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

Sonia Erattupuzha,Xinhua Xie,Markus Koch,Stefanie Gräfe,Kaoru Yamanouchi,Václav Hanus,Seyedreza Larimian,Arthur Russakoff,Andrius Baltuška,Kálmán Varga,Cody L Covington,Markus Kitzler,Sergiy Bubin,Erik Lötstedt

doi:10.1088/1361-6455/aa7098

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.

Highlights

IntroductionStrong-field ionisation of atoms is reasonably well understood [2]
We investigated the intensely debated mechanism behind the process of efficient laser-ionisation of polyatomic molecules that results in the emission of multiple electrons during the interaction with ultrashort intense laser pulses
By a combined experimental and numerical approach based on coincidence momentum spectroscopy and timedependent density functional theory (TDDFT) simulations and using the acetylene molecule as a model system we succeeded in establishing an intuitive explanation for the surprisingly high charge states measured in experiments conducted with quite moderate laser peak intensities [18, 29–33]

Summary

Introduction

Strong-field ionisation of atoms is reasonably well understood [2] For molecules this process is still under intense investigation, since it is much more complex. Laser-ionisation of molecules is complicated by their structure and its dynamical changes that are initiated by the action of the laser field and that may subsequently take place during the laser pulse. An important discovery in the ionisation behaviour of molecules is that for diatomics the ionisation rate shows a strong variation with the internuclear distance, R. This phenomenon, known as enhanced ionisation (EI), has been investigated in numerous experimental and theoretical works for different molecular species, e.g., H2/D2 [3–11], I2 [12–15], N2 [16–18], and Cl2 [19]. It was found that in many cases the ionisation rate is strongly enhanced around a critical internuclear distance, Rc

Results

Discussion

Conclusion