Monochromatic radiation pulsed excitation with

The basic principle is to observe the change in absorbance after an intense radiation pulse has created a significant population of short-lived reactive intermediates. Early experiments used xenon flash lamps as excitation sources and were able to detect intermediates with lifetimes >10 second. Modern experiments use pulsed lasers as excitation sources. The monochromatic output of a laser allows selective excitation the narrow pulse width allows detection of species with lifetimes as low as 10 second. A complementary experiment uses a pulse of electrons from a linear accelerator to generate the reactive species. More experimental detail is available in many reviews [139]. 

It is widely appreciated that modem NMR spectrometers use a short pulse of radiofrequency energy to excite nuclear resonances over a range of frequencies. This pulse is supplied as monochromatic radiation from the transmitter, yet the nuclear spin transitions giving rise to our spectra vary in energy according to their differing Larmor frequencies and so it would appear that the pulse will be unable to excite all resonances in the spectmm simultaneously. However, Heisenberg s Uncertainty principle tells us that an excitation pulse of duration At has associated with it a frequency uncertainty or spread of around 1/At Hz... 

Fig. 5. Pulsed-nozzle FT microwave measurements. A molecule-radiation interaction occurs when the gas pulse is between mirrors forming a Fabry-Perot cavity. If the transient molecule has a rotational transition of frequency vm falling within the narrow band of frequencies carried into the cavity by a short pulse (ca. 1 (is) of monochromatic radiation of frequency v, rotational excitation leads to a macroscopic electric polarization of the gas. This electric polarization decays only slowly (half-life T2 = 100 (is) compared with the relatively intense exciting pulse (half-life in the cavity t 0.1 (is). If detection is delayed until ca. 2 (is after the polarization, the exciting pulse has diminished in intensity by a factor of ca. 106 but the spontaneous coherent emission from the polarized gas is just beginning. This weak emission can then be detected in the absence of background radiation with high sensitivity. For technical reasons, the molecular emission at vm is mixed with some of the exciting radiation v and detected as a signal proportional to the amplitude of the oscillating electric vector at the beat frequency v - r , as a function of time, as in NMR spectroscopy Fourier transformation leads to the frequency spectrum [reproduced with permission from (31), p. 5631.

A pulsed N2 laser (2 = 337.1 nm, rp = 8 ns, v = 1 kHz) and a CW HeCd laser (2 = 325 nm) were used for photoluminescence (PL) and lasing measurements. The monochromatized radiation of a Xe lamp (7exc 10"4 W/cm2) was used for PL excitation (PLE) spectra measurements. Low-excitation time-resolved PL measurements were performed at RT using the time-correlated single-photon counting method. X-ray diffraction analysis was performed with a high-resolution diffractometer (PANalytical s X Pert PRO MRD). 

Lasers with short pulses are not used in Raman spectrometers, mainly because the detectors in Raman spectrometers are tuned to high sensitivity. Such detectors are very easy to saturate and this is a case where short and intense laser pulses are employed for excitation of Raman scattering. It must be noted, that gas lasers are not perfect sources of monochromatic radiation. Together with intense coherent radiation such lasers produce weak incoherent radiation, caused by a different transition between electronic energy levels of the gas. The intensity of this incoherent and noncollimated radiation can be suppressed by increasing the distance between the laser and the sample, by placing a spatial filter (consisting of two lenses and a pinhole) or a narrow-band filter (usually an interference filter) into the laser beam. 

Lasers are another source of excitation radiation used in fluorescence detection systems. The high-directional output of a laser maximizes the fraction of total output that can be easily focused down to a spot size compatible with the dimensions of CE detection cells. The output of a laser is also typically monochromatic, or a discrete set of spectrally narrow lines. This type of output makes it relatively easy to filter out low-level incoherent plasma radiation and undesired emission lines without greatly diminishing the overall output power. In addition, many lasers provide flexibility in terms of pulse width and repetition rate, which allows one to optimize excitation with respect to analyte photostability. 

By using either a continuous or pulsed source of radiation and by measuring the amount of radiation absorbed by the reaction products, it is possible to determine product state distributions. The source of radiation can either be monochromatic (resonance lamp or laser) or broad-band (flash lamp or arc lamp) used in conjunction with a form of monochromator at the detector. The amount of absorption is monitored by an appropriate photosensitive or energy-sensitive detector. Particular care must be taken in the case of resonance lamps to avoid self-reversal of the output of the source, as this will complicate the quantitative analysis of product densities [17]. Similarly, laser sources must not be operated at such high output powers that the transitions involved become saturated, as this also complicates the analysis. Absorption measurements can be used for a wide range of reaction products, both ground and excited states of atoms, radicals and molecules [9,17, 22]. 

The nitrogen laser is pumped with a high-vollagc spark source that provides a momentary (1 to 5 ns) puLse of current through the gas. I he excitation creates a population inversion that decays very quickly by spontaneous emission because the lifetime of the excited stale is quite short relative to the lifetime of the lower level. The result is a short (a few nanoseconds) pulse of intense (up to I MW) radiaiion at. 337,1 nm. This output is used for exciting fluorescence in a variety of molecules and for pumping dye lasers. I he carbon dioxide gas laser is used to produce monochromatic Infrared radiation at 10.6 pm. 

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