We describe how to separate all surface, interface, and bulk tensor components in optical secondharmonic generation experiments on isotropic thin films. Using in situ thickness scans and group theoretical arguments concerning the bulk tensor components, we demonstrate that the second-harmonic resonance of C60 at 26co =3.6 eV is of bulk character, and, quite remarkably, of magnetic dipole induced origin. PACS numbers: 78.66.w, 42.65.k, 61.46.+w Among the interesting properties of the fullerenes are their strong nonlinear optical properties. Quite some experimental eAort has been made to clarify the high second and third order optical response of C60 and some related materials [1-6]. In a previous paper we reported on optical second-harmonic generation (SHG) experiments on C6g thin films, showing a narrow resonance at 2hco=3. 6 eV [6].In this paper we present a systematic procedure to resolve all bulk, surface, and interface tensor components for an isotropic thin film system. This separation of surface vs bulk SHG contributions is known to be a fundamental difficulty in the field of surface SHG [7]. Our method makes use of the experimental data from SHG thickness scans, and an estimation of the ratio between two bulk tensor components. Using this procedure, the situation for C60 is clarified for the specific case of the resonant SHG at 26m =3.6 eV. From comparison with the linear optical spectrum of C60 [8,9] we expect the resonance to be dominated by the h~h" t &"diagram as indicated in Fig. 1. The forbidden h" t~"transition energy is of the order of 2 eV, so that the total SHG diagram is at, or close to, double resonance. We demonstrate that the SHG resonance is magnetic dipole (MD) induced, and show that this is due to the spherical character of the C60 molecule. To our knowledge this is the first direct evidence for such optically MD induced contributions in nonlinear optical experiments on nonconducting materials.For third order optical experiments on C60 [1-4] all involved electronic transitions are electric dipole allowed, so that the strong delocalization of the p molecular orbitals can give rise to high g values. The situation for the second order optical response is more complicated. Although relative high SHG e%ciencies for C60 have been reported [4][5][6], the process is symmetry forbidden within the electric dipole approximation.Therefore, other mechanisms become important, that is nonlocal bulk contributions, indicated by g~[ including both MD and electric quadrupole (EQ) processes], and electric dipole allowed or field gradient induced surface contributions, t, , 8=6

The electric dipole forbidden 1 T 1g excitonic state of solid C 60 athω=1.81 eV can be probed with a Second-Harmonic Generation (SHG) experiment [1].We show that the SHG line shape depends strongly on the degree of rotational order. We observe a splitting into two peaks below the rotational ordering phase transition temperature of 260 K. The origin of this splitting is discussed in terms of a possible Jahn-Teller effect, a possible Davydov splitting due to the four molecules per unit cell in the low temperature phase, and a mixing of the nearly degenerate 1 T 1g and 1 G g free molecule states because of the lower symmetry in the solid. The exciton band structure is calculated with a charge transfer mediated propagation mechanism as suggested by Lof et al.[2] and with one-electron (-hole) transfer integrals determined from band structure calculations. Comparison with our experimental SHG data leads to a reasonable agreement and shows that a mixing of 1 T 1g and 1 G g states may explain the splitting at low temperature.

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It is shown that the modulated phases of tetramethylammonium tetrachlorozincate, [(CH3)4N]2ZnC14, can be described by one superspace group: Pcmn(OO3")(lsi).This group is consistent not only with the properties of the diffraction pattern of the commensurate and incommensurate phases (and in particular with the corresponding space-group assignments found in the literature) but also with the crystal morphology, the latter being studied here by growth sphere experiments. The description of the morphology in terms of main and satellite faces, analogous to the description of the diffraction pattern, reveals a simple order in the crystal morphology of the different phases. Whereas the main faces remain relatively unperturbed, the position and appearance of satellite faces are directly related to the modulation wave. In fact, the evolution of the modulation wave vector can be monitored from the position of the satellite faces with respect to the main faces. Morphological extinction conditions even show compatibility with the proposed superspace group. Though the bonding structure of the satellite faces is not quite understood yet, a preliminary explanation is given in terms of a stabilized step structure.

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The magnetic phase transi tion Jml' ~ m with the Curie temperature Tc=<30K bas been studied by means of optical domain analysis and birefringence measurements using polarized light microscopy. The domain structure is consistent with the species 43ml'Fm with 24 partially ferromagnetic /fully ferroelectric I fully ferroelastic orientation states. The weakly ferromagnetic m phase possesses a spontaneous magneti. zation perpendicular to the (llO)cub planes (mirror m). A coupling effect between the ferromagnetic, ferroelectric and ferroelastic domains was observed and detected. The spontaneous birefringence of magnetic and possibly coupled structural origin vanishes above T c and exhibits a second order character of transition.

A rich phase chemistry has been predicted for pyridine, with a computational study by Price et al. [1] revealing 12 energetically similar structures. This, coupled with the simple nature of the molecule makes pyridine a good candidate for detailed structural studies. As well as being interesting in itself, it is hoped that the structural behavior of pyridine may shed light on more fundamental trends in structure and bonding in molecular materials. Previously only two crystal phases of pyridine have been identified by experiment. The Z´=4 phase (pyridine-I) first found by Mootz et al. [2] is unexpectedly complicated and has been found to arise from a range of crystallisation conditions. The simpler Z´=1 phase (pyridine-II) has previously only been grown for pyridine at high pressure [3], or for perdeuteropyridine from a pentane solution [4]. Using the HRPD instrument at the ISIS facility, we have performed a study of the crystallographic behaviour from 2K to melting of a pure perdeuteropyridine powder sample. Pyridine-I was found at lower temperatures, until a phase transition to pyridine-II was observed with increasing temperature. On further increase in temperature, a re-entry to the pyridine-II phase was seen.

We show that second-harmonic generation ͑SHG͒ and sum-frequency generation ͑SFG͒ are selective tools for probing excitons. In SHG and SFG measurements performed on C 60 and CuCl films we observe Frenkel and Wannier excitons, respectively. On the other hand, no second-harmonic ͑sum-frequency͒ intensity enhancement was observed at energies above the conductivity gap. This is in strong contrast to, for instance, one-photon and two-photon absorption experiments. The selectivity of SHG and SFG for excitons compared to interband transitions can be explained in terms of coherence of the respective excitation processes.