CHEMICAL PHYSICS LETTERS 20 July 1984 A PRECISE DETE~~~ATION OF THE FIRST IONrZATION POTENTIAL OF BENZENE S.G. GRUBB *. R.L. WHETTEN *‘. AC. ALBRECHT and E.R. GRANT Bah-rr laboratory of Clwmistry. Cornell Uttixwit_r. Irhaca. :Vm* York I4S.53. L’S,4 Rcceiued 30 April 1984:in final form 14 Map 1981 The first ionization potentials of benzene and benzeneil6 have been precisely determined by the extrapolation of three-photon resonant Rydberg states in the four-phoron ionization spectrum of the jet-cooled molecule. The convergence of resoived transitions in two Rydberg series for principal quantum numbers as high as 14 (-lrd and 15 (46) establish adiabatic thresholds of 74573.0 + 2.0 cm-’ and 74592.5 I 1.2 cm-‘, respectively. These results arc crucial for the under- stxtding of the many excited states of benzene in terms of quantum defect theory. Precise quantum defects have been ob- tained for seveml Rydbeg series and their variation with principal quantum number is reported. The results strongly sup- gest that the 11”’ scrics of Wil!&xon is derived front a n(et,) - nfkt Rydberg excitation. 1. Introduction The knowkdge of the higher excited states of benzene has been greatly espandcd by the recent ‘two- and four-photon resonant spectroscopic obscrva- tion of six new gerade Rydberg series [ l--3] _ These data reveal numerous perturbations and deviations from ideal Rydberg behavior which remain to be es- pfained. Such a complete and extensive accounting of excited states in benzene invites a deeper analysis_ One approach f’] to the study of perturbations in Rydbcrg series is the examination of systematic varia- tions in the quantum defect along a series and be- tween series converging to different ionization thrcsh- olds, either vibrational or electronic. In order to ob- tain inforii~ation on such variations with an accuracy sufficient for its iiiterpretation in terms ofsingle or muItichanne1 quantum defect theory (MQDT). one needs precise data on the positions of both vibronic origins of a Rydberg series and the ionization thresh- old to which it converges. Quantum defects for small term values (higher Rydbergs) are particularly sensi- tive to the choice of the ionization potential. * Present address: &son Research and Engineering Compa- ny, Chemical Pftysics Division, Annandalc, New Jersey 08801. USA. * National Science I:oundation Predoctoral Fellow. 420 Accurate ionization potentials are usually deter- mined by Rydberg series estrapolstion. For the case of benzene. confident application of this procedure has been limited by the facr that no high Rydberg states have been observed. The presently accepted value for the ionization potential of benzene, ob- tained by tabular fit IO each of the four known un- gerade Rydbcrg series. is 74557 * 15 cm-l or 9.247 k 0.002 eV [S ,6] _ The four separate ionization potentials determined by single-series extrapolation, however, have values that vary by more than IO0 cm-t [G] _ No less uncertain is the extrapolation of band origins for members of the new gerade series [l--3]. This large uncertainty prohibits a confident determination of the systematics of the quantum defect variations_ The decline expected [7] in core-Rydberg elec- tron interaction with increasing principat quantum number (12~~ Law) suggests that at high 11 the quan- tum defect. 8. as defined by the Rydberg formula: .OO = IP --4,./(lZ - 6)? !I (1) will be constant_ Under such conditions the usual two parameter fit of IP and 6 should yieid a unique ionization potential. Thus in order to achieve a pre- cise measurement of the ionization potential it is necessary to measure the excitation energies of high Rydberg states (11 = S-20) in at least one series dem- 0 009-26 14/84/S 03 -00 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
