Stark shift of rotational lines in the UV spectrum of the charge-transfer molecule benzonitrile

We present the resolved Stark splitting of rovibronic lines in the resonanc e enhanced multiphoton ionization spectrum of benzonitrile caused by an ele ctric field up to 700 V/cm. The Stark spectrum is theoretically reproduced yielding a value of 4.5(1) Debye for the dipole moment of the excited S-1 s tate. The same value is found from the Stark splitting of the 11(0)(1) band at an excess energy of 703 cm(-1). The slight increase of the dipole momen t compared to the ground-state value of 4.18 D and the absence of local per turbations in the spectra demonstrate that there is no detectable influence of the theoretically predicted charge-transfer state caused by a rehybridi zation of the carbon atom in the cyano group. Our results further show that the presence of electric fields during excitation can lead to a broadening of REMPI spectra and should be avoided in rotational resolution work on po lar molecules.