Nonperturbative Quantum Theory of Multiplasmonic Electron Emission from Surfaces: Gauge-specific Cumulant Expansions Vs. Volkov Ansatz over Plasmonic Coherent States

Energetic electromagnetic fields produce a variety of elementary excitations in solids that can strongly modify their primary photoemission spectra. Such is the plasmon excitation or pumping mechanism which, although indirect, is very efficient and hence may give rise to formation of plasmonic coherent states. In turn, these states may act as a source or sink of energy and momentum for escaping electrons. Starting from the model Hamiltonian approach we show that prepumped plasmonic bath of coherent states gives rise to ponderomotive potentials and Floquet electronic band structure that support multiple plasmon-induced electron emission or plasmoemission from metals. Theoretical description of multiple plasmoemission requires a nonperturbative approch which is here formulated by applying cumulant expansion and Volkov ansatz to the calculations of electron wavefunctions and emission rates. The calculations are performed in the standard length gauge as well as in the Pauli-transformed velocity gauge for electron-plasmon interaction. The applicability of two nonperturbative approaches to calculation of excitation amplitudes are examined in each gauge. They smoothly interpolate between the fully quantal first order Born approximation and semiclassical multiplasmon-induced electron excitation limit. This is illustrated on the example of plasmoemission from Floquet surface bands on Ag(111) from which this channel of electron yield has been detected. Our calculations indicate that even subsingle mode occupations of plasmonic coherent states can support multiplasmon electron emission from surface bands. A way of calibration of plasmonic coherent states is proposed.