Appl. Phys. A 49, 393-397 (1989) Applied so,,.. Physics A "' Surfaces 9 Springer-Verlag1989 High Resolution Photoabsorption Spectroscopy at the Carbon K-Edge D. Arvanitis 1, H. Rabus 1, M. Domke 1, A. Puschmann 1, G. Comelli 2, H. Petersen 3, L. Tr6ger 1, T. Lederer 1, G. Kaindl t, and K. Baberschke 1 t Institut ftir Atom- und Festk6rperphysik, FU Berlin, Arnimallee 14, D-1000 Berlin 33 2 Sincrotrone Trieste, Padriciano 99, 1-34012 Trieste, Italy 3 BESSY GmbH, Lentzealtee 100, D-1000 Berlin 33 Received 25 April 1989/Accepted 5 June 1989 Abstract. Using plane grating, ellipsoidal mirror, grazing incidence monochromators at the storage ring BESSY, a resolution of < 150 meV was achieved for a photon energy of 285 eV. This high energy resolution considerably extends the range of possible studies using core level spectroscopy. Some of the new opportunities are illustrated at the C K edge by resolving the vibrational fine structure of condensed C2H 4 multilayers in the (ls- 1, re*) state by means of photoabsorption spectroscopy. For the sake of comparison with other high resolution instruments, the vibrational fine structure of condensed N 2 multilayers at the N K edge (,-~ 400 eV) was also measured, yielding a resolving power of 3000. PACS: 07.85.+n, 33.20.Rm, 35.20.Pa X-ray photoabsorption spectroscopy at the carbon K edge has been used for the past eight years in order to determine the orientation of molecules adsorbed on surfaces [1]. The energy resolution of the Grasshopper monoehromator at SSRL where these measurements were started was, in the early days, of approximately 2eV. The SX700I monochromator at BESSY, operating since 1983, has an energy resolution of 1.2 eV in routine operation (100 gm exit slit, 5 x 109 photons/ 100 mA s at 285 eV). Under these conditions it is not possible to study the fine structure of the molecular excitations. Commonly the photon excitation of an 1 s electron into the lowest unoccupied n* molecular level was detected as a broad resonance line. For a line shape analysis a Gaussian profile was used to mimic the energy resolution of the monochromator. Recently, great progress has been made at NSLS, using a spherical element monochromator. Chen and Sette [2] achieved a resolving power close to 104 at the N K edge (~ 400 eV). Energy resolutions of the order of 100 meV are good enough in order to study molecular vibrations, with typical separations of 100-500meV between the vibrational eigenstates. In the present study we show that a comparable resolving power of 3 x 103 can be achieved at the C K edge (285 eV) using a plane grating, ellipsoidal mirror monochromator at BESSY. These energy resolutions are comparable to the ones obtained by other techniques, like electron energy loss spectroscopy (EELS) or valence level photoemission spectroscopy. Condensed N2, as in [2], or condensed C2tt 4 molecules (this work) can be used to demonstrate some of the new opportunities offered to core level spectroscopy. In the present work the (ls- 1, re*) excited bound state vibrational excitations of C2H 4 are investigated, yielding complementary information to EELS spectroscopy, which probes the excitations of the molecule in its ground state. Comple- mentary information is given also with respect to valence photoemission spectroscopy. There the excita- tions of the molecular ions are probed, whereas in the present work neutral species are studied. But probably the newest and most useful applica- tion in core level spectroscopy will be the use of the linearly polarized highly energy resolved X-rays to study oriented molecules on surfaces in monolayer coverage. However, the optimum energy resolution combined with high photon fluxes needed for such studies is only possible if the monochromator's optical