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The chemistry of interfaces plays a fundamental role in adhesion. Many analytical methods, such as SIMS, XPS, AES and SEM, can be used to study surfaces after adhesive failure. Only a few techniques exist, however, for the non-destructive, in situ chemical analysis of buried interfaces. Infrared-visible sum-frequency generation (SFG) is one such technique. Shen has demonstrated recently' that infrared spectra of interfaces can be obtained by sum-frequency spectroscopy, even in the presence of bulk phases on either side. SFG is a powerful technique still in its infancy; here, we demonstrate the application of SFG to the analysis of metals coated with monolayer films. These films are useful model systems and also have practical importance as adhesion promoters and wetting modifiers.In infrared-visible sum-frequency generation, a fixedfrequency visible laser and a tunable infrared laser are overlapped at an interface and the light emitted at the sum of the incident frequencies, u , , , , is detected (Scheme 1). In our experiments, laser light at 532 nm is provided by a frequencydoubled Nd : YAG laser, and the tunable infrared radiation is generated by difference frequency mixing of the Nd : YAG fundamental and a near-IR, Nd:YAG-pumped dye laser. The photons emitted at the sum frequency are separated from scattered light by a filter and monochromator, and detected with a photomultiplier tube. The intensity, S,,, , of the emitted sum frequency is given by' (photons per pulse) where Li are Fresnel field factors, I is the intensity of a laser, A the area of overlap of the laser beams, T the pulse duration and $, ) , , ( -us,,; wvis, urn) the second-order non-linear susceptibility of the sample.

An apparatus has been built which allows, by radiation pressure forces, the manipulation of a single colloid particle, immersed in water, close to a flat surface. The particle movement is monitored by total internal reflection microscopy (TIRM). Preliminary results are presented which illustrate the usefulness of the technique for studying the weak interactions between a 10-µ polystyrene latex particle and a quartz glass surface. An interaction potential energy curve as a function of the relative separation between the particle and the surface has been obtained.

The transition temperature of the leaf polar lipids and the critical temperature for chill-induced inhibition of photosynthesis was determined for three altitudinal ecotypes of the wild tomato Lycopersicon hirsutum. Photosynthesis was measured as C02-dependent 02 evolution at 25°C after leaf slices were exposed to chilling temperatures for 2 hours at a moderate photon flux density of 450 micromoles per square meter per second. The transition temperature of the leaf polar lipids was detected from the change in the temperature coefficient of the fluorescence intensity of trans-parinaric acid. Chill-induced photoinhibition was evident in the three tomato ecotypes when they were chilled below a critical temperature of 100, 110, and 130C, respectively, for the high (LA1777), mid (LA1625), and low (LA1361) altitudinal ecotypes. The temperature differential, below the critical temperature, required to produce a 50% inhibition was also similar for the three ecotypes. A transition was detected in the leaf polar lipids of these plants at a temperature similar to that of the critical temperature for photoinhibition. The results show that the three tomato ecotypes are similar with respect to their critical temperature for chilling-induced photoinhibition and the rate of their response to the chilling stress. They are, thus, similarly sensitive to chilling.Altitudinal ecotypes of the wild tomato Lycopersicon hirsutum have been used extensively in comparative studies of the susceptibility ofplants to chilling (1,10,12,18 It is important to note that in the terminology adopted here a distinction is being made between the terms 'sensitivity' to chilling and 'response' to a chilling stress. The basis for the distinction is that described by Raison and Lyons (15). For tropical plants that are sensitive to chilling there is usually a critical temperature below which injury develops and this can vary from about 70 to 1 5C depending on the species (7, 13). For plants with a critical temperature near 15°C and chilled at 0°C the chilling stress, determined as the temperature differential below the critical temperature, will be greater than that for plants with a critical temperature near 7°C. The term response is used in relation to the time or rate of symptom development. For any plant this will depend on the chilling stress (1), the type of tissue (19), its metabolic state prior to chilling (9), the time in a day/night cycle when chilling commences, the chilling temperature (9), and a number of environmental factors such as humidity (20). From the available data, it is not clear whether the tomato ecotypes adapted to the lower temperatures at high elevation are less sensitive to chilling, i.e. have a lower critical temperature, or are more resistant to the stress and thus show a slower response.Differences have been noted in both the response and sensitivity of photosynthetic reactions of plants to chilling for a few hours at a moderate PFD of less than 500 ,umol m-2 s-' (2, 4). For plants sensitive to chilling, such as olea...

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