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arXiv:0704.1921 · Physical Review A

Quantum molecular dynamics study of the pressure dependence of the ammonia inversion transition

Isabelle Herbauts, David J. Dunstan

Published Dec 14, 2007• 5 citations
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Abstract

The mechanism of the shift, broadening, and quenching of the ammonia inversion frequency with gas pressure has been a problem of lively interest for over $70\phantom{\rule{0.3em}{0ex}}\text{years}$. A simple quantum model of the ammonia molecule perturbed by collisions with ideal gas molecules displays the essential features of the experimental data for $\mathrm{N}{\mathrm{H}}_{3}$ and for $\mathrm{N}{\mathrm{D}}_{3}$. The model does not display the behavior expected from theories of environmental decoherence. On the other hand, models of perturbed classical oscillators do display behavior similar to our model and to experimental data. The quenching of the ammonia inversion transition cannot therefore be interpreted as spatial localization of the wave function.

DOI
10.1103/physreva.76.062506

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The mechanism of the shift, broadening, and quenching of the ammonia inversion frequency with gas pressure has been a problem of lively interest for over 70years70\phantom{\rule{0.3em}{0ex}}\text{years}70years. A simple quantum model of the ammonia molecule perturbed by collisions with ideal gas molecules displays the essential features of the experimental data for NH3\mathrm{N}{\mathrm{H}}_{3}NH3​ and for ND3\mathrm{N}{\mathrm{D}}_{3}ND3​. The model does not display the behavior expected from theories of environmental decoherence. On the other hand, models of perturbed classical oscillators do display behavior similar to our model and to experimental data. The quenching of the ammonia inversion transition cannot therefore be interpreted as spatial localization of the wave function.
The decoherence of a two-state tunneling molecule, such as a chiral molecule or ammonia, due to collisions with a buffer gas is analyzed in terms of a succession of quantum states of the molecule satisfying the conditions for a consistent family of histories. With $\ensuremath{\hbar}\ensuremath{\omega}$ the separation in energy of the levels in the isolated molecule and $\ensuremath{\gamma}$ a decoherence rate proportional to the rate of collisions, we find for $\ensuremath{\gamma}\ensuremath{\gg}\ensuremath{\omega}$ (strong decoherence) a consistent family in which the molecule flips randomly back and forth between the left- and right-handed chiral states in a stationary Markov process. For $\ensuremath{\gamma}<\ensuremath{\omega}$ there is a family in which the molecule oscillates continuously between the different chiral states, but with occasional random changes of phase, at a frequency that goes to zero at a phase transition $\ensuremath{\gamma}=\ensuremath{\omega}$. This transition is similar to the behavior of the inversion frequency of ammonia with increasing pressure, but will be difficult to observe in chiral molecules such as D2{}_{2}2​S2{}_{2}2​. There are additional consistent families both for $\ensuremath{\gamma}>\ensuremath{\omega}$ and for $\ensuremath{\gamma}<\ensuremath{\omega}$. In addition, we relate the speed with which chiral information is transferred to the environment to the rate of decrease of complementary types of information (e.g., parity information) remaining in the molecule itself.
We propose a model to describe a gas of pyramidal molecules interacting via dipole-dipole interactions. A cooperative effect induced by the interaction modifies the tunneling properties between the classical equilibrium configurations of the single molecule. The model suggests that, for sufficiently high gas density, the molecules become localized in these classical configurations. On this basis it is possible to explain the shift and the disappearance of the inversion line observed upon increase of the pressure in a gas of ammonia or deuterated ammonia. The same mechanism also accounts for the presence of stable optical activity of certain pyramidal molecules. We discuss the concept of environment induced superselection rule which has been invoked in connection with this problem.
I report the observation of population inversion by optical adiabatic rapid passage. These observations, on an NH3{\mathrm{NH}}_{3}NH3​ infrared transition with all the relevant parameters known, agree with theoretical expectations. The pressure dependence of T1{T}_{1}T1​, obtained by this technique, makes possible the first estimate of the collision-induced rotational lifetime of NH3{\mathrm{NH}}_{3}NH3​ in an excited band, which is significantly longer than the rotational lifetime in the ground vibrational band.
Purely quantum phenomena such as dynamical localization (DL) can be observed in a molecular rotor subjected to a periodic train of short and intense laser pulses in the gas phase. Using requantized Classical Molecular Dynamics Simulations, we investigate the impact of collisional effects for O2 molecules. We consider different trains of kicks for which the system exhibits quantum resonance, DL, or Bloch oscillations under collision-free conditions. Within achievable experimental conditions, we demonstrate that, as the gas pressure increases, all the dynamics become progressively similar, leading to a quantum-to-classical transition. We study how decoherence induced by intermolecular collisions reduces the quantum interferences through loss of memory. These findings pave the way for future experimental investigations and further theoretical studies on the influence of dissipation on quantum effects in rotational dynamics.