Vacuum ultraviolet pulsed field ionization-photoelectron and infrared-photoinduced Rydberg ionization study of trans-1,3-butadiene

The vacuum ultraviolet (VUV) laser pulsed field ionization-photoelectron (PFI-PE) spectrum of trans-1,3-butadiene (trans-CH₂=CHCH=CH₂) has been measured in the region of 0-1700 cm^-1 above its ionization energy (IE) to probe the vibrational modes v_i⁺ (i=1-18) of trans-CH₂=CHCH=CH₂⁺. The high-frequency vibrational modes v_i⁺ (i=19, 22, and 23) of trans-CH₂=CHCH=CH₂⁺ have also been probed by the VUV-infrared-photoinduced Rydberg ionization (VUV-IR-PIRI) measurement. On the basis of the semiempirical simulation of the origin VUV-PFI-PE band, the IE(trans-CH₂=CHCH=CH₂) is determined to be 73 150.1±1.5 cm^-1 (9.06946±0.00019 eV). This value has been used to benchmark the state-of-the-art theoretical IE prediction based on the CCSD(T,Full)/CBS procedures, the calculation of which is reported in the present study. The vibrational bands observed in the VUV-PFI-PE and VUV-IR-PIRI spectra were assigned based on ab initio anharmonic vibrational frequencies and Franck-Condon factor calculations for the photoionization transitions. Combining the VUV-PFI-PE and VUV-IR-PIRI measurements, 17 fundamental vibrational frequencies of trans-CH₂=CHCH=CH₂⁺ have been determined, including v₁⁺ = 182±3, v₂⁺ = 300±3, v₃⁺ = 428±3, v₄⁺ = 514±3, v₅⁺ = 554±5, v₆⁺ = 901±3, v₇⁺ = 928±5, v₈⁺ = 994±3, v₉⁺ = 1008±5, v₁₀⁺ = 1094±5, v₁₃⁺ = 1258±3, v₁₄⁺ = 1293±3, v₁₆⁺ = 1479±3, v₁₈⁺ = 1620±3, v₁₉⁺ = 2985±10, v₂₂⁺ = 3030±10, and v₂₃⁺ = 3105±10 cm^-1. © 2008 American Institute of Physics.