Coulomb Explosion Imaging for the Visualization of a Conical Intersection

Abstract

Coulomb explosion imaging is proposed as a method to directly map the presence of conical intersections encountered by a propagating wave packet in a molecular system. The case of choice is the non-adiabatic coupling between two dissociative surfaces in the methyl iodide molecule, probed by Coulomb explosion with short, intense near-infrared pulses causing multiple ionization. On-the-fly multidimensional trajectory calculations with surface hopping using perturbation theory and including spin-orbit coupling are performed to visualize the dynamics through the conical intersection and compare with experimental results. The possibilities and limitations of the technique are examined and discussed.

Photodissociation dynamics of methyl iodide probed using femtosecond extreme ultraviolet photoelectron spectroscopy

“…The UV driven dissociation of methyl iodide following excitation into the A-band has been extensively studied by numerous techniques and stands as a benchmark system against which new chemical dynamics methods are often tested. [10][11][12] The dynamics of the system following excitation on the red side of the A-band can be reasonably well explained with reference to the cuts through the potential energy surfaces along the dissociative coordinate, as plotted in Fig. 1.…”

“…Along the dissociative coordinate, the wavepacket passes a crossing between the 3 Q 0 and 1 Q 1 states where population transfer can occur, leading to formation of both the ground I( 2 P 3/2 ) and excited I*( 2 P 1/2 ) spin-orbit states of atomic iodine, in conjunction with the ground state methyl radical. This relatively simple picture of the dynamics has been shown to be consistent along a reasonable range of energies on the red side of the A-band, 10,11,[15][16][17][18][19][20][21][22] although recent photoelectron spectroscopy measurements suggest the dynamics on the blue side may not be so straight forward. 22 To date there have been surprisingly few measurements of the excited state dynamics of the A-band when compared with measurements of the product state distributions.…”

“…22 To date there have been surprisingly few measurements of the excited state dynamics of the A-band when compared with measurements of the product state distributions. The dynamics have been probed using Coulomb explosion imaging, 10,23,24 XUV transient absorption spectroscopy 11,25 and multiphoton ionisation photoelectron spectroscopy. 22 The CEI experiments show strong sensitivity to geometry changes at extended bond lengths, but struggle to capture the early time (o100 fs) dynamics due to limited temporal resolution and/or strong field effects in the probe process.…”

“…22 The CEI experiments show strong sensitivity to geometry changes at extended bond lengths, but struggle to capture the early time (o100 fs) dynamics due to limited temporal resolution and/or strong field effects in the probe process. 10,23,24 The XUV transient absorption experiments, meanwhile, are sensitive to the intermediate transition state region, providing a timeframe for transition through this region, but they do not show structural sensitivity. 11 The multiphoton ionisation photoelectron spectroscopy experiments provide timeframes for exiting the Franck-Condon region but the limited ionisation energy means the reaction cannot be followed to extended bond lengths.…”

NaI revisited: Theoretical investigation of predissociation via ultrafast XUV transient absorption spectroscopy

“…[16][17][18][19][20] Those elusive processes have been a subject of numerous spectroscopic studies for their ubiquitous role in steering the outcome of photochemical reactions. 15,[21][22][23][24][25][26][27][28][29][30][31][32][33][34] Despite the established concept of potential crossings, their experimental observation is still considered challenging, as it requires few-femtosecond time resolution and a capability to resolve closely-spaced electronic states by the probe method.…”