Generation of the simplest rotational wave packet in a diatomic molecule: Tracing a two-level superposition in the time domain

We introduce a time-domain approach to explore rotational dynamics caused by intramolecular coupling or the interaction with dissipative media. It pushes the time resolution toward the ultimate limit determined by the rotational period. Femtosecond pulses create a coherent superposition of two rotational states of carbon monoxide. The wave-packet motion is observed by subsequent Coulomb explosion, which results in a time-dependent asymmetry of spatial fragmentation patterns. The asymmetry oscillation prevails for at least 1 ns, covering more than 300 periods with no decoherence. Long time scans will allow weak perturbations of the order of $\Delta E/E={10}^{-4}$ to be discerned. Our conclusions are confirmed by a fully quantum-mechanical model.

Figure 1

Scheme of the experimental setup. The collinear near-infrared and XUV beams are both focused into the center of the ionization region of a velocity-imaging spectrometer. They intersect there, with the molecular beam entering the spectrometer through a 1-mm skimmer. The delay between the two laser pulses can be varied by a delay line in the infrared laser path (not shown). The ions created by the XUV pulse are projected to a MCP, where a mass gate can be selected by switching the applied voltage. The resulting images taken with a CCD camera are stored with 10 Hz.

Figure 2

Anisotropy observed in a pump-probe scan ($P_0$ $=$ 14 bars) averaged over 25 shots per delay. The inset shows a comparison of the measured signal with a theoretical prediction for a rotational temperature of 2.8 K.

Figure 3

Fourier transforms of two pump-probe scans under different expansion conditions. The frequency domain data clearly show that the signal is dominated by the $J(0\to 2)$ transition, with an increased $J(1\to 3)$ contribution at higher temperature and lower pressure, respectively.