Laser repetition rate

The experiment is performed with a spectrofluorometer similar to the ones used for linear fluorescence and quantum yield measurements (Sect. 2.1). The excitation, instead of a regular lamp, is done using femtosecond pulses, and the detector (usually a photomultiplier tube or an avalanche photodiode) must either have a very low dark current (usually true for UV-VIS detectors but not for the NIR), or to be gated at the laser repetition rate. Figure 11 shows a simplified schematic for the 2PF technique. 

Figure 6.4 displays the result of this statistical analysis for one single thunderstorm. At the location of the laser filaments (arrow head), 43% (3 out of 7) of the pulses are synchronized with the laser repetition rate, corresponding to a high statistical significance (1 — async = 0.987). The delay mismatch between the RF pulses detected on the different LMA detectors correspond to some tens of meters, typical of spatially spread events, such as a series of... 

AT quartz crystals, as those used in this work, are designed to display the smallest temperature dependence at room temperature, but frequency shifts due to temperature variation must be eliminated in order to get only the mass variation contribution. Although this is not a major problem, low laser repetition rates are systematically used, therefore avoiding any possible heat accumulation. 

Fluorescence is induced by a Molectron Nd YAG pumped dye laser. The laser repetition rate is 10 Hz, the bandwidth is -0.01 nm, and the maximum pulse energy and peak power at 309 nm are 3 mJ and. 5 MW, respectively. The laser is focused into the flame by a 15 cm focal length lens the focused spot size is about 100 ym, as determined from burn patterns on thermal paper. 

The excitation of the transition at 43.9 THz requires laser radiation at 6.8 pm. Such could be obtained by optical difference frequency generation and is in the reach of present laser technology, particularly for pulsed laser devices. Therefore the experiment would fit nicely into the environment of a pulsed muon facility with intense pulses of up to 1 ps duration and an ideal pulse separation given by the tolerable laser repetition rate which may be up to kHz. 

Frequency stabilisation and scanning is accomplished by use of a confocal cavity of free spectral range matched to the dye laser repetition rate. Phase modulated sidebands are put on to the mode spectrum of the mode-locked pulse train and used to lock the laser to the reference cavity. The frequency modulation technique is also used to lock the ultra-violet enhancement cavity to the mode-locked pulse train. 

The IR pulse is split into a weak probe beam, which passes down a computer-controlled variable delay line with up to 12 ns of delay and a strong pump beam. The pump and probe pulses are counterpropagating and focused into the center of the SCF cell. Typical spot sizes (1/e radius of E-field) were oj0 120 pm for the pump beam and oj0 60 pm for the probe beam. A few percent of the transmitted probe beam is split off and directed into an InSb detector. A reference beam is sent through a different portion of the sample. The reference beam is used to perform shot-to-shot normalization. The pump beam is chopped at half the laser repetition rate (900 Hz). The shot-to-shot normalized signal is measured with a lock-in amplifier and recorded by computer. 

In photocathode electron guns, the timing between the microwaves used for acceleration and the photocurrent-generating laser pulse is of critical importance. Precise synchronization between the laser and electron beams is obtained by using a MHz quartz master oscillator to control the cathode pump laser repetition rate and the microwave amplifier system seed frequency (Fig. 3). 

A photon wavelength of 266.2 nm is used in the experiments reported here, the fourth harmonic of a special neodymium laser system which can generate powerful ultra-violet pulses of a joule or more. " Experimental conditions are (see also table 1) molecular beam density at the interaction volume, 6x 10 molecules cm laser energy, 0.3 J per pulse (4x 10 photons per 10 ns pulse, i.e., 30 MW) laser repetition rate, one pulse per minute and flight path length, 5.63 cm. 

The laser repetition rate is a key variable in the LIBS technique. Laser breakdown produces a persistent mass of aerosol above the sample, the production rate increasing with increasing repetition rate and yielding a higher steady-state aerosol concentration above the sample. 

To study the influence of the laser repetition rate on the decomposition of Kapton, a SiC disk with abraded Kapton was irradiated with 80 mj cm"2 at a repetition rate of 0.086 Hz. This is well below the frequency where accumulative heating from the laser is important [296]. The spectra match very well the spectra from the experiments using 10 Hz. The only difference is an enhanced absorption of the band at 2270 cm"1, suggesting that in the experiments with higher repetition rates additional thermal decomposition of the isocyanate group takes place. 

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