Two femtosecond laser pulses

In the last few years Nelson and co-workers [63-65] have presented a new approach to light scattering spectroscopy, named impulsive stimulated light scattering (ISS), which seems to be able to detect one particle rotational correlation functions. In ISS, one induces coherent vibrational motion by irradiating the sample with two femtosecond laser pulses, and... 

The second scheme of coherent control, proposed by D.J. Tanner and A. Rice, utilizes the time difference between two femtosecond laser pulses interacting with a molecule on two different transitions sharing a common level, which was illustrated in Sect. 6.4.4 by the example of the Na2 molecule. Here the phase of the wave packet produced by the first pulse in the excited state develops in time, and the controlled time lag between the first and the second pulse selects a favorable phase for further excitation or deexcitation of the molecule by the second pulse. 

Two femtosecond laser pulses were used in the experiment of FS TR SEP ED. One pulse was used as a pump source to excite the molecule to the electronically excited state, to generate fluorescence. The other, with a specific delay time, was used to perform SEP from the electronic excited states to the ground state. The variation of the decrease in the fluorescence induced by the SEP with delay time reflects vibrational relaxation in the excited states. A homemade regenerative amplified self-mode-locking Ti sapphire femtosecond laser, whose oscillator and amplifier... 

Figure 9.41 Potential energy curves for the two lowest electronic states of Nal showing avoided level crossing and the effect of excitation with a femtosecond laser pulse. (Reproduced, with permission, from Rose, T. S., Rosker, M. J. and Zewail, A. H., J. Chem. Phys., 91, 7415, 1989)...

A suitable method for a detailed investigation of stimulated emission and competing excited state absorption processes is the technique of transient absorption spectroscopy. Figure 10-2 shows a scheme of this technique. A strong femtosecond laser pulse (pump) is focused onto the sample. A second ultrashort laser pulse (probe) then interrogates the transmission changes due to the photoexcita-lions created by the pump pulse. The signal is recorded as a function of time delay between the two pulses. Therefore the dynamics of excited state absorption as... 

Even more elegantly, the local resolution is improved by irradiation with very intense focused femtosecond laser pulses outside the absorption range of the fluoro-phore (e.g., in the near-infrared). The very intense focus of the laser beam—and only this—will excite the fluorophore by nonresonant two-photon absorption. Artifacts by scattered primary radiation are ruled out and the local resolution is comparable to a confocal microscope. In addition, the damage of the sample by laser light absorption is reduced to a minimum. 

Meshulach, D., and Silberberg, Y. 1998. Coherent quantum control of two-photon transitions by a femtosecond laser pulse. Nature 396(6708) 239 2. 

For the development of the measurement methodology, it is anticipated that the same mechanism of O2 A ) generation occurs under both one- and two-photon excitation. This expectation is reasonable due to the very short duration of the femtosecond laser pulse used for two-photon excitation. Thus,... 

Figure 6.9 Generic five-state system for ultrafast efficient switching. The resonant two-state system of Figure 6.6 is extended by three target states for selective excitation. While the intermediate target state 4) is in exact two-photon resonance with the laser pulse, both outer target states 3) and 5) lie well outside the bandwidth of the two-photon spectrum. Therefore, these states are energetically inaccessible under weak-field excitation. Intense femtosecond laser pulses, however, utilize the resonant AC Stark effect to modify the energy landscape. As a result, new excitation pathways open up, enabling efficient population transfer to the outer target states as well.

In Section 6.3.2, we presented experimental data from strong-field excitation and ionization of K atoms with shaped femtosecond laser pulses. Here we give a description of the apparatus and strategy used in the experiments presented in this contribution. Figure 6.12 gives an overview over the complete experimental two-color setup. For the experiments on strong-field control of K atoms (cf Sections 6.3.2.2, 6.3.2.3, and 6.5) only the one-color beamline was used. An... 

The experiments were carried out in a high vacuum chamber where a beam of atomic potassium K (4s) intersects perpendicularly with the femtosecond laser pulses leading to photoionization. The released photoelectrons are detected employing a magnetic bottle time-of-flight electron spectrometer. The 785 nm, 30 fs FWHM laser pulses provided by an amplified 1 kHz Ti sapphire laser system are split into two beams using a Mach-Zehnder interferometer. In the first experiment the time delay r is varied in a range of 80 to 100 fs with 0.2 fs resolution at a... 

G. Gerber By applying two-photon ionization spectroscopy with tunable femtosecond laser pulses we recorded the absorption through intermediate resonances in cluster sizes Na with n = 3,. 21. The fragmentation channels and decay pattern vary not only for different cluster sizes but also for different resonances corresponding to a particular size n. This variation of r and the fragmentation channels cannot be explained by collective type processes (jellium model with surface plasmon excitation) but rather require molecular structure type calculations and considerations. 

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. 

Figure 3. Selective vibrational transitions OH(l>, = 0) - OH(ty = 5) and OH(u, = 5)->-OH(iy = 10) induced by two individual IR femtosecond/picosecond laser pulses. The electric fields c(i) and the population dynamics Pv(t) are shown in panels (a) and (b), respectively. Sequential combination of the two individual laser pulses yields the overall transition OH(u = 0) - OH(u = 5) - OH(u/ = 10) cf. Fig. 1 and Table I. For the isolated system, the population of the target state Pv= fo(t) is constant after the series of IR femtosecond/picosecond laser pulses, i > 1 ps.

The experiments demonstrate that femtosecond laser pulses offer new opportunities for multiple-photon ionization of bioorganic molecules on surface. The fast femtosecond excitation makes it possible to produce molecular and fragmentation ions directly on the surface being irradiated. The two-photon excitation with an intense femtosecond pulse allows the selectivity of ionization of the chromophore (tryptophan in our case) in large molecules... 

The chemical specificity of CARS microscopy is readily combined with other nonlinear optical image contrast mechanisms, such as two-photon fluorescence (TPF), SHG, and THG, resulting in a multimodal CARS microscopy [88, 118, 117, 43]. In multimodal nonlinear optical imaging, TPF, SHG, and THG signals all benefit from the use of femtosecond laser pulses of high peak intensities, whereas the contrast and chemical selectivity of CARS benefits from the use of picosecond (narrow-bandwidth) pulses (see discussion in Sect. 6.2.3). As demonstrated by Pegoraro et al. [43], this apparent... 

Bandrauk s long-term research interests include the dressed-state representation of molecular spectroscopy. His contributions to the nonperturba-tive treatment of molecular spectroscopy from the weak field to strong field limits have been summarized in two chapters in a book he edited in 1993.286 Bandrauk and his coworkers published the first theoretical demonstration of the use of chirped pulses to effect laser bond breaking in less than a picosecond.287 His other firsts include the first prediction of molecular stabilization in intense laser fields288 and the first complete non-Born-Oppenheimer calculation of dissociative ionization of molecules in intense femtosecond laser pulses.289... 

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. 

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