Laser spectroscopy of free pentacene molecules (I): The rotational structure of the vibrationless S1←S transition

The rotational structure of the vibrationless S1←S0 transition of pentacene has been investigated using a strongly collimated seeded supersonic argon beam. Because single rotational lines could not be completely resolved, a band contour analysis was performed. The rotational constants of the electronic ground state X 1A1g were found to be under the asymmetric rotor approximation A″=1320.6(9), B″=117.97(9), C″=108.28(15) MHz, whereas the differences to the first electronic excited state A 1B2u are ΔA=A′−A″=13.2(3), ΔB=−0.764(45), ΔC=−0.54(6) MHz. A new value of the band origin was determined to be ν00=18 648.996(4) cm−1 and the band type was confirmed to be of type b as proposed by symmetry arguments. Good agreement between observed and calculated spectra was obtained assuming planarity in both ground and excited state. From the fit procedure a rotational temperature of about 7 K was deduced. The nuclear statistical weights of the electronic ground state are reported.