Rotational cooling

The rotational temperature is defined as the temperature that describes the Boltzmann population distribution among rotational levels. For example, for a diatomic molecule, this is the temperature in Equation (5.15). Since collisions are not so efficient in producing rotational cooling as for translational cooling, rotational temperatures are rather higher, typically about 10 K. 

It might be thought that the small number of molecules in a typical supersonic jet or beam would seriously limit the sensitivity of observation of the spectra. Flowever, the severe rotational cooling which may be produced results in a collapsing of the overall intensity of a band into many fewer rotational transitions. Vibrational cooling, which greatly increases the population of the zero-point level, concentrates the intensity in few vibrational transitions, and these two effects tend to compensate for the small number of molecules. 

An ingeneous method to circumvent this problem was first devised by Zewail and colleagues, who took advantage of the vibrational and rotational cooling properties and collision-free conditions of the supersonic... 

Friedrich, B. and Herschbach, D.R. (1991). On the possibility of orienting rotationally cooled polar molecules in an electric field, Z. Phys. D—Atoms, Molecules and Clusters, 18, 153-161. 

In Fig. 5, additionally the calculated and measured vibrational temperatures [50] are plotted. In contrast to the rotational cooling, there is vibrational heating indicating that there should be enhanced dissociation for vibrating hydrogen molecules on Pd(l 00). Vibrationally enhanced dissociation has been known for years in the gas phase dynamics community [53]. Usually it is associated with strongly curved reaction paths in activated systems [4]. However, the most favorable path towards dissociative adsorption in the system H2/Pd(l 0 0) is purely attractive and has a rather small curvature (see Fig. 2a). Therefore one would not expect any substantial influence of the vibrational state of H2 on the sticking probability. 

However, a slight decrease in So by 10-20% is observed at high nozzle temperature (Tn) which is thought to be due to rotational excitation of D2 molecules [86]. Microscopic reversibility would lead one to predict that rotational cooling should occur on Pt(l 11) upon associative desorption, although no such measurement has been made. Note that a small rotational cooling has been observed for desorption not only from Cu surfaces [98], but also from Pd(l 0 0) surface where hydrogen dissociation is un-activated [99]. 

Due to the jet cooling of the spectrum, the rotational congestion of the vibronic bands was greatly reduced from that in the room temperature spectrum. The rotational cooling caused the two isotopic components of each vibronic band to become distinct (see Figure 15), and sharp bandheads for each isotopomer resulted (see Figures 16 and 17). Thus one benefit of the rotational cooling was accurate relative vibronic band positions. In addition, in FTS the entire spectrum is measured at once, which insures correct relative intensities. As a result, relative vibronic band areas, and therefore relative band intensities, can be accurately measured. Correct... 

See Sheet also material solidified from a melt on a rotating, cooled drum (flaker) and removed by scrapers. 

The next example illustrates how the corresponding atom is chosen. Consider the molecules HI and HBr. The presence of the light H atom results in a very open rotational structure, so that, at high principal quantum numbers, successive Rydberg manifolds overlap. Since separated Rydberg members do not occur at very high n, they can only be sorted out by rotational cooling. 

Figure 9.5 The phase of a vibrational quantum beat depends on whether the bright state for the excitation step is bright or dark in the fluorescence detection step, A molecular beam of anthracene, rotationally cooled to 3K, is excited by a 15 picosecond pulse at 1420 cm-1 above the Si <— So 0q origin band. Fluorescence is detected in a selected wavelength region through...

For example, let us contrast the dissociations of OCS and H202 in a molecular beam. Because of the strong rotational cooling present in molecular beam expansions, the parent angular momentum will be very small. Thus, in the case of OCS, the dissociation can be considered to occur in a single plane. The OCS bending motion, which provides the torque to give... 

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