State-selective dissociation dynamics of an oxygen molecular ion studied with single-harmonic pump and infrared-probe pulses

Abstract

Laser-induced dissociation of a photoionized oxygen molecule is studied employing an extreme ultraviolet (EUV) pumpnear-infrared (NIR) probe technique. A combination of a narrow-band 11 th harmonic pump centered at 17.3 eV and a moderate-intensity NIR probe restricts the dissociation dynamics to the pair of low-lying cationic states, the 4 u a  and the 4 g f . The measured kinetic energies of the O + fragments reveal contributions from one-, two-and threephoton dissociation pathways (1 , 2 and 3) involving these two states. While the yields of the two-and three-photon channels initially rise and then decrease as a function of EUV-NIR delay, the yield of the single-photon pathway rises slower but keeps increasing over the whole delay range studied. This behavior reflects the evolving probability density of the ionic nuclear wave packet at the internuclear distances, where it can undergo resonant 3 and 1 transitions from the 4 u a  to the 4 g f  state of O2 + .

Molecular bond stabilization in the strong-field dissociation of O2+

“…Starting with pioneering pump-probe experiments in the 1980s [17], the evolution and ultimate fate of lightexcited molecular cations has been probed experimentally and modeled theoretically in a large number of studies. In many of these investigations, the molecular dynamics initiated by excitation and ionization in an ultrashort pump pulse is destructively imaged by dissociating the excited diatomic molecular cation in delayed ultrashort probe pulses [1,4,8,14,15,18,19]. Scanning the pump-probe delay over a sufficiently large delay range while recording KER spectra for each delay, allows the time-resolved mapping of the bound motion and dissociation pathways in excited molecules.…”

“…As an example, Fig. 1 shows the field-induced one-photon avoided crossing between the Floquet potential curves associated with the field-free a 4 Π u and f 4 2 for an applied external field with a wavelength of 800 nm and intensity of 10 13 W/cm 2 (solid red curve). The corresponding field-free BO potential energy curves are shown for the cationic states, a 4 Πu (dashed-dotted blue curve) and f 4 Πg (dashed black line).…”

“…More recently, noticeable progress was made in experimentally imaging and numerically modeling DI in heavier diatomic molecules, such as N 2 [14,36], O 2 [1,3,4,8,14,36], CO [14,36], and noble-gas dimers [22,37]. These investigations were carried out with IR -IR [3,14,22,36,37], XUV -IR [1,4], and XUV -XUV [8] combinations of pump and delayed probe pulses, and are being extended to IR-pump and soft X-ray probe pulse [38].…”

“…More recently, noticeable progress was made in experimentally imaging and numerically modeling DI in heavier diatomic molecules, such as N 2 [14,36], O 2 [1,3,4,8,14,36], CO [14,36], and noble-gas dimers [22,37]. These investigations were carried out with IR -IR [3,14,22,36,37], XUV -IR [1,4], and XUV -XUV [8] combinations of pump and delayed probe pulses, and are being extended to IR-pump and soft X-ray probe pulse [38]. In contrast to hydrogen molecules, manyelectron diatomic molecules have a complex electronic structure such that in general several adiabatic BO potential surfaces are populated during the initial ionization in the pump pulse.…”

“…In many cases, the intramolecular nuclear dynamics in the excited molecular ions can be described in terms of nuclear wave packets that are generated by ionization of the parent molecule and represented as coherent superpositions of ro-vibrational stationary states in several adiabatic Born-Oppenheimer (BO) electronic states of the external-fieldfree molecular ion (see, e.g., Refs. [1][2][3][4][5]…”