Research Interests

Current Research: Coherently controlled chemical reactions at low temperature

The quantum mechanism of molecular interactions controls all chemical reactions. Therefore, one of the important goals in quantum chemistry is to understand and control molecular forces at the quantum level. To reach this goal, we perform scattering experiments that map out the interaction potential by correlating the quantum state of the incoming particles with the measured state of the outgoing particles. State resolved scattering experiments studying single collision events provide experimental access to these potentials because the probability of scattering within a differential solid angle is directly proportional to the square of the scattering matrix element between the input and output states. While the output states can be measured with great precision using available state of the art technology, controlling over input states poses the greatest challenge. Moreover, to obtain a good signal-to-noise ratio in a scattering experiment, it is essential that we prepare a large population in a precisely defined energy eigenstate.

Quantum state preparation in our lab

To meet the outstanding challenge of preparing a large molecular ensemble in a desired quantum state, we have invented a coherent optical technique called Stark-induced adiabatic Raman passage (SARP). SARP utilizes partially overlapping single mode pump and Stokes pulses of nanosecond duration with fluence of a few Jules/mm2. Because SARP is based on off-resonant Raman excitation, it can prepare a broad range of molecular eigenstates using commercially available laser sources. We have demonstrated SARP preparing a single vibrationally excited rotational eigenstate, defined by the quantum numbers (v, j), as well as the projection of the rotational angular momentum j onto a fixed axis, given by the m quantum number. The preparation of states with specific (j, m) defines the molecular frame with respect to a known axis in the lab frame and provides the stereodynamical control necessary to determine the anisotropy of scattering force fields.

[doi: 10.1063/1.3599711; 10.1063/1.4790402; 10.1063/1.4818526; 10.1063/1.4964938]

Cold Collisions

Having prepared molecules in a single quantum state with sufficiently high density to conduct a scattering experiment, we need to select the orbital angular momentum states of the incoming particle. Unless the collision energy is reduced substantially, the interference of a large number of orbital angular momentum states in the input channel obscures interesting features of the interaction potential. To gain the clearest possible understanding of the molecular force field, it is therefore necessary to restrict the number of involved input orbital angular momentum states by reducing the collision temperature. To accomplish cold collisions, we supersonically co-expand both of the colliding species in a single molecular beam. This brings the collision energy down near 1 K, limiting the partial wave components to s (l = 0) and p (l = 1).

Quantum Control

By combining SARP with the coexpansion of the colliding partners in the molecular beam, we are able to control both the internal quantum state (v, j, m) and partial waves of the incoming molecules. Our complete control of the input state coupled with our partial wave analysis method reveals molecular dynamics at the quantum level, and provides deep insight into the anisotropic potential responsible for the observed scattering.

Ongoing Experiment: Quantum Control of Molecular Scattering near 1 K

We study fundamental collision between state prepared isotopologues of H2 molecules with other atoms (He, Ar) or molecules (H2, D2, HD) in a supersonically co-expanded molecular beam. Laser pulses are introduced in the reaction region to prepare reactants in specific quantum states and to probe the products by state selective ionization. High resolution scattering angular distribution is extracted from the measurement of time-of-flight spectrum and velocity map imaging in a mass-spectrometer connected to our high vacuum reaction chamber. From our experimental measurements of scattering angular distribution we are able to extract detailed information about the scattering potential. By fitting the angular distribution using a partial wave analysis, we have been able to measure the four-vector correlation, namely, k, J, , , where k and are the initial and final velocities and J and are the initial and final rotational angular momenta of the quantum state prepared molecular species (HD).

Two different collision geometries: The quantum state of HD is prepared using SARP optical fields polarized (A) parallel (H-SARP) or (B) perpendicular (V-SARP) to the molecular beam axis. The green and red arrows define the polarization direction of the pump and Stokes optical fields. For H-SARP, the m state refers to the quantization z axis parallel to the relative velocity of HD and its scattering partner. For V-SARP, the m′ state refers to the quantization z′ axis perpendicular to the relative velocity. For H-SARP the HD bond axis is preferentially aligned parallel to the relative velocity, and for the V-SARP the HD bond axis is preferentially aligned perpendicular to the relative velocity. Because the scattering partner (either H2 or D2) is not state prepared, its bond axis is distributed isotropically.


Complete Quantum Control by Preparing Both Colliding Partners in Precisely Defined (v, J, m) Quantum States

To gain full quantum control, our immediate goal is to prepare both molecular species in addressable quantum states. This goal is accomplished using SARP with a sequence of a single pump pulse partially overlapping with a pair of Stokes pulses as shown below. Each sequence of pump and Stokes pulses prepares each species in the desired quantum state. Such collision experiments will lead to deeper understanding of the interaction that drives chemistry at the quantum level and, will allow us, for the first time, to measure entanglement in a bi-molecular collision.

SARP with a three-pulse sequence prepares collision partners (H2 and HCl) in specific vibrationally excited quantum states. The pump and Stokes 1 prepares HCl. The pump and Stokes 2 prepares H2

Quantum Interference in Scattering: A Molecular Interferometer

One of our long-sought goals is to study interference in molecular scattering by preparing target in superposition of quantum states using SARP with specific polarization of the pump and Stokes waves. The purpose of such interference experiment is to measure the relative phase of the scattering matrix. In essence we construct a molecular interferometer by preparing H2 in a coherent superposition of quantum states such as:

The scattering angular distribution from such a target will show interference arising from the entangled states within the superposition. We have already demonstrated the preparation of such a target state using SARP.

[doi: 10.1063/1.4865131]

Coherent SARP Ladder to Prepare Molecules in Highly Excited Vibrational Level.

We would like to prepare a molecule high up in the vibrational ladder and study molecular collisions or chemical reactions using such highly excited quantum state. We know intuitively that highly vibrationally excited species are more reactive. By preparing a hydrogen molecule near its dissociation limit, we want to ask this fundamental question: at what level of vibrational stretch, the H-atoms in a hydrogen molecule no longer feel bound to each other and are able to act independently in a collision with another partner? While such a collision experiment is inspiring, it is immensely difficult to prepare a single highly excited vibrational level. We have shown theoretically that using multicolor SARP ladder the complete population of the ground H2 (v=0) level can be transferred to the highest (v=14) vibrational level of H2 just below the dissociation limit. Such excitation can be further utilized to fully dissociate a H2 molecule generating a pair of cold entangled H-atoms.

Temporal dynamics of a four-color ladder SARP preparing H2 (v =14, J=0) via the intermediate H2 (v=4, J=0) and H2 (v=9, J=0). Four-color ladder SRAP is achieved by combing a single pump pulse with partially overlapping three different Stokes pulses. The three Stokes pulses are combined on the left wing of the stronger pump pulse with a delay of 7 ns as shown in the upper panel of the figure. The middle panel shows the dynamic detuning (GHz) of the three different Raman transitions associated with the three steps of the vibrational ladder. The lower panel shows the dynamics of adiabatic population flow through the various vibrational levels of the ladder. The lower panel shows complete population transfer from H2 (v=0) to H2 (v=14) leaving no population stranded in the intermediate levels.

We will soon set up a multicolor SARP experiment to prepare molecules in selected highly excited vibrational levels. This will enable us to coherently control very low temperature collisions and observe exotic geometrical phase effects.