Time-resolved resonance Raman spectroscopy pulsed lasers

Biological Aspects.—The lowest excited triplet states of all-rran.s- -carotene produced by pulse radiolysis has been studied by time-resolved resonance Raman spectroscopy.Six transient Raman bands at 965, 1009, 1125, 1188, 1236, and 1496cm were observed and assigned to the triplet state of ) -carotene. The authors conclude that the molecule may be substantially twisted, presumably at the 15,15 band in the triplet state. Further work has also been carried out by the same workers on the triplet state of all-rran -retinal. The results indicate increased 7r-electron delocalization in the triplet state and propose that the relaxed excited triplet-state exists in either sAX-trans or 9-cis conformation. Das and Becker" have also employed pulse radiolysis and laser flash photolysis to study several photophysical properties of the triplet states of the series of polyenals (29)—(33) related to retinal (31) as homologues (Table 35). Results are presented... 

The most widely used vibrational spectroscopic technique is time-resolved resonance Raman spectroscopy (TR ) [65]. This has been used successfully to obtain structural information about organic excited states in SCCO2. McGar-vey and co-workers probed the excited triplet state of anthracene in SCCO2 [66]. However, TR experiments involve data collection over many laser pulses, with all of the problems associated with secondary photolysis. These problems have prevented TR being used effectively to follow chemical reactions apart from highly photoreversible processes. To our knowledge, TR has not yet been used to follow chemical reactions in SCFs. Recently, however. 

The conditions which determine whether flash photolysis can be used to smdy a given chemical system are (i) a precursor of the species of kinetic interest has to absorb light (normally from a pulsed laser) (ii) this species is produced on a timescale that is short relative to its lifetime in the system. Current technical developments make it easy to study timescales of nanoseconds for production and analysis of species, and the use of instrumentation with time resolution of picoseconds is already fairly common. In certain specific cases, as we will see in the last part of this chapter, it is possible to study processes on timescales greater than a few femtoseconds. Once the species of interest has been produced, it is necessary to use an appropriate rapid detection method. The most common technique involves transient optical absorption spectroscopy. In addition, luminescence has been frequently used to detect transients, and other methods such as time-resolved resonance Raman spectroscopy and electrical conductivity have provided valuable information in certain cases. 

Time-resolved resonance Raman (TR ) spectroscopy experiments were first reported in 1976 and used a 30 ns pulse radiolytic source to generate the intermediates that were then probed on the microsecond time-scale by a laser source to generate the TR spectrum. TR spectroscopy was then extended to study intermediates... 

Since there are a large number of different experimental laser and detection systems that can be used for time-resolved resonance Raman experiments, we shall only focus our attention here on two common types of methods that are typically used to investigate chemical reactions. We shall first describe typical nanosecond TR spectroscopy instrumentation that can obtain spectra of intermediates from several nanoseconds to millisecond time scales by employing electronic control of the pnmp and probe laser systems to vary the time-delay between the pnmp and probe pnlses. We then describe typical ultrafast TR spectroscopy instrumentation that can be used to examine intermediates from the picosecond to several nanosecond time scales by controlling the optical path length difference between the pump and probe laser pulses. In some reaction systems, it is useful to utilize both types of laser systems to study the chemical reaction and intermediates of interest from the picosecond to the microsecond or millisecond time-scales. 

Ultrafast time-resolved resonance Raman (TR ) spectroscopy experiments need to consider the relationship of the laser pulse bandwidth to its temporal pulse width since the bandwidth of the laser should not be broader than the bandwidth of the Raman bands of interest. The change in energy versus the change in time Heisenberg uncertainty principle relationship can be applied to ultrafast laser pulses and the relationship between the spectral and temporal widths of ultrafast transform-limited Gaussian laser pulse can be expressed as... 

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. 

A promising recent development in the study of nitrenium ions has been the introduction of time-resolved vibrational spectroscopy for their characterization. These methods are based on pulsed laser photolysis. However, they employ either time resolved IR (TRIR) or time-resolved resonance Raman (TRRR) spectroscopy as the mode of detection. While these detection techniques are inherently less sensitive than UV-vis absorption, they provide more detailed and readily interpretable spectral information. In fact, it is possible to directly calculate these spectra using relatively fast and inexpensive DFT and MP2 methods. Thus, spectra derived from experiment can be used to validate (or falsify) various computational treatments of nitrenium ion stmctures and reactivity. In contrast, UV-vis spectra do not lend themselves to detailed structural analysis and, moreover, calculating these spectra from first principles is still expensive and highly approximate. 

Recognizing the faet the seattering process occurs on a very short timescale of 10 s, it becomes evident that short-pulse-width lasers can be used to acquire RR spectra of transient species that can be generated by a photolysis pulse. In favorable cases the species of interest can be prepared and interrogated within the same laser pulse, whereas in other situations a short probe pulse is temporally delayed relative to an initial pump pulse, the latter protocol properly being called time-resolved resonance Raman or TR spectroscopy. 

Most of the early gas lasers emitted in the visible region. Continuous-wave (CW) lasers such as Ar+ (351.1-514.5 nm), Kr+ (337.4-676.4 nm), and He-Ne (632.8 nm) are now commonly used for Raman spectroscopy. More recently, pulsed lasers such as Nd YAG, diode, and excimer lasers have been used for time-resolved and ultraviolet (UV) resonance Raman spectroscopy. 

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