Femtosecond monitoring laser pulse

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.

In conclusion, although the increased propensity for photodamage by femtosecond pulses and the requirement for an additional delayed laser pulse can be disadvantageous, time-resolved CARS microspectroscopy not only provides a means for efficient and complete nonresonant background suppression but also offers the prospect for monitoring ultrafast processes of molecular species inside a sub-femtoliter sample volume [64, 152-154]. 

This approach has the potential to resolve the time evolution of reactions at the surface and to capture short-lived reaction intermediates. As illustrated in Figure 3.23, a typical pump-probe approach uses surface- and molecule-specific spectroscopies. An intense femtosecond laser pulse, the pump pulse, starts a reaction of adsorbed molecules at a surface. The resulting changes in the electronic or vibrational properties of the adsorbate-substrate complex are monitored at later times by a second ultrashort probe pulse. This probe beam can exploit a wide range of spectroscopic techniques, including IR spectroscopy, SHG and infrared reflection-adsorption spectroscopy (IRAS). 

Osorio et al. [134] performed TOF-MS measurements of TNT and RDX on soil surfaces. They used tunable UV radiation from a 130 fs laser to monitor the kinetic energy distribution of N0/N02 photofragments released by the dissociation of TNT and RDX. Analysis of the kinetic energy distribution of the photofragments revealed differences in the processes for NO and NOz ejection in different substrates. Mullen et al. [135] detected triacetone triperoxide (TATP) by laser photoionization. Mass spectra in two time regimes were acquired using nanosecond (5 ns) laser pulses at 266 and 355 nm and femtosecond (130 fs) laser pulses at 795, 500, and 325 nm. The major difference observed between the two time regimes was the detection of the parent molecular ion when femtosecond laser pulses were employed. 

In Zewail s laboratory a strong laser flash of a few femtoseconds duration shines on beams of molecules streaming into a vacuum chamber. The laser beam is tuned to excite all of the molecules to the same state where they are vibrating in unison. Subsequent, weaker laser pulses monitor the concentrations of the reactants, intermediates, and products as the reaction occurs. 

Fig. 4. Rise time of a 4 GPa shock in a thin film of A1 generated by a femtosecond laser pulse. The open squares are the experimentally measured phase shift of interference fringes generated by a pair of femtosecond probe pulses monitoring shock breakout at the free A1 surface. The solid circles are the data corrected for changes in the optical properties of Al. The shock front rise time tr = 6.25 ps. Reproduced with permission from ref. [32].

Transient grating spectroscopy is relatively easily handled compared with the transient absorption spectroscopy, and is often used to study carrier dynamics at semiconductor electrodes [32]. Figure 14 schematically shows the principle of transient grating spectroscopy. A femtosecond laser pulse for sample excitation is split into two beams, which are crossed again at the semiconductor surface to produce an optical striped interference pattern. The interference pattern produces a striped pattern of the densities of photo-generated electrons and holes near the semiconductor surface. The latter striped pattern gives rise to a striped pattern of optical refractive index near the semiconductor surface, which is monitored by measuring a diffraction pattern of a second probe laser... 

With a second laser electronic transitions in the neutral atoms or the ions are excited and the laser-induced fluorescence is monitored for specific known excitation lines. With a time gate in the detector system the spectra can be taken at different times and therefore different temperatures of the expanding plasma. The intensity of the LIF gives information about the atomic composition of the evaporated material and the abundance of the excited species, if the transition probabilities are known [198, 199]. The sensitivity of the technique depends on the peak intensity and the pulse duration of the excitation laser pulses. Typical laser pulse widths range from nanoseconds to picoseconds. Recently also femtosecond lasers have been used. Since only a tiny amount of material (nanograms to picograms) is evaporated the sample is not essentially damaged by this purely optical analysis [200]. 

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