Lasers, types infrared wavelength

Activation of the vibrational energy of ions can also be induced by the absorption of IR radiations. A popular type of IR radiation source is far-IR laser. In fact, many molecules have a broad IR absorption band. The most widely used IR source is a continuous wave (c.w.) CO2 laser, with the wavelength of 10.6 pm. This wavelength corresponds to an energy of 0.3 eV per laser photon. Because decomposition of a chemical bond requires >1 eV, laser excitation has to extended over hundreds of milliseconds to allow ions to absorb multiple IR photons. This method is known as infrared multiphoton dissociation (IRMPD). Another type of similar technique is black-body infrared radiative dissociation... 

The operating principle of this type of laser is rather simple and is illustrated in Fig. 1.1. The incoming radiation is produced by an infrared laser, usually a CO2 laser, and is absorbed on vibrational-rotational transitions in a molecular vapour. The vapour is thereby excited to states normally almost totally empty. The resulting population inversions provide several possibilities for laser emission, and as shown in Fig. 1.1 these can be either in the upper or lower vibrational level. It may be reasoned that, since stimulated absorption and emission have equal probabilities, the maximum quantum efficiency for the optically pumped laser is 0.5 in practice the value is seldom as high as 0.1. In terms of energy, the efficiency is lower, of course, by the ratio of the far-inffared to the infrared wavelengths, not to mention the efficiency of the infrared pump laser. Nevertheless the cleanness and simplicity of the system make it very attractive. 

Figure C3.3.4 shows a schematic diagram of an apparatus that can be used to study collisions of the type deseribed above [5, 9,12,16], Donor molecules in a 3 m long collision cell (a cylindrieal tube) are exeited along the axis of the eell by a short-pulse excimer laser (typically 25 ns pulse width operating at 248 nm), and bath moleeules are probed along this same axis by an infrared diode laser (wavelength in the mid-infrared with eontinuous light-output...

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