Simplification of complex absorption spectra

The V-type OODR scheme can be utilized for the simplification and analysis of infrared, visible, or UV molecular spectra. This is particularly helpful if the spectra are perturbed and might not show any regular pattern. Assume the pump laser Li, with intensity 7i, which has been tuned to the transition 1) 2) is chopped at the frequency f. The population densities N, N2 then show a corresponding modulation 

The LIF intensity /fi( 2) induced by the tunable probe laser L2 will be modulated at the frequency f if the wavelength I2 coincides with an absorbing transition 1) - m) from the optically pumped lower level 1 to an upper level m or with a downward transition 2) m) from the upper pump level 2) to a lower level m). 

If the probe laser-induced fluorescence Im(X2) is monitored through a lock-in amplifier at the frequency /i, one obtains negative OODR signals for all transitions 1) - m) and positive signals for the transitions 2) m) (Fig. 5.17). From the phase of the lock-in signal it is therefore, in principle, possible to decide which of the two possible types of probe transitions is detected. Since this double-resonance technique selectively detects transitions that start from or terminate at levels labeled by the pump lever, it is often called labeling spectroscopy. 

In reality, the situation is generally more complex. If the OODR experiments are performed on molecules in a cell that have a thermal velocity distribution and that may suffer collisions, double-resonance signals are also observed for probe transitions starting from other levels than 1 or 2. This is due to the following facts  

In order to avoid such secondary OODR signals, sub-Doppler excitation under collision-free conditions has to be realized. This can be achieved in a collimated molecular beam that is intersected by the two lasers Li and L2 either at two differ- 

In order to avoid such secondary OODR signals, sub-Doppler excitation under collision-free conditions has to be realized. This can be achieved in a collimated molecular beam that is intersected by the two lasers LI and L2 either at two different positions z and zi (Fig-10.18) or with two overlapping laser beams (Fig. 10.19). In the first arrangement the probe laser-induced fluorescence /fi( 2) can be imaged separately onto the detector, and chopping of the pump laser LI with phase-sensitive detection of yields 

The fluorescence intensity /fi( 2) induced by the probe laser with intensity h = /2o(l +cos2 t/20 on a transition starting from the lower level /) of the pump transition is 

The application of OODR in molecular beams to the analysis of complex perturbed spectra is illustrated by Fig. 10.20, which shows a section of the visible NO2 spectrum around X = 488 nm. In spite of the small residual Doppler width of 15 MHz, not all lines are fully resolved and the analysis of the spectrum turns out to be very difficult, because the upper state is heavily perturbed. If in the OODR experiment the pump laser LI is kept on line 1, one obtains the two OODR signals as shown in the upper left part of Fig. 10.20, which proves that lines 1 and 4 in the lower spectrum share a common lower level. The right part of Fig. 10.20 also shows that the lines 2 and 5 start from a common lower level. The whole lower spectrum consists of two rovibronic transitions 7 , K = 10, 5) (J K ) = (11,5) (each with three hfs components), which end in two closely spaced rotational levels with equal quantum numbers (./, K ) that belong to two different vibronic states coupled by a mutual interaction [10.39]. 

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The reduction of Trot and Tyib results in a drastic simplification of the molecular absorption spectrum because only the lowest, still populated levels contribute to the absorption. Transitions from low rotational levels become stronger, those from higher rotational levels are nearly completely eliminated. Even complex spectra, where several bands may overlap at room temperature, reduce at sufficiently low rotational temperatures in cold beams to a few rotational lines for each band, which are grouped around the band head. This greatly facilitates their assignments and allows... 

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