Dissociation dynamics of diatomic molecules in intense laser fields: A scheme for the selection of relevant adiabatic potential curves

Abstract

We investigated the nuclear dynamics of diatomic molecular ions in intense laser fields by analyzing their fragment kinetic-energy release (KER) spectra as a function of the pump-probe delay τ . Within the BornOppenheimer (BO) approximation, we calculated ab initio adiabatic potential-energy curves and their electric dipole couplings, using the quantum chemistry code GAMESS. By comparing simulated KER spectra as a function of either τ or the vibrational quantum-beat frequency for the nuclear dynamics on both individual and dipolecoupled BO potential curves with measured spectra, we developed a scheme for identifying electronic states that are relevant for the dissociation dynamics. We applied this scheme to investigate the nuclear dynamics in O 2 + ions that are produced by ionization of neutral O 2 molecules in an ultrashort infrared (IR) pump pulse and dissociate due to the dipole coupling of molecular potential curves in a delayed IR probe laser field.

Probing calculatedO2+potential-energy curves with an XUV-IR pump-probe experiment

“…Soon after that quantum simulations [8] confirmed the interpretation that in this experiment strong-field ionization to the binding O 2 + (a 4 u ) state by an intense laser * philipp.coerlin@mpi-hd.mpg.de † robert.moshammer@mpi-hd.mpg.de pulse launches a molecular wave packet. This state can be coupled to the weakly repulsive O 2 + (f 4 g ) state by an IR probe pulse which causes the molecule to dissociate (Fig.…”

“…The QB energy coincides with the expected vibrational level spacings E v=10 − E v=9 = 0.103 eV and E v=9 − E v=8 = 0.106 eV of the a 4 u state calculated in Ref. [8]. In the experimental data these two energies are not resolved because the time-delay range of 2 ps is too short to allow their separation in the frequency domain.…”

“…[7,8,11,20]). We apply the Franck-Condon approximation and assume that the vibrational ground state of the potential belonging to the neutral X 3 − g state (taken from Ref.…”

“…1). This complicates the interpretation of experimental kinetic-energyrelease (KER) spectra and might explain why the detailed wave-packet dynamics in atmospherically relevant diatomic molecules such as N 2 or O 2 [6][7][8][9] has been investigated less frequently than in H 2 + and its isotopes.…”

Molecular resolvent-operator method: Electronic and nuclear dynamics in strong-field ionization

“…As one of the properties of these harmonics is coherence, the harmonic spectrum contains information about the molecular structure and the induced dynamics [12,13]. The measurement of the kinetic energy of the electrons that escape forever (i.e., do not recombine) and the associated molecular cation (EKE and KER) allows one to study time-dependent processes such as the nuclear dynamics in, e.g., H 2 [14,15] or H 2 + [16][17][18][19]. The KER spectra for H 2 + ionization show peculiar structures, for which different mechanisms, such as CREI or above-threshold Coulomb explosion, have been proposed [20][21][22][23].…”