Transient spectroscopy picosecond lasers

Transient spectroscopy experiments were performed with a pump-probe spectrometer [7] based on a home-made original femtosecond Ti saphire pulsed oscillator and a regenerative amplifier system operated at 10 Hz repetition rate. The Tirsaphire master oscillator was synchronously pumped with doubled output of feedback controlled mode-locked picosecond pulsed Nd YAG laser. The pulse width and energy of Ti saphire system after the amplifier were ca. 150 fs and 0.5 mJ, respectively, tunable over the spectral range of 760-820 nm. The fundamental output of the Ti saphire system (790 nm output wavelength was set for present study) splitted into two beams in the ratio 1 4. The more intense beam passed through a controlled delay line and was utilized for sample... 

Picosecond spectroscopy enables one to observe ultrafast events in great detail as a reaction evolves. Most picosecond laser systems currently rely on optical multichannel detectors (OMCDs) as a means by which spectra of transient species and states are recorded and their formation and decay kinetics measured. In this paper, we describe some early optical detection methods used to obtain picosecond spectroscopic data. Also we present examples of the application of picosecond absorption and emission spectroscopy to such mechanistic problems as the photodissociation of haloaromatic compounds, the visual transduction process, and inter-molecular photoinitiated electron transfer. 

Picosecond spectroscopy provides a means of studying ultrafast events which occur in physical, chemical, and biological processes. Several types of laser systems are currently available which possess time resolution ranging from less than one picosecond to several picoseconds. These systems can be used to observe transient states and species involved in a reaction and to measure their formation and decay kinetics by means of picosecond absorption, emission and Raman spectroscopy. Technological advances in lasers and optical detection systems have permitted an increasing number of photochemical reactions to be studied in. greater detail than was previously possible. Several recent reviews (1-4) have been written which describe these picosecond laser systems and several applications of them... 

Conclusive evidence for the cation intermediate was obtained by detection of the absorption spectra of several diarylmethyl cations following nanosecond or picosecond laser flash photolysis of their respective diazo, diphenylazi-ridinylimine, or 3H-indazole precursors in acidic media [107-112], Photochemically generated vinyl carbenes were recently shown to similarly protonate by deuterium labeling experiments to give allylic cations that were detected by transient absorption spectroscopy [113],... 

In the systems for femto-picosecond transient spectroscopy it is used a special detection technique, known as pump and probe . The idea behind this technique is to use the same laser source to generate the excitation pulse (PUMP), and the analysis beam (PROBE). The path of the PROBE beam is varied in length by a delay line, i.e. a mobile platform on which are mounted mirrors that reflect the laser beam with high efficiency. The change in the optical path allows the control of the temporal distance between excitation and analysis (Fig. 8.14). 

The availability of lasers having pulse durations in the picosecond or femtosecond range offers many possibiUties for investigation of chemical kinetics. Spectroscopy can be performed on an extremely short time scale, and transient events can be monitored. For example, the growth and decay of intermediate products in a fast chemical reaction can be followed (see Kinetic measurements). 

The first laser Raman spectra were inherently time-resolved (although no dynamical processes were actually studied) by virtue of the pulsed excitation source (ruby laser) and the simultaneous detection of all Raman frequencies by photographic spectroscopy. The advent of the scanning double monochromator, while a great advance for c.w. spectroscopy, spelled the temporary end of time resolution in Raman spectroscopy. The time-resolved techniques began to be revitalized in 1968 when Bridoux and Delhaye (16) adapted television detectors (analogous to, but faster, more convenient, and more sensitive than, photographic film) to Raman spectroscopy. The advent of the resonance Raman effect provided the sensitivity required to detect the Raman spectra of intrinsically dilute, short-lived chemical species. The development of time-resolved resonance Raman (TR ) techniques (17) in our laboratories and by others (18) has led to the routine TR observation of nanosecond-lived transients (19) and isolated observations of picosecond-timescale events by TR (20-22). A specific example of a TR study will be discussed in a later section. 

Early picosecond studies were carried out by Schneider et al, [63] on the parent spiro-oxazine (NOSH in Scheme 8) and similar derivatives. In a back-to-back work, they also described a complimentary CARS (coherent anti-Stokes Raman spectroscopy) investigation [69], Simply put, these authors found that the closed spiro-oxazine ring opened in 2-12 psec after laser excitation. The reaction was slower in more viscous solvents. An intermediate state formed within the excitation pulse and preceded the formation of merocyanine forms. This transient was named X in deference to the X transient named by Heiligman-Rim et al. for the spiropyran primary photoproduct [8], (See also the previous section.) The name X has since been adopted by other workers for the spiro-oxazines [26,65],... 

The rotational reorientation times of the sample in several solvents at room temperature were measured by picosecond time-resolved fluorescence and absorption depolarization spectroscopy. Details of our experimental setups were described elsewhere. For the time-correlated single photon counting measurement of which the response time is a ut 40 ps, the sample solution was excited with a second harmonics of a femtosecond Ti sapphire laser (370 nm) and the fluorescence polarized parallel and perpendicular to the direction of the excitation pulse polarization as well as the magic angle one were monitored. The second harmonics of the rhodamine-640 dye laser (313 nm 10 ps FWHM) was used to raesisure the polarized transient absorption spectra. The synthesis of the sample is given elsewhere. All the solvents of spectro-grade were used without further purification. 

The reactive intermediates leading to the (charge-transfer) photodecomposition of the 6w(arene)iron(II) acceptor are revealed by picosecond time-resolved spectroscopy. For example, photoexcitation of the CT absorption band of the ferro-cene-(HMB)2Fe complex (HMB = hexamethylbenzene) with the second harmonic output (at 532 nm) of a mode-locked Nd YAG laser (25-ps pulse width) generates a transient spectrum with an absorption maximum at 580 nm (see Figure 11 A). Careful deconvolution of this absorption spectrum reveals the superposition of the absorption bands of ferrocenium (Imax = 620 nm, e = 360 cm [162]) and (HMB)2Fe+ (2 ,ax = 580 nm, = 604 M" cm" [163]). 

We summarize here two time-resolved spectroscopic methods, giving direct information on picosecond and nanosecond photodynamics of solid surface. One is a fluorescence spectroscopy which analyzes fluorescence behavior of the surface area excited by the evanescent laser pulse. The other is to get transient UV-visible absorption spectra by using the evanescent light as a probe beam. 

The homolysis product, Phl"+, has been observed directly by laser-flash photolysis probed with transient absorption spectroscopy on both the nanosecond [23,69] and picosecond [22] time scales. Klemm et al. [23] and Hacker and Dektar [70] suggest the possibility of electron transfer between the homolysis fragments leading to products identical with those of heterolysis. DeVoe et al. found no evidence for this process [22], Electron transfer... 

The methods discussed so far, fluorescence upconversion, the various pump-probe spectroscopies, and the polarized variations for the measurement of anisotropy, are essentially conventional spectroscopies adapted to the femtosecond regime. At the simplest level of interpretation, the information content of these conventional time-resolved methods pertains to populations in resonantly prepared or probed states. As applied to chemical kinetics, for most slow reactions (on the ten picosecond and longer time scales), populations adequately specify the position of the reaction coordinate intermediates and products show up as time-delayed spectral entities, and assignment of the transient spectra to chemical structures follows, in most cases, the same principles used in spectroscopic experiments performed with continuous wave or nanosecond pulsed lasers. 

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