by Alex Liu
Over the past eight weeks, I had the opportunity to intern in Professor Yamanouchi’s laboratory at the University of Tokyo Hongo Campus. I would like to give a huge thank you to FUTI for providing the financial assistance needed to make this dream a reality, as I never expected to be able to do research abroad in Tokyo for such a period.
As Professor Yamanouchi’s research largely focuses on using intense laser pulses to precisely extract the energy levels of the quantum eigenstates of various atoms or small molecules, I decided to try and determine the spin-orbit coupling for the Kr⁺ cation. Since Kr⁺ has a lone unpaired electron orbiting the nucleus, the spin and orbital angular momenta can couple with each other, causing the ground state to split into two energy levels. However, because the transition between these two states is a forbidden one-photon transition,previous methods to determine this energy difference have only yielded values with fairly large uncertainties; my goal was to significantly improve the accuracy and precision of this value.
At first, I was a bit nervous about this idea because
I didn’t know much about quantum mechanics. While my inorganic chemistry class did touch upon a few topics, the ideas of spin-orbit coupling or quantum eigenstates were still not very clear to me and I wasn’t sure whether I would be able to proceed. Thankfully, the other lab members were very kind and always willing to explain confusing topics to me, and I was soon able to understand, appreciate, and carry out my research.
My research can be briefly summarized as follows. Using a first, linearly-polarized “pump” laser pulse, some Kr atoms will have an electron ejected from the pz orbital and be ionized to the Kr⁺ cation. This causes the cation to be prepared in an excited state (<ml=0, ms=±1∕2∣), where it oscillates between that state and another excited state (<ml=±1, ms=∓1∕2∣). Since these are both a superposition of the two lowest-energy Kr⁺ eigenstates, the oscillation period is exactly the quantum beat between those two states, or the spin-orbit coupling that we are trying to measure. Following a time delay (varied between 0 and 530 ps), a second, linearly-polarized “probe” pulse is then introduced, further exciting the Kr⁺ cation into the Kr2⁺ dication. The yield of this dication is then measured with respect to the time delay, and since the two excited states have differing ionization probabilities, the yield also oscillates with this quantum beat period. After taking the Fourier transform of this spectrum, we were then able to observe a sharp peak at this precise spin-orbit coupling energy. Careful calibration and robust analysis of our data then allowed us to resolve this energy with a much higher precision than previously reported (at least two orders of magnitude higher). I also explored the difference between the various Kr isotopes (83Kr⁺ is especially interesting because it has nuclear spin, which creates hyperne splitting in the energy levels) as well as the spin-orbit coupling of Kr2⁺, which I hope to publish in a paper soon.
However, these eight weeks in Tokyo consisted of more than just research. I also joined our lab’s “unofficial” softball team and participated in a friendly tournament among all the Hongo campus labs (we got third overall!). During our lunch time and other periods of free time, I frequently talked to other lab members, both in English and in Japanese. On the weekends, I was also able to explore many different areas of Tokyo. Aside from visiting “touristy” locations such as Asakusa’s Sensoji Temple or the Ueno Zoo in Ueno Park, I also had a chance to walk along the quieter streets of Iriya, eat at local restaurants with staff that barely spoke English (which certainly tested my Japanese skills), and gain a better sense and appreciation of Japanese culture.
Furthermore, during the long weekend of 海の日, one of my new friends from lab invited me over to his home—it was in a quiet neighborhood around 20 minutes from both Yokohama and Kamakura—and offered to show me around Kamakura. And while Kamakura’s Big Buddha statue and Yokohama’s Sankeien Gardens were amazing, my true highlight was my first real experience of Japanese hospitality. My friend’s parents were extremely welcoming, cooked two amazing meals, took us to their local community festival, and even invited me to use their Japanese-style bath. As this was my first time using a Japanese-style bath (or any bath, period, in at least 15 years), I felt a bit uneasy at first, but the moment I entered the bath, all my worries and aches melted away into the hot water. It was so soothing and relaxing! Needless to say, I will never forget their warm smiles and courtesy, and it was unfortunate that I had to return to my apartment in Tokyo so soon. Next time I come to Japan, I’ll be sure to visit them again!
Overall, my experiences in Tokyo over the past two months have been truly amazing, and I would love to come back here next year as an intern again. Lastly, I would like to once again thank FUTI for allowing me to come here, and I am excited to hear more about the experiences of the other scholarship recipients as well!