We investigate laser induced nonsequential multiple ionization using a simple statistical model in which an electron recollides inelastically with its parent ion. In this collision, it thermalizes with the remaining $N-1$ bound electrons within a time inte

2Speed-upandslow-downcollisionsinlaser-inducednonsequentialmultipleionization

ThisprocesshasbeensuccessfullydealtwithusinganS-Matrixapproachanditsclassicalcounterpart[5–9].Inparticular,ifthelaser- eldintensityishighenough,bothquantum-mechanicalandclassicalmodelsyieldverysimi-larmomentumdistributions1.Wehaveshownthatthisagreementoccursforseveraltypesofelectron-electroninteraction,andintheabsenceorpresenceofCoulombrepulsionintheelectron nalstates[6–8].Furthermore,wehaveveri edthattheoutcomeoftheclassicalandquantummechanicalcomputa-tionsalsoexhibitastrikingagreementifthedriving eldisafew-cyclelaserpulse[9,12].Inparticular,boththeclassicalandquantummechanicaldistri-butionsarestronglydependentontheso-calledabsolutephase,i.e.,thephasedi erencebetweenthepulseenvelopeanditscarrieroscillation.Wehaveusedthisfactinordertoproposeaschemefordiagnosingtheabsolutephase[12],whichhasalsobeenexperimentallyrealized[13].

Ifoneisdealing,however,withmorethantwoactiveelectrons,therearemanyopenquestionsconcerningthephysicsbehindlaser-inducedmultipleionization.Thisisinparticulartruefortripleandquadrupleionizationofneonbynearinfra-red(ω=0.057a.u.)laser eldsinanintensityrangebe-low2×1015W/cm2[2,14,15].Thedistributionsobservedinsuchexperi- ments,approximatelypeakedatnon-vanishingionmomentaP||=±2N

however,nolongerholdsiftheintensityisnearthreshold,i.e.,justenoughforthesecondelectrontobereleased.Foradetaileddiscussionc.f.[10],andformeasurementsofnonsequentialdoubleionizationbelowthethresholdsee,e.g.,[11].1This,