Effects of Ultrafast Molecular Rotation on Collisional Decoherence

Using an optical centrifuge to control molecular rotation in an extremely broad range of angular momenta, we study coherent rotational dynamics of nitrogen molecules in the presence of collisions. We cover the range of rotational quantum numbers between $J=8$ and $J=66$ at room temperature and study a crossover between the adiabatic and nonadiabatic regimes of rotational relaxation, which cannot be easily accessed by thermal means. We demonstrate that the rate of rotational decoherence changes by more than an order of magnitude in this range of $J$ values and show that its dependence on $J$ can be described by a simplified scaling law.

Figure 1

Experimental setup. BS, beam splitter; DM, dichroic mirror; CP and CA, circular polarizer and analyzer, respectively, of opposite handedness; DL, delay line; L, lens. $N_2$ marks the pressure chamber filled with nitrogen gas under pressure $P$ and temperature $T$. An optical centrifuge field is illustrated above the centrifuge shaper with $\mathbf{k}$ being the propagation direction and $\mathbf{E}$ the vector of linear polarization undergoing an accelerated rotation.

Figure 2

Experimentally detected Raman spectrogram showing the rotational Raman spectrum as a function of the time delay between the beginning of the centrifuge pulse and the arrival of the probe pulse. Color coding is used to reflect the signal strength. Tilted white dashed line marks the linearly increasing Raman shift due to the accelerated rotation of molecules inside the 100 ps long centrifuge pulse. A one-dimensional cross section corresponding to the Raman spectrum at $t=270\mathrm{ps}$ is shown in white.

Figure 3

Logarithm of the intensity of the experimentally measured Raman lines as a function of time (normalized to 1 at $t=200\mathrm{ps}$, chosen so as to avoid the effects of the detector saturation at earlier times). Dots of the same color represent experimental data for one particular value of the rotational quantum number. Black solid lines show the numerical fit to the corresponding exponential decay. Data were collected at $P=0.75\mathrm{atm}$ and $T=294\mathrm{K}$.

Figure 4

Decay rate of the rotational coherence of ${\mathrm{N}}_2$ as a function of the rotational quantum number at two different temperature values, $T=294\mathrm{K}$ (blue triangles) and $T=503\mathrm{K}$ (red circles), expressed as the Raman linewidth (see the text). Solid curves correspond to the prediction of the simplified ECS approximation at $J>12$. Black squares represent the data from Ref. [11]. The dependence of the adiabaticity factor $a$ on $J$ is shown in the inset for both temperatures, as well as for $T=1100\mathrm{K}$. The solid horizontal line marks the adiabaticity threshold $a=\pi$. Black filled circles mark the rotational levels containing 0.2% of the total population at a given temperature.