We have examined the profile of an isolated autoionizing resonance driven by a pulse of short duration and moderately strong field. The analysis has been based on stochastic differential equations governing the time evolution of the density matrix under a stochastic field. Having focused our quantitative analysis on the 2s2p( 1 P ) resonance of Helium, we have investigated the role of field fluctuations and of the duration of the pulse. We report surprisingly strong distortion of the profile, even for peak intensity below the strong field limit. Our results demonstrate the intricate connection between intensity and pulse duration, with the latter appearing to be the determining influence, even for a seemingly short pulse of 50 fs. Further effects that would arise under much shorter pulses are discussed.PACS numbers: 32.80. Aa, 32.80.Hd, 32.70.Jz, 32.80.Rm Autoionizing (AI) states, also referred to as resonances, in atoms and molecules belong to a rich field of Atomic Molecular and Optical (AMO) physics, representing a paradigm of discrete states embedded in continua. Typically, they can be excited either by photoabsorption or by collisions. The literature on the subject is vast, but luckily a relatively recent review [1] provides an interesting guide to the origins of the field, as well as a substantial collection of references to both theoretical and experimental work. The simplest case in point is represented by a so-called "isolated AI resonance", which means that the width of its excitation profile is much smaller than the energy distance from the nearest AI resonance. A textbook example of an isolated resonance is provided by the doubly excited 2s2p( 1 P ) state of Helium, which has been studied in exhaustive detail, both theoretically and experimentally, over the last 60 or so years [1]. The field has more recently been enriched with fascinating details on the temporal evolution of AI through the exploitation of coherent few cycle pulses [2,3,4].Traditional photoabsorption studies of AI resonances have been carried out mainly by means of synchrotron radiation, where weak, practically monochromatic radiation excites the resonance. The most easily observed quantities, as a function of the arXiv:1707.09172v1 [physics.atom-ph]
The excitation of an autoionizing resonance by intense radiation requires a theoretical description beyond the transition probability per unit time. This implies a time-dependent formulation incorporating all features of the radiation source, such as pulse temporal shape and duration, as well as stochastic properties, for pulses other than Fourier limited. The radiation from short wavelength free electron lasers is a case in point, as it is the only source that can provide the necessary intensity. In view of ongoing experiments with such sources, we present a systematic study for an isolated autoionizing resonance. We find that intensity, pulse duration and field fluctuations conspire in producing unexpected excitation profiles, not amenable to a description in terms of the usual Fano profile. In particular, the role of intensity fluctuations turns out to pose challenging theoretical problems part of which have been addressed herein.
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