Rotational Echoes as a Tool for Investigating Ultrafast Collisional Dynamics of Molecules

We show that recently discovered rotational echoes of molecules provide an efficient tool for studying collisional molecular dynamics in high-pressure gases. Our study demonstrates that rotational echoes enable the observation of extremely fast collisional dissipation, at timescales of the order of a few picoseconds, and possibly shorter. The decay of the rotational alignment echoes in ${\mathrm{CO}}_2$ gas and ${\mathrm{CO}}_2\text{−}\mathrm{He}$ mixture up to 50 bar was studied experimentally, delivering collision rates that are in good agreement with the theoretical expectations. The suggested measurement protocol may be used in other high-density media, and potentially in liquids.

Figure 1

(a) Temporal trace of the alignment signal of ${\mathrm{CO}}_2$ kicked by two pump pulses (${\mathcal{P}}_1$ and ${\mathcal{P}}_2$) in the collision-free conditions depicting the echo ($E$), the second-order echo (SE), and the imaginary echo (IE). (b) Variation of the echo amplitude along the horizontal dashed line depicted in (c). Calculated peak-to-dip amplitude of the rotational echo of ${\mathrm{CO}}_2$, as a function of the delay ${\tau }_{12}$ and intensity of ${\mathcal{P}}_2$, as shown for two intensities of ${\mathcal{P}}_1$: $I_1=20\mathrm{TW}/{\mathrm{cm}}^2$ (c) and ${\mathit{I}}_1=60\mathrm{TW}/{\mathrm{cm}}^2$ (d).

Figure 2

(a) Low-pressure temporal traces of the echo recorded in ${\mathrm{CO}}_2$ for different time delays ${\tau }_{12}$ between the pulse ${\mathcal{P}}_1$ and ${\mathcal{P}}_2$ as a function of the probe delay $\tau$ defined with respect to ${\mathcal{P}}_1$. The intensity of ${\mathcal{P}}_2$ is set to the optimal value for the delay ${\tau }_{12}=2.4\mathrm{ps}$, i.e., $13\mathrm{TW}/{\mathrm{cm}}^2$ (see text). (b) High-pressure temporal traces in ${\mathrm{CO}}_2\text{−}\mathrm{He}$ gas mixture at a pressure of 43.5 bar with an enlargement of the echo decay shown in the inset. The estimated intensity of ${\mathcal{P}}_1$ is $20\mathrm{TW}/{\mathrm{cm}}^2$. The positions of the alignment peaks respective to ${\mathcal{P}}_1$ and ${\mathcal{P}}_2$ are marked with arrows.

Figure 3

(a) Peak-to-dip amplitude of the echo (filled circles) recorded in a ${\mathrm{CO}}_2\text{−}\mathrm{He}$ gas mixture ($P=43.5\mathrm{bar}$) as a function of ${\tau }_{12}$ compared with the CMDS simulations (dashed line). Least-squares fitting of the experimental data with an exponential law (solid line). (b) Decay rates $\gamma$ of the echo (filled circles) measured for various density of the gas mixture with the linear fit (solid line) of the data and the CMDS calculations (dashed line). The error bars reflect the dispersion of the measurements. The bottom (top) $x$ axis denotes the gas density (pressure) in units of amagat (bar). (c) Decay rates $\gamma$ measured in pure ${\mathrm{CO}}_2$.