LASER SPECTROSCOPY OF FREE PENTACENE MOLECULES (I) - THE ROTATIONAL STRUCTURE OF THE VIBRATIONLESS S-1[-S-0 TRANSITION

The rotational structure of the vibrationless S-1<--S-0 transition of pentacene has been investigated using a strongly collimated seeded sup ersonic argon beam. Because single rotational lines could not be compl etely resolved, a band contour analysis was performed. The rotational constants of the electronic ground state X (1)A(1g) were found to be u nder the asymmetric rotor approximation A '' = 1320.6(9), B ''=117.97( 9), C ''=108.28(15) MHz, whereas the differences to the first electron ic excited state A B-1(2u) are Delta A =A' - A '' = 13.2(3), Delta B = - 0.764(45), Delta C = - 0.54(6) MHz. A new value of the band origin was determined to be v(00) = 18 648.996(4) cm(-1) and the band type wa s confirmed to be of type b as proposed by symmetry arguments. Good ag reement between observed and calculated spectra was obtained assuming planarity in both ground and excited state. From the fit procedure a r otational temperature of about 7 K was deduced. The nuclear statistica l weights of the electronic ground state are reported. (C) 1998 Americ an Institute of Physics.