Femtosecond activation

Femtosecond activation of reactions and the concept of nonergodic molecules. Science 279, 847-851. 

Kim S K, Guo J, Baskin J S and Zewail A H 1996 Femtosecond chemically activated reactions concept of nonstatistical activation at high thermal energies J. Phys. Chem. 100 9202-5... 

Formaldonitrone, CH2=N(H)—O (3), the elusive simplest organic nitrone, has been prepared transiently in the gas phase by femtosecond collisional neutralization of its cation radical, CH2—N(H)—0+". The latter was generated by dissociative ionization of 1,2-oxazolidine. Nitrone 3 showed negligible dissociation upon collisional neutralization and was distinguished from its tautomers formaldoxime 2 and nitrosomethane 1 that gave different NR mass spectra. The enthalpy of formation was calculated from enthalpies of atomization and two isodesmic reactions as Af//29s(3) = 58 1 kJmol . The calculated, large activation barriers for isomerization of 3 (179 and 212 kJmoH for 3 anti-2 and 3 1, respectivelyindicate that once 3 is formed and diluted in the gas phase it should not isomerize unimolecularly to either 1 or (syn/anti) 2. 

Molecular hydrogen has a vibrational period of only 7.6 fs, while vibrational modes involving heavier atoms are slower. The motions of atoms in activated molecules, the various vibrations and rotations leading to chemical reactions, will generally take place over time periods of 10 to 100 or 1000 fs, the femtosecond time scale. One femtosecond is 10 s femton is Swedish for 15, making the femtosecond unit, the next beyond the nicely classical miUi-, micro-, nano-, picosecond sequence, easy to remember. 

This experimental work on the dissociation of excited Nal clearly demonstrated behavior one could describe with the vocabulary and concepts of classical motions.The incoherent ensemble of molecules just before photoexcitation with a femtosecond laser pump pulse was transformed through the excitation into a coherent superposition of states, a wave packet that evolved as though it represented a single vibrationally activated molecule. 

The new generations of experiments are aimed at linking dynamical studies of these and other processes to the function. We have already begun research in this direction. In a recent publication [9] we reported studies of the femtosecond dynamics of an RNA-protein complex and then compared the results with those obtained for in vivo (E. Coli) transcription anti-termination activities. In two other studies we measured the activity of the protein Subtilisin Carlsberg, discussed above, to a substrate, and the role of hydration in interfacial binding and function of bovine pancreatic phospholipase at a substrate site. The goal in all these studies is to relate structures to the dynamics and hopefully to key features of the (complex ) function. 

In studying this system, the first femtosecond pulse takes the ion pair M+X to the covalent branch of the MX potential at a separation of 2.7 A. The activated complexes [MX], following their coherent preparation, increase their intemuclear separation and ultimately transform into the ionic [M+ X ] form. With a series of pulses delayed in time from the first one the nuclear motion through the transition state and all the way to the final M + X products can be followed. The probe pulse examines the system at an absorption frequency corresponding to either the complex [M X] or the free atom M. 

Figure 13. Femtosecond dynamics of dissociation (Nal) reaction. Bottom Experimental observations of wavepacket motion, made by detection of the activated complexes [Nal] or the free Na atoms. Top Potential energy curves (left) and the exact quantum calculations (right) showing the wavepacket as it changes in time and space. The corresponding changes in bond character are also noted covalent (at 160 fs), covalent/ionic (at 500 fs), ionic (at 700 fs), and back to covalent (at 1.3 ps).

Active control of population transfer using the control relation displayed in Eq. (5.23) has been demonstrated experimentally by Sherer et al. [18]. In this experiment gaseous I2 was irradiated with two short (femtosecond) laser pulses the first pulse transfers population from the ground-state potential-energy surface to the excited-state potential-energy surface, thereby creating an instantaneous transition dipole moment. The instantaneous transition dipole moment is modulated by the molecular vibration on the excited-state surface. At the proper instant, when the instantaneous transition dipole moment expectation value is maximized, a second pulse is applied. The direction of population transfer is then controlled by changing the phase of the second pulse relative to that of the first pulse. 

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