Measurement of an Electronic Resonance in a Ground-State, Gas-Phase Acetophenone Cation via Strong-Field Mass Spectrometry.

A one-photon ionic resonance is measured in the strong-field regime in acetophenone by recording the mass spectra as a function of excitation wavelength from 800 to 1500 nm. The ratio of the benzoyl to parent ion signals in the mass spectrum varies significantly with excitation wavelength, where the highest ratio observed upon excitation at 1370 nm (0.90 eV) indicates the presence of a one-photon resonance. At the resonant wavelength, the ratio of the benzoyl to parent ion signals increases linearly with laser intensity over a range from 1.1 × 10(13) to 6.0 × 10(13) W cm(-2). The ratio increases by a factor of 5 at 1370 nm with increasing pulse duration from 60 to 100 fs. Calculations using the equation of motion coupled cluster method support the existence of a one-photon transition from the ground ionic to a dissociative excited ionic state (0.87 eV), where motion of the acetyl group from a planar to nonplanar structure within the pulse duration enables the otherwise forbidden transition.