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We extend the application of the Z-scan experimental technique to determine free-carrier nonlinearities in the presence of bound electronic refraction and two-photon absorption. We employ this method, using picosecond pulses in CdTe, GaAs, and ZnTe at 1.06 m and in ZnSe at 1.06 and 0.53 Am, to measure the refractive-index change induced by two-photon-excited free carriers (coefficient ar,), the two-photon absorption coefficient 1, and the bound electronic nonlinear refractive index n 2. The real and imaginary parts of the third-order susceptibility (i.e., n 2 and 13, respectively) are determined by Z scans with low inputs, and the refraction from carriers generated by two-photon absorption (an effecitve fifth-order nonlinearity) is determined from Z scans with higher input energies. We compare our experimental results with theoretical models and deduce that the three measured parameters are well predicted by simple two-band models. n 2 changes from positive to negative as the photon energy approaches the band edge, in accordance with a recent theory of the dispersion of n 2 in solids based on Kramers-Kronig transformations [

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lines are partially blended. Thus, it appears that Chiu's equations give very reasonable predictions for the intensities of this mixed magnetic-dipole-e1ectric-quadrupole transition.Wilkinson and Mulliken,2 using relatively poor uncalibrated photographic data on the a lIIgf-X ll;g+ system and employing line-strength formulas developed only for a ll;g+f-ll;g+ transition 9 (admittedly very tentatively), estimated Q/D=0.15, which is of the order of magnitude of the value obtained here. This presumably more precise value of 0.33 leads to a value of X in Eq. (1) of 9 a.u. instead of 4.5 a.u. given previously.The above determined ratio is certainly not in disagreement with the crude theoretical prediction of Q/ D"-'0.1 mentioned earlier. Nevertheless, Condon 7 has studied such transitions for p2, p3, and p4 configura-9 H. M. James and A. S. Coolidge, Astrophys. J. 85, 438 (1938).THE JOURNAL OF CHEMICAL PHYSICS 2683 tions in atoms, and has found quite variable values for this ratio. It does appear that a careful theoretical study of the vibronic matrix elements for this particular transition would be most helpful.It is perhaps worth mentioning that the line-intensity study reported here is one of the few experimental tests on intensities in forbidden transitions in diatomic molecules and only the fourth in the vacuum ultraviolet. lO ACKNOWLEDGMENTSThe authors wish to express their appreciation to Mr.The structures of ethylene and deuteroethylene were investigated by electron diffraction to resolve apparent discrepancies between earlier diffraction and spectroscopic studies. Satisfactory agreement with the previous diffraction study was found for five of the six molecular parameters determined but a difference of five standard deviations was encountered for the C-H bond length. Mean bond lengths and standard errors for C2H, obtained in the present investigation were rg (CH)=1.1030±0'()()l s A and r g (CC)=1.3369± 0.0016 A. Corresponding lengths for C2D, were r g (CD)=1.099±0.003 A and r g (CC)=1.338±0.003 A.Bond angles, corrected for shrinkage effects, were L CCH= 121.4±0.6° and L CCD= 121.4±0.8°. The new diffraction results are compared with mean bond lengths and angles calculated from spectroscopic rotational constants taking rotation-vibration interactions into account. Excellent agreement is found. Significant differences exist, however, between the conventional bond-length parameters derived by spectroscopy and diffraction. Root-mean-square amplitudes of vibration were also determined by diffraction, and these agreed satisfactorily with amplitudes calculated from vibrational frequencies.

Better utilization of research and evidence in development policy and practice can help save lives, reduce poverty and improve the quality of life. However, there is limited systematic understanding of the links between research and policy in international development. The paper reviews existing literature and proposes an analytical framework with four key arenas: external influences, political context, evidence and links. Based on the findings of stakeholder workshops in developing countries around the world, the paper identifies four key issues that characterize many developing countries. These are: (i) troubled political contexts; (ii) problems of research supply; (iii) external interference; and (iv) the emergence of civil society as a key player. Despite these challenges, two institutional models seem to be particularly effective: (i) think tanks and (ii) regional networks. Copyright © 2005 John Wiley & Sons, Ltd.