Picosecond absorption spectrometer

Investigation of excited states of polyatomic molecules in solution provides a good framework in order to study many classes of interactions. But their lifet ime is very short (10 s - 10 °s). So, in order to study such excited states we have built a picosecond absorption spectrometer which allowed us to study many types of molecular systems in solution. We present here two such studies. In the first one (1-4-diphenyl-butadiene) we have shown a solvent assisted electronic level inversion. In the second one we have studied interactions between an intramolecular charge transfer state and polar environment. Before discussing these results, we present the apparatus we have built in our lab. 

Techniques and equipment used to obtain data in this type of experiment are discussed and described in quite a number of papers published during the year. The generation and properties of ultrashort pulses has been clearly presented in an article published in American Scientist . An apparatus for carrying out pump-probe broad band spectroscopy by transient absorption in the subpicosecond region has been described in detail. This equipment has been used to observe the photodissociadon of bis-(p-aminophenyl) disulphide and also to show a biexponential frequency shift arising from solvent relaxation of the photo-generated p-aminophenyl thiyl radicals. Another picosecond time resolved absorption spectrometer system using a streak camera has been reported by Japanese workers . Okamoto and Yoshihara ... 

In RCs of Rb,capsulatus wild-type, mutation at amino acid residue TRP M250 was introduced by oligonucleotide-mediated mutagenesis, according to methods described in [4,5]. Quinone-titrations for determining the apparent dissociation constants in the mutant was performed as in [4]. Measurements of kg and the P Qi recombination rate k were done on RCs reconstituted with 2-methyl-1,4-naphthoquinone (NQ) in contrast to recombination studies on P H where the RCs were quinone-free. All experiments were performed on RCs in aqueous buffer/glycerol(60%) solution. Picosecond [10], nanosecond [6,7,11] and millisecond [12] transient absorption spectrometers were used for the measurements of kg, radical pair recombination parameters and k. . 

Recent developments in laser technology and fast detection methods now allow the kinetic behaviour of the excited state species arising from absorption of radiation by polymers to be studied on time-scales down to the picosecond region ( ). An example of a time-resolved fluorescence spectrometer which can be used to study such ultrafast phenomena is illustrated in Figure 5 Q). 

Figure 19.1. Schematic diagram of a general pump-probe-detect laser spectrometer suitable for picosecond electronic absorption, infrared (IR) absorption, Raman, optical calorimetry, and dichroism measurements. For picosecond fluorescence—a pump-detect method, no probe pulse needs to be generated.

More information about the sequential pathway versus one in which the 0—0 and C C bond homolyses occur conceitedly was obtained by means of picosecond IR absorption spectroscopy. While IR spectroscopy, in principle, can provide more structural information, hmitations stemming from the weakness of some IR intensities can be detrimental to the detection of a reactive intermediate. In this case, the CO2 photoproduct was monitored (near 2335 cm ) instead of radical intermediates 2 or 3. It was argued that neither 1, 2, nor 3 absorbed in this region. Excitation of 1 in CCLi was achieved with a 308-nm, 1-ps pulse. The 1.3-ps probe pulse may be tuned to cover a range from 2000 to 4300 cm with the spectrometer that was used, but in these specific experiments the interrogation window was from... 

Acridine has been studied by means of the echelon technique.(24,25) The changes in absorption (AA) were measured with a double beam picosecond spectrometer similar to the one shown in Figure 3. The difference absorption spectrum at one selected delay time after excitation was calculated according to... 

Using a mode-locked Nd + YAG laser system to generate picosecond sample excitation pulses and picosecond probing continuum pulses in their double beam spectrometer, Spalink et. al. (30) were able to measure difference absorption spectra of irradiated samples of 11-cis-rhodopsin and 9-cis-rhodopsin at selected times after excitation by means of a PAR OMA-2 optical multichannel detection system. The difference absorption spectral data were obtained over the entire spectral range from 410 nm to 650 nm at one time with an OMCD as opposed to the... 

Picosecond transient absorption was monitored on a spectrometer consisting of two dye lasers synchronously pumped by an actively mode-locked, actively Q-switched Nd-YAG laser (JK-AML 2000). Single pulses... 

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