Charge Resonance Enhanced Ionization ofCO2Probed by Laser Coulomb Explosion Imaging

Abstract

The process by which a molecule in an intense laser field ionizes more efficiently as its bond length increases towards a critical distance R(c) is known as charge resonance enhanced ionization (CREI). We make a series of measurements of this process for CO(2), by varying pulse duration from 7 to 200 fs, in order to identify the charge states and time scales involved. We find that for the 4+ and higher charge states, 100 fs is the time scale required to reach the critical geometry <R(CO)> ≈ 2.1 Å and <θ(OCO)> ≈ 163° (equilibrium CO(2) geometry is <R(CO)> ≈ 1.16 Å and <θ(OCO)> ≈ 172°). The CO(2)(3+) molecule, however, appears always to begin dissociation from closer than 1.7 Å indicating that dynamics on charge states lower than 3+ is not sufficient to initiate CREI. Finally, we make quantum ab initio calculations of ionization rates for CO(2) and identify the electronic states responsible for CREI.

Role of proton dynamics in efficient photoionization of hydrocarbon molecules

“…Therefore, EI can be suppressed by using pulse durations shorter than the time needed for the involved bonds to stretch to R c . This has been experimentally demonstrated for the very fast nuclear motion in molecular hydrogen [35] and deuterium [46], as well as for the considerably slower nuclear motion in CO 2 [41]. In our experiments we employ laser pulses with durations τ (FWHM) ranging from the few-cycle (∼ 4.5 fs) to the multicycle (25 fs) regime, and laser peak intensities between 2 × 10 14 and 8 × 10 14 W/cm 2 .…”

“…By the development of the concept of enhanced ionization it became possible to explain the reason why the kinetic energy of fragment ions recorded by Coulomb explosion imaging was considerably lower than the kinetic energy expected, when the Coulomb explosion takes place at the equilibrium position of the ground state electronic potential energy surface of the neutral parent molecule [29][30][31][32][33]. Since then, the EI mechanism has been investigated extensively for diatomic molecules, e.g., [34][35][36][37], as well as for triatomic molecules [38][39][40][41].…”

Tabletop imaging of structural evolutions in chemical reactions demonstrated for the acetylene cation

“…Unfortunately, these techniques are largely insensitive to the more subtle and irregular structural changes that can occur within a single small molecule undergoing a chemical reaction. Pump-probe Coulomb Explosion Imaging (CEI) allows observation of these changes on a femtosecond (fs) timescale with atomic resolution [10][11][12][13] . So far, however, important phenomena such as proton migration in the acetylene cation have only been observed using vacuum ultraviolet light (VUV) from a free-electron laser (FEL) 14 .…”

Correlated Electron and Nuclear Dynamics in Strong Field Photoionization ofH2+

“…At high intensity, the two processes coexist and the 2D plots show a very rich behavior, suggesting that the correlation between electron and nuclear dynamics in strong field ionization is more complex than one would have anticipated. DOI: 10.1103/PhysRevLett.110.113001 PACS numbers: 33.20.Xx, 33.60.+q, 33.80.Rv The interaction of atoms and molecules with intense infrared laser pulses has been the object of continuous research for more than two decades [1][2][3][4][5][6][7][8][9]. Since the potential induced by such lasers on the electrons is comparable to or even stronger than that generated by the nuclei, the resulting electron dynamics is significantly different from that of the isolated system, which makes these lasers ideal tools to achieve electronic control [10][11][12][13].…”