A NNALES DE L ’I. H. P., SECTION A
S. L. R OBINSON
The semiclassical limit of quantum dynamics.
II : scattering theory
Annales de l’I. H. P., section A, tome 48, n
o4 (1988), p. 281-296
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281
The semiclassical limit of quantum dynamics II:
Scattering Theory
S. L. ROBINSON (*) Department of Mathematics,
Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061
Ann. Henri Poincaré,
Vol. 48, n° 4, 1988, Physique ’ théorique ’
ABSTRACT. 2014 We
study
the h ~ 0 limit of thequantum scattering
determined
by
the HamiltonianH(h) = -
0 + V on for short2m
range
potentials
V. We obtainclassically
determinedasymptotic
behaviorof the
quantum scattering operator applied
to certain states of compactsupport.
Our main result is the extension of a theorem ofYajima
to theposition representation.
Thetechniques
involve convolution with Gaussian states. The error terms are shown to haveL2
norms oforder h1/2-~
foraribtrarily
smallpositive
B.RESUME. 2014 Dans cet article nous etudions la limite h ~ 0 de la diffusion
quantique
del’opérateur H(h) = -h2/2m0394 +
V sur ou V est unpoten-
- tiel a courte
portee.
Nous obtenons Iecomportement asymptotique
de1’ope-
rateur d’onde
applique
a certains etats asupport compact.
Notre résultatprincipal
est une extension d’un resultat deYajima
a larepresentation
deposition.
La methodeemployee
consiste a faire une convolution avec unetat Gaussien. Les termes de correction ont une norme
L2 qui
tend verszero
plus
vite que~1~2-E
pour tout nombrepositif
~.(*) Present address: Department of Mathematical Sciences, University of North Caro- lina, Wilmington, NC 28403.
Annales de 1’Institut Henri Poincaré - Physique theorique - 0246-0211
Vol. 48/88/04/281/16/X 3,60/(0 Gauthier-Villars 12
282 S. L. ROBINSON
1. INTRODUCTION
In this paper we
study
the relation between the classical andquantum descriptions
ofpotential scattering
in the semiclassical limit. We provea theorem on the small h
asymptotics
of thequantum scattering
operatorJ(h) applied
to states of the where S- is real- valued andf
is ofcompact support.
Therigorous scattering theory
insimilar contexts has been studied
by Yajima ( [Y1 ], [Y2 ])
andHage-
dorn
( [Hal ])
and the semiclassicallimit
of thescattering
cross section andscattering amplitudes
have also received somerigorous
attention( [ES ], [SY ], [Y3 ]).
Our theorem can be viewed as the extension of the theorem ofYajima ( [Y1 ])
to the more naturalposition representation,
a resultapparently
unobtainableby Yajima’s
Fourierintegral operator techniques.
Hagedorn’s
results are concerned withincoming
states of a certain Gaus- sian form and are basic to ourtechnique.
Ourproof
mimicks theproof
of our
previous
result on the semiclassical time evolution( [R ]).
We now introduce
enough
notation and definitions to allow us to state our main result. For technical reasons we must restrict our attention todimension n
3.ASSUMPTION 1.1.
Suppose
V ECI+2(~n~ IR)
for someinteger
l ~ 1+ !!.
and assume there exist constants
Ca
and v > 0 such that 2for all multi-indices x
with ~ ~
+ 2. ~2For such
potentials
the quantum HamiltonianH(~) = 2014
0 + V is self2m
.. ~ ... ,
~2
adjoint
on theself-adjoint
domain of the free HamiltonianH0(h) = - 2014A
( [RS 1 ]).
It is well known( [RS2 ])
that forpotentials satisfying Assump-
tion 1.1 the wave operators
exist and are
asymptotically complete
= = theabsolutely
continuousspectral subspace
of withrespect
to for each ~ > 0.Moreover,
thequantum
mechanicalscattering
matrix~(~)
=SZ-(~)*SZ+(~)
is aunitary operator
onThe
problem
of the classicalscattering
has been studied in some detail and thefollowing
facts can be found in or are consequences of facts found in[Hal], [Hu ], [Sie ], [Sim]
and[RS2 ] :
There exists a set ~o c of1’Institut Henri Poincaré - Physique theorique
283
THE SEMICLASSICAL LIMIT OF QUANTUM DYNAMICS. - II
Lebesgue
measure zero such that for any there is aunique
solution(a(a _, r~ _, t), ~-~-, t))
of the systemand o a
unique
vector(a + (a _, r~ _ ), r~ + (a _, ~ _ )) E IF~2"
withr~ + (a _, r~ _ ) ~ 0
1such that
and
The classical
scattering
matrix ~~1 defined onby
is a class
C2
measurepreserving
map. We now state our main result.THÉORÈME1 .2. -
Suppose
that the space dimension n is greater than 2 and assume V satisfiesAssumption
1.1. Let and let ~/be a closed subset of IRn
containing
the setSuppose /
EC)
is such that supp(/)
is contained in and let/),e(0,l/2).
DefineQ+ :
H IRnby Q+(~-)
=a+(~-,VS-(~-))
andassume det
2014~ (~-)
~ 0 for x E supp(f).
Then there exist 5 > 0 and aL~- J
constant C
independent
of h such thatfor 5. The
dependence
on x of the terminvolving
the summation isimplicit.
For fixed x the summation is over all x~ such that = x and is finite. isgiven by
and is an
integer multiple of 03C0
definedexplicitly
in theproof.
2 48, n° 4-1988.
284 S. L. ROBINSON
Remarks. - 1. The norm
appearing
above is of course the norm of,
2.
Yajima ( [Y1 ])
has shown a similar theoremusing
Fourierintegral operator techniques
asdeveloped by
Hormander and Asada andFujiwara
Not
only
do we feel that our methods are much lesscomplicated,
butYajima’s approach
is restricted to the momentumrepresentation
whileour methods are
applicable
even in mixedrepresentations.
2. NOTATION AND DEFINITIONS
Throughout
this paper n denotes the spacedimension.
is the Hilbert space of squareintegrable, complex
valued functions on !R" withthe usual inner
product .,.>
andnorm ~.~2.
The quantum mechanical Hamiltonian is the operator -2 m
ð. + V onHere,
ð. is then-dimensional
Laplacian
operatorfii is a small
positive
parameter(a
dimensionlessmultiple
of Planck’sconstant), m
is apositive
constant, and V is a real valued function on [R"viewed here as a
multiplication
operator on A multi-index 0: is anordered
n-tuple (Xi,X2, ...,xJ
ofnonnegative integers,
and we use thestandard notation for the
order a I
and the factorial oc! of 0:. Forn
x =
(jci,
x2, ...,xn)
E IRn thesymbol x03B1
is definedby x03B1
= Dcx standsfor the
partial
differential operatori = 1
For x E [R" or denotes the Euclidean norm of x. We denote the
n
usual inner
product
on Rn or C"by u, v> =
and bethe standard basis for [R" or en. For
sufficiently
smoothfunctions
~f ~x isthe matrix
/5f.B
We willwrite f(1)
to denotethe gradient
off and f(2)
to denote the Hessian matrix e will not
explicitly distinguish
row and column vectors in IRn or cn in our
computations.
Thesymbol
1Annales de l’Institut Poincaré - Physique theorique
285
THE SEMICLASSICAL LIMIT OF QUANTUM DYNAMICS. - II
will stand for the n n
identity
matrix. For an n ncomplex
matrix Awe will use the
symbol ~ A ~
for the norm of the matrix A = whichn
B1/2we choose to be
given by II A ~
=sup (I | aij !i .
We willuse | 1 A I
to denote the matrix
(AA *)1/2
where A* is theadjoint (complex conjugate transpose)
of A. will denote theunique unitary
matrixguaranteed by
the
polar decomposition
theorem such that A= I A I A I = (AA *)1/2.
Following Hagedorn ([Ha3 ])
we definegeneralized
Hermitepolynomials
on Rn
recursively
as follows: We set0(x)
= 1 and;c)
= 2 v,x>
where ~ is an
arbitrary
non-zero vector in en. For Vb ...,~arbitrary
non-zero vectors in en we set
The
polynomials
areindependent
of theordering
of the vectors..., Given a
complex
invertible n x n matrix A and a multi-index 0:we define the
polynomial
where the vector
uAei
appears ai times in the list of variables of Jf We will find it useful to considercomplex n
x n matrices A and B satis-fying
thefollowing
conditions :A and B are invertible
(2 .1 a)
BA -1 is
symmetric (2 .1 b)
Re
(BA-1)
isstrictly positive
definite(2 .1 c) (2. Id) (Here, symmetric
means(real symmetric)
+i(real symmetric)).
For
complex n
x n matrices A and Bsatisfying
conditions(2.1),
vectorsa
and ri
E multi-indices 0:, andpositive ~
we defineThe branch of the square root of det
(A)
will bespecified
in the contextin which the functions
03C603B1
are used. Whenever we writewe are
assuming
that the matrices A and Bsatisfy
conditions(2.1).
ForVol. 48, n° 4-1988.
286 S. L. ROBINSON
fixed
A, B, h,
a,and ~
the functionsB, h,
a, ~,x)
form an orthonormal basis of For aproof
of this fact and otherproperties
of these func-tions,
see[Ha3 ]. Using
Lemma 2 . 2 of[Ha3] we
obtain3. PROOF OF THE THEOREM
In this section we prove the technical details
required
for theproof
ofTheorem 1. 2. We do not
actually
prove the theorem but argueby analogy
to the
proof
of Theorem 1. 2 of[R] once
we haveproved
the technicalities.We first sketch the idea of our
proof.
For the purpose of
explaining
the idea behind theproof
we restrictourselves to the case in which
Q +
is adiffeomorphism
on some open ball~ =3
supp(/).
In this case the summationappearing
in the statement ofthe theorem contains
only
one term. Thegeneral
case is obtainedby introducing
a smoothpartition
ofunity
on supp( f )
andpatching
localdiffeomorphisms.
Forbrevity,
we writex)
=G(h, x)
+O(~~)
to meanthat there exist
positive
constants 5 and Cindependent
of h such that~ 5
implies !! F(~ -) - By
Lemma 3.1 of[R] we
haveWe then
apply
thescattering
operator and pass itthrough
theintegral sign.
Werequire
a lemma contained in[HI], [H2] and [H3] on
the semiclassical behavior of Gaussian
states
such as theexponential
termin the
integrand.
This lemma allows us toapproximate ~(~)~r _ by
arather
involved
integral expression
of the form-
where
h, x)
is apolynomial
in x with coefficientsdepending
on a-and h, P+ (a _ ) = ~+(a_ ,S(1)_(a_)), S(a _ )
is a real valued function related to the classical action associated with thetrajectory (a(a _, 5~1~(a _ ), t),
Annales de l’Institut Henri Poincaré - Physique theorique
THE SEMICLASSICAL LIMIT OF QUANTUM DYNAMICS. - II 287
r~(a _ , S~ 1 ~(a _ ), t))
andT(a _ )
is acomplex n
x n matrix valued function with rather niceproperties.
We thenchange
variables from a _ toQ+ = Q+(~_)
to obtain anintegral
which has a convolution structure.An estimate much like the first of the estimates in this sketch
along
withcertain
properties
of the function T thenyields
the desired result. We nowprovide
the details needed for theproof.
The classical
scattering theory
forpotentials satisfying Assumption
1.1is
quite complete
and thefollowing
facts are known( [Hal ], [Hu ], [RS2 ], [Sie ], [Sim ]).
There exists a subset Eo c ofLebesgue
measure zerosuch that
given (~-~-)e
there is aunique
solutionof the
system
such that
We note
that {(a, yy)
ER2n : ~
=0 }
c Eo. We denote the closure of Eoby ~.
The matrices
A(~-,~-,t)
andB(~-,~-~) satisfy
conditions(2.1)
forall
Moreover,
there existcomplex
n x n matricesA+(~-,~-)
andVol. 48, n° 4-1988.
288 S. L. ROBINSON
B+(~~_),
vectorsa+(a_, ~_)
andr~+(a_, r~_)
in IRn with~+(~_,~_) ~ 0,
and a real number
S+(~-,~-)
such thatThe
functions a,
a +, r~, and 11 + are of class andsatisfy
Annales de 1’Institut Henri Poincare - Physique theorique
THE SEMICLASSICAL LIMIT OF QUANTUM DYNAMICS. - II 289
The matrices
A + (a _ , r~ _ )
andB + (a _ , ~ _ ) satisfy
conditions(2 .1 )
andthe
following
relations hold :,
and o,
are "given by
Our next three
propositions
are concerned with the smoothness andlarge |t| I asymptotics
of the functions discussed above.PROPOSITION 3.1. The functions a, 11,
A, B,
a +, ~+,A+
andB +
are of class C2 in the variables(a -, ~ - ) E (~2n B E,
and the classicalscattering
matrix Ócl : defined
by 11-)
=(~+(~-. ~-). ~1 + (a -, ~1- ))
is a measure
preserving
classC2
transformation.Proof.
2014 We omit theproof
of thisproposition noting
that thearguments
of Section XI. 2 of
[RS2] can easily
be extended to this case. II PROPOSITION 3 . 2. Let Jf be a compact subset of and let v beVol. 48, n° 4-1988.
290 S. L. ROBINSON
as in
Assumption
1.1. Then there existpositive
constants C and Tdepending only
on Jf such that Timplies
for all
(~-,~-)eJf.
Forbrevity
we have denoted~-(a-~-)=~, r~_(a_, r~_) = r~_, A_(a_, r~_) _ ’0, B_(a_, r~_) _ ’0, S-(~-~_)=S-(~-)
and the
inequalities
above are understood to hold with ±replaced by - for t - T and
+for t T.
Proof
Theorem XI. 2(a)
of[RS2] and
itsproof imply
the existenceof T > 0 such that
if
and(a _, r~ _ ) E ~’.
Weagain emphasize
that when wewrite
inequalities containing
bothsubscripts
+ and - we meanthat
the
inequality
holds with the -subscript
forlarge negative
time and withthe +
subscript
forlarge positive
time. Asimple argument using
thetriangle inequality
and compactness shows that there exists a constantC 1
such
that r~ _ , t) ~ > I
forall
T and(a _ , r~ _ )
HenceI
is bounded from aboveby
a constantmultiple
ofuniformly
in forall
T.Equations (3 . 3a), (3. 5~),
and(3. 5b) imply
the existence of a constantC2
such thatAnnales de l’Institut Henri Poincaré - Physique " theorique "
291
THE SEMICLASSICAL LIMIT OF QUANTUM DYNAMICS. - II
uniformly
in(a _, r~ _ ) E ~’
forall
T.Equations (3 . 3b)
and(3 . Sb) imply
the existence of a constantC3
such thatuniformly
in(a _, r~ -) E ~’
forall
T. From conservation of energy,I ~l - I - I ~1 + (a - ~ ~1- ) ~ I
and henceequations (3 . 8) imply
the existence of aconstant
C4
such thatuniformly
in(a _, 11 -)
E Jf for T. The twoinequalities concerning
the matrices
A(a _, r~ _, t)
andB(a _, r~ _, t)
follow from the last four andequations (3 . 7).
We proveonly
one of theremaining inequalities involving
the derivatives of
a(a _, ~ _, t)
and~(a _, r~ _, t)
with respect to a _ and 11-,the others follow
by
similararguments. Following
an ideaby Yajima (see
theproof
of Lemma 2 . 7 of[Y1 ])
wechange
the order ofintegration
in
(3.4a)
to obtainBy
Gronwall’sLemma II ~a ~a(a_, t is
bounded aboveby
a constantfor all t and
(a
_,11 - )
E H.Hence, by (3 . 4a)
or(3 . 4a’)
there exists aconstant
CS
such thatfor all ~ - T and
(a _, ~ _ ) E K. By equations (3 . 4a), (3 . 6a), (3 . 6c)
andexplicit computation
and therefore there is a constant
Cs
such thatfor all ~ T and
(a _, r~ _ ) E K.
IIIt is worth
noting
that thekey ingredient
in theproof
ofProposition
3 . 2is the existence of T > 0 such
that ~(~-,~-, ~ I
is bounded from belowVol. 48, n° 4-1988.
292 S. L. ROBINSON
by
a constantmultiple of t ~ I and (D°‘v)(a(a _, r~ _, t)) ~ I
is bounded from aboveby
a constantmultiple of uniformly
infor
all
T.PROPOSITION 3.3. Let Jf be a
compact
subset of Then the functionS + (a _, r~ _ )
is of class C1 in the variables(~-,~-)eJf
andProof
- Forand t,
defineBy
the conservation of energy, i. e.and some
elementary manipulations involving differentiating
under theintegral sign
andintegrating by
parts we obtainand
We remind the reader that we do not
distinguish
row and column vectors,clearly
the vectors~-, r~ _, t)
andr~(a _, r~ _, to) appearing
in theexpressions
above are row vectors.
By Proposition 3.2,
and
Annales de Henri Poincaré - Physique - theorique -
293
THE SEMICLASSICAL LIMIT OF QUANTUM DYNAMICS. - II
where the limits are uniform on Jf. The
proposition
follows from the fact thatand standard theorems on the
interchange
of limits and derivatives(for example,
Theorem 28 . 5 of[B can
be extended to beapplied here).
ttFollowing
the idea of Section 4 of[R ],
we would like to restrict theincoming asymptotes (~-,~-)
to lie on a= S~ 1 ~(q _ ) ~ .
A
complication
arises in that we must avoid the set E. LetWe note that j~ is closed
by
virtue ofbeing
the inverseimage
of theclosed set E under the continuous
mapping q_
H(q-, S(1)_5(q_)).
We assumefor the rest of the
proof
that supp( f )
c Define aC2 mapping
Q + :
Hand assume
for all x E supp
( f ).
As in[R ],
thisgives
rise tofinitely
many open ballsc
covering supp(/)
such thatQ +,k
=Q + 1
is a classC2 diffeomorphism.
Wedefine, for q
EQ + [~V’k ],
-By Proposition 3 . 3,
forq E Q+ we
haveand,
since Ócl ismeasure-preserving,
the argument ofProposition
4.1 of[R] ]
showsWe note that
(3 . 9) implies
thatS + ,k
is of ClassC3 in q
EQ + [J~].
For q
EQ + [~] we
define the branch of the square root of det[A+~)]
Vol. 48, n° 4-1988.
294 S. L. ROBINSON
by
firstdemanding
j that the branchof ( det C’0
+it m 1 I 1/2 be
determined oby continuity
from t = 0. We then determineby continuity
in t and therequirement
Then,
det[A+ ~k~C~~ ~ 1 ~2
is determinedby continuity
and the tworequirements
and
We determine the branches of
and
by analytic
continuationalong ç
E[0, 1] ]
ofand
respectively, starting
withpositive
valuesfor 03BE
== 0. The branch of,
(det
2014 Q
+J
is then determinedby (3 .10).
As in[R ],
to eachNk
we can
assign
aunique (mod 4) index ~
such thatfor
all q
EQ + [~].
As in Section 4 of[R ],
we find that it is sufficient toconsider a
single
ball andC) function fk
and prove thefollowing analog
of Lemma 4 . 3 of[R ].
Poincaré - Physique theorique
THE SEMICLASSICAL LIMIT OF QUANTUM DYNAMICS. - II 295
LEMMA 3 . 4. Given
~e(0,1/2),
there existpositive
constants 8 and Cindependent
of h such thatLemma
3.4 (and hence,
Theorem1.2)
followsby mimicking
theproof
of Lemma 4 . 3 of
[R ] with
the references to the work ofHagedorn replaced by
thefollowing proposition
whichrepresents
a result contained in[Hal],
[Ha2 ],
and[Ha3] though
notexplicitly
stated there.LEMMA 3 . 5. -
Suppose
V satisfiesAssumption
1.1. Letbe
compact
and ~, E(0,1/2).
Then there exist apositive
number~,
functionsc+03B1(a_,
11 -,h),
and a constant Cindependent
of h such that h~(0, 5 ] implies
for all
(a _, r~ _ )
Moreover, the functionsc~(~-,~-, ~C)
are of classC1
in
(a _, ~ _ ) E ~2"B E
and the constant C can be chosen such thatand
To prove Lemma 3 . 5 we
simply
mimic thearguments
used in theproofs
of Theorem 1. 2 of
[Hal] and
Theorem 1.1 of[Ha3 ] using
the uniform estimates ofProposition
3.2 tokeep
track of thedependence
on t and to.Standard contraction
mapping
and fixedpoint
theorems(see,
e. g., Sec- tion 4 . 9 of[LS] ]
and Section XI. 2 of[RS2 ]) guarantee
the smoothness of the functions We omit the details of this argument since all of thesteps
are the obviousanalogues
of thearguments
ofHagedorn.
The crucialestimate
(the proof
of whichrequires
that n3)
is thehigher
order exten-sion of Lemma 3 . 2 of
[Hal ].
Lemma 3 . 4 follows
by using
Lemma 3 . 5 and theargument
of Lemma 4 . 3 of[R ].
Theorem 1. 2 then followsby
theargument
used toprove
Theo-rem 1. 2 of
[R ], using
Lemma 3 . 4.ACKNOWLEDGEMENTS
This paper is based on the author’s Ph. D. dissertation « The Semiclassi- cal Limit of
Quantum Dynamics »
atVirginia Polytechnic
Institute andVol. 48, n° 4-1988.
296 S. L. ROBINSON
State
University
under thedirection
ofGeorge
A.Hagedorn.
The authorwishes to thank Professor
Hagedorn
for hisguidance
and assistance inthe
preparation
of this paper.REFERENCES
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(Manuscrit reçu le 15 juillet 1987) (Version révisée reçue le 20 mars 1988)
Annales de l’Institut Henri Poincaré - Physique theorique