Laser double-resonance experiments

The infrared photochemistry of SF continues to attract considerable interest. One important study involves excitation of SFg in high-resolution diode-infrared laser double resonance experiments. At high intensities, and 30 ps infrared pulses, spectral hole burning is observed at energies corresponding to the absorption of between 4 and 5 photons per molecule (Reiser et al. cf. Sharp et al.). 

Our studies of the effect of velocity-changing collisions in an rf-laser double resonance experiment contribute to a new vista into the role of collisictis in laser spectroscopy of sub-level structures the limitation of the observation time of the active atoms due to narrow-bandwidth optical excitation and simultaneous velocity diffusion can be of importance for a variety of spectroscopic techniques that use a velocity-selective excitation and detection of either sublevel populations or sublevel coherence. On the other hand, the collisional velocity diffusion of sublevel coherence within an optical Doppler distribution can also give rise to new and surprising phenomena as will discussed in the next section. 

Fig. 11. Optical pumping of molecules and scheme for double resonance experiments on laser-pumped molecules in case of Zeeman splitting of rotational levels of the electronic ground state...

We report on saturated absorption experiments in Na2> realized with a tunable and stabilized argon laser. These experiments provide both spectroscopic and physical results, which help in understanding the behavior of optically pumped alkali dimer lasers. We briefly describe a new double resonance experiment which enables us to study the gain line-shapes of the dimer laser and to demonstrate the backward-forward gain competition. 

NO2 is one of the few molecules for which the dependence of the dissociation rate on the rotation of the excited complex has been investigated. In a double-resonance experiment. Bezel et al. [276] first excited NO2 with an IR photon to a particular rovibrational state, (1, 0,1 J, Ka), and a second laser with fixed frequency promoted the prepared molecules into the continuum. Here, J is the total rotational angular momentum quantum... 

Most double resonance experiments exploit one of the four energy level schemes shown in Fig. 1.17. The variety of detection schemes is enormous, but most schemes may be divided into those which result in a signal on an essentially dark background vs. those which result in a dip in an essentially constant background level. By definition, the frequency of the PROBE (or DUMP) laser is scanned and that of the PUMP or DETECT laser is held fixed while a double resonance spectrum is being recorded. 

Millimeter wave spectroscopy with a free space cell such as a Broida oven is more sensitive than lower frequency microwave spectroscopy. However, the higher J transitions monitored by millimeter wave spectroscopy often do not show the effects of hyperfine structure. In the case of CaOH and SrOH, the proton hyperfine structure was measured in beautiful pump-probe microwave optical double resonance experiments in the Steimle group [24,68], They adapted the classic atomic beam magnetic resonance experiments to work with a pulsed laser vaporization source and replaced the microwave fields in the A and C regions by optical fields (Fig. 15). These sensitive, high-precision measurements yielded a very small value for the proton Fermi contact parameter (bF), consistent with ionic bonding and a... 

Double-resonance experiments involving i.r. pumping followed by visible or u.v. laser probing of the vibrationally excited species have been reported by several groups. Laser-induced fluorescence of thiophosgene following MPA of COj radiation at intensities > 3 MW cm has shown that the vibrational populations of i = 0—4,6, and 8 of the V4 mode are depleted or not populated under collision-free conditions when the initially pumped mode is 2v, and only at lower laser intensities are population increases in these levels observed. It appears that all... 

In all these double resonance experiments the pump and probe beams excited transitions well separated in frequency. It is equally possible for both lasers to excite the same close group of levels. For example. 

In Fig. 8.19 the experimental setup for such time resolved double resonance experiments is schematically shown. The laser beam is split by the beam splitter BS into a pump pulse and a probe pulse which travel along different pathlengths before they enter the sample cell. The probe pulse is sent through a Raman shifter where its wavelength can be tuned to different transitions of the sample molecule. The time delay between pump- and probe pulse can be tuned by a movable retro-reflector. 

Very recently optical-optical double resonance experiments involving two lasers were performed for the study of highly excited electronic states. 

In a similar experiment Frenkel et at " investigated CHjCP and and measured the influence of P(26) CO2 laser radiation on the microwave absorption. As was also reported in the earlier experiments, e.g., by Ronn and Lide, and by Shimizu and Oka, Frenkel and co-workers observed that in such a double resonance experiment not only the participating levels but also other rotational levels, vibrational levels, and even species of different isotopic composition are affected via collisional transfer. Frenkel et al also determined relaxation times. This was accomplished by modulating the laser intensity with a chopper whose speed could be varied between 80 and 4000 Hz and by measuring the rise time of the double resonance signal from which vibrational relaxation rates can be deduced. 

Spectroscopy conducted on ions which have been trapped by an appropriate configuration of fields leads to results void of inhomogeneous broadening sources and allows measurements with high spectral resolution. In a recent laser-microwave double resonance experiment conducted on Yb trapped ions, Blatt and co-workers (1983) resolved the ground state hyperfine splitting of the 171 isotope with a Q-factor, i.e., Avlv, of 10 . 

Fig. 14. Increase in resonance fluorescence observed in laser-rf double resonance experiment on one of the hyperfine levels of the state of After Childs et al. (1983). Using similar methods a...

F. 1 Schematic representation of the molecular beam set-up equipped with a laser desorption source and a reflectron Time-of-Flight mass spectrometer used for IR-UV double resonance experiments at the FELIX Facility. The inset shows details of a typical laser desorption source... 

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