Structure and Dynamics from Time Resolved Absorption and Raman Spectroscopy

Abstract

Spectroscopy is the study of interaction between electromagnetic radiation with matter. To understand the architecture of atoms and molecules we use many spectroscopic techniques. For example X-rays are used to determine crystal structure of molecules, vibrational spectroscopy is used to study molecular structures. When a photon is incident on a molecule, the electromagnetic field will distort the electron cloud (charge density) around the molecule. In that process it can be absorbed or scattered by the molecule giving rise to different spectroscopic processes. When the frequency of the incident field νi, matches with the frequency of the energetics of the molecule νm, resonance is established and electromagnetic radiation is absorbed. In UV-Visible spectroscopy the incident radiation promote the molecule from ground electronic state to higher electronic states and providing information about the electronic energy levels. The nature of chemical bonding in molecules can be probed by infrared or Raman spectroscopy. Shape of the electronic absorption band is associated with vibrational transitions coupled to the electronic excitation are functions of equilibrium bond length. To understand the vibrational structure in the electronic spectra of molecules we apply the Franck-Condon principle. It states that as the nuclei are so heavier than electrons, electronic transition takes place much faster than the nuclei can respond. As a result of Franck-Condon principle, the higher vibrational levels of various modes of the singlet electronic excited state are populated during electronic transition. The higher vibrational level of the singlet electronic excited states then relaxes to the ground vibrational level of the electronic excited state of same multiplicity by Internal Conversion (IC) or to different multiplicity (triplet) by Inter System Crossing (ISC). Generally emission starts from lowest electronic excited state to ground state and it is called Kasha’s rule, albeit some exceptions are there. The lowest singlet electronic excited state relaxes to ground state, either radiatively by fluorescence or non-radiatively by IC and similarly the triplet electronic excited state relaxes to ground electronic state, either radiatively by phosphorescence or non-radiatively by ISC. In many of the organic chemical reactions, the intermediate and the transient species involved are short-lived which could not be detected by steady state spectroscopy. The time resolved spectroscopy (TRS) could give us insight about the electronic and structural aspects of the intermediates/transient species involved in organic chemical reactions. Time resolved spectroscopy is basically a pump-probe experiment. The pump pulse initiates the reaction while the probe beam, probes the intermediates/transient species formed during the course of the photo-initiated chemical reaction. Here we discuss about the basics and some applications of time resolved resonance Raman spectroscopy (TR3s) and ultrafast Raman loss spectroscopy (URLS) to probe excited state dynamics.