X-ray Transient Absorption Spectroscopy Study Of Molecular Auger States

Since the advent of the chirped-pulse amplification(CPA)in 1980s,pulse laser has formally entered into the period with ultra-short pulse duration and ultra-high power energy.Such great achievements have broadly served many disciplines such as physics,chemistry,material,medicine,and so on.The ultra-short pulse laser can provide a set of ultra-fast 'photo shutters' that can help us 'see' and study ultra-fast physical dynamics at the time scale of femtosecond or even sub-femtosecond.Presently,the high harmonic generation(HHG)has reported the shortest isolated extreme ultraviolet(XUV)pulse of 43 as,and the x-ray free electron laser(XFEL)can also supply soft/hard x-rays with pulse duration down to a femtosecond or a sub-femtosecond.Scientific applications of these extreme ultrashort pulse lasers have been one of the most promising areas in atomic and molecular physics.The transient absorption spectroscopy is one of the typical applications of ultrafast pulse lasers.For example,Attosecond Transient Absorption Spectroscopy(ATAS)uses sub-femtosecond XUV pulses to study the ultrafast dynamics of electrons.An attosecond XUV pulse generates broadband excitation,accompanied with a strong pump of infrared(IR)lasers,thus the ATAS can study the energy exchange process in laser-matter interactions with unprecedented accuracy.In addition,thanks to the development of the advanced XFEL facilities and tabletop attosecond laser sources from HHG,the technique of ATAS has been extended to the x-ray region.The transient absorption spectra of the ultra-fast Auger excited states by femtosecond x-ray pump,namely X-ray transient absorption spectrum(XTAS),have been proposed.In this paper,x-ray transient absorption spectra(XTAS)of molecules are theoretically investigated in a scenario of femtosecond x-ray pump and continuous wave(CW)infrared(IR)-control.The scheme is exemplified by CO molecule resonantly pumped into carbon and oxygen core-excited ls??*states by a weak femtosecond x-ray pulse,while dynamic Stark shifts are introduced by the CW IR-control radiation.As a result,significant shoulder structures appear in XTAS showing strong dependence on the phase of IR radiation relative to the envelope of the x-ray pulse.Due to a significant difference in the frequencies of the two pulses,the present XTAS scheme provides much clear interpretations of the dynamic Stark effects as compared to the scenario of attosecond UV transient absorption.Within a suggested two-level model,where the total spectrum is decomposed as incoherent superposition of contributions from different vibrational excitations weighted by the Frank-Condon Factors,all spectral structures can be well identified and interpreted in a good agreement with the full-scale molecular dynamic simulations.Furthermore,the well characterized XTAS in the proposed IR-control scheme could be applied for fine phase synchronization between IR and x-ray pulses,highly demanded in the modern experiments of x-ray free-electron lasers.