Differential Power Broadening

Perturbations often provide information about previously unknown perturbing states. Extra lines, arising from levels with predominant perturber character, are perhaps the most information-rich features of a perturbed band spectrum. When the perturbation interaction is very weak, one is lucky to locate an extra line and even luckier to be able to prove that this line is extra rather than extraneous (rare isotope or impurity). 

An optical-optical double resonance (OODR) scheme exists, utilizing two continuous-wave (cw), monochromatic, tunable lasers, whereby the rotational quantum numbers of all observed lines may be established without ambiguity, prior knowledge of B-values, trial-and-error searches for consistent combination differences, or redundant confirmation lines that are weak because of level population may be distinguished from those that are weak because of intrinsic linestrength and forbidden transitions may be made to appear with comparable peak intensities but considerably narrower widths than allowed transitions. 

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Figure 6.13 OODR differential power broadening in BaO. The PUMP laser is tuned to the A1 E+ <— X1 E+ (1,0) R(50) line the PROBE is scanned in the region near P(51) of C1 E+ <— A1E" "(3,1). The main line (P3) and two of four extra lines (P2 and P4) are shown. The unassigned collisional satellite line marked with an arrow has area equal to that of the P4 extra line [from Gottscho and Field (1978).]...

An extremely sensitive MODR scheme, microwave optical polarization spectroscopy (MOPS), was introduced by Ernst and Torring (1982). The most important features of MOPS are that it requires respectively 100 and 10 times lower laser and microwave intensities than MODR and results in 10 times narrower lines. This means that it will be possible to take full advantage of differential power broadening effects (Section 6.5.1) and to utilize low-power, frequency-doubled dye lasers and low-power, broadly tunable microwave sources (backward wave oscillators) in order to gain access to and systematically study perturbations. 

There are also several perturbation-sensitive non-isolated molecule properties, such as collision-induced population anomalies (Radford and Broida, 1963) (Section 6.5.5) and differential pressure and power (Gottscho and Field, 1978) broadening efforts (Section 6.5.1). Since all of the isolated molecule properties are explicitly related to p, the following discussion focuses on p. Note, however, that the nature of a perturbation related intensity anomaly is profoundly dependent on whether /x 2, r, or (A, B, /, a) is being measured and on the state selectivity and spectral resolution of the specific experiment. 

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