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Register a new userLaser-induced rotation of iodine molecules in He-nanodroplets: revivals and breaking-free
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Rotation of molecules embedded in He nanodroplets is explored by a combination of fs laser-induced alignment experiments and angulon quasiparticle theory. We demonstrate that at low fluence of the fs alignment pulse, the molecule and its solvation shell can be set into coherent collective rotation lasting long enough to form revivals. With increasing fluence, however, the revivals disappear -- instead, rotational dynamics as rapid as for an isolated molecule is observed during the first few picoseconds. Classical calculations trace this phenomenon to transient decoupling of the molecule from its He shell. Our results open novel opportunities for studying non-equilibrium solute-solvent dynamics and quantum thermalization.
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We show that a 450 fs nonresonant, moderately intense, linearly polarized laser pulse can induce field-free molecular axis alignment of methyliodide molecules dissolved in a helium nanodroplet. Time-resolved measurements reveal rotational dynamics much slower than that of isolated molecules and, surprisingly, complete absence of the sharp transient alignment recurrences characteristic of gas phase molecules. Our results presage a range of new opportunities for exploring both molecular dynamics in a dissipative environment and the properties of He nanodroplets.
Iodine (I$_2$) molecules embedded in He nanodroplets are aligned by a 160 ps long laser pulse. The highest degree of alignment, occurring at the peak of the pulse and quantified by $langle cos^2 theta_{2D} rangle$, is measured as a function of the laser intensity. The results are well described by $langle cos^2 theta_{2D} rangle$ calculated for a gas of isolated molecules each with an effective rotational constant of 0.6 times the gas-phase value, and at a temperature of 0.4 K. Theoretical analysis using the angulon quasiparticle to describe rotating molecules in superfluid helium rationalizes why the alignment mechanism is similar to that of isolated molecules with an effective rotational constant. A major advantage of molecules in He droplets is that their 0.4 K temperature leads to stronger alignment than what can generally be achieved for gas phase molecules -- here demonstrated by a direct comparison of the droplet results to measurements on a $sim$ 1 K supersonic beam of isolated molecules. This point is further illustrated for more complex system by measurements on 1,4-diiodobenzene and 1,4-dibromobenzene. For all three molecular species studied the highest values of $langle cos^2 theta_{2D} rangle$ achieved in He droplets exceed 0.96.
Dimers and trimers of carbonyl sulfide (OCS) molecules embedded in helium nanodroplets are aligned by a linearly polarized 160 ps long moderately intense laser pulse and Coulomb exploded with an intense 40 fs long probe pulse in order to determine their structures. For the dimer, recording of 2D images of OCS$^+$ and S$^+$ ions and covariance analysis of the emission directions of the ions allow us to conclude that the structure is a slipped-parallel shape similar to the structure found for gas phase dimers. For the trimer, the OCS$^+$ ion images and corresponding covariance maps reveal the presence of a barrel-shaped structure (as in gas phase) but also other structures not present in the gas phase, most notably a linear chain structure.
Acene molecules (anthracene, tetracene, pentacene) and fullerene (C$_{60}$) are embedded in He nanodroplets (He$_N$) and probed by EUV synchrotron radiation. When resonantly exciting the He nanodroplets, the embedded molecules M are efficiently ionized by the Penning reaction $mathrm{He}_N^*+mathrm{M}rightarrowmathrm{He}_N + mathrm{M}^+ + e^-$. However, the Penning electron spectra are broad and structureless -- showing no resemblance neither with those measured by binary Penning collisions, nor with those measured for dopants bound to the He droplet surface. The similarity of all four spectra indicates that electron spectra of embedded species are substantially altered by electron-He scattering. Simulations based on elastic binary electron-He collisions qualitatively reproduce the measured spectra, but require the assumption of unexpectedly large He droplets.
We demonstrate the experimental realization of impulsive alignment of carbonyl sulfide (OCS) molecules at the Low Density Matter Beamline (LDM) at the free-electron laser FERMI. OCS molecules in a molecular beam were impulsively aligned using 200 fs pulses from a near-infrared laser. The alignment was probed through time-delayed ionization above the sulphur 2p edge, resulting in multiple ionization via Auger decay and subsequent Coulomb explosion of the molecules. The ionic fragments were collected using a time-of-flight mass spectrometer and the analysis of ion-ion covariance maps confirmed the correlation between fragments after Coulomb explosion. The analysis of the CO+ and S+ channels allowed us to extract the rotational dynamics, which is in agreement with our theoretical description as well as with previous experiments. This result opens the way for a new class of experiments at LDM within the field of coherent control of molecules with the possibilities that a precisely synchronized optical-pump XUV-probe laser setup like FERMI can offer.
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