FEMTOSECOND TIME-RESOLVED UV-VISIBLE ABSORPTION-SPECTROSCOPY OF TRANS-AZOBENZENE - DEPENDENCE ON EXCITATION WAVELENGTH

Femtosecond time-resolved UV-visible absorption spectroscopy has been used to study the photochemistry of trans-azobenzene in n-hexane. Exci tation to the S-1(n pi) state results in transient absorption bands a t ca. 400 nm (strong) and 550 nm (weaker) which decay with a lifetime 2.5 +/- 0.2 ps on excitation at 503 nm, close to the S-1 origin, and w ith an additional fast component of ca. 0.6 ps on excitation at 390 or 420 nm, both well above the S-1 origin. Excitation to the S-2(pi pi) state results in transient absorption at 400 nm which decays with a d ominant component of ca. 0.9 ps and a weaker component of ca. 15 ps; t his 400 nm band itself is observed to rise synchronously as a transien t band at 475 nm decays with a lifetime of < 200 fs. These results are discussed in terms of the dual mechanism proposed for azobenzene phot oisomerization. (C) 1998 Elsevier Science B.V. All rights reserved.