Pulsed laser Raman technique

A tunable pulsed laser Raman spectrometer for time resolved Raman studies of radiation-chemical processes is described. This apparatus utilizes the state of art optical multichannel detection and a-nalysis techniques for data acquisition and electron pulse radiolysis for initiating the reactions. By using this technique the resonance Raman spectra of intermediates with absorption spectra in the 248-900 nm region, and mean lifetimes > 30 ns can be examined. This apparatus can be used to time resolve the vibrational spectral o-verlap between transients absorbing in the same region, and to follow their decay kinetics by monitoring the well resolved Raman peaks. For kinetic measurements at millisecond time scale, the Raman technique is preferable over optical absorption method where low frequency noise is quite bothersome. A time resolved Raman study of the pulse radiolytic oxidation of aqueous tetrafluoro-hydroquinone and p-methoxyphenol is briefly discussed. 

In summary, we have combined state of the art optical multichannel analysis techniques with the well established method of electron pulse radiolysis to construct a pulsed laser Raman spectrometer for time resolved studies of transient intermediates in solution. This apparatus can be applied to time resolve the vibrational spectral overlap between transients decaying at differ-... 

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. 

The background resulting from Raman and Rayleigh scattering can be drastically reduced using a pulsed laser and the single-photon timing technique (see Chapter... 

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. 

Methods such as nuclear magnetic resonance (NMR), electron spectroscopy for chemical analysis (ESCA), electron spin resonance (ESR), infrared (IR), and laser raman spectroscopy could be used in conjunction with rate studies to define mechanisms. Another alternative would be to use fast kinetic techniques such as pressure-jump relaxation, electric field pulse, or stopped flow (Chapter 4), where chemical kinetics are measured and mechanisms can be definitively established. 

An alternate approach is to perform coherent Raman spectroscopy in the time domain rather than in the frequency domain. In this case, a single laser that produces short pulses with sufficient bandwidth to excite all of the Raman modes of interest is employed. One pulse or one pair of time-coincident pulses is used to initiate coherent motion of the intermolecular modes. The time dependence of this coherence is then monitored by another laser pulse, whose timing can be varied to map out the Raman free-induction decay (FID). It should be stressed at this point that the information contained in the Raman FID is identical to that in a low-frequency Raman spectrum and that the two types of data can be interconverted by a straightforward Fourier-transform procedure (12-14). Thus, whether a frequency-domain or a time-domain coherent Raman technique should be employed to study a particular system depends only on practical experimental considerations. 

Such single-mode lasers, often pulse amplified by dye laser amplifiers pumped by injection-locked Nd YAG lasers, are used in nonlinear Raman techniques by which an instrumental resolution better than 0.001 cm is achieved (Esherick and Owyoung (1982), Schrotter et al. (1988a)). 

Nonlinear vibrational spectroscopy provides accessibility to a range of vibrational information that is hardly obtainable from conventional linear spectroscopy. Recent progress in the pulsed laser technology has made the nonlinear Raman effect a widely applicable analytical method. In this chapter, two types of nonlinear Raman techniques, hyper-Raman scattering (HRS) spectroscopy and time-frequency two-dimensional broadband coherent anti-Stokes Raman scattering (2D-CARS) spectroscopy, are applied for characterizing carbon nanomaterials. The former is used as an alternative for IR spectroscopy. The latter is useful for studying dynamics of nanomaterials. 

---