Resonant uv pump-probe spectroscopy of dipicolinic acid via impulsive excitation

Abstract

We present experimental evidence of coherent wave packet motion in dipicolinic acid $({\mathrm{C}}_{7}{\mathrm{H}}_{5}\mathrm{N}{\mathrm{O}}_{4})$ which is an important marker molecule for bacterial spores. Resonant impulsive excitation is achieved by applying a uv pump pulse ($267\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, $16\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$) which has a duration that is shorter than the vibrational period of the molecules. The resulting dynamics is then probed with a weaker pulse of the same width and frequency. Evidence of the important ``fingerprint'' region for this molecule (between $1000\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ and $1500\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$) is found in the transient absorption of the probe. We present simulations of the pump-probe experiment, based on the Liouville equation for the density matrix, and predict the optimal pulse width and detuning.