11 Optical Diagnostics with Ultrafast and Strong Field Raman Techniques
279
assume that the orientation of the angular momentum is preserved by the collisions
(more precisely the M/J value is conserved). The use of classical molecular dynamics calculations could be a way to assess the validity of either one or the other
assumption [45].
11.4 Conclusion
In this chapter, we have reviewed some time-resolved ultrafast nonlinear coherent techniques and their application to the diagnostic of concentration/temperature,
and their reliability to provide information about collisional relaxation processes,
the knowledge of the latter being crucial for the success of practical applications. Owing to the large spectral width of femtosecond pulses, a superposition
of rotational or rovibrational states can be prepared through a non-resonant Raman excitation resulting in a coherent wavepacket that can be probed over time
and that provides dynamical information on the system. Besides, strong fields
values offer the possibility to investigate specific processes such as molecular
alignment. Due to the absence of resonance conditions, Raman Induced Polarization Spectroscopy (RIPS) is particularly suitable to the detection of several
number of molecules simultaneously, providing they have a anisotropic polarizability. In this case, a simultaneous thermometry procedure is also possible. The
RIPS signal being sensitive to pure rotational relaxation, the accuracy of the diagnostic will directly depends on that of the relaxation processes description. At
the same time it can be a powerful tool to precisely get information on collisional relaxation by allowing to test rotational energy transfer models. When RIPS
measurements are not possible, as for molecular hydrogen, femtosecond Coherent Anti-Stokes Raman Spectroscopy (fs-CARS) is an alternative technique which
proves to be not only a tool for accurate temperature diagnostics, but also capable to study sophisticated effects such as the consequences of collision-induced
radiator velocity changes and of the speed dependence of collisional parameters.
When using strong fields values, the production of molecular alignment happens
to be a unique way of probing the dissipative properties of the studied media and
yielding information which are difficult if not inaccessible to obtain by conventional
frequency-resolved and low field techniques. In particular, it should be possible to
disentangle the elastic and inelastic contributions of the relaxation rates within a
single measurement, and get insight on the reorientation of the angular momentum
under collisions.
Acknowledgements The authors would like to thank Ha Tran, P. Joubert, L. Bonamy, D. Robert,
R. Saint-Loup, D. Sugny, Th. Vieillard, and V. Renard for their contributions to the results presented here.
279
assume that the orientation of the angular momentum is preserved by the collisions
(more precisely the M/J value is conserved). The use of classical molecular dynamics calculations could be a way to assess the validity of either one or the other
assumption [45].
11.4 Conclusion
In this chapter, we have reviewed some time-resolved ultrafast nonlinear coherent techniques and their application to the diagnostic of concentration/temperature,
and their reliability to provide information about collisional relaxation processes,
the knowledge of the latter being crucial for the success of practical applications. Owing to the large spectral width of femtosecond pulses, a superposition
of rotational or rovibrational states can be prepared through a non-resonant Raman excitation resulting in a coherent wavepacket that can be probed over time
and that provides dynamical information on the system. Besides, strong fields
values offer the possibility to investigate specific processes such as molecular
alignment. Due to the absence of resonance conditions, Raman Induced Polarization Spectroscopy (RIPS) is particularly suitable to the detection of several
number of molecules simultaneously, providing they have a anisotropic polarizability. In this case, a simultaneous thermometry procedure is also possible. The
RIPS signal being sensitive to pure rotational relaxation, the accuracy of the diagnostic will directly depends on that of the relaxation processes description. At
the same time it can be a powerful tool to precisely get information on collisional relaxation by allowing to test rotational energy transfer models. When RIPS
measurements are not possible, as for molecular hydrogen, femtosecond Coherent Anti-Stokes Raman Spectroscopy (fs-CARS) is an alternative technique which
proves to be not only a tool for accurate temperature diagnostics, but also capable to study sophisticated effects such as the consequences of collision-induced
radiator velocity changes and of the speed dependence of collisional parameters.
When using strong fields values, the production of molecular alignment happens
to be a unique way of probing the dissipative properties of the studied media and
yielding information which are difficult if not inaccessible to obtain by conventional
frequency-resolved and low field techniques. In particular, it should be possible to
disentangle the elastic and inelastic contributions of the relaxation rates within a
single measurement, and get insight on the reorientation of the angular momentum
under collisions.
Acknowledgements The authors would like to thank Ha Tran, P. Joubert, L. Bonamy, D. Robert,
R. Saint-Loup, D. Sugny, Th. Vieillard, and V. Renard for their contributions to the results presented here.