Raman pulsed lasers

With the advent of short pulsed lasers, investigators were able to perfonn time resolved coherent Raman scattering. In contrast to using femtosecond pulses whose spectral widtii provides the two colours needed to produce Raman coherences, discussed above, here we consider pulses having two distinct centre frequencies whose difference drives the coherence. Since the 1970s, picosecond lasers have been employed for this purpose [113. 114], and since the late 1980s femtosecond pulses have also been used [115]. Flere we shall briefly focus on the two-colour femtosecond pulsed experiments since they and the picosecond experiments are very similar in concept. 

In our tip-enhanced near-field CARS microscopy, two mode-locked pulsed lasers (pulse duration 5ps, spectral width 4cm ) were used for excitation of CARS polarization [21]. The sample was a DNA network nanostructure of poly(dA-dT)-poly(dA-dT) [24]. The frequency difference of the two excitation lasers (cOi — CO2) was set at 1337 cm, corresponding to the ring stretching mode of diazole. After the on-resonant imaging, CO2 was changed such that the frequency difference corresponded to none of the Raman-active vibration of the sample ( off-resonant ). The CARS images at the on- and off- resonant frequencies are illustrated in Figure 2.8a and b, respectively. 

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

High-power pulsed lasers offer the possibility of studying nonlinear phenomena such as stimulated Raman scattering, the inverse Raman effect and the hyper-Raman effect. These investigations have contributed much to our knowledge of the solid-state and liquid stucture of matter and its higher order constants. 

We have previously investigated ligand release in the 6-coordinate piperidine complexes of Ni octaethylporphyrin (10). For Ni(OEP), formation of the 6-coordinate complex is not complete, and so, the picture is complicated by the presence of both 4- and 6-coordinate species in the initial sample. However, upon excitation with the pulsed laser the relative proportions of the two Ni(OEP) species change as determined by changes in the relative intensities of the 4- and 6-coordinate sets of Raman marker lines. Thus, axial ligand release is observed in the excited state generated during the... 

Another possible solution that has been under development for three decades is to use a pulsed laser and time-resolved detection to allow the Raman photons to be discriminated from the broad luminescence background. The Raman interaction time is virtually instantaneous (less than 1 picosecond), whereas luminescence emission is statistically relatively slow, with minimum hundreds of picoseconds elapsing between electronic excitation and radiative decay. If we illuminate a sample with a very short (= 1 ps) laser pulse, all of the Raman... 

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. 

Raman spectra of adsorbed species, when obtainable, are of great importance because of the very different intensity distributions among the observable modes (e.g., the skeletal breathing frequency of benzene) compared with those observed by infrared spectroscopy and because Raman spectra of species on oxide-supported metals have a much wider metal oxide-transparent wavenumber range than infrared spectra. Such unenhanced spectra remain extremely weak for species on single-crystal surfaces, but renewed efforts should be made with finely divided catalysts, possibly involving pulsed-laser operation to minimize adsorbate decomposition. Renewed efforts should be made to obtain SER and normal Raman spectra characterizing adsorption on surfaces of the transition metals such as Ni, Pd, or Pt, by use of controlled particle sizes or UV excitation, respectively. 

Not long after the discovery of the stimulated Raman effect in liquids 63> it was also detected in single crystals 64), namely diamond, calcite, and a-sulfur. Only much later could it be shown that the effect can also be observed in crystal powders 651. The stimulated Raman effect 99 > is excited by giant-pulse lasers with a power of several MW. The strongest Raman lines of a substance are amplified until their intensity is of the same order of magnitude as that of the exciting line furthermore second, third, etc. Stokes lines of the fundamentals in question are observed with twice, thrice, etc. the frequency shift. 

The inverse Raman effect was detected in liquids 93> soon after the discovery of the stimulated Raman effect. When a medium is irradiated simultaneously by intense monochromatic light from a giant-pulse laser and by a continuum, sharp absorption lines are observed on the anti-Stokes side of the laser line, and under special conditions also on the Stokes side 94 >. McLaren and Stoicheff 95) used the intense fluorescence from a dye solution excited by frequency-... 

The time-resolved Raman approach proved effective in isolating the Raman spectra of deep layers however, the complexity of the instrumentation required and the high peak intensities associated with short-pulse lasers precluded its wider use. These problems were addressed by utilising the spatial... 

Maguire, J.F. Busbee, J.D. Liptak, D.C. Lubbers, D.P. LeClair, S.R. 8c Biggers, R.R. Process Control for Pulsed Laser Deposition Using Raman Spectroscopy US 6,038,525 Assigned to Southwest Research Institute Filed in 1997. 

We present here preliminary results for the (temperature x vj ljjcity) probability density function shown in this paper as , where the quantities within the average brackets are instantaneous values. These data have been obtained from a coordinated experimental program utilizing pulsed laser vibrational Raman scattering and cw real fringe laser velocimetry (LV). 

Optical multipass cells have been used for the enhancement of CW Raman scattering(4) however, these cells are typically not well-suited for use with high power, pulsed lasers. A new multipass cell for use with a pulsed Nd YAG laser is proposed whereby the 1.06 micron laser output is admitted into a multipass cell cavity where it is partially converted to 532nm with a Brewster s angle cut second harmonic generating crystal The 532nm pulse is trapped in the mirrored cavity while the 1.06 micron pulse is dumped. This multipass cell concept has been demonstrated with the experimental set-up shown in figure 1. 

Pulsed laser-Raman spectroscopy is an attractive candidate for chemical diagnostics of reactions of explosives which take place on a sub-microsecond time scale. Inverse Raman (IRS) or stimulated Raman loss (.1, ) and Raman Induced Kerr Effect (2) Spectroscopies (RIKES) are particularly attractive for singlepulse work on such reactions in condensed phases for the following reasons (1) simplicity of operation, only beam overlap is required (2) no non-resonant interference with the spontaneous spectrum (3) for IRS and some variations of RIKES, the intensity is linear in concentration, pump power, and cross-secti on. 

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