Two-photon vibronic spectroscopy of allene at 7.0–10.5 eV: experiment and theory

Abstract

2 + 1 resonance-enhanced multiphoton ionization (REMPI) spectra of allene at 7.0–10.5 eV have been observed. The excited vibronic symmetry has been determined from polarization-ratio measurements. Based on the vibronic energies and peak intensities calculated using ab initio MO and time-dependent density functional theory, the very congested REMPI spectra have been assigned as due to π* ← π, 3p ← π, 4s ← π, 4p ← π, and 4d ← π transitions. Vibrational progressions related to the CH2 twisting (ν4 ∼770 cm−1) have been observed for several excited electronic states. Calculated Franck–Condon factors also confirm that CH2 twisting is the most active mode in the vibronic spectra of allene. In this study, theoretical calculations of two-photon intensities and polarization ratios have been made through the ab initio computed one-photon transition dipole moments to various electronic states as intermediates. As a starting point to interpret the complicated vibronic spectrum of allene, the theoretical approach, without vibronic couplings, has been applied to predict the peak positions, spectral intensities, and polarization ratios of Rydberg states, and qualitatively shows a considerable agreement with experimental observations.