Vacuum ultraviolet laser pulsed field ionization photoelectron study of trans-2-butene.

The single-photon pulsed field ionization photoelectron (PFI-PE) spectrum of trans-2-butene (trans-CH[sub 3]CH=CHCH[sub 3]) in the energy range of 73 500–75 850 cm-1 has been measured using vacuum ultraviolet laser sources. The semi-empirical simulation of fine structures resolved in the original PFI-PE band yields a value of 73 624.7±2.0 cm[sup -1] for the ionization energy (IE) of trans-2-butene. The vibrational bands for trans-CH[sub 3]CH=CHCH[sub 3][sup +] resolved in the PFI-PE spectrum are assigned based on ab initio calculations of the vibrational frequencies and Franck-Condon factors (FCFs) for ionization transitions. This assignment has provided reliable vibrational frequencies (ν[sub 1][sup +]=104 cm[sup -1], ν[sub 2][sup +]=127 cm[sup -1], ν[sub 3][sup +]=131 cm[sup -1], ν[sub 5][sup +]=484 cm[sup -1], ν[sub 8][sup +]=798 cm[sup -1], ν[sub 13][sup +]=1164 cm[sup -1], ν[sub 14][sup +]=1264 cm[sup -1], ν[sub 16][sup +]=1307 cm[sup -1], ν[sub 20][sup +]=1407 cm[sup -1], and ν[sub 22][sup +]=1567 cm[sup -1]) for trans-CH[sub 3]CH=CHCH[sub 3][sup +]. The PFI-PE spectrum is compared to the recently reported PFI-photoion (PFI-PI) spectrum for trans-2-butene. The major difference observed between the PFI-PE and PFI-PI spectra is that the intensities for excited vibrational bands were significantly suppressed or indiscernible in the PFI-PI spectrum, suggesting that the lifetimes for high-n Rydberg states associated with these excited vibrational bands were greatly reduced under the conditions used in the PFI-PI study. The experimental conditions used in the PFI-PI study also led to an IE value of about 20 cm-1 lower than that obtained in the PFI-PE measurement. We have also reassigned the vibrational bands resolved in the PFI-PE spectrum for cis-2-butene based on the FCF calculation and a more reliable set of theoretical vibrational frequencies. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]