Time-resolved photodissociation of oxygen at 162 nm

Abstract

Oxygen was excited by 10-fs pulses in the Schumann-Runge continuum at 162 nm, which is by 0.57 eV above the dissociation limit. It was probed by high-intensity ionization at 810 nm with 10 14 W cm −2 , measuring the ion yields. The O 2 + signal decays in 4.3 fs, which is much shorter than the expected time for dissociation. It is ascribed to a rapid decay of the ionization probability. In a similar time, the ion in the second excited state (with excess energy taken over from the neutral) reaches the dissociation limit, whereas this time would be much longer from the two lower ion states. In fact, the O + signal rises to a (first) maximum at 6 fs. The preference for the higher ion state is rationalized by an intermediate resonance in the neutral molecule, for which the polarization dependence also provides evidence. But the shape of the O + signal is very odd: it exhibits three maxima (at 6, 29 and 53 fs) of increasing intensity, before decaying rapidly (≤3.5 fs) to a pedestal. In contrast to the first maximum, the others appear at times when there is practically no interatomic force left in the excited state. We postulate a highly repulsive doubly excited state as a resonance for interpreting the second maximum, and for the third an ion-pair state lying further outside. Comparison is made with enhanced ionization, which has often been found at large interatomic distances on multiple ionization in strong laser fields. Consistent with this mechanism is the absence of similar observations at negative delay times, where five fundamental photons act as a pump and the fifth harmonic as a probe.

Tracing few-femtosecond photodissociation dynamics on molecular oxygen with a single-color pump-probe scheme in the VUV

“…With this state being reached with a single photon, we are able to avoid the use of a strong multiphoton IR probe transition and its associated complications as pointed out in Refs. [11,14,18], at comparably low intensities (≈3 × + from Refs. [2,8].…”

“…The calculation, also consulted in Ref. [11], allows an estimation of the time it takes for the system to reach a specified internuclear distance. Following the same analysis as in Ref.…”

“…The excitation of O 2 in the vicinity of 160 nm leads to the first fully allowed transition in the so-called Schumann-Runge continuum, ultimately resulting in molecular dissociation. Still, the temporal progress of the reaction in the considered dissociation channel is the subject of an ongoing discussion [10][11][12]. Most studies so far have used continuous-wave VUV sources or multiphoton excitation schemes with visible laser pulses, while single-photon experiments on the relevant femtosecond time scale are rare.…”

“…Regarding the earlier investigations [10] they noted that due to a longer probe wavelength different states were reached, changing the ionization detection window. In a recent study, Thrushin et al [11] measured a decay time of 4.3 fs extracted from the O 2 + signal, upon pumping with 10-fs pulses at 160 nm and probing the ionic state with multiphoton absorption at 810 nm. The reported value was interpreted as the time until an internuclear distance on the molecular potential is reached from where more than four to five photons are needed for ionization.…”

Efficient generation of below-threshold harmonics for high-fidelity multi-photon physics in the VUV spectral range

“…Commonly, molecular electronic excitations lie in the DUV-VUV spectral region, and in many cases, the corresponding dynamics evolve in subpicosecond time scales [1]. Intense, ultrashort pulses in the vicinity of 160 nm are advantageous for a plethora of applications ranging from time-resolved studies of ultrafast molecular dynamics [2][3][4][5][6][7], to low-energy nuclear isomeric state excitation [8,9], paving the way for a solidstate nuclear clock [10,11].…”