Abstract
The separation of electronic and nuclear dynamics due to differing timescales is a useful concept for understanding ground-state molecular systems. However, coupling between these degrees of freedom is critical to understanding the evolution of most excited-state systems. We measure two-photon ionization delays of ${\mathrm{H}}_{2}$ and compare to calculations of the same measurement in a frozen-nuclei approximation. We find discrepancies between the vibrationally resolved measurement and bond-length-dependent theory, suggesting that nuclear motion affects ${\mathrm{H}}_{2}$ photoionization on attosecond timescales. We ascribe our observation to nuclear-electronic channel coupling between continuum vibrational states. Our results demonstrate that nuclear-electronic coupling cannot be neglected in the sudden ionization of molecules containing light atoms.
Highlights
The coupling of electronic and nuclear motion is a fundamental problem in photochemistry
The separation of electronic and nuclear dynamics due to differing timescales is a useful concept for understanding ground-state molecular systems
We investigate how electron dynamics are influenced by nuclear motion in hydrogen-containing systems, on their natural timescale of attoseconds
Summary
The coupling of electronic and nuclear motion is a fundamental problem in photochemistry. The photoionization time delay is a differential measurement of the phase of the two-photon ionization matrix element, and can reveal ionization dynamics when compared to a set of models Correlated electronic phenomena such as autoionization resonances [1,2,3,4,5], continuum channel coupling [6,7,8], and shake-up ionization [9] have been studied in atomic systems using photoionization time delays. Continuum-channel coupling in molecular photoionization can occur in a vibrationally excited system where nuclear and electronic timescales match This is an example of the breakdown of the Born-Oppenheimer, or adiabatic, approximation (BOA), which allows for the separation of nuclear and electronic degrees of freedom in the Hamiltonian. Our work builds on existing knowledge of non-BOA physics in H2 photoionization [17] by revealing continuum channel coupling between discrete H2+ vibrational states, with high resolution in both time and energy
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