  Ultrafast laser pump-probe spectroscopy is an important and growing field of
physical chemistry that allows the measurement of chemical dynamics on their
natural timescales, but undergraduate laboratory courses lack examples of such
spectroscopy and the interpretation of the dynamics that occur. Here we develop
and implement an ultrafast pump probe spectroscopy experiment for the
undergraduate physical chemistry laboratory course at the University of
California Berkeley. The goal of the experiment is to expose students to
concepts in solid-state chemistry and ultrafast spectroscopy via classic
coherent phonon dynamics principles developed by researchers over multiple
decades. The experiment utilizes a modern high-repetition-rate 800 nm
femtosecond Ti:Sapphire laser, split pulses with a variable time delay, and
sensitive detection of transient reflectivity signals using the lock-in
technique. The experiment involves minimal intervention from students and is
therefore easy and safe to implement in the laboratory. Students first perform
an intensity autocorrelation measurement on the femtosecond laser pulses to
obtain their temporal duration. Then, students measure the pump-probe
reflectivity of a single-crystal antimony sample to determine the period of
coherent phonon oscillations initiated by an ultrafast pulse excitation, which
is analyzed by fitting to a sine wave. Students who completed the experiment
in-person obtained good experimental results, and students who took the course
remotely due to the COVID-19 pandemic were provided with the data they would
have obtained during the experiment to analyze. Evaluation of student written
and oral reports reveals that the learning goals were met, and that students
gained an appreciation for the field of ultrafast laser-induced chemistry.
