Subpicosecond laser excitation

Pump-probe Spectroscopy with Subpicosecond Laser Excitation... 

The dynamics of the SHL intensity after subpicosecond UV laser excitation of RuC18B LB films is shown in Figure 32[115,116]. The SHL intensity decreased to 70 % of its initial value upon excitation and returned to almost the initial value within several hundred picoseconds as shown by a bold line. The fluorescence decay of RuC18B LB films measured by the single photon-counting... 

Figure 32 Temporal profile of relative SHL intensity from alternate LB films of RuC18B and 2C18NB with subpicosecond laser pulses at 378 nm bold curve (A) experimental data, fine curve (B) calculated SHL intensity based on the excited-state lifetime of RuC18B in LB films.

Figure 1. Diagram of the intensity / (W/cm2) vs. duration of laser pulse tp(s) with various regimes of interaction of the laser pulse with a condensed medium being indicated very qualitatively. At high-intensity and high-energy fluence 4> = rpI optical damage of the medium occurs. Coherent interaction takes place for subpicosecond pulses with tp < Ti, tivr. For low-eneigy fluence (4> < 0.001 J/cm2) the efficiency of laser excitation of molecules is very low (linear interaction range). As a result the experimental window for coherent control occupies the restricted area of this approximate diagram with flexible border lines.

Terahertz spectroscopy uses continuous wave (CW) and short pulsed laser excitation in the spectrum region between infrared and microwave frequencies. Pulsed laser excitation using pulse widths in the range of 10-100 femtoseconds has enabled the use of time-resolved terahertz spectroscopy, which is capable of capturing dynamic information at subpicosecond time scales. 

Low temperature experiments have shown the formation of hypso intermediates from several species [99,103,105-107]. The study of early photoconversion processes in squid [108], which also involved the evaluation of the relative quantum yields among the four pigments (squid rhodopsin, squid batho-, hypso- and isorhodopsin) showed that hypsorhodopsin is a common intermediate of rhodopsin and isorhodopsin there is no direct conversion between rhodopsin and isorhodopsin bathorhodopsin is not converted directly to hypsorhodopsin and both rhodopsin and isorhodopsin convert more efficiently to bathorhodopsin than to hypsorhodopsin. While a temperature dependence of the relaxation processes from the excited state of rhodopsin, and an assumption that batho could be formed from one of the high vibrational levels of the ground state hypso have been invoked to explain these findings [108], the final clarification of this matter awaits results from subpicosecond laser photolysis experiments at liquid helium temperature. 

The nature of the excitation has a profound influence on the subsequent relaxation of molecular Uquid systems, as the molecular dynamics simulations show. This influence can be exerted at field-on equiUbrium and in decay transients (the deexdtation effect). GrigoUni has shown that the effect of high-intensity excitation is to slow the time decay of the envelope of such oscillatory functions as the angular velocity autocorrelation function. The effect of high-intensity pulses is the same as that of ultrafast (subpicosecond laser) pulses. The computer simulation by Abbot and Oxtoby shows that... 

A spectrometric set-up to measure transient spectra in the subpicosecond range is shown in Fig. 12. Half of the amplified pulses from the subpicosecond laser (L) is split into a cell of H20. The residual pulses pass through a variable delay line and are focused to excite the sample (beam 2). The delay is automatically scanned by a stepping-motor-driven stage (M). The emerging continuum beam is split into two parts (beams 0 and 1). Both are focused into the sample (E) and after that on the entrance slit of a spectrograph (S). The resulting two dispersed spectra are recorded by an optical multichannel analyser (D). The description of the passively mode-... 

Texier, I. Delouis, J. F. Delaire, J. A. Gionnotti, C. Plaza, P. Martin, M. M. Dynamics of the first excited state of the decatungstate anion studied by subpicosecond laser spectroscopy. Chem. Phys. Lett. 1999, 311, 139-145. 

In order to treat these observations and hypotheses in a theoretical framework as successfully as the case of electron localization in helium, we must first probe the dynamical properties of the IR absorptions in the subpicosecond regime. What perhaps is surprising and stimulating for future studies is the wealth of microscopic details that can be obtained on intermolecular interactions and electron transfer in liquids through picosecond spectroscopy, information of fundamental interest to chemical dynamics in the condensed phase. In this vein, we will conclude this chapter by an example of photoselective chemistiy in electron transfer processes that occur following laser excitation of e in the cluster. 

The NO2 dissociation rate was measured by a two-color picosecond pump-probe method in which the product NO was monitored by LIF. Of particular significance in this study is that the NO2 density of states at the dissociation limit of 25,130.6 cm is relatively well established from an extrapolation of experimentally determined densities at an energy of 18,500 cm . This density (for cold samples where the rotations do not contribute significant densities) is 0.3 states per cm , (Miyawaki et al., 1993) which leads to a minimum rate constant l/h p( ) = 1 x 10 sec . The experimentally measured rate increases from 0 to 1.6 x 10 sec at the dissociation limit. It is interesting that the subpicosecond laser pulses with their transform limited resolution of about 20 cm do not excite individual NO2 resonance states (see section 8.3, p. 284) but, instead, prepare a superposition of those states that are optically accessible within the laser bandwidth. It is thought that all resonance states in this bandwidth are... 

Real-time experiments (Khundkar and Zewail, 1990 Zewail, 1991) with a subpicosecond resolution have probed the unimolecular dynamics of NO2 NO + O (Ionov et al., 1993a) and H + CO2 HOCO -> HO 4- CO (Scherer et al., 1987, 1990 Ionov et al., 1993b). The NO2 experiment is described and discussed in section 6.2.3.1 (p. 196). The H + CO2 reaction and ensuing formation of HOCO is initiated by photodissociation of HI in the HI—CO2 van der Waals complex (Fig. 8.8). A subpicosecond laser pulse is used to initiate the reaction while a second laser pulse probes the product formation. The reactants are vibrationally and rotationally cold prior to excitation, and the experiments demonstrate that the H + CO2 reaction proceeds... 

A. Andreev et al. Excitation and decay of low-lying nuclear states in a dense plasma produced by a subpicosecond laser pulse. J. Exp. Theor. Phys. 91, 1163 (2000)... 

Fig. 12.8 Diagram of radiation intensity I (W/cm ) versus laser pulse duration Tp (s), with the various laser-pulse-condensed-medium interaction regimes being indicated very qualitatively. At high radiation intensities I and energy fluences

There are two approaches for time-resolved RR spectroscopy of these retinal proteins. Using cw-excitation a time-resolution down to 100 ns can be achieved by rapidly moving the sample through the laser focus (rotating cell, capillar flow system). Pulsed laser excitation can provide a time-resolution even in the subpicosecond range depending on the... 

The dynamics of the interfacial electron-transfer between Dye 2 and TiOz were examined precisely by laser-induced ultrafast transient absorption spectroscopy. Durrant et al.38) employed subpicosecond transient absorption spectroscopy to study the rate of electron injection following optical excitation of Dye 2 adsorbed onto the surface of nanocrystalline Ti02 films. Detailed analysis indicates that the injection is at least biphasic, with ca. 50% occurring in <150 fsec (instrument response limited) and 50% in 1.2 0.2 psec. 

While such a device has yet to be constructed, Debreczeny and co-workers have synthesized and studied a linear D-A, -A2 triad suitable for implementation in such a device.11641 In this system, compound 6, a 4-aminonaphthalene monoimide (AN I) electron donor is excited selectively with 400 nm laser pulses. Electron transfer from the excited state of ANI to Ai, naphthalene-1,8 4,5-diimide (NI), occurs across a 2,5-dimethylphenyl bridge with x = 420 ps and a quantum yield of 0.95. The dynamics of charge separation and recombination in these systems have been well characterized.11651 Spontaneous charge shift to A2, pyromellitimide (PI), is thermodynamically uphill and does not occur. The mechanism for switching makes use of the large absorption cross-section of the NI- anion radical at 480 nm, (e = 28,300). A second laser pulse at 480 nm can selectively excite this chromophore and provide the necessary energy to move the electron from NI- to PI. These systems do not rely on electrochemical oxidation-reduction reactions at an electrode. Thus, switching occurs on a subpicosecond time scale. 

Pt(l 11) [6-8], Cu(l 1 1) [9] and Ag(l 1 1) [9], and CO fromPt(00 1) [10] andPt(l 1 1) [11,12]. On the other hand, these molecules are not desorbed from Ni and Pd metal surfaces in spite of the isoelectronic character of the metals Ni, Pd and Pt [13,14]. Desorption induced by subpicosecond-pulsed laser takes place via multiple correlated (and partially coherent) electronic transitions DIMET. DIMET is a very different mechanism from DIET [15-17] and in DIMET the vibrational excitation during the multiple electronic transitions leads to the desorption. Desorption via multiple vibrational transitions has also been observed using an infrared laser [18]. However, these topics are not described in this review. 

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