Session G7: Coherent Molecular Spectroscopy

Laser-induced rotation of molecules in He-nanodroplets: Revivals and breaking-loose

High resolution infrared and microwave spectra of molecules dissolved in liquid helium nanodroplets display discrete rotational structure -- a unique feature explained as the result of frictionless rotation of molecules adiabatically followed by a local solvation shell of He atoms [1]. The frictionless behavior did, however, not manifest itself in recent time-resolved experiments, based on femtosecond laser-induced molecular alignment techniques. In particular, the transient recurrences of alignment characteristic of freely rotating gas phase molecules were absent [2,3]. In this talk we present experiments on femtosecond laser-induced alignment of iodine molecules embedded in helium nanodroplets showing striking new phenomena [4]: 1) At low to moderate fluences the alignment pulse sets the molecule and a non-superfluid fraction of the He droplet into coherent motion. The coherence, although decaying, persists for many hundreds of picoseconds -- long enough to allow the composite molecule-He-shell system to exhibit rotational revivals. Our experimental observations are rationalized by classical considerations and quantum theory based on the angulon quasiparticle [5]. 2) With increasing fluence 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. \textbf{References} [1] M. Y. Choi, \textit{et al.}, Int. Rev. Phys. Chem. \textbf{25}, 15 (2006). [2] D. Pentlehner \textit{et al.}, \textit{Phys. Rev. Lett. }\textbf{110}\textbf{\textit{, }}093002 (2013). [3] L. Christiansen \textit{et al.}, \textit{Phys. Rev. A. }\textbf{92}\textbf{\textit{, }}053415 (2015). [4] B. Shepperson \textit{et al., submitted} (2017). [5] R. Schmidt and M. Lemeshko, Phys. Rev. Lett. \textbf{114} 203001 (2015).   

Opto-Optical Phase Control of Coherent Extreme Ultraviolet Light Pulses

We present an experimental and theoretical study of opto-optical phase modulation of extreme ultraviolet (XUV) free induction decay (xFID)[1]. Coherent XUV light, from high-order harmonic generation [2], is used to promote an ensemble of atoms to a superposition of the ground state and a series of excited states. The technique is demonstrated for a number of different target atoms and includes both bound states and higher lying auto ionizing states. When an ensemble of atoms is exposed to a short, coherent light pulse it will respond collectively and the excited atoms will act as oscillating dipoles. These dipoles may continue to oscillate coherently for a long time after the excitation pulse has passed, resulting in forward scattered light known as free induction decay (FID) [3,4]. This forward scattered light has the same spatial properties as the excitation pulse, but the phase is shifted by $\pi$. The overlap between the two fields will therefore yield the normal absorption spectrum observed in optical spectroscopy. Applying an infrared probe pulse after the excitation pulse we can control the phase of the emitted light. If the delay controlled IR pulse is co-linear, but non-coaxial, with the XUV pulse a Stark induced phase gradient can be induced resulting in a precise control of the direction and timing of the xFID emission. With a single IR control pulse we can form an opto-optical switch, or with multiple pulses an opto-optical modulator. Applications for the opto-optical phase modulator include reducing the temporal jitter in optical-FEL pump-probe experiments, background free 2D spectroscopy in the XUV, and ultrafast ‘which-way’ interferometry. [1] S. Bengtsson et al. submitted [2] M. Ferray et al. J. Phys. B $\bf{21}$, L31 (1988) [3] R. G. Brewer and R. L. Shoemaker, Phys. Rev. A $\bf{6}$, 2001 (1972) [4] F. A. Hopf, R. F. Shea, and M. O. Scully, Phys. Rev. A $\bf{7}$, 2105 (1973)   

A self referencing attosecond interferometer with zeptosecond precision

We present a controlled interferometric measurement of two beating train of attosecond pulses. The attoseond pulse train is generated by higher order harmonics from two sources in a gas phase. By controlling the offset phase between the two train of attosecond pulses we are able to measure the phase of all the harmonics relative to the offset phase of the fundamental $f_0$. Somewhat surprisingly we find that the phase evolution for all the measured harmonics follows the linear relation $\delta \phi_{q} = (2n+1)f_0$. This represents an ideal source for heterodyne spectroscopic measurements in the XUV regime. Phase measurements were performed with a resolution of 12.5 attoseconds or half of the atomic unit of time. The precision of the measurement is in the hundreds of zeptoseconds which can be enhanced in further experiments. Finally, no carrier-envelope phase stabilization nor generation of isolated attosecond pulses is required for the presented measurements, thus reducing the complexity of future experiments.