Volume 64, number 3 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA CHEMICAL PHYSICS LETTERS 15 JuI> 1979 THE 51000 cm-l STATE IN BENZENE: Erg RYDBERG OR Ezg VALENCE? * Karsten KROGHJESPERSEN, Richard P. RAVA z and Lionel GOODMAN zyxwvutsrqponmlkjihgfedcbaZYXWVUTS Department of Chemistry. Rutgers. l7”e State Unil ersity of Xew zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONML Jersey. Xew Brunswick. New Jersey 08903. US-4 Received 5 ApriI 1979 A previoudy unobserved eIectronic state in fhrorobenzene appears as a tuo-photon resonance in the multiphoton reso-‘ nance ionization spectrum. The band system is assigned to the 3s Rydberg -ha(n) HOUO transition with a quatitum defect of 0.83 and an origin at 50 914 k I5 cm-t. Its correspondence to the benzene state characterized by Johnson in the same re- gion assigns that state to Etg 3s Rydberg. I- Introduction Renewed interest in the lower Iying electronically excited states of benzene has arisen with the discovery by Johnson [ 11, using multiphoton resonance ioniza- tion (MPRI) spectroscopy, of a two-photon allowed bound state energetically positioned between the lBr,, and 1 El, states. The vibrationaI analysis indicates this new state to be degenerate of symmetry Er, or Ezg with origin 51085 cm-r _ Only two such states can he in this region [I J , the theoreticahy predicted and much sought low lying valence (sr, x’) E2e state [2] or the 3s El, Rydberg arising from excitation out of the HOMO el8(rr)_ The s Rydbergs are conspicuous- Iy unobserved in most benzenoid molecules, whereas Rydberg states of p and d symmetry have been easiry identified [3] _ The symmetry designation of the new state in benzene might be settled by examining the spectra under a perturbation. Rydbeg states often appear weak, broad, and shifted to the bIue under pressure from an extermd gas or in a condensed phase, whereas valence states are only slightly broadened and red-shifted under sin&r conditions [3] _ The two-photon absorption spectrum of liquid benzene obtained by Monson and McCIain f Supported by National Science Foundation Grant CHE76- 23813_ $ Henry Rutgers Undergraduate Scholar 1978-1979. [4] did not reveal any features around 51 000 cm-1 Twarowski and KIiger [5] carried out a thermal blooming experiment on Iiquid benzene and located the r Bzu and 1 B,, states, but again no band system was observed around 5 1 000 cm-l : instead, increasing continuous absorption showed up below 4000 A peak- ing to the blue of 3600 a (one-photon wavelengths)_ They interpreted these observations as showing the Johnson system to be Rydberg in nature and their new transition as involving the EZg valence state in two-photon absorption. This interpretation was dis- puted by Vaida et al. 161, who pointed out that the ionization potential in the liquid wouid be significant- ly lowered relative to the gas phase value of 9.24 eV. No Rydberg states would be stable in the Iiquid, and the bandwidth (at least 11000 cm-r) showed that this transition couId not represent the Ezg state- These authors assigned the continuous absorption as direct two-photon excitation into the conduction band, and the condensed phase test as an external per- turbation is thus not conclusive_ In pyridiue and pyrazine two-photon band systems have recentry been observed in the 50 500 cm-l re- gion using MPRI spectroscopy and have been assigned as transitions to 3s Rydberg orbitak [7] _ The polariza- tion measurements of Berg et al. [S] show the states to be totally symmetric and consequently arise from excitation from nitrogen lone pair, not rr orbitals. With a transferable 3s term value of =24 000 cm-l 413