OODR spectroscopy

The combination of supersonic-jet-laser spectroscopy and high-level ab initio calculations provides a powerful method for probing the geometrical structure and photophysical properties of small molecules. In this chapter, we described the experimental and theoretical characterizations of geometrical structures and elementary photoprocesses in supersonic free jet. The major focus of the review was on the OODR spectroscopy, which is designed to identify and characterize dark electronic states of the molecule. The main conclusions that emerge from the review can be summarized as follows. 

Fig. 5.19 OODR spectroscopy in a molecular beam with spatially separated pump and probe beams. The probe transition L2 can be monitored by ionizing the excited levels m with L3...

Fig. 5.20 OODR spectroscopy in a collimated molecular beam with overlapping pump and probe beams and detection at the sum frequency...

Fig. 5.33 A-type OODR spectroscopy illustrated for the example of the Cs2 molecule (a) term diagram (b) two OODR signals of the same transition measured with collinear and anticollinear propagation of pump and probe beams. For this example the homogeneous width of the upper (predissociating) level is y2 100 MHz [605]...

Questions of linkage are posed and answered by asking the molecule to satisfy successively two resonance conditions. Schemes which accomplish this include Dispersed Fluorescence Spectroscopy (DF, Section 1.2.2.2 a laser is tuned to excite a single line and the spectrum of the resulting molecular fluorescence is recorded), Modulated Population Spectroscopy (MPS, Section 1.2.2.3) an intense, fixed frequency, amplitude modulated PUMP laser is used to modulate the population in the upper and lower levels connected by the laser excited transition the modulation is then detected by a frequency scanned PROBE laser), which is an example of Optical Optical Double Resonance (OODR, Section 1.2.2.3). 

Two-color, Resonance Enhanced Multi-Photon Ionization (REMPI) spectroscopy is similar to OODR, differing only in that the... 

A perturbation acts as a window through which normally unobservable states may be viewed. Anticrossing, quantum-beat, and MODR spectroscopies are capable of examining the states in the immediate vicinity of the perturbed level. Optical-optical double resonance (OODR) allows systematic exploration of states far above and below the perturbed level. Gottscho, (1979) utilized an... 

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. 

The downward transitions can be probed by the change in transmission of the probe wave by OODR-polarization spectroscopy or by ion-dip spectroscopy (Sect. 5.5.1). 

Similar to polarization spectroscopy with a single tunable laser (Sect. 2.4), in OODR-polarization spectroscopy the sample cell is placed between two crossed polarizers and the transmitted probe laser intensity /t( 2) is measured as a function of the probe laser wavelength A2. 

Fig. 5.42 Triple-resonance spectroscopy (a) population of high vibrational levels in the ground state by stimulated emission pumping and absorption of a third laser radiation, resulting in the population of levels just below and above the dissociation energy of the excited A Eu state of Na2 [617] (b) stepwise excitation of Rydberg levels by OODR and microwave-induced transitions to neighboring levels...

Doppler-free two-photon spectroscopy or with two-step excitation (Sect. 5.4). For illustration, Fig. 8.5 illustrates pressure broadening and shifts of a rotational transition to a Rydberg level of the Li2 molecule measured with Doppler-free OODR polarization spectroscopy (Sect. 5.5) in a lithium/argon heat pipe [980], where the intermediate level B(v, J ) was pumped optically by a circularly polarized pump laser. For the chosen temperature and pressure conditions the argon is confined to the cooled outer parts of the heat pipe, and the center of the heat pipe contains pure lithium vapor (98 % Li atoms and 2 % Li2 molecules) with a total vapor pressure p(Li) = p(Ar) up to argon pressures of 0.7 mbar. The observed pressure broadening and shift in this range p < 0.7 mbar are therefore caused by Li + Li collisions. 

F. ByUcki, G. Persch, E. Mehdizadeh, W. Demtroder, Saturation spectroscopy and OODR of NO2 in a collimated molecular beam. Chem. Phys. 135, 255 (1989)... 

PERTURBATION FACILITATED OODR RESOLVED FLUORESCENCE SPECTROSCOPY OF THE 3,... 

Resolved OODR Ilg - Fluorescence Spectroscopy, J. Chem. Phys. 

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