Pulse sampling technique

Abstract

Many early pulse sampling instruments used a nitrogen laser as the fluorescence excitation source, since they provide a high-enough pulse energy to yield sufficient detected fluorescence photons for every excitation. e molecular nitrogen laser (Lengyel 1971) operates at a wavelength of 337.1 nm and, as its name indicates, uses nitrogen gas as active medium at pressures between 100 and 105 Pa. Today, most commercial designs use flowing gas, but sealed tube systems do exist, although these operate at low repetition rates. e nitrogen laser is a three-level system, but since the upper laser level of nitrogen is directly pumped, population inversion can take place. Pumping is normally provided by direct electron impact where the gain medium is usually pumped by a transverse electrical discharge. Laser pulses with energy range from microjoules to millijoules, and a peak power in the range of kilowatts to more than3 MW can be achieved. e main disadvantages are the long pulse lengths-between a few hundred picoseconds and a maximum of approximately 30 ns depending on the gas pressure-and the low repetition rate, although this is not necessarily a problem for point spectroscopy measurements unless significant averaging over multiple pulses is required to improve the signal-to-noise ratio. Nitrogen lasers only offer limited control of pulse energy, have poor beam quality, need regeneration of the laser discharge electrodes after a few million pulses, and require a bulky gas supply that can compromise portability. However, the nitrogen gas supply is economical, and modern systems have a lower complexity compared with other subnanosecond pulsed nitrogen laser systems used in the past (Mycek et al. 1998).