Enhanced ionisation of polyatomic molecules in intense laser pulses is due to energy upshift and field coupling of multiple orbitals

Abstract

We present the results of a combined experimental and numerical study on strong-field ionisation of acetylene performed with the aim of identifying the mechanism behind the previously reported surprisingly large multi-electron ionisation probabilities of polyatomic molecules. Using coincidence momentum imaging techniques and time-dependent density functional simulations, we show that the reported efficient ionisation is due to the combined action of a significant geometrically induced energy upshift of the most relevant valence orbitals as the C–H distance stretches beyond about two times the equilibrium distance, and a strong increase in the coupling between multiple molecular orbitals concomitant with this stretch motion. The identified enhanced ionisation mechanism, which we refer to as EIC-MOUSE, is only effective for molecules aligned close to parallel to the laser polarisation direction, and is inhibited for perpendicularly aligned molecules because of a suppression of the C–H stretch motion during the onset of ionisation.

Quantum bridges in phase space: interference and nonclassicality in strong-field enhanced ionisation

“…Since its first prediction [11], enhanced ionisation has been calculated and measured in myriad systems. These include diatomic [12,13] molecular species such as H 2 [14][15][16][17][18][19], I 2 [20][21][22][23], and Cl 2 [24], tri- [17,25,26] and polyatomic molecules [27,28]. It has also been used as a means to highlight electron nuclear coupling of degrees of freedom in photoelectron holography [29].…”

“…Nearly degenerate coupled charge-resonant states that occur for large inter-nuclear separations also facilitate a strong population transfer to the continuum [11,15,33]. On the other hand, for extended systems, multielectron and non-adiabatic effects play an increasingly important role [28,34,35] (for a recent review see [36,37]). Loosely speaking, non-adiabaticity implies that the electron probability density as a function of time does not 'follow' the field gradients, and instead may be related to population trapping, resonances, multielectron effects and coupling of different degrees of freedom.…”

Time-resolved nuclear dynamics in bound and dissociating acetylene

“…This enhancement process is mediated by the stretching motions of the CH-bonds and occurs at bond lengths, which are about twice as large as the equilibrium value 18,19 . Recently, Erattupuzha et al found that CREI in acetylene originates not only from coupling of two states but rather due to an energy upshift and field coupling of multiple orbitals 20 . Following this argument, EI in complex molecules such as acetylene is not limited to a single critical inter-nuclear distance but can include a broader range of inter-nuclear distances, which are possibly assumed during a longer time interval during molecular motion initiated by, e.g., the ionization of the neutral molecule.…”

Electron correlation effects in enhanced ionization of diatomic molecules in near-infrared fields

“…For an accurate description of the correlation effects we take orbitals with higher m-quantum numbers as described in Eq. (12) and in the present calculations we have considered up to m = ±1 which produces a converged EI results. In Ref.…”

“…EI describes the phenomenon that when a molecule is exposed to a strong laser field, the ionization probability increases significantly at certain critical internuclear separations. This enhancement is also known as charge-resonance enhanced ionization and has been studied extensively both experimentally [5][6][7][8][9][10][11][12] and theoretically [13][14][15][16][17][18]. The quantum mechanical study of simple diatomic molecules with double-well potentials leads to many interesting features which are absent in atomic processes.…”