A general method of imaging organic and biological surfaces based on the photoelectric effect is reported. For the experiments, a photoelectron emission microscope was constructed. It is an ultrahigh vacuum instrument using electrostatic electron lenses, microchannel plate image intensifier, cold stage, hydrogen excitation source9 and magnesium fluoride optics. The organic surfaces examined were grid patterns of acridine orange, fluorescein, and benzo(a)pyrene on a Butvar surface. A biological sample, sectioned rat epididymis, was also imaged by the new photoelectron microscope. Good contrast was obtained in these initial low magnification experiments. These data demonstrate the feasibility of mapping biological surfaces according to differences in ionization potentials of exposed molecules. A number of technical difficulties, such as the intensity of the excitation source, must be solved before high resolution experiments are practical. However, it is probable that this approach can be useful, even at low magnifications, in determination of the properties of organic and biological surfaces.Spectroscopic labeling techniques are becoming increasingly useful in studies of membranes and other biological surfaces. Labeling or tagging with organic dye molecules has long been recognized as a useful approach (1). The techniques are, of course, becoming more refined and the useful region of the electromagnetic spectrum has been greatly extended. The common techniques now include fluorescence (2, 3), optical absorption (3), electron spin resonance (3, 4), and nuclear magnetic resonance spectroscopy (3, 5). All of these techniques can yield information regarding molecular motion and orientation of molecules, and the polarity of specific binding sites. However, these spectroscopic methods do not determine the positions of the labels or distinguish between surface and bulk properties of the specimen. This is especially troublesome when dealing with biological surfaces (e.g., cell surfaces, nerve endings, and membranes of organelles). Understanding mechanisms of drug action, cell adhesion, membrane structure, immunological responses, and loss of contact inhibition in malignant cells require a knowledge of the relative positions, environments, and population densities of binding sites on the surface. It is clear that new microscopic techniques are needed that can be combined with existing spectroscopic methods in studies of biological surfaces. It was to develop new microscopic techniques that we began several years ago to examine the photoelectric effect of organic and biological surfaces.A typical experiment is depicted in Fig. 1. The specimen is placed in a vacuum chamber and is then subjected to ultraviolet light. If the energy of the light source (ha) is sufficiently high, the sample surface can emit electrons (photoionize) as well as fluoresce. This intrinsic photoionization depends on the ionization potentials of various functional groups on or very near the surface. The process for individual molecules is desc...

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The photoelectron quantum yields of 21 common amino acids and 15 polyamino acids were measured in the 180-240 nm wavelength region. On the average, the quantum yields of these two groups exhibit quite similar wavelength dependence. For lambda > 220 nm all amino acid and polyamino acid quantum yields are

Abstract— The application of photoelectron microscopy as a general method of imaging organic and biological surfaces requires a knowledge of the photoelectric effect of very thin organic films. In this study, low magnification images of a 7 Å thick pattern of copper phthalocyanine were obtained, demonstrating that it is possible to visualize a monolayer of organic compound in photoelectron microscopy. Relative photoelectron currents were measured for a series of copper phthalocyanine films ranging in thickness up to 1900 Å. The relative photoelectron currents were independent of temperature (90–298°K), suggesting that electron‐electron and not electron‐phonon scattering is the dominant mechanism. The photoelectric properties measured are determined primarily by the large organic ring structure and not the central metal atom, as evidenced by the fact that substitution of metal‐free phthalocyanine for copper phthalocyanine did not substantially alter the values of observed photoelectron currents. An analysis of the data indicates the depth resolution is 15 ű 5 Å, and equals the electron mean free path. This very good depth resolution is a result of the low kinetic energy associated with electrons released by irradiation near the photoemission threshold.

The photoelectric effect can provide the physical basis for a new method of mapping organic and biological surfaces. The technique, photoelectron microscopy, is similar to fluorescence microscopy using incident ultraviolet light except that photoejected electrons form the image of the specimen surface. In this work the minimum wavelengths of incident light required to produce an image were determined for the molecules 3,6-bis(dimethylamino)acridine (acridine orange) (I), benzo[a]pyrene (II), N,N,N',N'-tetraphenylbenzidine (III), and copper phthalocyanine (IV). The photoelectron image thresholds for these compounds are 220 (I), 215 (II), 220 (III), and 240 nm (IV), all +/-5 nm. Contrast of I-IV with respect to typical protein, lipid, nucleic acid, and polysaccharide surfaces was examined over the wavelength range 240-180 nm. The low magnification micrographs exhibited bright areas corresponding to I-IV but dark regions for the biochemical surfaces. The high contrast suggests the feasibility of performing extrinsic photoelectron microscopy experiments through selective labeling of sites on biological surfaces.