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

The rotational structure of the vibrationless $S_1{\mathrm{\leftarrow S}}_0$ 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 ${\mathrm{X\hspace{0.17em}}}^1{A}_{\text{1g}}$ were found to be under the asymmetric rotor approximation $A^{″}\text{=1320.6(9),}$ $B^{″}\text{=117.97(9),}$ $C^{″}\text{=108.28(15)\hspace{0.17em}}\mathrm{MHz},$ whereas the differences to the first electronic excited state ${\mathrm{A\hspace{0.17em}}}^1{B}_{\text{2u}}$ are ${\mathrm{\Delta A=A}}^{\prime }{\mathrm{-A}}^{″}\text{=13.2(3),}$ $\text{ΔB=−0.764(45),}$ $\text{ΔC=−0.54(6)\hspace{0.17em}}\mathrm{MHz}.$ A new value of the band origin was determined to be ${\nu }_{00}\text{=18\hspace{0.17em}648.996(4)\hspace{0.17em}}{\mathrm{cm}}^{\mathrm{-1}}$ 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.