Non-Markovian Dynamics inaDensePotassium Vapor

Using transient four-wave mixing, wehaveobserved therelaxation dynamics ofadense potassium vapor whosedephasing timeislarge compared tothecollision duration, revealing a clear transition from non-Markovian toMarkovian dynamics. 02005 Optical Society ofAmerica OCIScodes: (320.7150) Ultrafast spectroscopy; (300.6240) Coherent transient spectrosopy Relaxation phenomena incondensed matter often occur intime scales ontheorder ofapicosecond orless. Inthese short timeregions, thedynamics depart from Markovian behavior, meaning thedynamics arealtered duetothe phase memoryofcollisions. Nonlinear optical time-domain techniques using two- andthree-pulse excitation are powerful tools that canreveal characteristics ofthedynamics. Suchexperiments have beenperforned inmolecular systems (1). Inthose experiments, theform oftheinteractions isoften difficult tocalculate theoretically because the time scale ofthelocal structural transitions isclose tothat ofthesolvent interactions andbeing pulse-width limited canbeanissue. Wehaveobserved inatwo-pulse transient four-wave mixing experiment thenon-Markovian dynamics inasimple atomic system whose dephasing time islarge compared tothecollision duration, allowing clear separation oftime scales, making accurate theoretical description possible. Inmolecular systems, thetime scales corresponding toelectronic andvibrational dynamics areoften difficult to separate. Asimple atomic system hasinteractions well-separated into short- andlong-time regimes, making it possible tocalculate theformoftheinteractions andconfirm itexperimentally. Westudy thephase relaxation dynamics ofadense potassium vapor using thetime-domain technique oftransient four-wave mixing (TFWM). Inself-diffracted TFWM,twopulses with wavevectors k,andk2interact nonlinearly inasample, resulting ina background-free signal indirection 2k,-k1 which istime-integrated byaslowdetector. Theloss ofcoherence ofthe system isprobed byvarying thedelay between thepulses. Intheshort timelimit, thesystem canbeconsidered inhomogeneously broadened duetothetemporally static butspatially varying distances between atoms. Atan intermediate time, which isfinite butless than thecollision duration between atoms, therelaxation dynamics are dominated bytheperturbations ofthetransifion frequency oftheatoms asthey collide. Thecollision canbethought ofasthetransient formation ofapotassium dimer(KA2) Interns ofthepotential energy surfaces ofthepotassium dimer, showninFig. 1,this timescale reveals dynamics modified bypotential energy fluctuations atsmall intermolecular spacings, r.Attimes muchgreater than thecollision duration, wecanestimate theperturbations as delta functions intime andpositions cannolonger beconsidered fixed duetothemofions oftheatoms, making the system homogeneously broadened. This limit inthedimer picture corresponds totheatoms traversing therange of theintermolecular potential intimes that canbeconsidered infinitely fast. \ ~~~~K2' 4