TA & FSRS
Unveiling the geometry of transient species and thereby corroborating the mechanism of a chemical reaction continues to be the choice goal of every chemist. Among the plethora of experimental techniques that we have now in the wake of advancing technology in creating short femtosecond pulses, novel methods have proven successful in this mission. Femtosecond Stimulated Raman Spectroscopy (FSRS) is one such novel method dominating in spectral resolution in both time and frequency domains. Our team has also developed this technique and captures the "SNAPSHOTS" during the making and breaking of chemical bonds during a reaction’s course. This method provides a convenient window to directly view the geometry of a transition state and thus aides in getting an unambiguous reaction mechanism.

With this technique, we can obtain dynamic information in sub-picosecond time resolution coupled with spectral resolution in a few tens of cm-1 simultaneously. Hence it is genuinely a state-of-the-art technique working in the limit of the "Heisenberg Uncertainty Principle".

The vibrational information obtained by FSRS with the above mentioned resolutions enables us to make real-time movies of the chemical reactions
In this set up, we use three different femtosecond pulses. With the interplay of the three pulses, we carry out FSRS (Femtosecond Transient Absorption Spectroscopy and Stimulated Raman Spectroscopy). In case of FSRS, one pulse (actinic pump) initiates the reaction in the desired system and the other two pulses (Visible and continuum) are used to observe the stimulated Raman gain and loss signals emanating from the system. The details of FSRS are given above and the other two experiments are briefly introduced below:
a. Femtosecond Transient Absorption Spectroscopy (TA)
TA is a complementary technique to time resolved fluorescence method. Since absorption is a more general property, all molecules do not exhibit fluorescence, TA spectroscopy has wider scopes in studying the dynamics of the excited state of a chromophore. Several intra-molecular physical vibrational energies flow in condensed phase decay channels of the excited state of a fluorophore. Other photophysical properties can be studied in sub-picosecond time scale using this spectroscopic technique.
Here we use one pulse (UV/Vis) for excitation and then use continuum light.
b. Stimulated Raman Spectroscopy (SRS):
SRS is a combination of Raman scattering and stimulated emission. The intensity of the inelastic scattered light is extremely low such that the Raman signal is very weak. With the combination of stimulated emission, the signal intensity can be enhanced. This technique has wide application. It is carried out using the above set-up.
1. Seung Min Jin, Young Jong Lee, Jongwan Yu, and Seong Keun Kim, "Development of Femtosecond Stimulated Raman Spectroscopy: Stimulated Raman Gain via Elimination of Cross Phase Modulation", Bull.Kor.Chem.Soc. 25, 1829-1832 (2004).

2. Jaeyun Kim, Sungjin Park, Ji Eun Lee, Seung Min Jin, Jung Hee Lee, In Su Lee, Il Seung Yang, Jun-Sung Kim, Seong Keun Kim, Myung-Haing Cho, and Taeghwan Hyeon, "Designed Fabrication of Multifunctional Magnetic Gold Nanoshells and Their Applications to Immunotargeted Magnetic Resonance Imaging and Rapid Noninvasive Photothermal Therapy", Angew.Chem.Int.Ed. 45, 7754-7758 (2006).
2PPE
Electron transfer across interfaces is of fundamental importance in many areas of physics, chemistry and biology ranging from electronic devices to surface reactions to photosynthesis. A very critical issue common to all these fields is the phenomena of carrier transport across the interface.
On a microscopic level the dynamics of charge transfer across the interface is governed by the strength of the electronic coupling between the molecule and the underlying substrate. This problem is closely related to surface photochemistry which is often believed to be driven by hot electron transfer to the adsorbate as shown in our photochemical study of phenol on Ag(111).
We have investigated the photochemical problem on metal surfaces. Molecular electronic structure and its geometry can be altered by adsorption or when the adsorbate is excited, the substrate presents additional efficient pathways for energy exchange and dissipation. On the other hand, the substrate itself can efficiently absorb the incident photon energy; the dissipation of the captured energy can cause chemical changes in the adsorbed molecules. The two-photon photoemission(2PPE) spectroscopy is used to characterize the transient anionic state involved in photodissociation of molecules adsorbed on a metal surface.
These experiments are carried out in a 3-stage UHV chamber with a base pressure of ~10-11 Torr. The chamber is equipped with low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) to investigate the surface long range order and surface composition or surface cleanliness, respectively. An ion sputter gun is used for sample cleaning. Two quadropole mass spectrometers (QMS) are installed for temperature programmend desorption (TPD) and laser induced thermal desorption (LITD). Finally, an electron time-of-flight (e-TOF) spectrometer is installed for 2PPE spectroscopy.
In the 2PPE process, the first photon excites an electron from the initial state (Ei) to an unoccupied intermediate state (Em). Subsequently, a second photon ionizes the electron to the final state (Ef) above the vacuum level. By analyzing the kinetic energy of the photoemitted electron, the energetics of the relevant state can be elucidated. Compared to conventional photoemission and inverse photoemission spectroscopy, 2PPE spectroscopy can probe both the occupied state below the Fermi level and the unoccupied state above the Fermi level. In addition, time-resolved experiments on the dynamics of the excited electrons is possible by using ultrashort laser pulses.
In an angle-resolved manner, 2ppE can determine the effective mass of the electronic state parallel to the surface by analyzing the angle dependence of the kinetic energy of the photoelectron.
Currently, we are designing novel surface material compositions and structures to probe energetic alignment with barrier at the interface for light harvesting in next generation photovoltaics.

FT-IR
FT-IR spectrometer is well known for analyzing the vibrational spectrum to identify the structure of organic molecules including biomaterials, and to determine the intra- or intermolecular interactions in a reaction pathway. The basic principle based on the molecular absorption for a specific vibration is powerful for not only native species but also for isotope exchange and labeled species with IR probes. Recently, progressive developments beyond the Fourier Transform technique with an interferometer can be used to achieve a more sensitive and efficient way to do experiments than previously. Our team has been working on a time domain FT-IR spectrometer with a step-scan method that can measure up to several nanoseconds. Briefly, the step-scan method is one of the temporal scan modes that uses a preamplifier. The other is a rapid scan mode with a microsecond time scale as its limit using an air-floating type moving mirror system.

Time resolved FT-IR spectroscopy can be applied to the stop the flow injection method (microsecond to millisecond), and can be used to measure fast dynamic reactions with a pulsed laser system (nanosecond to microsecond). To connect with a spectroscopic system, we are modifying the FT-IR system to optimize the sample injection stage for a rapid scan spectrometer. The theme of our project is focused on understanding the biological phenomena in a cell, especially the protein misfolding pathway and cellular interaction in drug delivery.

Figure FT-IR Spectrometer (Vertex 70) and their microscope (Hyperion 1000 system)

Single Molecule Spectroscopy
Super-resolution Optical & Nano-optical Nanoscopy
Real-time Femtosecond Dynamics