Hot Chemistry with Cold Molecules

Abstract

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