The room-temperature adsorption of thiophene on Si(111)7X 7 has been investigated using photoemission with monochromatic synchrotron radiation at 340-eV photon energy. The presence of two C 1s photopeaks at room temperature is consistent with the observation of two molecular-desorption states and with an earlier proposed adsorption model of thiophene on Si(111)7X7 (that involves a surfaceparallel m-bonded geometry and a cr-bonded geometry through the sulfur atom). The C 1s photoemission spectra have also been collected after annealing the sample to different temperatures. Although the observed core-level chemical shifts and the temperature dependence of the two photopeaks are consistent with the thermal-desorption data in the framework of a simple charge-transfer model, correlation between the photopeaks and thermal-desorption data cannot be used to unambiguously identify the nature of the proposed adsorption structures for the adsorption states. Some of the potential difficulties involved in this type of common practice will be discussed.The studies of cyclic hydrocarbons on Si surfaces have attracted recent attention because of the unexpected surface reactivity observed. In particular, recent studies of Si(111)7X 7 carried out in our laboratory' and of Si(111)ZX1 by other groups have demonstrated the high reactivity of these semiconductor surfaces towards cyclic olefins. Not only adsorption of a series of homologous five-membered and six-membered cyclic olefins, including thiophene, furan, ' and benzene has been observed at room temperature (RT), strong evidence of catalytic dehydrogenation surface reactions of cyclohexene and cyclohexadienes as well as their methyl-substituted derivatives has also been obtained for Si(111)7X 7. Furthermore,~-conjugated heteroarornatic compounds such as thiophene and pyrrole have attracted practical interest as plausible candidates as the basic building blocks of the so-called conductive polymers, which are found to have valuable technological properties.Very recently, we reported a thermal desorption spectrometric (TDS) study of the room-temperature adsorption of thiophene and related five-membered cyclic olefins on clean and modified Si(111)7X 7 surfaces. ' By comparing the desorption profiles of these olefins, information about their interactions with the surface has been obtained. This work also provides an interesting comparison with the results of a series of photoemission and electron energy loss studies of Piancastelli et aI. , who reported desulfurization of thiophene ' and deoxygenation of furan on Si(111)2X1 for room-temperature exposure.At low temperature (85 K), however, no fragmentation was found and two adsorption states were reported.It was further proposed that the adsorption states for thiophene at low temperature involved a m-bonded geometry and a a. -bonded geometry resulting from the breakage of a C -H bond.In the case of roomtemperature adsorption of thiophene on Si(111)7X7, we have found two molecular desorption states which, unlike the proposed adsorptio...