Microcanonical thermodynamic properties of helium nanodroplets

Lehmann, Kevin K.

doi:10.1063/1.1588991

Cited by 16 publications

(10 citation statements)

References 22 publications

(34 reference statements)

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“…While ripplons have no density change, except for the surface, there is a hydrodynamic flow field that scales as (r/R(N)) L and thus becomes increasingly surface localized for higher L [87]. Ripplons are the lowest energy excitations of a nanodroplet and dominate its thermodynamics below 1 K [83,88].…”

Section: Helium Droplet Excitationsmentioning

confidence: 99%

Spectroscopy and dynamics in helium nanodroplets

Stienkemeier

Lehmann

2006

J. Phys. B: At. Mol. Opt. Phys.

397

555

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Abstract. This article provides a review of recent work in the field of helium nanodroplet spectroscopy with emphasis on the dynamical aspects of the interactions between molecules in helium as well as their interaction with this unique quantum solvent. Emphasis is placed on experimental methods and studies introducing recent new approaches, in particular including time-resolved techniques. Corresponding theoretical results on the energetics and dynamics of helium droplets are also discussed.

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Section: Helium Droplet Excitationsmentioning

confidence: 99%

Spectroscopy and dynamics in helium nanodroplets

Stienkemeier

Lehmann

2006

J. Phys. B: At. Mol. Opt. Phys.

397

555

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“…For some examples of such work, see Refs. [20][21][22][23][24][25][26][27][28][29][30][31][32][33]. The developments of helium-nanodroplet spectroscopy have been reviewed by several authors [34][35][36][37][38][39][40][41][42][43].…”

Section: Unique Features Of Helium-nanodroplet Spectroscopymentioning

confidence: 99%

Superfluid helium nanodroplets: 2006 Benjamin Franklin Medal in Physics presented to Giacinto Scoles and J. Peter Toennies

Szalewicz¹

2010

Journal of the Franklin Institute

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“…The form for ρ(n, E, J) for fixed n, E is that of a thermal distribution of a spherical rotor with a mean value of J(J + 1) that grows as E 8/7 . 11 For the initial values of n, E, the rms of the distribution of ρ(n, E, J) occurs for J = 345, an order of magnitude below the mean value of the angular momentum deposited in pickup of a large molecule, such as pentacene. It can be expected that during evaporation, the density of states will bias evaporation such that the J moves towards the rms value, which will result in the average angular momentum being reduced during evaporation.…”

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confidence: 97%

“…For droplets of the size and energy range of interest to experiments, the density of states is dominated by quantized capillary waves on the surface of the droplets ( ripplon modes). 4 Simple, but highly accurate, analytical approximations for N (n, E ′ , J ′ ) and ρ(n, E ′ , J ′ ) = (∂N/∂E) J ′ have recently been published, 11 thus allowing calculation of k(n, E, J). Using an ensemble that conserves n, E, J, the droplet temperature can be calculated using (k B T (n, E, J)) −1 = (∂ ln ρ(n, E, J)/∂E) J .…”

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confidence: 99%

“…For a canonical distribution at 0.38 K, the mean terminal value of E = 18.5 K and J(J + 1) = 39 are predicted for droplets of n = 10 4 . 11 Figure 1 shows a plot of droplet energy, angular momentum quantum number, temperature, and number of evaporated helium atoms as a function of time for five representative trajectories with initial condition E int = 1700. K and J int = 3000.…”

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confidence: 99%

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Evaporative Cooling of Helium Nanodroplets with Angular Momentum Conservation

Lehmann

Dokter

2004

Phys. Rev. Lett.

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Evaporative cooling of helium nanodroplets is studied with a statistical rate model that includes, for the first time, angular momentum conservation as a constraint on the accessible droplet states. It is found that while the final temperature of the droplets is almost identical to that previously predicted and later observed, the distribution of total droplet energy and angular momentum states is vastly more excited than a canonical distribution at the same temperature. It is found that the final angular momentum of the droplets is highly correlated with the initial direction, and that a significant fraction of the alignment of the total angular momentum should be transferred to the rotational angular momentum of an embedded molecule.The study of helium nanodroplets has received considerable attention in the past decade. 1,2,3 Such droplets rapidly cool by helium atom evaporation while they travel inside a vacuum chamber. Brink and Stringari 4 used a statistical evaporation model and predicted a terminal temperature of 4 He nanodroplets of 0.4 K, in excellent agreement with the value of 0.38 K later deduced from the rotational structure in vibrational spectra of SF 6 and other molecules. 5,6 Despite the obvious success of this theoretical work, the model used is clearly incomplete in that the constraint of angular momentum conservation was not imposed. The need for a more complete evaporative cooling study was made evident by the recent observation of a polarization anisotropy in the absorption spectrum of pentacene in helium droplets. 7 The authors of this work suggested that the total angular momentum deposited in the droplets by the pickup of a heavy molecule is aligned perpendicular to the droplet velocity, and that this droplet alignment survives the evaporative cooling and is transferred to the embedded molecule. The present study was undertaken to test the reasonableness of this conjecture.We model the evaporative cooling with a statistical rate approach, analogous to phase space theory in unimolecular dissociation, which explicitly includes the constraints of angular momentum conservation. 8 We use Monte Carlo sampling to follow cooling 'trajectories' as the droplets lose much of their initial energy and angular momentum by helium atom evaporation. It is found that the droplets cool to final temperatures close to those predicted without angular momentum constraints. 4 However, the distribution of terminal droplet states (where the evaporative cooling lifetime becomes longer than typical flight times in experiments) cover a vastly broader range of energy (E) and total angular momentum (J) than was previously expected. Further, it is found that the final angular momentum vector of the droplet is highly correlated with the initial value, such that much of the alignment remains, and that a sizable fraction of this alignment is transfered to an embedded rotor.Evaporative Cooling Model -Consider a helium nanodroplet, D, with initial values of the conserved quantities n (number of helium atoms), E ′ (total int...

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Microcanonical thermodynamic properties of helium nanodroplets

Cited by 16 publications

References 22 publications

Spectroscopy and dynamics in helium nanodroplets

Spectroscopy and dynamics in helium nanodroplets

Superfluid helium nanodroplets: 2006 Benjamin Franklin Medal in Physics presented to Giacinto Scoles and J. Peter Toennies

Evaporative Cooling of Helium Nanodroplets with Angular Momentum Conservation

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