Molecular Physics, Vol. 105, Nos. 5–7, 10 March–10 April 2007, 755–769 A study of the CF radical with PE and CIS spectroscopy: investigation of Rydberg states above the first ionization threshold F. INNOCENTIy, L. ZUINy, M. L. COSTAx, A. A. DIASx, M. GOUBETx, A. MORRISy, R. I. OLERIUx, S. STRANGESz and J. M. DYKE*y yUniversity of Southampton, UK zISMN-CNR sez. Roma La Sapienza, and INSTM Unita di Roma La Sapienza, Italy xUniversidade Nova de Lisboa, Portugal (Received 29 August 2006; in final form 9 October 2006) The CF radical has been studied with photoelectron (PE) and constant-ionic-state (CIS) spectroscopy using synchrotron radiation. By scanning the photon energy in the region between the first and second ionization onsets, while monitoring the intensity of selected vibrational components in the first photoelectron band, excitations to Rydberg states, which are part of a series which converge to the second ionization limit, were revealed. By comparing PE spectra recorded at selected resonance positions with calculated Franck–Condon PE vibrational envelopes to establish the excited state vibrational numbering, fitting the observed resonance positions to Rydberg series, and comparing the results obtained with results from a preceding paper on the isoelectronic molecule NO (1), the structure obtained could be assigned to excitation to np Rydberg states with a CF þ (a 3 ) core. The Rydberg series fits led to an improved adiabatic ionization energy (AIE) from CF(X 2 ) to the CF þ (a 3 ) state of (13.942  0.003) eV. 1. Introduction As part of a programme to investigate the photo- ionization dynamics of reactive intermediates and to study their low-lying ionic states and highly excited neutral states with synchrotron radiation using angle resolved photoelectron spectroscopy (PES) and constant-ionic-state (CIS) spectroscopy, an investigation of the CF radical is reported. It was hoped that with assistance from the results obtained in an earlier paper on NO [1], which is isoelectronic with CF, that analysis and assignment of the spectra obtained could be achieved. CF þ is one of the most abundant ions present in semiconductor plasma processing [2–5]) and therefore obtaining information on the electronic states of CF þ is important as it can be used to understand such processes. The ground state electronic configuration of CF may be written as 1 2 2 2 3 2 4 2 1 4 5 2 2 1 . This is the same as NO but with the 5 and 1 orbitals interchanged in energy order. Some studies have already been made on CF with photoelectron spectroscopy (PES) [6, 7]. The 2,5 and 1 valence orbitals are accessible with the HeI (21.22 eV) radiation but only the (2) 1 and (5) 1 ionizations have been observed by PES [6, 7]. The CF þ (X 1  þ )  CF(X 2 ) ionization arising from one-electron removal from the 2 level has been recorded as a vibrationally structured band with an adiabatic ionization energy (AIE) of 9.11  0.02 eV and a vertical ionization energy (VIE) of 9.55  0.02 eV (6). For one-electron removal from the 5 level, the CF þ (a 3 )  CF(X 2 ) ionization has been observed as a vibrationally structured band with the AIE equal to the VIE at 13.94  0.02 eV [7]. The associated CF þ (A 1 )  CF(X 2 ) ionization has not been observed because the A 1  ionic state is effectively dissociative [7]. Bands associated with the six ionizations CF þ ( 1,3  þ )  CF(X 2 ), CF þ ( 1,3 )  CF(X 2 ) and CF þ ( 1,3   )  CF(X 2 ) arising from the 1 level have not been observed. They are expected above 18.5 eV [6–8] and the ionic states are expected to be dissociative [8]. Far fewer theoretical studies have been performed on CF compared to NO. There appears to be no calculations of CF photo-ionization cross-sections or *Corresponding author. Email: jmdyke@soton.ac.uk Molecular Physics ISSN 0026–8976 print/ISSN 1362–3028 online ß 2007 Taylor & Francis http://www.tandf.co.uk/journals DOI: 10.1080/00268970601075279