Collision-induced rotational transfer

Fig. 8 Collision-induced rotational transfer in collisions between Ar and N2(0 10) squares), O2(0 12) circles) and OF1(0 3) triangles). Collision conditions are Maxwell-Boltzmann distribution of velocities at 300 K. Note that there is a Aj = 2n selection rule in collision-induced processes for homonuclear diatomic molecules...

Collision induced rotational energy transfer. A new scaling law probed by fs CARS... 

Figure 2. Schematic of the energy levels for the OH molecule. The collision-induced energy-transfer transitions are denoted by double-line arrows. The rotational quantum number is denoted by K or K". Both spin doubling and lambda doubling have been suppressed for clarity.

The cross-section for collision-induced rotational energy transfer is very large, of the order of magnitude of the hard sphere cross-section or larger. 

Unlike the case of collision-induced vibrational energy transfer, collision induced rotational energy transfer seems to be free of strong restrictions on the changes in the rotational quantum numbers. When account is taken of the spectral widths of the excitation sources used, the nature of the rotation-vibration structure in the fluorescence and absorption spectra, and the possibility of resonant ener f transfer in the collision, it is concluded that the studies of Bj aniline are the weakest, those of B2 benzene better, and those of glyoxal the best available. With this hierarchy of quality of information kept in mind, the following weaker conclusions can also be obtained from the studies cited. 

Collision-induced rotational relaxation is very much more efficient when resonant transfer of electronic energy is possible than when it is not possible. The cross-sections reported for rotational energy transfer in collisions of Bjy and benzene molecules, and in collisions of... 

Collision induced rotational energy transfer. A new scaling law probed by fs CARS G. Knopp, P. Beaud, P. Radi, M. Tulej and T. Gerber 69... 

H. Klar, Theory of collision-induced rotational energy transfer in the 1/4-state of diatomic molecule. J. Rhys. B 6, 2139 (1973)... 

This computational routine for predicting state-to-state probabilities has been shown to give quantitative agreement with experiment for a wide variety of collision-induced processes. These include rotational transfer (RT) [24, 26, 36, 38—40], i.e., state change within a vibrational manifold, vibration-rotation transfer (VRT) [41, 42], i.e., transitions between vibrational states and electronic energy transfer [43], i.e., transitions between discrete levels of different electronic states. An example is given in Fig. 6 in which the calculated data were computed using the... 

Fig. 5 Plot of rotational transfer rate coefficients for state-to-state collision-induced transfer in A(X) Li2 with Ne, Ar and Xe as collision partners from experiment (symbols) and via (5) (full line). Numerous other data sets on a wide range of molecules were found [36] to give fits of similar quality to (5)...

Early optically detected microwave transition linewidth studies (Pratt and Broida, 1969) suggested that CN A2II(u = 10) —> B2E+(u = 0) collision-induced transitions through perturbed levels occur at gas-kinetic collision rates, faster than pure rotational relaxation. The pressure dependence of chemiluminescence in the Ba + N2O system also implied BaO b3II(u = 10) —> A1E+(u = 1) collisional transfer through spectroscopic doorways (Field, et al., 1974). 

Collision-induced intramolecular vibration-to-rotation energy transfer appears to be inefficient. The evidence for this inference comes from the study of rotational contours in the one collision-induced transition 7 0° in glyoxal. It is found that the emission from 0° has a distribution over rotational transitions that is close to the thermal distribution. But the vibration v-j in glyoxal is a torsional motion, and the axis of torsion very nearly coincides with the smallest axis of inertia of the molecule, so if collision-induced intramolecular vibra-tion-to-rotation transfer were efficient the emission from 0 should have a nonthermal distribution in the quantum number K (which describes quantization of the motion about the smallest axis of inertia). Note, however, that the collision partner used in this experiment was... 

Such determinations of rotationally inelastic integral cross sections ajcm for collision-induced transitions in excited molecules obtained from measurements of satellite lines in the fluorescence spectrum have been reported for a large variety of different molecules, such as I2 [983, 984], Li2 [985, 986], Na2 [987], or NaK [988]. For illustration, the cross section a (A7) for the transition 7 7 + A7 in excited NaJ molecules induced by collisions Na + He are plotted in Fig. 8.8. They rapidly decrease from a value a(AJ = 1) 0.3 nm to a(AJ = 8) 0.02 nm. This decrease is essentially due to energy and momentum conservation, since the energy difference AE = E(J A7) — E(J) has to be transferred into the kinetic energy of the collision partners. The probability for this energy transfer is proportional to the Boltzmann factor exp[—A /(A r)] [989]. 

While collision-induced transitions in excited electronic states can be monitored through the satellite lines in the fluorescence spectrum (Sect. 8.2.2), inelastic collisional transfer in electronic ground states of molecules can be studied by changes in the absorption spectrum. This technique is particularly advantageous if the radiative lifetimes of the investigated rotational-vibrational levels are so long that fluorescence detection fails because of intensity problems. 

One example of this pump-and-probe technique is the investigation of collision-induced vibrational-rotational transitions in the different isotopes HDCO and D2CO of formaldehyde by an infrared-UV double resonance [1041]. A CO2 laser pumpes the V6 vibration of the molecule (Fig. 8.18). The collisional transfer into other vibrational modes is monitored by the fluorescence intensity induced by a tunable UV dye laser with variable time delay. 

Since the scattering angle 0 is related to the impact parameter b, the experiment indicates which impact parameters contribute preferentially to collision-induced vibrational or rotational transitions, and how the transferred angular momentum A 7... 

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