Molecular events in the light of strong fields: A light‐induced potential scenario

Abstract

A new perspective on how to manipulate molecules by means of very strong laser pulses is emerging with insights from the so-called light-induced potentials, which are the adiabatic potential energy surfaces of molecules severely distorted by the effect of the strong field. Different effects appear depending on how the laser frequency is tuned, to a certain electronic transition, creating light-induced avoided crossings, or very off-resonant, generating Stark shifts. In the former case it is possible to induce dramatic changes in the geometry and redistribution of charges in the molecule while the lasers are acting and to fully control photodissociation reactions as well as other photochemical processes. Several theoretical proposals taken from the work of the authors are reviewed and analyzed showing the unique features that the strong-laser chemistry opens to control the transient properties and the dynamics of molecules.

Circularly polarized light-induced potentials and the demise of excited states

“…When working with electromagnetic fields, rather than static or ac electric fields, we expect some differences to occur. The field frequency selects the geometry of the excited potential that is resonant with the ground potential, forcing a stronger reshaping of the light-induced potentials 30–35 (LIPs) and trapping population at selected internuclear distances in the excited state. This is not the case, however, when the molecular axis is perpendicular to the polarization and θ = π/2.…”

“…Then the coupling is zero and the molecule dissociates, while some population may be transferred to the ground potential by non-adiabatic processes in the vicinity of θ = π/2. In the representation of the LIPs, this process can be seen as fast internal conversion through a light-induced conical intersection 35–41 or LICI, using the language of Photochemistry, and it is possible even in diatomic molecules, including the rotational degree of freedom. There have been a plethora of studies concerning how to probe the LICIs and how to use them as a resource to control photochemical or photophysical processes, but if anything, one expects that the presence of LICIs will make it harder to stabilize the excited states of single electron molecules.…”

Anti-alignment driven dynamics in the excited states of molecules under strong fields

“…Bond softening and bond hardening are then manifestations of the mixing of the bonding and anti-bonding orbitals that characterize the ground and excited molecular orbitals under the coupling of the field. In these studies, the assumption that the molecule is aligned, of at least that its axis remains mostly fixed with respect to the polarization field, has been implicitly used in the proposal of many quantum control scenarios using strong non-resonant fields [15,16], which are based on the dynamic Stark effect [17][18][19][20][21].…”

Protecting and accelerating adiabatic passage with time-delayed pulse sequences

“…In particular, the potential energy surface of the molecule can be severely deformed in the presence of the fields, generating light-induced potentials [26][27][28][29]. Then, the energy eigenstates of the field-free Hamiltonian are not a very useful basis to follow the dynamics of the system.…”

“…In this work we will analyze what happens for non-resonant two-photon transitions using strong fields, where many assumptions used to prove the decoupling of the electronic excitation do not operate. In particular, the potential energy surface of the molecule can be severely deformed in the presence of the fields, generating light-induced potentials [26][27][28][29]. Then, the energy eigenstates of the field-free Hamiltonian are not a very useful basis to follow the dynamics of the system.…”