Rovibrationally selected and resolved state-to-state photoionization of ethylene using the infrared-vacuum ultraviolet pulsed field ionization-photoelectron method

By preparing ethylene C2H4(X1Ag)] in selected rotational levels of the nu11(b1u), nu2+nu12(b1u), or nu9(b2u) vibrational state with infrared (IR) laser photoexcitation prior to vacuum ultraviolet (VUV) laser photoionization, we have recorded rotationally resolved pulsed field ionization-photoelectron (PFI-PE) spectra for C2H4+(X2B3u) in the energy region of 0-3000 cm(-1) above the ionization energy (IE) of C2H4(X1Ag). Here, nu2(ag), nu9(b2u), nu11(b1u), and nu12(b1u) represent the C-C stretching, CH2 stretching, CH2 stretching, and CH2 bending modes of C2H4(X1Ag), respectively. The fully rovibrationally resolved spectra have allowed unambiguous symmetry assignments of the observed vibrational bands, which in turn have provided valuable information on the photoionization dynamics of C2H4. The IR-VUV photoionization of C2H4(X1Ag) via the nu11(b1u) or nu2+nu12(b1u) vibrational states is found to predominantly produce vibrational states of C2H4+(X2B3u) with b1u symmetry, which cannot be observed in single-photon VUV-PFI-PE measurements of C2H4(X1Ag). The analysis of the observed IR-VUV-PFI-PE bands has provided the IE(C2H4) = 84,790.2(2) cm(-1) and accurate vibrational frequencies for the nu4+(au)84.1(2) cm(-1)], nu12+(b1u)1411.7(2) cm(-1)], nu4+ +nu12+(b1g)1482.5(2) cm(-1)], nu2+(ag)1488.3(2) cm(-1)], nu2+ + nu4+(au)1559.2(2) cm(-1)], 2nu4+ + nu12 +(b1u)1848.5(2) cm(-1)], 4nu4+ + nu12 +(b1u)2558.8(2) cm(-1)], nu2+ + nu12 +(b1u)2872.7(2) cm(-1)], and nu11+(b1u)2978.7(2) cm(-1)] vibrational states of C2H4+(X2B3u), where nu4+ is the ion torsional state. The IE(C2H4) and the nu4+(au), nu2+(ag), and nu2+ + nu4+ (au) frequencies are in excellent accord with those obtained in previous single-photon VUV-PFI-PE measurements. The other ion vibrational frequencies represent new experimental determinations. We have also performed high-level ab initio anharmonic vibrational frequency calculations for C2H4(X1Ag) and C2H4+(X2B3u) at the CCSD(T)/aug-cc-pVQZ level for guidance in the assignment of the IR-VUV-PFI-PE spectra. All theoretical vibrational frequencies for the neutral and ion, except the ion torsional frequency, are found to agree with experimental vibrational frequencies to better than 1%.