Stilbenes picosecond lasers

Platinum porphyrin complexes can be prepared by reaction with PtCl2(PhCN)2. Purification of the final complex is by medium pressure liquid chromatography on alumina. The strongly phosphorescent platinum(II) porphyrin complexes are efficient sensitizers for stilbene isomerization. The quantum yields for the cis to trans process are greater than unity because of a quantum chain process in which the metalloporphyrin serves both as an energy donor and an acceptor.1110 Picosecond laser spectroscopy has been used to obtain time-resolved excited-state spectra of platinum octaethylporphyrin complexes, and to probe the excited-state energy levels.1111 Tetrabenzoporphyrin complexes have been prepared for platinum in both the divalent and tetravalent oxidation states. The divalent complex shows strong phosphorescence at 745 nm.1112... 

Let us mention the rotational beats which have recently been observed (with a trans-stilbene molecule in a gasodynamic jet) and interpreted by Felker and Zewail [54, 145]. The essence of this interesting phenomenon may be described in a simplified way as follows. Let a picosecond laser pulse be capable of coherently exciting from some rotational state J f of a... 

A different type of environmental effect is manifested in the photoisomerization of trans-stilbene complexed to cyclodextrin. In picosecond laser studies a single decay in a-cyclodextrin shows binding occurs at one site, whilst in the (3-forra... 

Fluorescence Lifetimes and Picosecond Dynamics. The fluorescence lifetime (ts) of fran.s -stilbenes at room temperature is rather short. A value of approximately lOOps may be estimated from radiative rate constant ( r) using ts = f//rr [295]. Direct lifetime measurements with picosecond laser pulses confirm this finding (Table 14). Important progress in understanding of the excited state dynamics was achieved in several laboratories [314-375], especially by Yoshihara [314 319], Hochstrasser [262, 320 335], Fleming [343-348], Troe [352 355], Zewail [361-364] and their co-workers. Saltiel and Sun [28] extensively discussed the literature con-... 

Here, /lfexp —(E(/RT) and k0 are rate constants for activated and nonac-tivated decay steps of t, respectively. For stilbene, halogenated, and some other stilbenes (4-R CN, CH3, OCH3) the activated step has been assigned to twisting in the excited singlet state [27, 105, 114, 357, 358], whereas for nitrostilbenes intersystem crossing is involved in the activated step [31]. At low temperatures lifetimes of 1-2 ns have been reported for several trans-stilbenes [314, 337, 342], Below — 150°C an enhanced frans-stilbene was excited by a 532-nm picosecond laser pulse, in addition to a 266-nm pulse [317]. 

Picosecond laser measurements with trans-stilbene in solution (and even in the gas phase) agree in the respect that the fluorescence decay is single exponential (Table 14). Earlier results assuming a slow and a fast fluorescence decay component [376] have not been confirmed in at least four different laboratories [314, 321-323, 342, 349]. Because of the monoexponential decay, the back reaction 1 p —> 11 has to be much slower than the forward reaction t -. Elowever, the adiabatic formation of t from c indicates that the rate constants of both processes in solution have the same order of magnitude, independent of temperature. The low efficiency, %0.3%, of the p -> t process reflects very rapid p - p decay rather than a low p energy [81]. 

The characterization of the laser pulse widths can be done with commercial autocorrelators or by a variety of other methods that can be found in the ultrafast laser literature. " For example, we have found it convenient to find time zero delay between the pump and probe laser beams in picosecond TR experiments by using fluorescence depletion of trans-stilbene. In this method, the time zero was ascertained by varying the optical delay between the pump and probe beams to a position where the depletion of the stilbene fluorescence was halfway to the maximum fluorescence depletion by the probe laser. The accuracy of the time zero measurement was estimated to be +0.5ps for 1.5ps laser pulses. A typical cross correlation time between the pump and probe pulses can also be measured by the fluorescence depletion method. 

This Synchroscan [68] streak camera system has been used to study the time resolved fluorescence of trans-stilbene in the picosecond time regime. The experimental arrangement [69] is shown in Fig. 20. An acousto-optically mode-locked argon ion laser (Spectra Physics 164), modulated at 69.55 MHz was used to pump a dye laser. The fundamental of this dye laser, formed by mirrors M, M2, M3 and M4, was tunable from 565 to 630 nm using Rhodamine 6G and second harmonic output was available by doubling in an ADP crystal placed intracavity at the focal point of mirrors M5 and M6. The peak output power of this laser in the ultraviolet was 0.35W for a 2ps pulse which, when focused into the quartz sample cell of lens L, produced a typical power density of 10 KW cm-2. Fluorescence was collected at 90° to the incident beam and focused onto the streak camera photocathode with lens L3. The fluorescence was also passed through a polarizer and a bandpass filter whose maximum transmission corresponded to the peak of the trans-stilbene fluorescence. 

Two types of time resolved experiments can be carried out on such a system. The less ambiguous approach is that employed in the stilbene—He study (fig. 10.5) in which a second laser, time delayed from the pump laser, is used to excited the state. Smith and Knee (1993) have used picosecond time resolved TPES spectroscopy in which zero-energy electrons are collected as a function of the probe laser wavelength. [Laser based TPES has also been called zero kinetic energy (ZEKE) electron spectroscopy or pulsed field ionization (PFI) (Muller-Dethlefs and Schlag, 1991)]. Figure... 

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