Dissecting strong-field excitation dynamics with atomic-momentum spectroscopy

Strong, focussed linearly-polarized infra-red fields electronically excite and accelerate atoms in the laser propagation and the transverse directions. We develop a numerically-tractable, quantum-mechanical treatment of correlations between internal and centre-of-mass (c.m.) dynamics, and apply it to the hydrogen atom. The propagation-direction c.m. momentum carries no information on the internal dynamics. The transverse momentum records the time spent in the field, allowing femtosecond reconstruction of the strong-field excitation process. The ground state becomes weak-field seeking, an unambiguous signature of the Kramers-Henneberger regime.