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ics share in common similar techniques, which rely on the coherent superposition of rovibrational states, created by a very short laser pulse, resulting in quantum beats that can be recorded over time. The different dephasing processes experienced by the molecules affect the dynamics of such wave packets that can
be tracked on a picosecond time scale in order to access to the relevant information.
In the first part of this chapter, we will discuss the use of such coherent techniques for temperature and concentration diagnostics in the low field limit by focusing on Raman Induced Polarization Spectroscopy (RIPS) [1, 2] and femtosecond Coherent Anti-Stokes Raman Spectroscopy (fs-CARS) [3]. The principle of
the two techniques relies on the impulsive electronically non-resonant Raman excitation of a manifold of rotational states with an ultrashort polarized pump laser.
In both cases, the signal contains information about the molecular structure, the
density, the temperature, and the concentration. In the former, the initial vibrational level of the molecule (generally the ground one) is not changed and the
rotational superposition of states leads to transient alignments appearing with a
time period determined by the rotational constant B. In the latter, the signal arises
from the interferences between rovibrational transitions [4] and exhibits recurrences with a time period, which in the case of Q-branch transitions is given by
T r = 1/2α e , where α e is the first order rotational anharmonicity of the potential.
RIPS is suitable for molecules such as N 2 , CO 2 , O 2 , or N 2 O with small rotational constants and for which rotational transitions are in the range of tenths of
wavenumbers, whereas CARS will be preferred in the case of lighter molecules
such H 2 with a larger rotational constant and a rotational spectrum spreading over
a large wave number domain. Moreover, in contrast to RIPS or other techniques,
CARS is able to probe not only the ground state, but also the dynamics of excited
states.
Besides the oscillating contribution coming from the interferences, the molecules
undergo collisions and collisional relaxation, which manifests itself on the signal by
a time decay. Taking precisely into account these complex mechanisms is of great
importance for temperature and pressure measurements.
The case of hydrogen will be discussed apart. Indeed, this molecule exhibits
unusual behaviours due to the speed-dependence of the collisional parameters and
requires specific models to describe the CARS time response.
In the second part of this chapter, we will explore the extension of short pulse
induced alignment to dissipative environments. Although there are a real fundamental interest and practical applications of alignment in a dissipative medium,
field-free alignment has been mainly studied in low-density conditions. We will
show that performing measurement in a dissipative medium is a way to get independent information about the rotational population relaxation and the pure phase
decoherence effects. In this context, a quantum mechanical density matrix formalism, using the Liouville equation and the relaxation matrix, will be described. Results obtained on the CO 2 molecule in mixture with Ar at room temperature will be
presented.
F. Chaussard et al.
ics share in common similar techniques, which rely on the coherent superposition of rovibrational states, created by a very short laser pulse, resulting in quantum beats that can be recorded over time. The different dephasing processes experienced by the molecules affect the dynamics of such wave packets that can
be tracked on a picosecond time scale in order to access to the relevant information.
In the first part of this chapter, we will discuss the use of such coherent techniques for temperature and concentration diagnostics in the low field limit by focusing on Raman Induced Polarization Spectroscopy (RIPS) [1, 2] and femtosecond Coherent Anti-Stokes Raman Spectroscopy (fs-CARS) [3]. The principle of
the two techniques relies on the impulsive electronically non-resonant Raman excitation of a manifold of rotational states with an ultrashort polarized pump laser.
In both cases, the signal contains information about the molecular structure, the
density, the temperature, and the concentration. In the former, the initial vibrational level of the molecule (generally the ground one) is not changed and the
rotational superposition of states leads to transient alignments appearing with a
time period determined by the rotational constant B. In the latter, the signal arises
from the interferences between rovibrational transitions [4] and exhibits recurrences with a time period, which in the case of Q-branch transitions is given by
T r = 1/2α e , where α e is the first order rotational anharmonicity of the potential.
RIPS is suitable for molecules such as N 2 , CO 2 , O 2 , or N 2 O with small rotational constants and for which rotational transitions are in the range of tenths of
wavenumbers, whereas CARS will be preferred in the case of lighter molecules
such H 2 with a larger rotational constant and a rotational spectrum spreading over
a large wave number domain. Moreover, in contrast to RIPS or other techniques,
CARS is able to probe not only the ground state, but also the dynamics of excited
states.
Besides the oscillating contribution coming from the interferences, the molecules
undergo collisions and collisional relaxation, which manifests itself on the signal by
a time decay. Taking precisely into account these complex mechanisms is of great
importance for temperature and pressure measurements.
The case of hydrogen will be discussed apart. Indeed, this molecule exhibits
unusual behaviours due to the speed-dependence of the collisional parameters and
requires specific models to describe the CARS time response.
In the second part of this chapter, we will explore the extension of short pulse
induced alignment to dissipative environments. Although there are a real fundamental interest and practical applications of alignment in a dissipative medium,
field-free alignment has been mainly studied in low-density conditions. We will
show that performing measurement in a dissipative medium is a way to get independent information about the rotational population relaxation and the pure phase
decoherence effects. In this context, a quantum mechanical density matrix formalism, using the Liouville equation and the relaxation matrix, will be described. Results obtained on the CO 2 molecule in mixture with Ar at room temperature will be
presented.