by 16 Pheos. The spectra are analyzed with respect to the molecular orientation at the MNP surface. While all studies on P16 only account for the Pheo Qy-transition we also present data on the MNP induced change of the single Pheo Qx-absorption.

The synthesis and fluorescence of a new biphenyl ethylene derivative sensitive to DNA induced by single- and two-photon excitation

A new biphenyl ethylene derivative, 4,4′-bis-(2-(4-N-ethyl-N-ethoxyl-amino) styryl] biphenyl (BEEASP) was successfully synthesized and studied as a potential fluorescent probe of DNA detection. The linear absorption and fluorescent spectra of the dye in different solvents were investigated. The absorption, one- and two-photon fluorescent spectra of the free dye and dye–DNA complex were also examined to evaluate their photophysical properties. The binding constant of dye–DNA was calculated with a moderate value. Two-photon active cross-sections (Фδ) were finished by the reference method, and the value at a stable complex of dye–DNA was about 37GM. The experimental results showed that BEEASP can be used as an efficient fluorescent probe to detect DNA by single-photon excitation and two-photon excitation. The fluorescent method date indicated that the mechanism of dye–DNA complex interaction may be the groove binding.

Photoinduced excitation energy transfer in hexapyropheophorbide a

Comparative theoretical and experimental studies are reported on excitation energy transfer dynamics and linear absorption spectra of the supramolecular hexapyropheophorbide-a-complex in tetrahydrofuran solution. A mixed quantum–classical methodology was applied based on molecular dynamics simulation of the dissolved complex. The obtained nuclear trajectories together with precalculated potential energy surfaces and transition charges result in a time-dependent electronic Hamiltonian. The solutions of the related time-dependent Schrödinger equation together with an appropriate ensemble averaging allows to determine the linear absorption spectrum as well as the energy transfer. The strength of the method is demonstrated by good agreement between measured and computed data.

The effect of charge transfer on the ultrafast and two-photon absorption properties of newly synthesized boron-dipyrromethene compounds

New BODIPY compounds containing nitro, diphenylamine and triphenylamine groups were designed and synthesized to enhance two photon absorption properties at NIR region by increasing charge transfer mechanism. Charge transfer mechanisms were investigated with linear absorption, fluorescence and ultrafast pump-probe absorption spectra. Solvents with various polarities were used to better understand the charge transfer properties of investigated compounds. Open aperture Z-scan experiments showed that charge transfer enhanced to two photon absorption properties. Two photon absorption cross section value was measured as 220 GM at 800 nm wavelength for the 3TFABDP compound. This value at 800 nm wavelength is considerably high compared to similar BODIPY compounds in literature.

Selection rules and linear absorption spectra of carbon nanotubes in axial magnetic fields

We derive a transparent and easy-to-use analytic expression for the selection rules and the optical dipole matrix elements for carbon nanotubes of arbitrary chirality in the presence of axial magnetic fields using a single-orbital \ensuremath{\pi}-electron tight-binding model. From this, we calculate the linear absorption spectrum for arbitrary polarization directions of the incident light, providing insight into all optically allowed transition. We show that the transverse absorption peaks can be selectively excited with circularly polarized light and spectrally resolved in an axial magnetic field.