Probing ultrafast thermalization with field-free molecular alignment

Abstract

International audienceThe rotation-translation thermalization of CO2 gas is investigated 500 ps after its preheating by a nonresonant short and intense laser pulse. The temperature of thermalization is optically determined with two additional short laser pulses enabling a field-free molecular alignment process and its probing, respectively. The measurements are performed for various intensities of the preheat pulse, leading to the observation of different temperatures which are in very good agreement with classical molecular dynamics simulations. The results can be regarded as a step towards real-time tracking of ultrafast relaxation pathways in molecular motion

Using molecular alignment to track ultrafast collisional relaxation

“…As the relaxation of the sample proceeds and equilibrium among different degrees of freedom 063411-3 is approached, the shape of the revival transients resembles more the one recorded when the preheat pulse is switched off. However, as we have shown and analyzed in a previous publication [19], significant changes in the experimental traces obtained with and without the presence of the preheat pulse are still observed when the gas sample is thermalized. These changes are due to the increase of the sample's temperature and manifest in Fig.…”

“…The launch of the preheat pulse prior to the arrival of the aligning results in the alignment of the molecules and consequently in the increase of their rotational energy. As has been already experimentally and theoretically shown in [19] this leads to a significant modification of the revival transients produced by the aligning pulse, once thermalization is reached, i.e., for preheat-aligning delay times τ of about 500 ps. Furthermore, theoretical calculations [19] indicate that these transients depend strongly on the latter time delays, thus enabling one to "follow" and probe the relaxation process.…”

“…On the same grounds as the above, the scope of the present work is to use molecular alignment as a tool for probing the relaxation dynamics taking place in dissipative environments. The theoretical models were proposed by Ramakrishna and Seideman [16,17] and Hartmann and Boulet [18], while the primary experimental demonstration was reported in [11,19,20]. Our goal in the current paper is to extend these previous studies further by showing how field-free alignment can be used to probe the relaxation dynamics in a molecular ensemble that has been previously irradiated with a strong laser pulse.…”

“…Since the experimental setup used here is very similar to the one described previously [19] only a few major points will be recalled below. As we can see from Fig.…”

From Gyroscopic to Thermal Motion: A Crossover in the Dynamics of Molecular Superrotors

“…Transient molecular alignment has first been proposed [18] and later implemented as a powerful probe of collisional relaxation in a series of pioneering experiments [19][20][21]. The three relaxation steps, associated with (i) rotational decoherence, (ii) rotational reorientation, and (iii) rotation-translation (RT) thermalization, have been identified in the theoretical model [22], yet found to overlap in time too closely to enable an individual experimental study of each process separately [21].…”