Pulsed Gas Lasers

The lasers we have considered so far have a solid material as the active medium. Generally, flash-lamp pumping is employed and short pulses are obtained at a repetition rate of typically 10 Hz. High peak powers are obtained in the Q-switched mode (MW-GW). Gaseous media can also be used for the generation of short laser pulses. We shall here consider the nitrogen laser, the excimer laser and the copper vapour laser. The relevant parts of 

Pulsed CO2 lasers, which were available long before excimer lasers, have a similar construction. These infra-red lasers are of great technological importance. CO2 lasers will be discussed later. 

In the copper vapour laser the discharge tube must be heated to high temperatures in order to produce a sufficient Cu vapour pressure. This laser emits at 510 and 578 nm corresponding to the (4p P3/2 4d and 

Whereas population inversion can often be achieved relatively easily with a pulsed pumping source, it is considerably more difficult, and sometimes impossible, to continuously maintain population inversion while laser action prevails. Clearly, lasing means that excited atoms decay with the emission of stimulated radiation and thus the laser action itself will cause the lasing action to stop. Efficient pumping mechanisms are required. We will now study some important types of continuous fixed-frequency lasers which all work with gaseous laser media. 

The nitrogen laser has such a high gain that a laser beam can be obtained even without cavity mirrors tlnough amphfied spontaneous emission (sometimes the term super-radiant laser is used). Normally, a totally reflecting mirror is used at one end of the gain tube while the window at the opposite 

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The time profile of the laser pulse is not only determined by the amplification per round trip G t) (Vol. 1, Sect. 5.2) but also by the relaxation times t,-, Xk of the upper and lower laser levels. If these times are short compared to the rise time of the pump pulse, quasi-stationary laser emission is reached, where the inversion AN(t) and the output power FL(r) have a smooth time profile, determined by the balance between pump power Pp t), which creates the inversion, and laser output power Fl(0. which decreases it. Such a time behavior, which is depicted in Fig. 6.1a, can be found, for instance, in many pulsed gas lasers such as the excimer lasers (Vol. 1, Sect. 5.7). 

If the flash lamp is pulsed very rapidly, the emergent beam appears at a rate governed by the lifetime of the inverted population. The resulting laser beam becomes almost continuous because the pulses follow each other so rapidly. However, such a solid-state laser should not be pulsed too rapidly because, if it is, the rod heats to an unacceptable extent, causing distortion and even fracture. Generally, solid-state lasers are not used in continuous mode because of this heating aspect. Liquid or gas lasers do not suffer from this problem. 

The apparatus and techniques of ion cyclotron resonance spectroscopy have been described in detail elsewhere. Ions are formed, either by electron impact from a volatile precursor, or by laser evaporation and ionization of a solid metal target (14), and allowed to interact with neutral reactants. Freiser and co-workers have refined this experimental methodology with the use of elegant collision induced dissociation experiments for reactant preparation and the selective introduction of neutral reactants using pulsed gas valves (15). Irradiation of the ions with either lasers or conventional light sources during selected portions of the trapped ion cycle makes it possible to study ion photochemical processes... 

Figure 4. The sample cell arrangement in the DCSHG experiment, where the sample solution was inserted between two glass slips (lop), and the optical design for the DCSHG dispersion experiment, where the compressed H gas medium was pumped by a tunable pulsed dye laser source for Stokes generation by stimulated Raman scattering (bottom). (E° is the static electric field.) Key beam guiding prisms P, Stokes...

Pulsatile drug delivery systems, 9 57-61 Pulsating heat pipes (PHP), 13 235-236 Pulse combustion heat sources, 9 104-105 Pulse cycles, 9 778 Pulsed baffle reactors, 15 709-710 Pulsed discharge detector (PDD) gas chromatography, 4 614 Pulsed dye lasers, 23 144 Pulsed electrochemical machining (PECM), 9 604-605... 

The explosive phenomena produced by contact of liquefied gases with water were studied. Chlorodifluoromethane produced explosions when the liquid-water temperature differential exceeded 92°C, and propene did so at differentials of 96-109°C. Liquid propane did, but ethylene did not, produce explosions under the conditions studied [1], The previous literature on superheated vapour explosions has been critically reviewed, and new experimental work shows the phenomenon to be more widespread than had been thought previously. The explosions may be quite violent, and mixtures of liquefied gases may produce overpressures above 7 bar [2], Alternative explanations involve detonation driven by phase changes [3,4] and do not involve chemical reactions. Explosive phase transitions from superheated liquid to vapour have also been induced in chlorodifluoromethane by 1.0 J pulsed ruby laser irradiation. Metastable superheated states (of 25°C) achieved lasted some 50 ms, the expected detonation pressure being 4-5 bar [5], See LIQUEFIED NATURAL GAS, SUPERHEATED LIQUIDS, VAPOUR EXPLOSIONS... 

Gas lasers operate mainly in the cw regime, although in the case of singly ionized gases, both the pulsed and the continuous regime are used. 

Fig. 9. Experimental arrangement for laser photolysis, using the frequency-doubled output from a giant-pulse ruby laser as pump pulse and the wavelength continuum from a laser-induced high-temperature gas plasma as analysing pulse. (From Novak, J.R., Windsor, M.W., ref. 15 ))...

Photodissociation of molecules may also be achieved by depositing energy directly in the vibrational degrees of freedom. With hi -power pulsed CO2 lasers dissociation of molecules which absorb CO2 -laser radiation has been observed to proceed at an initial rate that far exceeds the measured thermal rate 169 ). The appearance of luminescence spectra of dissociation products preceding the occurrence of gas breakdown 169b) indicates that a considerable degree of dissociation exists for some time before breakdown. 

The velocity, density and temperature of a streaming gas can be determined by measuring the magnitude, frequency and spectral distribution of Rayleigh-scattered light from two simultaneously pulsed ruby lasers with parallel beams and slightly different frequencies 246)... 

It is more difficult to assign lines in laser systems which use polyatomic molecules in pulsed gas discharges 355) since here it is not always easy to decide whether the observed lines belong to transitions of the parent molecule or of dissociation products. 

Photoexcitation was achieved by using a triggerable nitrogen gas laser (LGI-21). The laser output pulse was of duration 8ns at a wavelength of 337nm. 

Gas-discharge lamps are used to optically pump the metastable helium atoms into a higher excited electronic state, which has a dipole-allowed transition to the ground state. Only He (2 S) can be pumped selectively, thereby producing pure He(23S) beams. For the heavier rare gases, both metastable states are equally pumped by gas-discharge lamps. The use of cutoff filters to selectively pump one state is not adequate because of the temperature dependence of the filter transmission and the low / numbers of the pumping transition. Metastable neon can be selectively pumped by a continuous wave (cw) dye laser,60 whereas Ar, Kr, and Xe have so far only been selectively pumped by pulsed dye lasers.61... 

Schmidt et al. [111] developed an atmospheric pressure laser ionization (APLI) source based on REMPI in pulsed gas expansions close to the inlet nozzle orifice (at high... 

In practice this condition may be fulfilled not only in excitation, e.g. by means of a pulsed laser or a continuous dye laser with insufficient frequency selectivity, but also by means of fines from a continuous gas laser working in simultaneous axial mode u>i (multimode) generation regime see Fig. 3.10(a). Let Au>i = u>i+1 — uii = itc/L denote the mode separation in a laser, L being the resonator length. Then, as pointed out in [110, 127, 231], broad line approximation works if Awj is smaller than the width of the Bennet holes r en [268, 320] in the absorption contour see Fig. 3.10(6). The positions of the Bennet holes are determined by the condition ujq — w/ + kv = 0, where luq is the central transition frequency, k is the wave vector and v is the velocity of the absorbing particle. The broad fine approximation is valid if the following conditions are fulfilled (see Fig. 3.10) ... 

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