The influence of out-of-plane modes on non-radiative transition rates in benzene

✍ Scribed by H. Hornburger; J. Brand

• Publisher: Elsevier Science
• Year: 1982
• Tongue: English
• Weight: 321 KB
• Volume: 88
• Category: Article
• ISSN: 0009-2614
• DOI: 10.1016/0009-2614(82)83358-7

✦ Synopsis

The shw increase in internal conversion rste St (Bzu) -S&I ,gj on cwss nbratlonti cnc~y III cwtcd clcctramc status in bemenc Ls controlled by out-of-plane modes At 3000 cm-* c~css energy more than 70% goes mto the vJ(b2g) out_~f-Pkme vibration and the relatwc rstc increases by about 7 orders of magnitude wtbm the harmonic-mode approulmallon. I. tntroduction If the excess vtbrational energy in the first ewted singlet state S1(Bzu) of benzene is mcreased under colhsion-free. conditions the v1bratlona.l bands become broad and diffuse and fiially form a structureless quasicontinuum [l] . At the same time a sharp increase of the non-radiative rate at about 3000 cm-l excess energy can be observed accompanied by decreasing tluorescence and phosphorescence yields [2,3]. These observations suggest internal conversion or reversible isomerisation as possible deactivation channels [4,5] .

Many different theoretical models have been developed to account for these effects [5-13 1. These models include for instance the effects of distortion, anharmonicities (Morse oscdlators), nonCondon treatment and coupling of excited states.

By introducing coupled excited states, km and coworkers [14] have shown how the S, + So decay in pyrazine ultimately surpasses the S, + T, decay channel at high excess vibrational energies and it is expected that benzene also manifest this decay pattern. However, all authors have assumed the C-H, C-C stretchingand the promoting modes to be the main acceptors