物理动力学
作者：E.M.Lifshitz/等 整理日期：2019-03-12 10:26:01
片断：
Theaboveargumentsare,ofcourse,generalones,andapplytoanyrapidly
varyingquantities(phases)whichtakeva luesinfiniteranges.
Returningtotherotationaldegreesoffreedomofmolecules,letusnotethatin
polyatomicgasesthedistributionfunctionmayalsodependontheangleswhich
specifythefixedorientationoftheaxesofthemoleculesrelativetothevectorM.
Forexample,inmoleculesofthesymmetrical-toptypethisistheprecessionangle
betweenMandtheaxisofthetop,whereasthedistributionfunctionmayagainbe
regardedasindependentoftherapidlyvaryinganglesofrotationofthetopabout
itsownaxisandprecessionofthisaxisaboutM.t
Thevibrationalmotionoftheatomswithinthemoleculeispracticallyalways
quantized,sothatthevibrationalstateofthemoleculeisspecifiedbytheap-
propriatequantumnumbers.Underordinaryconditions(atnottoohightem-
peratures),however,thevibrationsarenotexcitedatall,andthemoleculeisatits
groundvibrationallevel.
InthischapterweshalldenotebyFthesetofallvariablesonwhichthe
distributionfunctiondepends,otherthanthecoordinatesofthemoleculeasa
whole(andthetimet).Weseparatefromthephasevolumeelementdrthefactor
dV=dxdydz,anddenotebydFtheremainingfactorintermsofthevariables
used(andintegratedovertheanglesonwhichfdoesnotdepend).ThequantitiesF
haveanimportantcommonproperty:theyareintegralsofthemotion,andremain
constantforeachmoleculeduringitsfreemotion(intheabsenceofanexternal
field)betweensuccessivecollisions;buttheyareingeneralalteredbyeach
collision.Thecoordinatesx,y,zofthemoleculeasawholevary,ofcourse,during
itsfreemotion.
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