Laser intensity, high

A large number of atoms and molecules have been meanwhile investigated by level-crossing spectroscopy using laser excitation. Because of the available high laser intensity highly excited states can be studied, which have been populated by stepwise excitation (see Sect.8.8). Often resonance lamps are used to excite the first resonance level and a dye laser pumps the next step. Many experiments have been performed with two different dye lasers, either in a pulsed or a cw mode [10.91]. These techniques allow measurement of... 

From about 1970, but before the availability of suitable lasers, Parmenter and others obtained SVLF spectra, particularly of benzene, using radiation from an intense high-pressure xenon arc source (see Section 3.4.4) and passing it through a monochromator to select a narrow band ca 20 cm wide) of radiation to excite the sample within a particular absorption band. 

Nonlinear refraction phenomena, involving high iatensity femtosecond pulses of light traveling in a rod of Tfsapphire, represent one of the most important commercial exploitations of third-order optical nonlinearity. This is the realization of mode-locking ia femtosecond Tfsapphire lasers (qv). High intensity femtosecond pulses are focused on an output port by the third-order Kerr effect while the lower intensity continuous wave (CW) beam remains unfocused and thus is not effectively coupled out of the laser. 

SNMS sensitivity depends on the efficiency of the ionization process. SNs are post-ionized (to SN" ) either hy electron impact (El) with electrons from a hroad electron (e-)heam or a high-frequency (HF-) plasma (i.e. an e-gas), or, most efficiently, hy photons from a laser. In particular, the photoionization process enables adjustment of the fragmentation rate of sputtered molecules by varying the laser intensity, pulse width, and/or wavelength. 

As illustrated in Fig. 3.41, several laser schemes can be used to ionize elements and molecules. Scheme (a) in this figure stands for non-resonant ionization. Because the ionization cross-section is very low, a very high laser intensity is required to saturate the ionization process. Scheme (b) shows a simple single-resonance scheme. This is the simplest but not necessarily the most desirable scheme for resonant post-ionization. Cross-... 

A laser beam highly focused by a microscope into a solution of fluorescent molecules defines the open illuminated sample volume in a typical FCS experiment. The microscope collects the fluorescence emitted by the molecules in the small illuminated region and transmits it to a sensitive detector such as a photomultiplier or an avalanche photodiode. The detected intensity fluctuates as molecules diffuse into or out of the illuminated volume or as the molecules within the volume undergo chemical reactions that enhance or diminish their fluorescence (Fig. 1). The measured fluorescence at time t,F(t), is proportional to the number of molecules in the illuminated volume weighted by the... 

Clearly RR is attractive as a technique for detecting intermediates. It has, for example, been very successful with intermediates in organic and bioinorganic reactions (55). Unfortunately the high laser intensities that are needed in RR may be incompatible with the photosensitivity of most organometallic intermediates. For example, in an attempt to detect Cr(CO)5 in solution by RR, the only detectable transient signal was emission from excited Cr atoms (55). 

Photoionization ti me-of-fli ght mass spectrometry is almost exclusively the method used in chemical reaction studies. The mass spectrometers, detectors and electronics are almost identical. A major distinction is the choice of ionizing frequency and intensity. For many stable molecules multi photon ionization allowed for almost unit detection efficiency with controllable fragmentation(20). For cluster systems this has been more difficult because high laser intensities generally cause extensive dissociation of neutrals and ions(21). This has forced the use of single photon ionization. This works very well for low i oni zati on potential metals ( < 7.87 eV) if the intensity is kept fairly low. In fact for most systems the ionizing laser must be attenuated. A few very small... 

In practical application, Raman sensors exclusively use frequency-stabilised laser sources to compensate for the low intensity of the Raman radiation. For Raman sensors, prevalently compact high-intensity external cavity laser diodes are used, operated in CW (continuous wave) mode. These diode lasers combine high intensity with the spectral stability required for Raman applications and are commercially available at various wavelengths. 

Recently, even practically, single-mode lasing of a droplet has been reported for quantum dot doped droplets14, when the droplet diameter was sufficiently small and pump laser intensities were high enough as shown in Fig. 17.10. Interestingly, the... 

Fig. 17.10 Schafer et al.14 have observed single mode lasing from core shell CdSe/ZnS nano crystal quantum dots in a glycerine water mixture. The fluorescence spectrum (black line) showed clear peaks of WGM and single mode lasing (grey line) was observed for sufficiently small droplets ( 10 pm) and high pump laser intensities (53 mJ crrT2 in 10 ns pulses at 532 nm). Insert shows the droplet trapped between the electrodes. Reprinted from Ref. 14 with permission. 2008 American Chemical Society...

J. A. Bloom and W. W. Webb, Photodamage to intact erythrocyte membranes at high laser intensities methods of assay and suppression,/. Histochem. Cytochem. 32, 608-616 (1984). 

For comparison the output power of a high-pressure mercury lamp (Osram HBO 200) also is listed. The reader has to consider, however, that the mercury lamp radiates this power into the unit solid angle (= 60°) distributed over the spectral range from 2000 to 6000.A, whereas the laser intensity is concentrated at a single wavelength and collimated in a beam with a very small divergence between 10 and 10" sterad. 

Recent advances in constructing single-mode tunable cw dye lasers with a bandwidth of 20 Mc/sec or less 98a) open the way for high resolution absorption sepectroscopy without the limitations to fixed wavelengths 98b) One may observe either the total fluorescense intensity, which is proortional to the total absorption, or the decrease of the transmitted laser intensity. 

Because of the high laser intensity, a large population density, comparable to that of the initial ground states, can be obtained in these excited states. With these strongly pumped excited molecules many interesting experiments may be performed, a selection of which will be discussed below. 

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