Laser subpicosecond

However, time-resolved X-ray diffraction remains a young science. It is still impossible, or is at least very difficult, to attain time scales below to a picosecond. General characteristics of subpicosecond X-ray diffraction and absorption are hardly understood. To progress in this direction, free electron laser X-ray sources are actually under construction subject to heavy financial constraints. Nevertheless, this field is exceptionally promising. Working therein is a challenge for everybody ... 

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.

Electrons have not been detected by optical absorption in alkanes in which the mobility is greater than 10 cm /Vs. For example, Gillis et al. [82] report seeing no infrared absorption in pulse-irradiated liquid methane at 93 K. This is not surprising since the electron mobility in methane is 500 cm /Vs [81] and trapping does not occur. Geminately recombining electrons have, however, been detected by IR absorption in 2,2,4-trimethyl-pentane in a subpicosecond laser pulse experiment [83]. The drift mobility in this alkane is 6.5 cm /Vs, and the quasi-free mobility, as measured by the Hall mobility, is 22 cm /Vs (see Sec. 6). Thus the electron is trapped two-thirds of the time. 

The subpicosecond pulse radiolysis [74,77] detects the optical absorption of short-lived intermediates in the time region of subpicoseconds by using a so-called stroboscopic technique as described in Sec. 10.2.2 ( History of Picosecond and Subpicosecosecond Pulse Radiolysis ). The short-lived intermediates produced in a sample by an electron pulse are detected by measuring the optical absorption using a very short probe light (a femtosecond laser in our system). The time profile of the optical absorption can be obtained by changing the delay between the electron pulse and the probe light. 

Figure 11 The components of the timing jitter of the laser synchronized subpicosecond pulse radiolysis, crj is the length of the electron pulse (rms), c,- is the length of the probe light (rms), and ctj is the timing fluctuation (rms).

Initial laser pulse generation is achieved with the use of a twin-tube excimer laser in which one channel is a XeCl laser oscillator dehvering 15-ns, 308-nm, 80-mJ pulses for the driving of two dye lasers needed for difference frequency generation. The second channel is used for amphfication of subpicosecond 308-nm pulses that become pump pulses. 

M solutions of 6HQ (Kodak) in distilled, deionized water containing distilled perchloric acid (Aldrich 99.999%) or potassium hydroxide pellets (Aldrich 99.99%) were circulating in a flow cell (2.5 mm optical path length). The subpicosecond transient absorption experiment has been already described [5]. The polarizations of the laser pump (266 nm) and continuum probe pulses were set at the magic angle (overall time resolution 300 fs). 

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.

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. 

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