[Application of wavenumber-linear scaling to the calculated Raman frequencies of polyenes and carotenoids].

Raman spectra of two typical carotenoids (beta-carotene and lutein) and some short (n = 2-5) polyenes were calculated using density functional theory. The wavenumber-linear scaling (WLS) and other frequency scaling methods were used to calibrate the calculated frequencies. It was found that the most commonly used uniform scaling (UFS) method can only calibrate several individual frequencies perfectly, and the systematic result of this method is not very good. The fitting parameters obtained by the WLS method are upsilon(obs)/upsilon(calc)) = 0.999 9-0.000 027 4upsilon(calc) and upsilon(obs)/upsilon(calc)= 0.993 8-0.000 024 8upsilon(calc) for short polyenes and carotenoids, respectively. The calibration results of the WLS method are much better than the UFS method. This result suggests that the WLS method can be used for the frequency scaling of the molecules as large as carotenoids. The similar fitting parameters for short polyenes and carotenoids indicate that the fitting parameters obtained by WLS for short polyenes can be used for calibrating the calculated vibrational frequencies of carotenoids. This presents a new frequency scaling method for vibrational spectroscopic analysis of carotenoids.

Measurement of infrared level lifetime by upconversion luminescence.

Based on repetition frequency-dependent excited state absorption (ESA) upconversion (UC) luminescence, a method to measure the lifetime of an IR intermediate level is proposed so long as ESA UC luminescence can occur in the rare earth ions. The feasibility of this idea is demonstrated via a theoretical simulation. A Er(3+):LiNbO3 crystal ESA UC luminescence under femtosecond laser excitation is used to illustrate this measurement method, and the obtained 1.5 µm lifetime of 2.31 ms is shorter than previous reported values. This method can obviate the influence of radiation trapping effect on lifetime measurement, which is crucial in the traditional pulse sampling technique.

Effect of end groups on the raman spectra of lycopene and ß-carotene under high pressure.

The Raman spectra of all-trans-lycopene in n-hexane were measured under high pressure, and the results compared with those of ß-carotene. The different pressure effects on Raman spectra are analyzed taking into account the different structures of lycopene and ß-carotene molecules. It is concluded that: (a) the vibronic coupling between the S1 and S0 states of ß-carotene is stronger than that of lycopene, (b) the diabatic frequency increment of the ν1 mode is more susceptible to pressure than that of the ν2 mode for lycopene, and (c) ß-rings rotation can relieve the pressure effect on the C=C bond length in ß-carotene. This work provides some insights for elucidating the structural and environmental effects on Raman spectra of carotenoids.

Red upconversion emission in LiNbO3 codoped with Er3+ and Eu3+.

Red upconversion (UC) emission at 626 nm is obtained from a LiNbO(3) crystal codoped with Er(3+) and Eu(3+) under 800 nm femtosecond laser excitation. Energy transfer from ((2)H(11/2,),(4) S(3/2)) levels of Er(3+), which are excited by excited state absorption, to (5)D(1) of Eu(3+) followed by rapidly relaxing to (5)D(0) nonradiatively leads to this red UC emission. The energy transfer efficiency and Er-Eu transfer microparameter of approximately 30% is obtained in LiNbO(3):Er(3+)(1.0 mol%),Eu(3+)(0.1 mol%). These initial experimental results indicate that the red UC emission can be obtained from Er(3+)/Eu(3+) codoped system under diode laser excitation.

Effect of beta-ring rotation on the structures and vibrational spectra of beta-carotene: density functional theory analysis.

The effect of beta-ring rotation on the structures and vibrational spectroscopic characteristics of beta-carotene, including infrared (IR) intensities and Raman activities, is analyzed using density functional theory. Two stable isomers having Ci symmetry are obtained. The reversion of bond lengths is ascribed to the hyperconjugation effect. The natural bond orbital (NBO) charge analysis suggests that the NBO charges of C5 can be used to estimate the degree of pi-electron delocalization. These structural variations are used to analyze and assign the vibrational spectra. It is concluded that (a) the similar rotational angle dependencies of nu1 and nu2 frequencies justify the contribution of C=C stretch vibrations to the nu2 mode and explain the same conjugation length dependencies of nu1 and nu2 frequencies in polyenes, (b) the nu1 mode can be assigned to the C=C stretching in the central part of polyene chain, whereas beta-rings play an important role in nu2 and IR1 bands, especially for the all-trans isomer, and (c) the transition dipole moment of the calculated IR1 absorption band is relevant to the conjugation degree and the crossing angle between the eigenvectors of the polyene chain and the C5=C6 stretching vibration. This theoretical analysis, together with our previous Raman spectral experiments, suggests that the C6-C7 bond is easier to be twisted than other parts of beta-carotene molecule and so provides an insight into the structures of carotenoids and the properties of binding sites in carotenoproteins.

Two-photon-excited luminescence in a Tb3+ -doped lithium niobate crystal pumped by a near-infrared femtosecond laser.

Blue (487.6 nm), green (544.1 nm), yellow (582.1 nm), and red (623.6 nm) upconversion (UC) luminescences are achieved in a Tb(3+)-doped lithium niobate crystal when an 800 nm femtosecond laser is loosely focused onto the sample at room temperature. The relationship between UC luminescence intensity and the pump energy indicates that a two-photon excitation process is dominant in this UC luminescence phenomenon. The Tb(3+) sensitive temperature dependence of the luminescence intensity is demonstrated via an obvious reduction of luminescence intensity with durative laser irradiation.

Enhanced green upconversion emission of Er(3+) through energy transfer by Dy(3+) under 800 nm femtosecond-laser excitation.

Er(3+) green upconversion (UC) emission corresponding to the transition of (4)S(3/2) ((2)H(11/2))-->(4)I(15/2) is enhanced in a Er/Dy-codoped LiNbO(3) crystal compared with Er-doped LiNbO(3) under 800 nm femtosecond-laser excitation at room temperature. The upconversion mechanisms are proposed based on spectral, kinetic, and pump-power dependence analyses. The energy-transfer efficiency from Dy(3+)((4)F(9/2)) to Er(3+)((4)F(7/2)) is 33%, which results in the enhancement of green UC emission. This energy transfer is advantageous for the Er(3+) UC emission sensitized by Dy(3+), especially in a low-phonon-energy host matrix.

Effect of pressure and solvent on Raman spectra of all-trans-beta-carotene.

The ground state Raman spectra of all-trans-beta-carotene in n-hexane and CS2 solutions are measured by simultaneously changing the solvent environment and molecular structure under high hydrostatic pressure. The diverse pressure dependencies of several representative Raman bands are explained using a competitive mechanism involving bond length changes and vibronic coupling. It is therefore concluded that (a) the in-phase C=C stretching mode plays an essential role in the conversion of energy from S1 to S0 states in carotenoids, (b) internal conversion and intramolecular vibrational redistribution can be accelerated by high pressure, and (c) the environmental effect, but not the structural distortion or pi-electron delocalization, is responsible for the spectral properties of a given carotenoid species. These findings revealed the potential of high pressure in exploring the nature of the biological functions of carotenoids.