Volume 125, number 2 CHEMICAL PHYSICS LETTERS 28 March 1986 zyxwvutsrqpon OBSERVATION OF RESONANCE EFFECTS IN THE RELATIVE PARTIAL PHOTOIONIZATION CROSS SECTIONS OF THE d BANDS OF M(CO),, M = Cr, MO AND W Glyn COOPER, Jennifer C. GREEN, Martin P. PAYNE Inorganic Chemistry Laboratory, Oxford OXI 3QR, UK Barry R. DOBSON and Ian H. HILLIER The Chemistry Department, Manchester Unioersrty, Manchester MI3 9PL, UK Received 21 January 1986 Relative partial photoionization cross sections as a function of photon energy, over the range 20-110 eV, have been measured for the valence bands of Cr(CO),, Mo(CO), and W(CO),. All three tlg -’ bands show a very pronounced increase in intensity at photon energies (hv) corresponding to np resonant absorption (Cr(CO),, hv = 52.5 eV, n = 3; Mo(CO),,;,hv = 48 eV, n = 4; W(CO),, hv = 44 and 53 eV, n = 5). The other valence bands show a small intensity increase at similar energies. Observation of such resonant photoemission provides an unambiguous method for assignment of nd bands in the photoelec- tron spectra of gas-phase molecules. 1. Introduction Early in the development of gas-phase ultraviolet photoelectron spectroscopy (UPS), it became clear that band intensity changes with photon energy could be used in assigningvalence bands [ 11.Since then He I and He II radiation have been used routinely in the investigation of a wide variety of molecules. How- ever, restriction to two photon energies limits both the experimental information and the conclusions that may be drawn from such data. In a similar manner, the photon energy dependence of differential cross sections is used in photoemission with synchrotron radiation, in order to disentangle the contribution from different atomic states to the valence bands of com- pound solids [2]. Resonant photoemission on excita- tion of inner shells has been observed for rare earth [3] and transition metals [4]. Synchrotron radiation was obtained from the 2 GeV electron storage ring at SERC Daresbury Laboratory. A full account of the apparatus and its performance is given elsewhere [S]. For the present experiments the analyser was rotatable in a plane at right angles to the photon beam and had an angular acceptance of +2”. The analyser was positioned at the magic angle in order to eliminate variations in intensity due to the asym- metry parameter, 0. The cross section as a function of angle is given by da/da = (o/4$ [ 1 t $ fl(3P cos 28 t l)] , In order to place on a fumer basis the use of band intensity variation with photon energy as a method of valence band assignment in UPS, we have undertaken a study of the relative partial photoionization cross sec- tions (RPPICS) of the hexacarbonyls of chromium, molybdenum and tungsten using synchrotron radia- tion in the energy range 20-l 10 eV. where u is the cross section, 0 the angle of the ionized electrons with respect to the principle electric vector of the elliptically polarized synchrotron radiation, P the polarization of the photon beam, and a the ac- ceptance angle of the analyser. The polarization of the radiation was measured both by using a four-reflection polarization analyser [6] and by measurements on the photoelectron (PE) spectra of Ar and He in combina- tion with literature values for u [7,8]. The value taken for the magic angle was 123 So. The variation in col- 0 009-26 14/86/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) 2. Experimental 97