Cold Chemistry: Rotationally Inelastic Collisions of HD(v=1,j=2) with H2 and D2

In our lab, the goal is to understand experimentally molecular forces at the quantum level.

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Apr 23, 2018
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The paper in Nature Chemistry is here: https://go.nature.com/2qMTml1

In our lab, the goal is to understand experimentally molecular forces at the quantum level. Experiments performed with thermal molecules are unable to obtain the detailed quantum mechanical information because of averaging over many states. We perform fully quantum state resolved scattering experiments on simple, theoretically tractable molecules to directly access as much information about the scattering potential as possible. To achieve this, we need to control fully the input state, which is experimentally challenging. The difficulty here is twofold, we must select with the greatest possible precision both the internal quantum state and the orbital angular momentum state defining the relative motion of the two interacting bodies.

Although simple molecules such as H2 are ideally suited for theoretical investigation, it is particularly difficult to prepare a large ensemble of them in a precisely defined internal quantum state. To confront this challenge, we developed a new coherent optical technique, known as Stark induced adiabatic Raman passage (SARP), which is capable of producing large numbers of simple molecules in specific quantum states.1,2 SARP’s ability to generate high densities of excited molecules is integral for scattering experiments, where signal-to-noise ratios often barely exceed unity. Our first step toward reaching our goal of complete quantum control of molecular scattering was to show that SARP, using a pair of single-mode, nanosecond laser pulses, is able to transfer nearly 100% of the ground state population of H2 to a single rotational vibrational quantum state while simultaneously controlling the orientation of the bond axis in space.3,4

However, control over the internal quantum states of the input molecules is not sufficient for complete understanding of the scattering potential. We must also define the input angular momentum and energy arising from the relative motion of the colliding partners. In our lab, we accomplish this by supersonically co-expanding the two colliding species in the same molecular beam, which achieves low collision temperatures (0-5 K) without sacrificing molecular density.5 With this technique in hand, we are able to observe inelastic collisions with high angular and state resolution. The angular resolution is particular important for stereodynamic experiments, which are necessary to understand the angle-dependent portions of the interaction potential. 

Having developed the experimental tools necessary for full quantum-state control of molecular scattering, we also needed an analytical method capable of extracting information on the scattering potential directly from our experimental data. Using partial wave analysis based on the conservation of angular momentum, we are able to determine the dominant outgoing orbital angular momentum states, as well as the interaction potentials responsible for coupling these outgoing states with the input states. In our paper in the current issue of Nature Chemistry, as well as in our recent work in Science,6 we have shown the power of this analytical technique when coupled with complete quantum control over molecular scattering to elucidate experimentally the inner dynamics of molecular interactions.

In our recent scattering experiments, HD molecules were prepared in the rovibrationally excited (v = 1, j = 2, m) state. While even this state would have been inaccessible without SARP, our aim in the future is to conduct scattering experiments using even more exotic states. We have recently demonstrated that SARP can directly prepare HD molecules in the (v = 4) with near complete population transfer from the ground state.7 Additionally, we have shown theoretically that a SARP ladder can be used to prepare very highly excited vibrational levels close to the dissociation limit.8,9 We feel that access to these states at scattering-relevant densities will open a completely new frontier in the study of quantum chemistry.

References

1.        Mukherjee, N. & Zare, R. N. Can stimulated Raman pumping cause large population transfers in isolated molecules. J. Chem. Phys. 135, 1–7 (2011).

2.        Mukherjee, N. & Zare, R. N. Stark-induced adiabatic Raman passage for preparing polarized molecules. J. Chem. Phys. 135, 1–10 (2011).

3.        Dong, W., Mukherjee, N. & Zare, R. N. Optical preparation of H2 rovibrational levels with almost complete population transfer. J. Chem. Phys. 139, 74204 (2013).

4.        Mukherjee, N., Dong, W. & Zare, R. N. Coherent superposition of M-states in a single rovibrational level of H2 by Stark-induced adiabatic Raman passage. J. Chem. Phys. 140, 74201 (2014).

5.        Perreault, W. E., Mukherjee, N. & Zare, R. N. Supersonic beams of mixed gases: A method for studying cold collisions. Chem. Phys. (2018). doi:10.1016/j.chemphys.2018.02.017

6.        Perreault, W. E., Mukherjee, N. & Zare, R. N. Quantum control of molecular collisions at 1 kelvin. Science. 358, 356–359 (2017).

7.        Perreault, W. E., Mukherjee, N. & Zare, R. N. Preparation of a selected high vibrational energy level of isolated molecules. J. Chem. Phys. 145, 154203 (2016).

8.        Mukherjee, N., Perreault, W. E. & Zare, R. N. Stark-induced adiabatic Raman ladder for preparing highly vibrationally excited quantum states of molecular hydrogen. J. Phys. B At. Mol. Opt. Phys. 50, 144005 (2017).

9.        Mukherjee, N., Perreault, W, E., & Zare, R. N. Stark-Induced Adiabatic Passage Processes to Selectively Prepare Vibrationally Excited Single and Superposition of Quantum States, Chapter 1, pp. 1- 46 in Frontiers and Advances in Molecular Spectroscopy, J. Laane, ed. Elsevier, Amsterdam, Netherlands (2017).

Go to the profile of Richard N. Zare

Richard N. Zare

Marguerite Blake Wilbur Professor of Natural Philosophy, Stanford University

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