The lasers that we have considered so far operate at fixed or discrete frequencies. To obtain tunable lasers (wherein the laser frequency can be adjusted to be any value in a given range), we need a broad range of frequencies wherein the laser can be made to operate. The solvent broadening of the vibrational spectrum of dye molecules is used to accomplish this. Due to salvation of dye molecules, the fluorescence band is red shifted (shifted to higher wavelengths) relative to the absorption band. The range of frequencies where laser action is possible is shown in Fig 31.6
Figure 31.6 Wavelength region for laser action in a dye laser.
In the laser cavity of a dye laser, a diffraction grating is placed . By changing the angle of the grating relative to the pump radiation, shifts in laser wavelengths are achieved. To avoid heating, the dye solution is
made
to flow through the cavity
continuously. An example of
a
dye is Rhodamine 6G in methanol.
In additional to the above three types of lasers, other modes of laser action have been developed. Band gaps of semiconductors and the metastable vibrationally excited states of molecules formed during chemical reactions are two examples in this group. Let us now consider some chemical applications of lasers.
