Vibrational spectra and trace determination of p-difluorobenzene and ethylbenzene.

In this work, the structures and normal vibrations of p-difluorobenzene and ethylbenzene in the first excited state have been studied using resonant two-photon ionization spectrum and ab initio quantum chemical calculations. A vibronic spectrum of p-difluorobenzene has been reinvestigated in some detail and three new vibrational modes, 260(1), 130(1) and 30(1), are assigned respectively. The band origin of ethylbenzene of the S1←S0 transition appeared at 37,586 cm(-1), and more vibrational modes are observed. These appropriate structures and vibrational frequencies of the S0 and S1 states of both molecules are calculated using Hartree-Fock and configuration interaction singles methods with 6-311++G(2d, 2p) basis set. All spectral bands of both molecules have been successfully assigned with the help of our computed results and analogy with the reported spectra for similar molecules. The strongest transition 50(1) of p-difluorobenzene is selected to measure a concentration detective limit of the setup system and the achieved concentration detective limit is 3.125 ppb.