an even  lavie valve is used as a pulsed enantiopure fenchone 
 source with helium as carrier gas to avoid cluster formation . the 
 experiment was performed using the aurore laser system at celia , which 
 delivers 20 mj , 25 fs , 800 nm pulses at 1 khz . the presented results are obtained by scanning 
 the pump  probe delays typically 16 times , and at each delay , 
 one helicity image is a record over 45  000 laser shots . for 
 each scan , two background images corresponding to the probe pulse 
 alone in each helicity are recorded and subtracted from each image 
 of each delay for a given helicity . this background subtraction is 
 required for the longest pump  probe delay when the signal reaches 
 zero ( see figure s4 of the supporting information ) . high - level quantum chemistry calculations , using gaussian 09 , 
 molpro , and gamess - us quantum chemistry packages , have been performed 
 on the ground and first five electronic excited states of fenchone 
 to help in the interpretation of the experimental data .
unravelling the main 
 initial dynamics responsible for chiral recognition 
 is a key step in the understanding of many biological processes . 
 however , 
 this challenging task requires a sensitive enantiospecific probe to 
 investigate molecular dynamics on their natural femtosecond time scale . 
 here we show that , in the gas phase , the ultrafast relaxation dynamics 
 of photoexcited chiral molecules can be tracked by recording time - resolved 
 photoelectron circular dichroism ( tr - pecd ) resulting from the photoionization 
 by a circularly polarized probe pulse . a large forward  backward 
 asymmetry along the probe propagation axis 
 is observed in the photoelectron 
 angular distribution . its evolution with pump  probe delay reveals 
 ultrafast dynamics that are inaccessible in the angle - integrated photoelectron 
 spectrum or via the usual electron emission anisotropy parameter (  ) . 
 pecd , which originates from the electron scattering in the chiral 
 molecular potential , appears as a new sensitive observable for ultrafast 
 molecular dynamics in chiral systems .