Tapas Goswami

Probing the Ultrafast Solution Dynamics of a Cyanine Dye in an Organic Solvent Interfaced with Water

Dependence of ultrafast dynamics on the excited state evolution and ground state recovery of a cyanine dye (IR125) in dichloromethane (DCM) solvent interfaced with neat water is presented. We use degenerate pump-probe transient absorption spectroscopy to show that the excited-state dynamics of the dye molecule is strongly dependent on the position of the measurements from bulk DCM solution to the solution near the water layer. The decay component of the transient corresponding to the excited state lifetime increases from bulk DCM solution to its interface with water. Such results show that the effect of the presence of water layer over the dye solution in DCM extends several micrometers, indicating the surfactant nature of the IR125 molecules, and provides us a measure of the penetration of water into the DCM layer. The initial ultrafast decay component (coherent spike) directly correlates to the pulse-width of our near-transform limited pulses used in these experiments. This approach of measuring the excited state decay of a dye across an immiscible liquid interface can provide important characteristics of microtransport across such interfaces.

Hot Chemistry with Cold Molecules

Control of chemical reactions that focuses on the selective cleavage or on the formation of chemical bonds in a polyatomic molecule is a long-sought-after goal for chemists. Attempts have been made since the early days of lasers (1960) to make this dream come true (Rousseau, 1966). In most of the experiments, selectivity is lost because of the rapid intramolecular vibrational energy redistribution (IVR) which occurs on picosecond time scales. This simply leads to heated molecules. Supersonic molecular beam techniques have proven to be an excellent method for producing isolated cold molecules in the gas phase, where the molecules are in their lowest rotational and vibrational states and as a result several relaxation rates like the collisional and the IVR rates are much slower (Smalley et al., 1977; Levy, 1981). By combining ultrafast laser technology with supersonic molecular beam technique in a novel way, several control schemes known as ‘coherent control’, have been proposed that make use of the coherent nature of laser radiation (Zewail, 1980; Bloembergen & Zewail, 1984). Furthermore, study of control is typically pursued in molecular beams in order to isolate the elementary processes to be studied from surrounding solvent perturbations. In a chemistry laboratory, however, the conventional control that we generally use in increasing the yield of the desired products in the chemical synthesis are macroscopic variables, such as, the temperature, pressure, concentration, etc. Sometimes catalysts are also used to control the chemical reactions. But the methods involved in this type of conventional cases are based on the incoherent collision between collision and we cannot get direct access to the quantum mechanical reaction pathway. On the other hand, in the quantum control of chemical reactions, molecular dynamics involved can be altered by specifically designed external light fields with different control parameters, namely, the intensity, phase, frequency, and polarization, which can vary with time. Using such methods, one can reach a user-defined chemical reaction channel more selectively and efficiently (Brixner & Gerber, 2003). The short temporal duration of ultrafast laser pulses results in a very broad spectrum (Fig.1). The output of our amplified laser system has pulse duration around 50 femtosecond and spectral bandwidth of around 18 nm. Possibilities of manipulating such an ultrafast coherent broadband laser pulses have brought forth the exciting field of ultrafast pulse shaping. Pulse shaping involves the control over amplitude, phase, frequency, and or inter-pulse separation (Goswami, 2003). Control of chemical reactions by laser can have various applications in diverse industrial and biological or

Control of femtosecond laser driven retro-Diels-Alder-like reaction of dicyclopentadiene

Using femtosecond time resolved degenerate pump-probe mass spectrometry coupled with simple linearly chirped frequency modulated pulse, we elucidate that the dynamics of retro-Diels-Alder reaction of diclopentadiene (DCPD) to cyclopentadiene (CPD) in supersonic molecular beam occurs in ultrafast time scale. Negatively chirped pulse enhances the ion yield of CPD, as compared to positively chirped pulse. This indicates that by changing the frequency (chirp) of the laser pulse we can control the ion yield of a chemical reaction.