Femtosecond laser technology

V. S. Letokhov My answer to Prof. Quack is that it is indeed difficult to predict theoretically the effect of intense femtosecond IR pulses on the IVR rate of polyatomic molecules, which is important for the transfer of vibrationally excited molecules from low-lying states to the vibrational quasi-continuum. We are developing the relevant theoretical mechanisms of IR MP E/D of polyatomics since the discovery of this effect for isotopic molecules BC13 and SF6 in 1974-1975.1 hope that it will become more realistic to study experimentally the influence of intense IR pulses on IVR due to the great progress of femtosecond laser technology. 

A current description of femtosecond laser technology, with a discussion of ultrafast spectroscopic applications. 

With the advent of femtosecond laser technology [72], it is now possible to study, in real time, the dynamics of elementary chemical reactions. The first example of an application of this technique to a chemical dynamical problem comes from sub-picosecond experiments [73] on the photodissociation of ICN. In the most recent work [73b], a 40-fs pulse (k = 307 nm) excited the ICN molecule to a repulsive state and a delayed, second femtosecond pulse probed the CN fragment (via LIF) as a function of time. By varying the probe wavelength to the red of the free CN bandhead, they were able to observe fluorescence from the CN in the I-CN molecule in the process of falling apart. They found that the transition state lives for about four times the vibrational period of the I-CN bond. In this time interval there is negligible rotation of the parent ICN molecule. 

THz-TDS was initiated by the pioneering research on the emission of terahertz radiation by Auston et al. [5] and subsequent studies on the coherent detection of the terahertz radiation by van Exeter and Grischkowski [6], Arjavalingram et al. [7], and Nuss et al. [8], Since then, THz-TDS, enabled by the advent of femtosecond laser technology, has become a new spectrometric method covering the region from the millimeter wavelength to the far-infrared. 

Since most of the dissociation reaction of an isolated molecule occurs in a very short timescale ( 100 fsec), the direct observation of its transition from reagents to products has to wait until the discovery of femtosecond laser technology. With femtosecond wave packet spectroscopy, the trajectories of particles can be monitored during their motions on a potential energy surface. Zewail s group in 1989, for the first time, directly followed the evolution in space and time of such trajectories during the breakage of a chemical bond in the dissociation of sodium iodide. 

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