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Computation of Matrix Elements and Expectation Values

Values

The semiclassical propagation scheme allows for the analytical calculation of expectation values from the coherent states that comprise the final density matrix. For the unified sampling according to Chap. 3, each pair of trajectories provides a contribution to the final result which is incoherently (i. e., on the level of the density matrix) added to the other

D.4 Computation of Matrix Elements and Expectation Values 93 ones. Inserting the propagator from Eq. (3.5) into the density propagation scheme Eq. (3.4) results in D b AE = tr bAρf = Z dP Z dxidx′i dxf dx′f D x′f bA xf E b Kξ,νa (xf, xi) bKξ,νb∗ x′f, x′i  ρ xi, x′i  = Z dP 1 (2π~)2 D gσ p′f, q′f bA gσ(pf, qf) E R (pi, qi, t) R∗ p′i, qi′, t  e~i(S(pi,qi,t)−S(p ′ i,qi′,t)) hg σ(pi, qi) | Ψ0iΨ0 gσ p′i, qi′  (D.51) where the integrations over the forward and backward, initial and final coordinate spaces have been eliminated and the remaining one is over the noise force distribution and ini- tial forward and backward phase spaces P = (ξ, ν, pi, qi, p′i, qi′). Since the semiclassically

propagated wave function is represented by a number of (phase-corrected) Gaussians, any expectation value is composed of a set of such Gaussians sandwiching the respective observ- able. Calculation of the overlaps, prefactors (R (pf, qf)), and action expressions has already

been dealt with in Sec. 3.1. The Gaussian expectation valuesDg bA gEfor a few observables will be given here.

The first moment of position evaluates to gσ p′, q′ bx gσ(p, q) = = Z dx r σ πexp n −σ 2 x − q ′∗2 − ip′∗ x − q′∗−σ 2 (x − q) 2+ ip′′ (x − q)ox = Z dx r σ πexp n −σx2+ σq′∗+ σq − ip′∗+ ipx + ip′∗q′∗− ipq −σ 2q ′∗2σ 2q 2ox = Z dx r σ πexp n −σ (x − ¯r)2+ σ¯r2+ ip′∗q′∗− ipq − σ 2q ′∗2 σ 2q 2ox = ¯r expnσ¯r2+ ip′∗q′∗− ipq − σ 2q ′∗2σ 2q 2o (D.52)

with ¯r = q+q2′∗ + ip−p′∗. Similarly, the second moment of position results in gσ p′, q′ bx2 gσ(p, q) =  1 2σ + ¯r 2  expnσ¯r2+ ip′∗q′∗− ipq − σ 2q ′∗2σ 2q 2o. (D.53) The energy expectation value is calculated from the sum of the kinetic and the potential part. With the second derivative of a Gaussian

hx| △ |gσ(p, q) i = = △ expn−σ2 (x − q)2+ ip (x − q)o = ▽ (−σ (x − q) + ip) hx| gσ(p, q) i = −σ hx| gσ(p, q) i + (−σ (x − q) + ip)2hx| gσ(p, q) i = σ2x2− 2σxq + σ2q − 2σ (x − q) ip − p2− σhx| gσ(p, q) i = σ2x2− 2 σ2q + σipx + σ2q2+ 2iσqp − p2− σhx| gσ(p, q) i , (D.54)

the kinetic energy results in  gσ p′, q′ − 1 2m△ gσ(p, q)  = −2m1  σ2  1 2σ + ¯r 2 − 2 σ2q + σipr + σ¯ 2q2+ 2iσqp − p2− σ  expnσ¯r2+ ip′∗q′∗− ipq − σ 2q′∗2− σ 2q 2o. (D.55)

Inserting the Morse oscillator discussed in Chap. 5, i.e.,

V (x) = D [1 − exp {−κx}]2 = D − 2D exp {−κx} + D exp {−2κx} (D.56) into a Gaussian bracket results in

D gσ p′, q′ bV gσ(p, q) E = − 2D exp  σr −¯ κ 2σ 2 + ip′∗q′∗− ipq − σ 2q′∗2− σ 2q 2  + D expnσ¯r2+ ip′∗q′∗− ipq − σ 2q ′∗2σ 2q 2o + D exp  σ¯r − κ σ 2 + ip′∗q′∗− ipq − σ 2q ′∗2 σ 2q 2  . (D.57) With the potential and kinetic contribution the total energy expectation value yields

D gσ p′, q′ bE gσ(p, q) E =  −2D exp  −¯rκ 2 + κ2 4σ  + D + D exp  −¯rκ + κ 2 σ  −2m1  σ 2 q − q ′∗+ i1 2 p + p ′∗2σ 2 !! expnσ¯r2+ ip′∗q′∗− ipq − σ 2q′∗2− σ 2q 2o. (D.58)

For the harmonic oscillator potential V (x) = mω22x2 studied in Chap. 4, the Gaussian

bracket is much simpler D gσ p′, q′ bV gσ(p, q) E = mω2 2  1 2σ + ¯r 2  expnσ¯r2+ ip′∗q′∗− ipq − σ 2q′∗2− σ 2q 2o. (D.59)

With this expression, the total energy expectation value in the harmonic case results in D gσ p′, q′ bE gσ(p, q) E = −2m1  σ 2 q − q′∗  + i1 2 p + p′∗ 2 − σ2 ! +mω 2 2  1 2σ + ¯r 2  . (D.60)

D.4 Computation of Matrix Elements and Expectation Values 95 Since the Gaussians are supposed to be coherent states of the harmonic oscillator, i.e., σ = ωm, this can be simplified to

D gσ p′, q′ bE gσ(p, q) E = 1 2m  σ2  q + i σp   q′+ i σp ′  + σ  . (D.61)

All of the expressions given in this section can now be computed during the trajectory evaluation. Compared with the propagation of the EOMs of the trajectories, their com- putation is relatively costly as it involves the evaluation of complex exponential functions. Since the propagation does not depend on them, they do not need to be evaluated at every time step but only when needed.

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List of Publications

Scientific Journals

[1] Brisker, D., Cherkes, I., Gnodtke, C., Jarukanont, D., Klaiman, S., Koch, W., Weissman, S., Volkovich, R., Toroker, M.C., and Peskin, U.

Controlled electronic transport through branched molecular conductors. Molecular Physics, 106:281–287, 2008.

[2] Koch, W., Grossmann, F., Stockburger, J.T., and Ankerhold, J. Non-Markovian Dissipative Semiclassical Dynamics.

Phys. Rev. Lett., 100(23):230402, 2008. [3] Grossmann, F. and Koch, W.

A finite-difference implementation of the Caldeira–Leggett master equation. J. Chem. Phys., 130(3):034105, 2009.

[4] Koch, W., Großmann, F., Stockburger, J.T., and Ankerhold, J.

Semiclassical non-Markovian Brownian motion in anharmonic potentials. Chemical Physics, 370(1-3):34 – 41, 2010, ISSN 0301-0104.

Dynamics of molecular systems: From quantum to classical. [5] Koch, W., Großmann, F., Ankerhold, J., and Stockburger, J.T.

Semiclassical formulation of non-Markovian quantum Brownian motion. Physica E, 42(3):388 – 393, 2010, ISSN 1386-9477.

Proceedings of the international conference Frontiers of Quantum and Mesoscopic Ther- modynamics FQMT ’08.

[6] Goletz, C.M., Koch, W., and Grossmann, F.

Semiclassical dynamics of open quantum systems: Comparing the finite with the infinite perspective.

Chemical Physics, In Press, Corrected Proofs:–, 2010, ISSN 0301-0104. [7] Koch, W., Lubk, A., Grossmann, F., Lichte, H., and Schmidt, R.

Coherent and incoherent effects on the imaging and scattering process in transmission electron microscopy and off-axis electron holography.

Ultramicroscopy, 110(11):1397 – 1403, 2010, ISSN 0304-3991.

Proceedings

[1] Koch, W. and Grossmann, F.

Non-Markovian thermalization in the Morse oscillator.

In D.V. Shalashilin and M.P. de Miranda, editors, Multidimensional Quantum Mechanics with Trajectories. CCP6, Daresbury Laboratory, 2009, ISBN 978-0-9545289-8-0.

Danke

Wenn ich die vergangenen drei Jahre Revue passieren lasse, fallen mir eine ganze Menge Menschen ein, die mir in dieser Zeit wichtig waren. Einigen davon m¨ochte ich an dieser Stelle im Speziellen meine Dankbarkeit aussprechen, da ohne sie ein erfolgreicher Abschluss dieser Arbeit kaum m¨oglich gewesen w¨are.

Vor drei Jahren habe ich als ambitionierter Handwerker angefangen, dieses Projekt zu bearbeiten. Frank Großmann hat wesentlich dazu beigetragen, dass ich es als Physiker zu Ende bringen konnte. Er hat immer Zeit gefunden, Fragen zu beantworten, interessante Vorschl¨age zu machen und generell Probleme aus dem Weg zu r¨aumen. Dar¨uber hinaus hat er den Besuch von Konferenzen und Workshops m¨oglich gemacht und durch seinen umfangreichen Bekanntenkreis bei diesen Veranstaltungen immer wieder neue Kontakte hergestellt. Schließlich w¨are ohne seine Erfahrung das Schreiben von Ver¨offentlichungen und nicht zuletzt dieser Arbeit bedeutend komplizierter geworden.

F¨ur die Einf¨uhrung in die Thematik und umfangreiche Diskussionen ¨uber Probleme, neue Ideen und sich ergebende Resultate m¨ochte ich J¨urgen Stockburger, Joachim Anker- hold und David Tannor danken. Die pers¨onlichen Unterhaltungen und l¨angeren email Ko- rrespondenzen haben sich aus meiner Sicht immer gelohnt. Auch f¨ur die Einladungen nach Ulm und Rehovot bin ich dankbar.

Professor R¨udiger Schmidt hat mir in seiner Arbeitsgruppe schon zum Diplom einen Platz gew¨ahrt. Ich bin sehr froh, dass er sich bereit erkl¨art hat, mir auch als Doktorvater zur Verf¨ugung zu stehen. Die Arbeit hier in der Gruppe habe ich immer als angenehm empfunden. Dass ich dabei nicht an dem Umgang mit den b¨urokratischen Notwendigkeiten gescheitert bin, liegt wohl auch daran dass Gundula Sch¨adlich meiner Naivit¨at den Wirren der institutionalisierten Forschung gegen¨uber immer mit einem L¨acheln begegnete. Dar¨uber hinaus haben die Arbeitsgruppenausfl¨uge, erhitzte Diskussionen beim Kaffeetrinken und andere nicht-physikalische Aktivit¨aten ihren Teil dazu beigetragen, dass der Kopf wieder frei wurde, um ihn sich ¨uber den zu l¨osenden Problemen zerbrechen zu k¨onnen. Außerdem m¨ochte ich mich bei allen f¨ur die Unterst¨utzung bei der Verbesserung des Stils dieser Arbeit bedanken.

In den vergangenen acht Jahren, w¨ahrend des Studiums und der Promotion habe ich an der Universit¨at und vor allem auch außerhalb viele neue Menschen kennengelernt. Einige davon haben mich fast die gesamte Zeit begleitet. Es ist beruhigend zu wissen, dass man nach einem Tag lang Gleichungsjonglieren nach Hause kommen kann und immer jemanden findet, der willens ist, sich ¨uber etwas anderes zu unterhalten, gemeinsam zu essen oder irgend etwas zu unternehmen. Ohne meine Freunde w¨aren die letzten Jahre viel weniger farbenfroh und viel einseitiger geworden. Ich bitte um Vergebung f¨ur den ¨Arger den ich stellenweise verursacht habe. Im Speziellen sch¨atze ich mich ausgesprochen gl¨ucklich, dass ich immer mit netten und, meinen teilweise weltfremden Gedanken gegen¨uber, toleranten Mitbewohnern zusammenleben durfte.

Noch viel l¨anger als Freunde habe ich eine Familie. Und auch dabei habe ich Gl¨uck gehabt. Eine Schwester zu haben ist was tolles. Man kann sich ¨uber Gott und die Welt unterhalten, sich ¨uber die Vergangenheit und die Zukunft auseinandersetzen. Egal wer gerade das Land verlassen hat, danach sind wir um Erfahrungen reicher aber auch weiterhin immer noch Geschwister. Meine Schwester hat mir nicht nur erkl¨art, dass es vielleicht keine so gute Idee ist, als Deutscher einen englischen Text verfassen zu wollen, sondern mir auch dabei geholfen, die Konsequenzen davon weitestgehend zu verbergen.

Tja und schließlich bin ich erst seit ein paar Jahren Physiker, aber seit fast drei Jahrzehn- ten Sohn. Im Prinzip habe ich alles was ich kann und weiß, in irgendeiner Form meinen Eltern zu verdanken. Sie haben mir beigebracht nach den Sternen zu greifen. Dass ich mich heute glaube Wissenschaftler nennen zu k¨onnen ist ein Resultat davon, dass sie nie ein kindliches Warum oder Wieso unbeantwortet gelassen haben und dass sie mir sp¨ater beigebracht haben, Antworten selber zu suchen. Sie haben mich immer unterst¨utzt und M¨oglichkeiten geschaffen, weiter zu kommen; vom Besuch der Manos ¨uber das Austausch- jahr in den USA bis zum Studium und dar¨uber hinaus. Egal wohin es mich in der Zukunft

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