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

By preparing ethylene [C2 H4 (X̃ Ag1)] in selected rotational levels of the 11 (b1u), 2 + 12 (b1u), or 9 (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 C2 H4+ (X̃ B 3u 2) in the energy region of 0-3000 cm-1 above the ionization energy (IE) of C2 H4 (X̃ Ag1). Here, 2 (ag), 9 (b2u), 11 (b1u), and 12 (b1u) represent the C-C stretching, C H2 stretching, C H2 stretching, and C H2 bending modes of C2 H4 (X̃ Ag1), 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 C2 H4. The IR-VUV photoionization of C2 H4 (X̃ Ag1) via the 11 (b1u) or 2 + 12 (b1u) vibrational states is found to predominantly produce vibrational states of C2 H4+ (X̃ B 3u 2) with b1u symmetry, which cannot be observed in single-photon VUV-PFI-PE measurements of C2 H4 (X̃ Ag1). The analysis of the observed IR-VUV-PFI-PE bands has provided the IE (C2 H4) =84 790.2 (2) cm-1 and accurate vibrational frequencies for the 4+ (au) [84.1 (2) cm-1], 12+ (b1u) [1411.7 (2) cm-1], 4+ + 12+ (b1g) [1482.5 (2) cm-1], 2+ (ag) [1488.3 (2) cm-1], 2+ + 4+ (au) [1559.2 (2) cm-1], 2 4+ + 12+ (b1u) [1848.5 (2) cm-1], 4 4+ + 12+ (b1u) [2558.8 (2) cm-1], 2+ + 12+ (b1u) [2872.7 (2) cm-1], and 11+ (b1u) [2978.7 (2) cm-1] vibrational states of C2 H4+ (X̃ B 3u 2), where 4+ is the ion torsional state. The IE (C2 H4) and the 4+ (au), 2+ (ag), and 2+ + 4+ (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 C2 H4 (X̃ Ag1) and C2 H4+ (X̃ B 3u 2) 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%. © 2006 American Institute of Physics.