Abstract
Molecular wave-packet dynamics in oxygen are studied in the time domain, using a single-color VUV-pump--VUV-probe scheme. 17-fs VUV pulses, centered at 161 nm are generated via high-order harmonic generation driven by an intense 800-nm pulse leading to VUV pulse energies that reach 1.$1\ensuremath{\mu}\text{J}$ per pulse. An all-reflective interferometric pump-probe scheme is used for studying the delay dependence of the molecular oxygen ion signal with simultaneous nonresonant photoionization of krypton as a precise timing-reference. Access to the excited dissociative state lifetime is provided by the resulting delay-dependent ${{\mathrm{O}}_{2}}^{+}$ signal, ultimately limited by the molecular ionization window. The ability to use a two-photon VUV probe provides the delay-dependent detection of ${\mathrm{O}}^{+}$ as an additional observable, extending the dissociation observation window.
Highlights
On the basis of its significance for many photochemical processes in nature, molecular oxygen has been the subject of several studies in the past decades
We report on a VUV-pump–VUV-probe experiment on molecular oxygen with intense and well-characterized sub-20-fs pulses
VUV-probe experiment utilizing a short, energetic pulse at 161 nm yields a delay-dependent O2+ ion signal resulting in an extracted time constant of (5.6 ± 1.6) fs, modeled using a symmetric exponential decay function
Summary
On the basis of its significance for many photochemical processes in nature, molecular oxygen has been the subject of several studies in the past decades. The absorption exhibited both in the ultraviolet (UV) and the vacuum ultraviolet (VUV) spectral range leads to attenuation of solar radiation and most importantly the formation of the ozone layer in the stratosphere [1]. Thrushin et al [11] measured a decay time of 4.3 fs extracted from the O2+ 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
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