A Culmination of Stanford Chemistry’s Nandini Mukherjee’s Research Path

(Left to Right) Willie Perreault, Nandini Mukherjee, Haowen Zhou

Left to right: Willie Perreault, Nandini Mukherjee, and Haowen Zhou in lab

Nandini Mukherjee, researcher of 13 years in Stanford University’s Department of Chemistry, received Editor’s Choice for her recent ACS Publication, “Quantum-Controlled Collisions of H2 Molecules.”

Bringing a Big Dream to Stanford

Before starting at Stanford University, Nandini Mukherjee was a physicist at University of California, Los Angeles where she had the opportunity to build a special laser, called a quantum cascade laser, which operates in the range of infrared to terahertz. These wavelengths are able to excite vibrations and rotations in molecules. “Slowly, I started trying to see if these coherent optical techniques could be applied…When you go for dancing, you put on a certain dress and are looking pretty, and people are attracted, and you dance. I started thinking, how can I dress up the molecules with lasers to entice them in a quantum dance, thus tweaking their chemical bond?” She hoped to accelerate chemical reactions or stretch and break bonds – then get more sophisticated to be able to begin to control the chemical reaction.

After three and a half years at UCLA, Mukherjee realized she needed a chemistry lab. Around that same time, her son graduated high school and planned to attend UC Berkeley in the Bay Area. He told his mother she needed to come to the Bay Area with him. “I was very excited. I mean, he doesn’t say these things anymore,” Mukherjee said with a laugh. She and her spouse quit their jobs and began looking towards life in the Bay Area.

She contacted several labs in the Bay Area to discuss her ideas and set up tours, one of which was Stanford Chemistry’s Professor Richard Zare’s lab. In a phone call with Zare, she told him her dream was to control chemical reactions at the finest level and that she had the ability to do so, though it had not been done before.

Research at Stanford

Upon arrival to the Zare lab, Mukherjee saw that his group was doing collisions with hydrogen molecules, however, the molecules were not quantum state controlled – until then, controlling quantum states of H2 was a notoriously difficult challenge. She noted, “This is very important because H2 is the most theoretically tractable molecule that makes the ideal test bed for high level quantum chemistry computations. So, I was determined to find a way to control the quantum states for H2.” Mukherjee first mathematically showed the possibility of Stark-induced adiabatic Raman passage (SARP) as a powerful control process preparing single or superposition of quantum states. The work was published as a theoretical paper in The Journal of Chemical Physics.

Photo of Nandini Mukherjee and Willie Perreault
Nandini Mukherjee and Willie Perreault in lab

Mukherjee had a team of researchers she provided mentorship to. Previous Stanford Chemistry graduate student and postdoctoral scholar Willie Perreault worked with Mukherjee for 9 years and remembers his initial meeting with Mukherjee: “She spent 90 minutes explaining everything to me and I was really excited about it. She is just so incredibly brilliant and animated, and she loved her science so much – I think that came through immediately and only deepened my excitement.”

Mukherjee and her team were later able to demonstrate SARP experimentally after a long wait to purchase and receive exotic laser equipment. She explains, “Using two or more partially overlapping laser pulses, SARP not only prepares a large population (reacting agents) in a single or superposition of quantum states, it also fixes the geometry of collisions at the quantum scale by aligning molecular axes. Because the molecular forces, the electrostatic forces of the electrons and nuclei, are inherently anisotropic, to correctly map or understand these interaction forces one must orient the molecules in a specific way. Moreover, SARP prepares the coherent superposition states…Indeed, by preparing such non-classical states of H2 molecules, for the first time we were able to demonstrate the quantum interference in molecular collisions.” This work came out in Science and was also highlighted by Chemistry World and Physics Today.

Mukherjee mentioned that SARP has been “the game changer” because of its ability to control both the internal quantum states and the geometry of a collision at the same time. “SARP provides the field-free control – that means it dresses up the molecules and leaves them alone, so you can use the prepared sample for reaction without requiring the lasers to stay on during their interactions. You see, if the intense lasers were needed to stay on as in many other techniques, the intense electric fields of the lasers could also perturb the collision process, but surely we don't want that. We do not want to disturb their dance. Indeed, once SARP dresses up the molecules, they are free to dance in their own style. Without SARP, we could not have captured the quantum dance of the molecules!”

From there, Mukherjee and her team controlled various interactions at the quantum scale capturing two molecules intimately “talking” with each other. Mukherjee said, “If I produce a hydrogen molecule in a quantum superposition of two states, then from these two states there will be a probability of reaction via each of these two states. The situation is very similar to that existing in a double-slit interferometer where two possible indistinguishable paths interfere. Then, you should be able to show effect of quantum interference in molecular collisions, and even control it by changing the relative phase of the two states in the superposition.” This work resulted in a Science publication in 2021.

She remarked, “Imagine being able to implement your dream into action. This was that moment. This dream I had for decades; I remember saying this on the phone to Dick all those years ago that this was my dream… What we have done in Stanford Chemistry is we have shown the world that there are a lot of subtle quantum effects in the molecular level chemistry that we are just beginning to understand.” “Our research has opened many new possibilities for experimental quantum chemistry which are not limited to just H2 molecules,” she adds.

A Doubly Fulfilled Dream

The research demanding the fine control of quantum states required a very sophisticated experimental platform involving a large number (eleven) of high-power frequency stabilized and time synchronized laser systems. “I have been completely absorbed developing this new and exciting field of research, quantum controlled cold collisions, working 24/7 for over 14 years,” says Mukherjee, “and now a break is needed.”

In 2021, she initially turned down the requests to write an ACS article due to her heavy engagement in the laboratory. One of her unnamed but “rather popular” friends and colleagues was upset that she wouldn’t write the article and pointed out that, “If I can make time to write, why can’t you?” Needless to say, Mukherjee started on the ACS article shortly thereafter. The article, a culmination of her incredible work, was hugely successful and earned the ACS Editor’s Choice. Mukherjee remarked, “If I can inspire young researchers’ minds and excitement in quantum chemistry, that is an additional accomplishment. If this article can do that, I have doubly fulfilled my dream.”

Perreault also remarked, “It was an honor to spend so much time learning from her, not only because she is such an exceptional physicist with an unparalleled understanding of the workings of the quantum mechanical world, but also because she was a deeply caring mentor who took great pains to make sure that I understood every aspect of the research project.”

When asked what Mukherjee is going to do next, her response was, “I think my husband deserves a vacation with me. Then, I will definitely be back. I have this crazy passion for science.”