Molecules, ultrafast fragmentation

This type of ultrafast dynamics will now be investigated in detail for several alkali aggregates, beginning with the model molecule Nas excited to its C state (Sect. 4.1) and D state (Sect. 4.2). The ultrafast fragmentation of sodium clusters (Sect. 4.3) and potassium clusters (Sect. 4.4) rounds off these studies. 

Following a description of femtosecond lasers, the remainder of this chapter concentrates on the nuclear dynamics of molecules exposed to ultrafast laser radiation rather than electronic effects, in order to try to understand how molecules fragment and collide on a femtosecond time scale. Of special interest in molecular physics are the critical, intermediate stages of the overall time evolution, where the rapidly changing forces within ephemeral molecular configurations govern the flow of energy and matter. 

Figure 1.3. Real-time femtosecond spectroscopy of molecules can be described in terms of optical transitions excited by ultrafast laser pulses between potential energy curves which indicate how different energy states of a molecule vary with interatomic distances. The example shown here is for the dissociation of iodine bromide (IBr). An initial pump laser excites a vertical transition from the potential curve of the lowest (ground) electronic state Vg to an excited state Vj. The fragmentation of IBr to form I + Br is described by quantum theory in terms of a wavepacket which either oscillates between the extremes of or crosses over onto the steeply repulsive potential V[ leading to dissociation, as indicated by the two arrows. These motions are monitored in the time domain by simultaneous absorption of two probe-pulse photons which, in this case, ionise the dissociating molecule.

Figure 6.2 Steering of photochemical reactions by coherent control of ultrafast electron dynamics in molecules by shaped femtosecond laser pulses. Ultrafast excitation of electronic target states in molecules launches distinct nuclear dynamics, which eventually lead to specific outcomes of the photochemical reaction. The ability to switch efficiently between different electronic target channels, optimally achieved by turning only a single control knob on the control field, provides an enhanced flexibility in the triggering of photochemical events, such as fragmentation, excited state vibration, and isomerization.

Hankin et al. [133] demonstrated femtosecond ionization following 266 nm desorption of solid samples of trinitrobenzene (TNB), TNT, and trinitrophenol (TNP). They confirmed the advantages of ultrafast ionization, namely, the formation of characteristic precursor and structure-specific fragment ions. The optimum intensities for efficient LD without ionization were determined for the compounds studied. Differences between femtosecond ionization of vapor samples of explosives [131,132] and laser desorbed molecules were also discussed. 

Fig. 1.1. Schematic view of the Coulomb explosion imaging of nuclear dynamics. Molecules exposed to an intense laser field undergo structural deformation in response to the formation of light-dressed potential energy surfaces, and decompose into fragment ions after multiple ionization. Since the momentum vectors of fragment ions sensitively reflect the geometrical structure just before the Coulomb explosion, the ultrafast nuclear dynamics of a molecule in an intense laser field can be elucidated through measurements of the momenta of fragment ions...

Fig. 3 shows the DNB mass spectrum for using femtosecond laser pulses. Throughout our studies, the use of ultrafast ionization produces mass spectra with fragmentation patterns characteristic of individual molecules being investigated. In addition, the presence of the parent molecular ion improves the prospects for molecule-specific identification. 

The exploration of ultrafast molecular and cluster dynamics addressed herein unveiled novel facets of the analysis and control of ultrafast processes in clusters, which prevail on the femtosecond time scale of nuclear motion. Have we reached the temporal boarders of fundamental processes in chemical physics Ultrafast molecular and cluster dynamics is not limited on the time scale of the motion of nuclei, but is currently extended to the realm of electron dynamics [321]. Characteristic time scales for electron dynamics roughly involve the period of electron motion in atomic or molecular systems, which is characterized by x 1 a.u. (of time) = 24 attoseconds. Accordingly, the time scales for molecular and cluster dynamics are reduced (again ) by about three orders of magnitude from femtosecond nuclear dynamics to attosecond electron dynamics. Novel developments in the realm of electron dynamics of molecules in molecular clusters pertain to the coupling of clusters to ultraintense laser fields (peak intensity I = lO -lO W cm [322], where intracluster fragmentation and response of a nanoplasma occurs on the time scale of 100 attoseconds to femtoseconds [323]. 

Farmanara P, Stert V, Radloff W (1998) Ultrafast internal conversion and fragmentation in electronically excited C2H4 and C2H3CI molecules. Chem Phys Lett 288 518... 

The benzene radical cation represents another intermediate-size molecule where the vibronic-coupling concepts described in the present article have been fruitfully applied. Experimentally, it has been characterized by various types of photoelectron spectroscopy displaying significant vibronic stmcture. In addition, the investigation of the fluorescence and fragmentation dynamics has provided evidence of ultrafast, i.e., sub-picosecond, internal-conversion processes. Both phenomena are addressed in the following two subsections. 

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