Abstract
The ultrafast electronic decay of HCl molecules in the time domain after resonant core excitation was measured. Here, a Cl-2p core electron was promoted to the antibonding σ* orbital initiating molecular dissociation, and simultaneously, the electronic excitation relaxes via an Auger decay. For HCl, both processes compete on similar ultrashort femtosecond time scales. In order to measure the lifetime of the core hole excitation, we collinearly superimposed 40 fs soft x-ray pulses with intense terahertz (THz) radiation from the free-electron laser in Hamburg (FLASH). Electrons emitted from the molecules are accelerated (streaked) by the THz electric field where the resulting momentum change depends on the field's phase at the instant of ionization. Evaluation of a time-shift between the delay-dependent streaking spectra of photo- and Auger electrons yields a decay constant of (11 ± 2) fs for LMM Auger electrons. For further validation, the method was also applied to the MNN Auger decay of krypton. Reproduction of the value already published in the literature confirms that a temporal resolution much below the duration of the exciting x-ray pulses can be reached.
Highlights
When an electron of a molecule is resonantly excited to an antibonding orbital, the molecule will start to dissociate, and if the excitation has created a core-hole, the latter will typically relax electronically
A Cl-2p core electron was promoted to the antibonding rà orbital initiating molecular dissociation, and simultaneously, the electronic excitation relaxes via an Auger decay
In order to measure the lifetime of the core hole excitation, we collinearly superimposed 40 fs soft x-ray pulses with intense terahertz (THz) radiation from the free-electron laser in Hamburg (FLASH)
Summary
When an electron of a molecule is resonantly excited to an antibonding orbital, the molecule will start to dissociate, and if the excitation has created a core-hole, the latter will typically relax electronically. Lifetimes much shorter than the duration of the exciting pulses can be measured This technique is regularly used in attosecond physics, where, e.g., photoemission delays of (21 6 5) as have been observed after ionization with 200 as XUV pulses and streaking with NIR.[9,10,11] To obtain meaningful streak spectra, streak-fields with oscillation periods long compared to both the processes studied and the ionizing light pulses have to be used. The characteristic time constant of dissociating HCl molecules after resonant excitation of 2pCl electrons to the antibonding rà orbital was investigated, which to our knowledge is the first time-resolved measurement of this process
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