Third order nonlinear optical properties of some organic nitroxyls

The degenerate four-wave mixing experiments have been performed in the dichloromethane solutions of 2,2,6,6-tetramethyl-pipridine-4-ol-N-oxyl p-hydroxybenzaldehyde oxime benzoic ester (TPOHBOBE) and bis-(TPOHBOBE) cobalt (II), respectively. The result is in agreement with the theoretical prediction of depending on the molecular chain length l as l ⁵. A large off-resonant third order hyperpolarizability _xxxx of 1.3 × 10^–30 esu for bis-(TPOHBOBE) cobalt (II) was obtained. The corresponding value of the third order nonlinear optical susceptibility ⁽³⁾ _xxxx for its solid state is estimated to be 7.8 × 10^–9 esu.     

Third-order processes and their relation to structural symmetry

We have studied the various nonlinear optical processes that can be described by a fourth-rank ⁽³⁾-tensor: signals of frequency in degenerate four-wave mixing (DFWM), harmonics of frequency 2 and 3, and ⁽³⁾-type difference-frequency generation (DFG) with observation of anti-Stokes emission of a signal of frequency 2₁ — ₂. Structural information in terms of normalized anisotropies is derived in all frequency domains by analysis of the elements of the respective orientation-dependent susceptibility tensor. A novel laser-based technique for the remote orientation analysis of crystalline structures is introduced.     

Calculation of the dispersion of the third-order optical nonlinearity in C60 films

Using wave functions determined from ground-state local-density calculations, we have calculated the wave-dispersed free response of the optical nonlinear polarizability ⁽³⁾(–3;,,), for the C₆₀ molecule and ⁽³⁾(–3;,,) i.e. Third-Harmonic Generation (THG) for films using a sum-over-states approach. The influence of screening was determined by applying an external static electric field in separate selfconsistent calculations to evaluate induced dipole moments which was used to determine the static linear and nonlinear polarizabilities. The polarizabilities calculated in the static limit were used to determine an effective screening parameter which was, in turn, used together with an RPA approach to calculate screened wave-dispersed, third-order nonlinear optical properties such as ⁽³⁾(–3;,,) and ⁽³⁾(–3;,,). Comparing evaluated polarizabilities with experimental values we found that the non-resonant free polarizability compares well in absolute magnitude with experimental results. Inclusion of screening results in a polarizability about two orders of magnitude below the experimental values.Paper presented at the 129th WE-Heraeus-Seminar on Surface Studies by Nonlinear Laser Spectroscopies, Kassel, Germany, May 30 to June 1, 1994     

Microfacetting of Au(110) electrodes: An optical second-harmonic generation study

The formation and lifting of microfacetting at an Au(110) electrode surface was studied by using in-situ optical Second-Harmonic Generation (SHG). In contrast to the unreconstructed (1×1) Au(110) surface which has aC _2v symmetry, the reconstructed surface exhibits aC _s symmetry due to essentially (111)-oriented microfacets. Hence, it shows SHG-anisotropy patterns fundamentally different to those of a non-reconstructed surface. Apparently, the nonlinear susceptibility tensor contains an additional threefold symmetry element. Its amplitude was determined using Fourier analysis of SHG-anisotropy curves and, thus, served as a measure of microfacetting of the Au(110) surface. We observed that adsorption of an organic molecule, such as pyridine, has little effect on the electrode-surface crystallography. In contrast, adsorption of bromide ions results in the lifting of microfacetting as indicated by the disappearence of the threefold symmetry term. Potential-step experiments gave time constants for the lifting of microfacetting in the range of < _1/2<150 ms.Paper presented at the 129th WE-Heraeus-Seminar on Surface studies by Nonlinear Laser Spectroscopies, Kassel, Germany, May 30 to June 1, 1994.     

Phase-matched third-harmonic generation of Nd: Glass-laser picosecond pulses in a new cyanine-dye solution

The phase-matched direct tripling of picosecond light pulses of a mode-locked Nd: glass laser in a new cyanine dye PMC is studied. The solvents trifluoroethanol (TFE) and hexafluoroisopropanol (HFIP) are applied. The S ₀–S ₁ absorption peak of the dye is around =480 nm and the absorption cross section at the third-harmonic wavelength of ₃=351.3 nm is only ₃1×10^–19 cm². Phase-matching occurred at concentrations of C_PM=0.0874 mol/dm³ in HFIP and 0.1088 mol/dm³ in TFE. A third-harmonic energy conversion efficiency of _E0.01 was achieved at a pump-laser peak intensity of I _0L2.5×10¹¹ W/cm² in a 5 mm long sample of PMC in TFE. The conversion efficiency is limited by destruction of phase-matching due to the intensity-dependent nonlinear refractive index of the dye solutions.     

Excited-state absorption and third-order optical nonlinearities in symmetric π-electron organic molecules

Excited-State Absorption (ESA), Two-Photon Absorption (TPA) and the third-order polarizability (;,, – ) have been investigated for a model dichloride derivative of a symmetrically substituted benzylidene analine (SBAC), using a multielectron configuration-interaction procedure. The calculations indicate that SBAC exhibits ESA across the visible region of the spectrum, but that it is not as extensive as for molecules such as the phthalocyanines. The magnitude of the third-order polarizability is dominated by resonance enhancement from a very strongA _g B _u one-photon absorption. The calculated off-resonance value for (;,, – ) suggests that SBAC is a potential candidate for ultrafast switching applications.     

Anisotropy of the nonlinear optical susceptibility χ(3) in polydiacetylene single crystals

The anisotropy of ⁽³⁾ in Poly-bis(p-Toluene Sulfonate) of 2.4-hexadiyne-1.6-diol (PTS) PolyDiAcetylene (PDA) single crystals is investigated by Degenerate Four-Wave Mixing (DFWM) at a wavelength of 720 nm. The symmetries of ⁽³⁾(–;, –,) identify the non-vanishing and independent components of the tensor. For the calculation of the ⁽³⁾ elements from the measured values, the anisotropy of both the linear optical index of refraction and the absorption coefficient is included in the coupled wave approach. A magnitude of 10^–10 esu is measured for the ⁽³⁾ component if all beam polarizations are parallel to the polymer chains. The least upper bound for all other measurable components is estimated to be 10^–12 esu. The results for the intensity dependence of the DFWM signal are explained in terms of a thermooptic effect. Measurements on the orientational dependence of the DFWM signal in the samples are carried out, and the influence of the anisotropy of the linear optical parameters on the measured curves is discussed. A method for the preparation of thin layered polymer single crystals of poly-bis (4-ButoxyCarbonyle-Methylene-Urethane) of 4.6-decadiyne-1.1-diol (4-BCMU) and the first DFWM measurements on these samples are presented.     

Separation of interface and bulk contributions in second-harmonic generation from magnetic and non-magnetic multilayers

We discuss the separation problem of bulk vs interface contributions in optical Second-Harmonic Generation (SHG). A new method is presented, which — in special cases — provides a full separation of all interface and bulk tensor components. The method is based on measurements in a thin-film geometry, and the relation between two bulk tensor components obtained from group-theoretical arguments. The separation problem in its strict sense is shown to be absent in Magnetization-induced SHG (MSHG). Though, in practice, also in the MSHG case interface and bulk contributions can be closely resemblant. The approach is illustrated by second-harmonic experiments on C₆₀ thin films. SHG in this system is found to be completely dominated by bulk processes.Paper presented at the 129th WE-Heraeus-Seminar on Surface Studies by Nonlinear Laser Spectroscopies, Kassel, Germany, May 30 to June 1, 1994     

Temperature dependence of the effective work function for the BaO - W system

The temperature dependence of the work function has been examined for the BaO-W system for degrees of coating less than 1 and about 1 in the range 700–1400 °K; it is found that a monomolecular film gives a negative sign of this coefficient in this temperature range ( /T 10^–4 –10^–3 eV/deg). At 1000–1200 °K, the temperature coefficient is almost zero, while at 1200–1400 °K there is a positive value. If the covering is less than monomolecular, the sign is positive throughout the entire temperature range (/T 10^–4 eV/deg).Deceased.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 9, pp. 65–70, September, 1971.We are indebted to N. I. Éngovatova for direct assistance with the measurements, and to V. Rumyantsev for advice.     

Feynman integral and complex classical trajectories

Let _t be an analytic solution of the Schrödinger equation with the initial condition . Let _t be the solution of the Schrödinger equation with the initial condition =, where is an analytic function. When 0, then _t (x) _t (x)¹ ( _t (x)), where _t (x) trajectory starting from x. We relate this result to Feynman's sum over trajectories and complex stochastic differential equations.     

Experimental survey of the sticking problem in muon catalyzed dt fusion

The sticking process dt + n, which constitutes the most severe limit to the number of fusions which a muon can catalyze, is reviewed. Many attempts were made to determine by calculations and measurements the probability for initial sticking _s ⁰ (immediately after dt fusion) and for final sticking _s (after the came to rest). Previous results based on neutron disappearance rates and on the observation of -X-rays were controversial and also in some disagreement with theory. New data are reported from PSI on direct observation of final sticking, using a setup with the St. Petersburg ionization chamber. These data mark a significant improvement in reliability and may clarify questions concerning previous discrepancies. The new results is _s(0.56±0.04)%, lower than the theory prediction _s=(0.65±0.03)%, at medium density.     

Parametric relaxation of spectra

We study the change of an quasienergy spectrum upon variation of the weight of a perturbation in the Floquet operatorF=F ₀e^–iV . Employing ideas from level dynamics and random matrix-theory we show that the distribution of nearest-neighbor spacings can display effectively irreversible behavior. Small deviations from equilibrium relax in a certain collision time which scales with the numberN of levels as _collN ^–3/2.     

Stimulated IR- and vacuum-UV emission following two-photon-excitation of molecular hydrogen using an ArF laser

Hydrogen molecules are excited by two 193 nm-photons [X¹ _g ⁺ E,F¹ _g ⁺ ;Q₀₂(1)] generated with a commerical oscillator-amplifier ArF laser. Stimulated emission is subsequently observed in the IR (=752.4 nm to 836.6 nm;E, FB) and in the VUV (=134.0 nm to 160.9 nm;BX) with a VUV conversion efficiency of 0.2%. The rotational fine-structure, the saturation behaviour, pump depletion as well as SRS-onset on and off two-photon resonance are investigated.     

Fock representations of the affine Lie algebraA 1 (1)

The aim of this note is to show that the affine Lie algebraA ₁ ⁽¹⁾ has a natural family _, ,v of Fock representations on the spaceC[x _i,y _j;i andj ], parametrized by (,v) C ². By corresponding the highest weight _, of _, to each (,), the parameter spaceC ₂ forms a double cover of the weight spaceC₀C –₁ with singularities at linear forms of level –2; this number is (–1)-times the dual Coxeter number. Our results contain explicit realizations of irreducible non-integrable highest wieghtA ₁ ⁽¹⁾ -modules for generic (,v).     

Phase transition in an ideal bose gas in an external field

Bose-Einstein condensation is analyzed in an ideal Bose gas in an external field at temperatures at which the energy spectrum can be assumed continuous. The nature of the phase transition depends on the behavior of the potential near its minima. Criteria are derived which show in which fields the condensation is a second-order phase transition, a third-order phase transition with an infinite discontinuity in C_V/T, or a third-order phase transition with a finite discontinuity in C_V/T.Translated from Izvestiya VUZ. Fizika, No. 5, pp. 110–114, May, 1971.     

On the Davey–Stewartson and Ishimori Systems

We study the initial value problem for the two-dimensional nonlinear nonlocal Schrödinger equations i u_t + u = N(v), (t, x, y) R³, u(0, x, y) = u₀(x, y), (x, y) R² (A), where the Laplacian = ² _x + ² _y, the solution u is a complex valued function, the nonlinear term N = N₁ + N₂ consists of the local nonlinear part N₁(v) which is cubic with respect to the vector v=(u,u_x,u_y,\overline{u},\overline{u}_{x},\overline{u}_{y}) in the neighborhood of the origin, and the nonlocal nonlinear part N₂(v) =(v, ^{– 1} _x K_x(v)) + (v, ^{– 1} _y K_y(v)), where (, ) denotes the inner product, and the vectors K_x (C⁴(C⁶; C))⁶ and K_y (C⁴(C⁶; C))⁶ are quadratic with respect to the vector v in the neighborhood of the origin. We assume that the components K⁽²⁾ _x = K⁽⁴⁾ _x 0, K⁽³⁾ _y = K⁽⁶⁾ _y 0. In particular, Equation (A) includes two physical examples appearing in fluid dynamics. The elliptic–hyperbolic Davey–Stewartson system can be reduced to Equation (A) with , and all the rest components of the vectors K_x and K_y are equal to zero. The elliptic–hyperbolic Ishimori system is involved in Equation (A), when , and . Our purpose in this paper is to prove the local existence in time of small solutions to the Cauchy problem (A) in the usual Sobolev space, and the global-in-time existence of small solutions to the Cauchy problem (A) in the weighted Sobolev space under some conditions on the complex conjugate structure of the nonlinear terms, namely if N(eⁱ v) = eⁱ N(v) for all R.     

Role of Activator and Vacancy Centers in the Decay of the Gamma Scintillators of CsI–Na Crystals

The influence of the concentration of an activator (C _NaI) and of plastic deformation on a change in the contribution of a slow component to the decay of the -scintillations of CsI–Na crystals was investigated, as well as the influence of C _NaI on a change in the shape of the luminescence excitation spectrum in the region of absorption of activator centers (AC) and of vacancy-related centers (VRC) and also on a change in the intercenter time of deexcitation of the centers indicated. It is shown that AC and VRC participate in the photoluminescence and -scintillations of CsI–Na crystals. In the -scintillations, AC are responsible for the component ₁ = 370 nsec, whereas the components ₁ = 460 nsec and ₂ 2 sec are associated with VRC. The reduction of ₁ from 770 to 560 nsec with an increase in C _NaI from 2·10^–3 to 3·10^–2 mole% and from 570 to 470 nsec after plastic deformation of the crystals ( = 5%) along the 111 axis is caused by a decrease in the number of VRC. The mechanisms underlying the -scintillations of the CaI–Na crystals containing AC and VRC and also the decrease in the number of VRC are discussed.     

Four-wave mixing in CO via the C 1Σ+ (υ′=0) state

Coherent vacuum ultraviolet (VUV) radiation was generated by four-wave difference frequency mixing (_VUV=2₁–₂) of pulsed dye laser radiation in carbon monoxide (CO). The frequency ₁ was tuned to the C ¹⁺(=0)X ¹⁺(=0) two-photon transition, while the dye laser frequency ₂ was scaned around 17650 cm^–1 which corresponds to the A ¹(=7)«C ¹⁺(=0) transition energy. The VUV intensity was found to be strongly wavelength dependent. The analysis of the spectrum revealed (i) that the VUV intensity was enhanced by the rotational levels of the A ¹(=7) state and (ii) that the off-resonance excitation in the C ¹⁺(=0)X ¹⁺(=0) two-photon transition greatly contributed to the present four-wave mixing process. The effects of pumping laser detuning, saturation and foreign gases are briefly discussed.     

A Comparative study of p-Em and C12-Em interactions at 4·5 GeV/c per nucleon

In this paper we present a comparative study of inelastic interactions of C¹²-Em and p-Em at 4·5 GeV/c per nucleon. The multiplicity distributions and their correlations have been studied. The results show a strong correlation of n _s onn _h in the case of C¹²-Em but not in the case of p-Em. It is also shown that the n _s increases with the increase of the projectile mass. The angular distribution of the target fragments in C¹²-Em shows a massive forward collimation. Comparison is also made in the pseudorapidity distribution of both C¹²-Em and p-Em, the result shows many interesting features.     

Evidence for incipient O dimerization and absence of Cu+3 in temperature-dependent XPS of highT c superconducting oxides

From temperature-dependent X-ray photoemission (XPS) spectra between 300 and 10 K of Sr doped La₂CuO₄ and 123 compounds (YBa₂Cu₃O_7–) we deduce the absence of the Cu3d ⁸(Cu³⁺) configuration in the ground state of these Mott insulators. The metallic character of these samples and involving those Cu atoms which are relevant for the superconducting properties arises from (d ⁹ p ⁶)+(d ¹⁰ p ⁵) valence fluctuations, while the superconducting transition is indicated to be closely related to an incipient oxygen dimerization which is best detected upon cooling far belowT _c .