One-photon Absorption Spectra Research Articles

Excited states of an electron-hole pair in spherical quantum dots and their optical properties

The energy-level structure of a correlated electron-hole pair confined in spherical quantum dots and their optical properties are theoretically investigated. An efficient method of numerical calculation is presented for the eigenvalue problem of the excited states of the electron-hole pair with an arbitrary total angular momentum, which employs the Hylleraas coordinate system and a correlated-basis-set expansion. It is shown that the picture of exciton confinement holds generally when the dot radius is several times larger than the effective Bohr radius of the exciton, but the amount of blue shift in the excited states of the relative motion is larger than that in the lowest S-like state. In addition to the one-photon absorption spectra, the excited-state absorption and the two-photon absorption spectra are investigated. The calculated spectra show good agreements with those observed in CuCI nanocrystals.

Two photon absorption properties of 1,3,5-tricyano-2,4,6-tris(styryl)benzene derivatives.

Two-photon absorption (TPA) properties of 1,3,5-tricyano-2,4,6-tris(styryl)benzene derivatives have been investigated. Comparison of the absorption and fluorescence spectra reveals that these compounds show large Stokes shifts, which increase gradually as the conjugation length increases. One-photon absorption and excitation spectra are similar except that the latter exhibit several peaks near lambda(max). It is also found that the one- and two-photon-induced fluorescence excitation spectra are quite similar, which indicates that the one- and two-photon allowed-excited states are the same. The peak TPA cross section values (delta(max)) measured with nanosecond pulses by the two-photon-induced fluorescence method are in the range (50-2620) x 10(-50) cm4 s/photon. The delta(max) value increases as the donor strength and conjugation length increase. A linear relationship is observed between delta(max) and beta, and this delta-beta relationship is found to serve as a useful synthetic strategy for the design of novel TPA dyes with the octupolar structure.

Upconversion processes in Nd 3+-doped fluorochloride glass

Infrared (IR) to visible (VIS) and ultraviolet (UV) upconversion in Nd 3+-doped fluorochloride glass has been observed between 4.2 and 300 K under continuous wave (cw) and pulsed IR laser excitation in resonance with the 4 F 3/2 and 4 F 5/2 levels. The emission spectra have UV bands at 359 and 383 nm and VIS bands located around 416, 452, 527, 589, and 660 nm. The green, yellow, and red emissions are attributed to transitions from the 4 G 7/2 excited state. The excitation spectra of the upconverted emissions which are similar to the one-photon absorption spectrum, together with the features appearing in the decays of these emissions, indicate that an energy transfer upconversion (ETU) process seems to be responsible for the VIS luminescence observed, though other possible mechanisms cannot be disregarded. Referring to the blue and UV bands, the most probable origin of the upconverted fluorescence seems to be the 4 D 3/2 , 4 D 5/2 , 4 D 1/2 levels. In the last, the participation of three neodymium ions is necessary in the ETU process.

Upconversion processes in Nd 3+-doped fluoroarsenate glasses

In this work we have studied the infrared to visible upconversion luminescence of Nd 3+-doped fluoroarsenate glasses with different neodymium concentrations (0.1, 0.5, 2, 3, 4, and 5 mol%) under continuous wave and pulsed laser excitation. Green, orange, and red emissions are observed with excitation at 802 and 874 nm within the 4F 5/2 and 4F 3/2 levels. In addition, a weak blue emission appears under continuous wave (cw) laser excitation which under pulsed laser excitation only occurs for the most concentrated sample at 77 K. The green, orange, and red emissions can be attributed to transitions from the 4G 7/2 excited state. The pump power dependencies of these upconverted emissions together with the shape of the upconverted excitation spectra which are similar to the one-photon absorption spectrum and the temporal behavior of the decays, indicate that an ETU process seems to be responsible for the observed visible luminescence, though other possible mechanisms cannot be disregarded.

Remarkable Temperature Dependence of Two-Photon Absorption Spectrum of Poly(dihexylsilylene) in Solution

Abstract The nonresonant two-photon absorption spectrum of poly(dihexylsilylene) (1) in 3-methylpentane solution showed an abrupt thermochromism with a similar transition temperature to the one-photon absorption spectrum of 1. The parallel temperature dependence between the one-photon and two-photon absorption spectra suggests that the two-photon excited state involves an important contribution of the σ–σ* excited electronic configurations.

Rydberg states of propyne at 6.8–10.5 eV studied by two-photon resonant ionization spectroscopy and theoretical calculation

The vibronic spectra of jet-cooled propyne at 6.8–10.5 eV have been observed using 2+1 resonance-enhanced multiphoton ionization (REMPI) spectroscopy. The ns (n=4–13), np (n=3–4), and 3dz2 Rydberg states of propyne have been identified, of which seven are newly discovered. The symmetries of the excited vibronic states have been determined directly from polarization-ratio experiments applying linearly and circularly polarized lasers. Under a C3V group, the observed s Rydberg series are of E symmetry and the p Rydberg states belong to A1 or E. Clear doublet splittings in the ns Rydberg states (n=4–9) are observed for the first time. The splittings, 306 cm−1 at 4 s, decrease with increasing n. The doublets of A′ and A″ symmetries, identified from the polarization-ratio measurement, are that due to CS molecular geometry, rather than C3V, for the ns Rydberg states. The term values for the ns Rydberg series (n=6–13) converge to an adiabatic ionization energy of 83 625±2 cm−1 with a quantum defect of δ=0.95. Comparing with one-photon absorption spectrum of propyne, the absence of π→π*, np (n⩾4) and nd (n⩾3, except 3dz2) Rydberg states in the REMPI spectra suggests a strong predissociation character for these states. Calculations for the vertical excitation energies of π→π*, ns, np, and nd (n=3,4) Rydberg states of propyne were performed using time-dependent density functional theory and ab initio methods to compare with experimental results and to test the computational accuracy.

A theoretical study of the electronic spectrum of cis-stilbene

The valence and Rydberg electronic excited singlet states of cis-stilbene have been studied using multiconfigurational second-order perturbation theory (CASPT2). The study includes a geometry determination of the ground state. The geometry of the lowest 1B state has also been optimized, leading to a structure where the two benzyl groups are nearly perpendicular. The calculated vertical spectrum is in agreement with the occurrence of three main bands in the observed one-photon absorption spectrum. Below the relatively intense transition to the 2 1B (HOMO→LUMO) state at 4.61 eV, two weak transitions have been computed at 4.11–4.45 eV, which correspond to transitions to the 1 1B and 2 1A states, respectively. The 3 1A state, placed at 5.56 eV above the ground state, has a pronounced doubly excited character. In addition, the lowest vertical singlet–triplet transition has been characterized. Features of some static aspects along the trans↔ cis photoisomerization process in the singlet manifold are discussed. The lowest singlet excited state at a near-perpendicular conformation is of B symmetry with the next state (of 1A symmetry) 0.4 eV higher.

Optical spectroscopy of the Ag+ ion in NaF: Experimental results and analysis of manifestations of the Jahn–Teller effect

The 4d10→4d95s1 transitions of the Ag+ ion in single crystals of NaF were investigated by one- and two-photon spectroscopy and luminescence experiments. The one-photon absorption spectrum extends from 238 to below 180 nm. At low temperature, several bands show resolved fine structure, but compared to NaF:Cu+, there is much less, and the lines are broader. The Jahn–Teller effect is much larger in Ag+ than in Cu+ and has been identified in three of the excited states. Trapping in Jahn–Teller minima is shown to change the emission kinetics radically compared to Cu+. The off-center force from the d9p states has much less effect on Ag+ than on Cu+. All of the evidence shows that Ag+ is more strongly coupled to the lattice than is Cu+.

Characterization of the two-photon absorption resonance that is due to the internal charge-transfer transition of a push–pull molecule, 4-(diethylamino)-β-nitrostyrene

The two-photon absorption resonance that is due to the internal charge-transfer transition of an organic push-pull molecule has been characterized. A nonlinear absorption spectrum of the 4-(diethylamino)- beta-nitrostyrene molecule was measured in a tetrahydrofuran solution by optical Kerr ellipsometry. The shape and the amplitude of the two-photon absorption spectrum are well described by a model that uses the relevant one-photon absorption spectrum related to the internal charge-transfer transition.

Theoretical Study of the Electronic Spectrum of trans-Stilbene

The electronic spectrum of trans-stilbene in the energy range up to 6 eV has been studied using multiconfigurational second-order perturbation theory (CASPT2). The study includes a geometry determination of the ground state. In all, 12 singlet and one triplet excited states were studied. The calculated spectrum makes it possible to assign the valence excited singlet states corresponding to the three bands observed in the low-energy region of the one-photon absorption spectrum. The most intense feature of the calculated spectrum corresponds to the 11Ag → 21Bu transition at 4.07 eV. The weakly allowed 11Bu state was found 0.3 eV below 21Bu. Transition to the 31Ag state, computed at 4.95 eV, is responsible for the main band observed in the two-photon absorption spectrum. This state has a large weight (around 43%) of doubly excited configurations. The first singlet−singlet Rydberg transition (3s) appears at 5.33 eV and is weak. Implications of the present findings on the trans → cis photoisomerization process...

Detection of DCl by multiphoton ionization and determination of DCl and HCl internal state distributions

A study of the 2+1 resonantly enhanced multiphoton ionization (REMPI) spectrum of DCl is reported. Transition energies for excitation of the F 1Δ–X 1Σ+ (0,0) and (1,0) bands, as well as the V 1Σ+–X 1Σ+(v′,0) bands, for v′=15–19, are presented. The derived molecular constants for the F–X (0,0) and the V–X bands agree well with those previously obtained from analysis of the one-photon VUV absorption spectrum [A. E. Douglas and F. R. Greening, Can. J. Phys. 57, 1650 (1979)]. The ion signals for excitation through various rotational lines in the E–X (0,0) and F–X (0,0) and (1,0) bands are compared with theoretical two-photon line strengths. Extensive power- and J′-dependent ion fragmentation is observed for the former band. No fragmentation is observed in the F–X bands; however, the ion signal strengths are found to vary strongly with J′. This variation of REMPI signal strengths vs J′ was shown to be due to an indirect predissociation, as in HCl. Tables of experimental line strength factors for the F–X (0,0) and (1,0) bands of HCl and DCl are reported. Finally, the relative REMPI detection sensitivities for HCl and DCl, through their respective F–X (0,0) R(1) lines, are compared.

Resonant hyper-Raman and second-harmonic scattering in a CdS quantum-dot system.

Excitation profiles of resonant second-harmonic and hyper-Raman scattering in a CdS quantum-dot system are reported. Two maxima have been found in the related two-photon excitation spectra near the fundamental absorption region. Relative to the one-photon absorption spectrum, the maxima are more pronounced and shifted in energy. The result has revealed quantum-confined electron-hole pair states with a total angular momentum of odd number. Effective masses in the quantum dots have been estimated.

Revived interest on yellow-exciton series in Cu 2C: An experimental aspect

The yellow-exciton series in natural single crystals of Cu 2O are studied by one- and two-photon spectroscopy in high precision at 2K. The energies of the p-exciton series are resolved up to 12p state in one-photon absorption spectra, while those of the (s-d) excitons are resolved up to 7(s-d) states in two-photon spectra. The band gap energy is evaluated to be 2.1722eV. The different effective Rydberg constants are obtained to be 0.0981 ± 0.0005 eV for p-series, 0.1083±0.0138 eV and 0.1066±0.0014 eV for (s-d) series, respectively.

Identification of Two 3Πu ← X3Σ−g Transitions of 16O2 near 93 850 and 95 360 cm−1

The one-photon absorption spectrum of molecular oxygen in the photoionization threshold region has been recorded at high resolution (λ/Δλ = 2 × 105) against the background continuum of a synchrotron radiation source. The room temperature spectrum as well as the rotationally cold absorption of a supersonically expanding jet (Trot ≍ 20 K) have been registered on photographic plates. The drastically reduced complexity of the jet spectra has led to the unambiguous identification of two 3Πu vibronic levels of 16O2 at 93 850 and 95 360 cm−1; rotational and Λ-type doubling parameters have been determined. In addition, the upper state of a previously reported strong band of 16O2 (18O2) at 86 733 (86 697) cm−1 (Ogawa et al., J. Mol. Spectrosc.55, 425-429, 1975) has tentatively been attributed to ν = 0 of the bound second adiabatic 3Πu potential arising from the strong interaction of the · · · π4uπg · 3pσu Rydberg with the · · · π4uπgσu valence configuration (Buenker et al., Chem. Phys. Lett.34, 225-231, 1975; 42, 383-389, 1976).

Electronic spectrum of porphyrins. CS INDO CI study

The electronic spectra of porphin, chlorin and their magnesium derivatives have been studied quantum-mechanically with the CS INDO CI method. Both the S 0-S n and T 1-T n one-photon absorption spectra have been analyzed together with the correlation effects. The main result regards the T-T spectrum of porphin. To obtain a correct description of the complete spectrum it is necessary to take account of the correlation effects. Gouterman's model of four orbitals is too simple to describe the region of the Soret band.

Laser Two-Photon Ionization Spectrometry of Perylene, Naphthacene and Methylanthracene on Water Surface

The two-photon ionization spectra (wavelength dependence of the photoionization charge) of the three title molecules have been measured on the water surface in the region of 340-440nm using a Ti-sapphire laser. The signal was quadratic to the laser pulse energy. The peaks in the observed spectra are closely correlated with those in the one-photon absorption spectra in solution. The detection limit depended on the wavelength used and was better at band peaks in the photoionization spectrum.

Identification and role of two-photon excited states in a pi -conjugated polymer.

We have measured the full two-photon absorption (TPA) spectrum in the transparency range of single-crystal polydiacetylene paratoluene-sulfonate (PTS) using $z$ scan. The spectrum consists of three well-defined two-photon excited states superimposed on a continuum of even-parity states. Comparison of the TPA and the one-photon absorption spectra identifies a two-photon accessible state as the lowest excited state of PTS. The accompanying dispersion in the intensity-dependent refractive index coefficient ${n}_{2}$ is also reported.

Two-photon absorption in liquid xenon.

Excitation spectra of two-photon excited xenon near the triple point have been obtained in order to study exciton structure near the band edge. While the band gap is suppressed in the low-temperature liquid, absorption due to even-parity exciton states is presented and discussed. The even-parity ${\mathrm{\ensuremath{\Gamma}}}_{3/2}$,n=2 Wannier excitons observed in the liquid are shifted approximately 0.12 eV to higher energies relative to the energy of odd-parity excitons observed in the liquid by Beaglehole. The absorption linewidths (full width at half maximum) are measured to be 0.15 eV. Comparisons are made to one-photon absorption spectra in xenon.

Spectra of one-dimensional excitons in polysilanes with various backbone conformations.

Linear and nonlinear optical spectra (one-photon and two-photon absorption, photoluminescence, and electroabsorption spectra) have been investigated for solid thin films of polysilanes with a variety of conformations, such as trans-planar, alternating trans-gauche (TGTG' type), 7/3 helical, and disordered backbone structures. Optical spectra have revealed features of one-dimensional exciton states characteristic of the respective conformations. In particular, the lowest allowed and second parity-forbidden (or two-photon allowed) transitions can be assigned to the ground ${(}^{1}$${\mathit{B}}_{1\mathit{u}}$) and first excited ${(}^{1}$${\mathit{A}}_{\mathit{g}}$) states of one-dimensional Wannier-type exciton, respectively. Furthermore, the higher excited exciton states and subsequent band-to-band transitions show up in the electroabsorption spectra for polysilanes with various conformations of the Si backbones. Quantitative analysis based on a microscopic theory for the one-dimensional excitons is also presented.

Two-photon absorption spectra of quasi-low-dimensional exciton systems.

We present a unified theory for two-photon absorption (TPA) spectra of Wannier excitons in a low-dimensional semiconductor of an arbitrary dimension d=0, 1, 2, and 3. It is found that the spectra strongly reflect the anisotropies of both the band structure and the quasi-d-dimensional exciton envelope functions. Therefore the low dimensionality can be sensitively detected by the TPA spectroscopy. This is in striking contrast to the one-photon absorption spectra, anisotropies of which cannot be taken as evidence of quantum confinements.