Report for Analytical Chemists
SPECTRAL IMAGING,
INC.
Heavy horizontal lines represent vibrational states; lighter lines represent rotational fine structure. Excita tion and laser emis sion are represented by transitions A —* b and Β -*~ a, respec tively. Other transi tions represent losses in laser process
MODEL HTS-255-15 Hadamard - Transform Analytical Spectrometer
first off-the-shelf HTS instrument
multiplex power with dispersive economy
multislit efficiency: 120x monochromator, 3 0 x fourier-transform
4 0 0 0 to 6 6 6 cm'' at 3.5 cm',' average
fully-automatic operation; integral computer optional
SPECTRAL I M A G I N G ,
Figure 3. Sche matic representa tion of ττ-orbital en ergy levels of dye molecule
INC.
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singlet-to-singlet or triplet-to-triplet. Radiative transitions other t h a n electric dipole are too improb able to compete with other relaxa tion processes and will be neglected. Some insight into dye laser opera tion can be gained from following the dye through an absorptionemission cycle. Initially, most of the dye molecules exist in thermal equilibrium in low-lying vibrational or rotational states a t or near level A in the electronic ground singlet manifold S0. I n accordance with the Franck-Condon principle, the most probable optical interaction (absorption) is between these A levels in »S0 and rotationally and vibrationally excited states b in the first excited electronic manifold S x . (A-B transitions are much less probable.) These excited molecules can re lax by several routes: by reemitting a photon and returning to S 0 , by thermally equilibrating within Si to vibrational and rotational levels near B, by radiationless "in ternal conversion" back to the S0 manifold, or by radiationless "intersystem crossing" from the excited singlet manifold Si to the triplet manifold ΤΊ with a rate constant
CIRCLE 185 O N READER SERVICE CARD
32 A .
ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
kST. Thermal equilibration is the most rapid relaxation process, oc curring on a picosecond time scale, so the bulk of the excited dye mole cules ends up in B, i.e., in the lowest vibrational and rotational states of the first electronic excited mani fold. Laser action occurs through am plification (by stimulated emission) of naturally occurring, spontaneous emission from level Β back to the ground electronic manifold. Once more, the most probable transition is to an excited vibrational state, a, in S 0 rather t h a n to the ground vi brational state A. Thermal equil ibration occurs rapidly from these laser-terminus a states, and the dye molecule population in the S 0 mani fold maintains itself in a Boltzmann distribution, i.e., with only the lowest levels around A significantly populated. This fast thermal equilibration in the ground electronic manifold means t h a t the molecular terminal states of the laser, the a states, will always be emptied. This rapid emptying of the terminal states means t h a t population inversion be tween a—B levels is achieved rela tively easily ; laser threshold for
