When strong optical fields interact with molecules, electron cloud polarization is induced. If a molecule’s polarizability is asymmetrical along different axes, the optical field will tend to align the molecules. If the pulse duration is longer than the rotational period of the molecules, adiabatic alignment will trap molecules in pendular states. By contrast, if the optical pulse is much shorter than the rotational period of the molecules, transient field-free alignment can be achieved. We use femtosecond/picosecond CARS signals as a sensitive probe to quantify the degree of alignment induced within the probed molecules.
The combination of adiabatic and nonadiabatic alignment fields has been studied in molecular H2, for example, and shown to exhibit a higher degree of molecular alignment than is achievable with either adiabatic or nonadiabatic alignment alone. Figure 1 displays a 1D fs/ps CARS image that is sensitive to the degree of molecular alignment. The vertical axis of this image corresponds to position along a line in space. The horizontal axis is the frequency of the signal. As the two populated rotational states of H2 are emitted at different frequencies, the signal from each rotational level is isolated in the two bright vertical signals observed. At the center of the probed region, a focused nanosecond nonadiabatic alignment field interacted with the molecules. A bright enhancement of the signal, signifying increased molecular alignment, is shown on the J”=1 rotational line within the image. This enhancement corresponds to the spatial location where the adiabatic aligning field was focused.
- Coherent nonlinear optical spectroscopy is used both to control and probe the degree of molecular alignment.
- The combination of adiabatic and nonadiabatic molecular alignment approaches yields a higher degree of molecular control. We are presently implementing such approaches in the development of a refined optical centrifuge with increased control over the final rotational state distribution.