Laser induced fluorescence line width

In addition to measuring total recombination coefficients, experimentalists seek to determine absolute or relative yields of specific recombination products by emission spectroscopy, laser induced fluorescence, and optical absorption. In most such measurements, the products suffer many collisions between their creation and detection and nothing can be deduced about their initial translational energies. Limited, but important, information on the kinetic energies of the nascent products can be obtained by examination of the widths of emitted spectral lines and by... 

The molecular jet of molecules is crossed with a tunable dye laser and the laser-induced fluorescence is collected with a lens and focused on a PMT detector (Fig. 5). In the original experiments, a standard pulsed dye laser was used to match the 10-Hz duty cycle of the pulsed valve and the pulsed Nd YAG vaporization laser. Although this approach provides a high S/N ratio and wide spectral coverage, the resolution is limited by the laser line width of typically 0.5cm-1 (no etalon) to 0.05cm-1 with an etalon. 

Timable visible and ultraviolet lasers were available well before tunable infrared and far-infrared lasers. There are many complexes that contain monomers with visible and near-UV spectra. The earliest experiments to give detailed dynamical information on complexes were in fact those of Smalley et al [22], who observed laser-induced fluorescence (LIF) spectra of He-l2 complexes. They excited the complex in the I2 B band, and were able to produce excited-state complexes containing H-state I2 in a wide range of vibrational states. From line widths and dispersed fluorescence spectra, they were able to study the rates and pathways of dissociation. Such work was subsequently extended to many other systems, including the rare gas-Cl2 systems, and has given quite detailed information on potential energy surfaces [23]. 

With a second laser electronic transitions in the neutral atoms or the ions are excited and the laser-induced fluorescence is monitored for specific known excitation lines. With a time gate in the detector system the spectra can be taken at different times and therefore different temperatures of the expanding plasma. The intensity of the LIF gives information about the atomic composition of the evaporated material and the abundance of the excited species, if the transition probabilities are known [198, 199]. The sensitivity of the technique depends on the peak intensity and the pulse duration of the excitation laser pulses. Typical laser pulse widths range from nanoseconds to picoseconds. Recently also femtosecond lasers have been used. Since only a tiny amount of material (nanograms to picograms) is evaporated the sample is not essentially damaged by this purely optical analysis [200]. 

The narrow spectral line of a DL enables isotope selective analysis. For light and heavy elements (such as Li and U) the isotope shifts in spectral lines are often larger than the Doppler widths of the lines, in this case isotopically selective measurements are possible using simple Doppler-limited spectroseopy - DLAAS or laser induced fluorescence (LIF). For example, and ratios have been measured by Doppler-limited optogalvanic. spectroscopy in a hollow cathode discharge. DLAAS and LIF techniques have been combined with laser ablation for the selective detection of uranium isotopes in solid samples. This approach can be fruitful for development of a compact analytical instrument for rapid monitoring of nuclear wastes. 

A 112 2 ->(0,0,0)X 112 2 transitions, only those excitation lines are monitored which excite the upper levels of these R fluorescence lines. The laser-induced fluorescence spectrum reveals, however, that in spite of this filtering technique each excitation line still consists of two rotational lines which overlap within their Doppler width. For a complete resolution therefore Doppler-free techniques are demanded (see Chap.10). 

This optical-optical double-resonance technique has already been used for other Doppler-free techniques [10.25], such as polarization spectroscopy (see Sect.10.3). Its applications to molecular beams has, however, the following advantages compared to spectroscopy in gas cells. When the chopped pump laser periodically depletes the level E. and populates level Ej, there are two relaxation mechanisms in gas cells which may transfer the population modulation to other levels. These are collision processes and laser-induced fluorescence (see Fig.8.39). The neighboring levels therefore also show a modulation and the modulated excitation spectrum induced by the probe laser includes all lines which are excited from those levels. If several absorption lines overlap within their Doppler width, the pump laser simultaneously excites several upper states and also partly depletes several lower levels. 

The first observations of the faint emission spectra of CIO2 induced by various argon- ion laser lines have been reported by Sakurai, Clark and Broida /3/. Their attempt to measure the lifetime of the excited CIO2 with a pulsed dye laser was not successful due to the 10 ns width of the laser pulse. The fluorescence decayed almost within the excitation pulse. Later Curl et al. /4/ assigned the 4765 A excitation savely to the... 

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