Bond-­length dependence of attosecond ionization delays in O₂arising from electron correlation to a shape resonanc

We experimentally and theoretically demonstrate that electron correlation can cause the bond-­length sensitivity of a shape resonance to induce an unexpected vibrational state–dependent ionization delay in a nonresonant channel. This discovery was enabled by a high-­ resolution attosecond-­ interferometry experiment based on a

400-­ nm driving and dressing wavelength. The short-­wavelength driver results in a 6.2–electron volt separatio between harmonics, markedly reducing the spectral overlap in the measured interferogram. We demonstrate the promise of this method on O_2, a system characterized by broad vibrational progressions and a dense photoelec-tron spectrum. We measure a 40-­attosecond variation of the photoionization delays over the X²Π_g vibrational progression. Multichannel calculations show that this variation originates from a strong bond-­ length dependence of the energetic position of a shape resonance in the b⁴Σ⁻g channel, which translates to the observed effects through electron correlation. The unprecedented energy resolution and delay accuracies demonstrate the prom- ise of visible-­ light–driven molecular attosecond interferometry.