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Volumn 125, Issue 11, 2006, Pages

The molecular origins of nonlinear response in solute energy relaxation: The example of high-energy rotational relaxation

Author keywords

[No Author keywords available]

Indexed keywords

INTERNAL ENERGY; LINEAR-RESPONSE BEHAVIOR; ROTATIONAL RELAXATION; SOLVENT FLUCTUATIONS;

EID: 33749017062     PISSN: 00219606     EISSN: None     Source Type: Journal    
DOI: 10.1063/1.2336780     Document Type: Article
Times cited : (44)

References (66)
  • 23
    • 33749008352 scopus 로고    scopus 로고
    • note
    • The simplest (and for the purposes of this paper, the most general) derivations of linear response for solvation dynamics choose the final-state solute Hamiltonian (and its associated dynamics) as the reference system for a perturbation expansion. In this analysis, it is the fluctuations about the final state that should match the nonequilibrium response. See Ref. 2. The overall response seen in simulations agrees quite well with this expectation. However, as one would expect, it takes a finite amount of time for these fluctuations to evolve. Hence when the fluctuations about the two solute states do differ, it is common to find that the initial solvation response is governed by the initial-state fluctuations. See Refs. 4 and 5. As to the larger issue of what constitutes a violation of linear-response theory, perhaps a useful way to reconcile the various definitions would be to note that one can either define it narrowly or broadly starting from the observation that linear-response predictions are obeyed exactly if the liquid's fluctuations obey Gaussian statistics. The narrowest definition assumes that these Gaussian statistics are an intrinsic property of the liquid and thus have to be independent of the solute. We shall take the some-what broader perspective that the dynamics is still linear if we find different, but still Gaussian, solvent statistics with our different solute states. Geissler and Chandler (Ref. 23) have pointed out that this perspective is equivalent to simply allowing for nonstationary Gaussian statistics.
  • 27
    • 33749024576 scopus 로고    scopus 로고
    • note
    • Our motivation for this work consists mainly of solute relaxation examples in which the liquid dynamics is accurately described by classical mechanics. Accordingly, although we have described our systems as being prepared in specific "eigenstates," from this point on we will only be concerned with the equivalent classical problem of a system prepared with a precisely defined value of some component of its energy. We should also point out that our analytical approach of representing the initial preparation by saying that the average solute energy is fixed at some nonequilibrium value is actually a better match to the experimental distributions used in Ref. 25 than a delta-function initial distribution would have been. Our approach would also probably be well suited to treating solvation and vibrational relaxation experiments initiated by electronically exciting a solute. However, it might be less appropriate for modeling the vibrational relaxation that occurs from a single excited vibrational state.
  • 50
    • 33748989389 scopus 로고    scopus 로고
    • note
    • This bond length was chosen to mimic a vibrationally excited state of CN and is about 8% larger than the experimental ground-state bond length of 1.172 A.
  • 51
    • 33749001306 scopus 로고    scopus 로고
    • note
    • There is, of course, an entropie driving force for our kinetic energy relaxation. A more precise statement would be that there is no driving force in configuration space - no direction defined by a potential of mean force.
  • 52
    • 33748996063 scopus 로고    scopus 로고
    • note
    • These in-plane radial distribution functions are described in detail in Ref. 25 (in the Supporting Online Material). Briefly, these functions are stan
  • 53
    • 33749000703 scopus 로고    scopus 로고
    • note
    • dard N-Ar radial distribution functions, except that they exclude those Ar atoms lying within 60° of the axis defined by the instantaneous angular momentum vector. This criterion corresponds to including exactly half of the argon atoms in an infinite system, so the radial distribution functions are normalized to have an asymptotic value of 0.5. That the structural transitions we observe largely do take place within the instantaneous rotational plane is supported by the fact that radial distribution functions with no in-plane restrictions (not shown) exhibit a much less pronounced structural transition.
  • 54
    • 0000703221 scopus 로고    scopus 로고
    • The central role of midsection solvent atoms in coupling to solute rotation is discussed by J. Jang, Ph.D. thesis, Brown University, 2000
    • J. Jang and R. M. Stratt, J. Chem. Phys. 112, 7538 (2000). The central role of midsection solvent atoms in coupling to solute rotation is discussed by J. Jang, Ph.D. thesis, Brown University, 2000.
    • (2000) J. Chem. Phys. , vol.112 , pp. 7538
    • Jang, J.1    Stratt, R.M.2
  • 61
    • 33749013408 scopus 로고    scopus 로고
    • note
    • NN correlation function is the "true" friction in a generalized-Langevin-equation sense; we are simply noting that it represents a plausible way to generalize the friction concept to the nonlinear-response regime and has sensible limiting behavior when linear response is obeyed.
  • 62
    • 33748988308 scopus 로고    scopus 로고
    • note
    • As we have been emphasizing, nonstationary behavior of an arbitrary correlation function does not automatically imply nonlinear response. However, the friction correlation function plays a special role in mediating the energy relaxation.
  • 63
    • 33749033759 scopus 로고    scopus 로고
    • note
    • Schwartz and co-workers (Refs. 12-15) also have an extensive discussion of this issue in the context of solvation dynamics. But since those authors construed linear response in the narrow sense of Ref. 22 - their comparisons are between the nonequilibrium dynamics and the equilibrium fluctuations on the ground (rather than excited) state - their analysis is somewhat different from ours.


* 이 정보는 Elsevier사의 SCOPUS DB에서 KISTI가 분석하여 추출한 것입니다.