Observation of Quantum Interferences via Light-Induced Conical Intersections in Diatomic Molecules

Abstract

We observe energy-dependent angle-resolved diffraction patterns in protons from strong-field dissociation of the molecular hydrogen ion H + 2 . The interference is a characteristic of dissociation around a laser-induced conical intersection (LICI), which is a point of contact between two surfaces in the dressed 2-dimensional Born-Oppenheimer potential energy landscape of a diatomic molecule in a strong laser field. The interference magnitude and angular period depend strongly on the energy difference between the initial state and the LICI, consistent with coherent diffraction around a cone-shaped potential barrier whose width and thickness depend on the relative energy of the initial state and the cone apex. These findings are supported by numerical solutions of the time-dependent Schrödinger equation for similar experimental conditions.The Born-Oppenheimer approximation (BOA) represents intramolecular dynamics as the motion of nuclear wave packets on potential energy surfaces (PES) of electronic eigenvalues embedded in the space of nuclear geometries. A molecule on a single PES remains there so long as the adiabatic condition is obeyed, i.e. so long as nuclear kinetic energies are small compared to electronic state separations. This assumption must break down, however, if two or more PESs approach each other [1][2][3]. When this happens non-adiabatic couplings between the nearly-degenerate surfaces become important. The true eigenvalues can then be calculated by diagonalizing the Hamiltonian in the reduced space of the near degeneracy.According to simple geometrical arguments, molecules with at least two dimensions of internal nuclear motion (i.e. three or more atoms) must have some points where two or more PESs become degenerate, a condition known as a conical intersection (CI) [4]. These CIs play a key role in the relaxation dynamics of most polyatomic molecules including important biochemical processes such as the photostability of DNA [5], and the preliminary process of vision [6]. The dimension of the CI manifold is two less than the internal nuclear geometry. Thus for the simplest case of a tri-atomic molecule, the CI manifold has dimensionality of 3N − 8 = 1. The topological nature of CIs allows the nonadiabatic couplings to diverge and thus display related phenomena such as a geometric or Berry's phase [4,7] in wavepackets that circumnavigate the CI.Naturally occurring CIs cannot exist for a free diatomic molecule because the internuclear separation vector R is the only internal nuclear degree of freedom, and this is insufficient to fulfil the crossing condition. The nonadiabatic terms in the full Hamiltonian cause the BOA states to repel according to the so-called "no-crossing" rule. A diatomic molecule in a strong laser field, however, has a second degree of freedom defined by the laser polarization ε. When viewed in a Floquet basis of laserdressed electronic states, a molecule coupled by this field can exhibit a point of degeneracy called a light-induced conical intersection (LICI) in the 2−dimension...

Direct Signatures of Light-Induced Conical Intersections on the Field-Dressed Spectrum of Na2

“…18 The first experimental observation of LICIs in diatomic molecules was made by Bucksbaum et al. 20 Besides the diatomic studies, a few results are available for polyatomics as well. [21][22][23][24] In this case, due to the presence of several vibrational degrees of freedom, LICI can form without rotation, which opens up the door for manipulating and controlling nonadiabatic effects by light.…”

Zero-energy proton dissociation of H2+ through stimulated Raman scattering

“…We further-more show that the laser field-induced asymmetry of H + 2 dissociation in the (near-)zero energy region is due to the combined action of several processes rather than only pathway interferences, with bond-hardening taking a particularly important role. This finding opens up possibilities for detailed investigations of vibrational trapping and the influence of rotational states in molecular dynamics [33][34][35].…”

“…However, even for this simplest of all molecules there exist still a number of issues awaiting clarification, in particular in the family of bond-hardening phenomena. Examples include direct experimental confirmation of light-induced conical intersections (LICIs) [32][33][34][35], or a generally accepted picture of the concept of trapping that has been challenged by McKenna et al [36].Here, we focus on a bond-hardening process that leads to protons with (near-)zero kinetic energy during the dissociation H + 2 → H + + H. This dissociation pathway has been predicted [25] and observed [28] decades ago. It has been explained as bond-hardening at the zero-photon crossing of the Floquet ladder through dissociation involving the net-absorption of zero photons (zero photon dissociation, ZPD) [25,27,28].…”

Comparison between the analysis of the asymptotic wavepacket and the associated flux for the calculation of kinetic‐energy‐releases function

“…Recently, the correlation of the KER spectra with the wavelength and intensity of the driving laser field for the dissociation of reveals the mechanism of selective excitation of vibrational levels of the target molecule . The characteristic of dissociation around a laser‐induced conical intersection has been observed via the diffraction patterns of the angle‐resolved KER distribution for the dissociated in strong laser field . By taking advantage of the joint energy spectrum of the coincidentally measured electron and ion ejected from the same molecule, an unambiguous evidence of high‐order ATD could be observed in the nuclear KER spectrum, which indicates that the electron and nuclei of the molecule as a whole absorb the multiphoton energy .…”