The alternative to dynamic alignment explanation of experimental results on spatial-asymmetric dissociation of molecules in a laser field is proposed. The concept of geometrical alignment is sufficient for explanation of these results. In this case the spatial anisotropy of interaction of molecules with laser radiation is transferred from one field to another through the ordinary mechanism of nonlinear optical interactions. Thus laser radiation does not create alignment, but only registers it. A physical basis of such nonlinear processes is inequality of forward and reversed optical transitions that corresponds to a concept of time invariance violation in electromagnetic interactions. Directions of the further researches in the field of alignment spectroscopy are discussed.
In this paper, we studied the optimal alignment and anti-alignment of O2 molecules by femtosecond laser pulse under different input energy. The results show that there is an energy limit in molecular alignment. The optimal molecular alignment and anti-alignment degree will never increase with the increase of laser energy when the input laser energy exceeds the energy limit. Besides, under the energy limit, the molecular optimal alignment (or anti-alignment) degree by using a shorter pulse duration (under 100 fs) will be about same when the input laser energy remained constant (different pulse …
We establish a generalized picture of the phase-sensitivity of laser-induced directional bond-breaking using the H$_2$ molecule as the example. We show that the well-known proton ejection anisotropy measured with few-cycle pulses arises as an amplitude-modulation of an intrinsic anisotropy that is sensitive to the laser phase at the ionization time and determined by the molecule's electronic structure. Our work furthermore reveals a strong electron-proton correlation that may open up a new approach to experimentally accessing the laser-sub-cycle intramolecular electron dynamics also in large m…
We show, both classically and quantum mechanically, enantioselective orientation of gas phase chiral molecules excited by laser fields with twisted polarization. Counterintuitively, the induced orientation, whose direction is laser controllable, does not disappear after the excitation, but stays approximately constant long after the end of the laser pulses, behavior unique to chiral systems. We computationally demonstrate this long-lasting orientation using propylene oxide molecules (${\rm CH_{3}CHCH_{2}O}$, or PPO) as an example, and consider two kinds of fields with twisted polarization: a p…
We study ionization dynamics of aligned diatomic molecules N$_2$ in strong elliptical laser fields experimentally and theoretically. The alignment dependence of photoelectron momentum distributions (PMDs) of N$_2$ measured in experiments is highlighted with comparing to Ar measured synchronously. Our results show that the PMDs of N$_2$ depend strongly on the alignment of the molecule, relative to the main axis of the laser ellipse. In particular, the most-probable electron-emission angle which is often used in attosecond measurement, differs remarkably when changing the molecular alignment. We…
We have theoretically investigated the molecular orientation by a asymmetric potential created by the superposition of two-color laser fields. The time-dependent Schrodinger equation is solved numerically for different field parameters. We have shown how enhancement or suppression of the molecular orientation can be manipulated by the laser field parameters, such as time between laser pulses, the different intensity of the pulses, etc.
Based on the frequency-domain theory, we put forward a method of imaging the complex symmetrical polyatomic molecules by using IR+XUV two-color laser fields. Although the wave function of a polyatomic molecule, such as SF6, is quite complex in momentum space, it can be simplified by several terms in the region of high value of momentum, where the destructive interference fringes from these terms carry the information of the molecular structure. In this work, we find that, since the XUV laser field may increases the kinetic energy of the photoelectron dramatically in the above-threshold ionizat…
Molecular chirality is an omnipresent phenomenon of fundamental significance in physics, chemistry and biology. For this reason, search for novel techniques for enantioselective control, detection and separation of chiral molecules is of particular importance. It has been recently predicted that laser fields with twisted polarization may induce persistent enantioselective field-free orientation of chiral molecules. Here we report the first experimental observation of this phenomenon using propylene oxide molecules ($\mathrm{CH_{3}CHCH_{2}O}$, or PPO) spun by an optical centrifuge - a laser pul…
Bound states of solitons (molecules) occur in various settings, playing an important role in the operation of fiber lasers, optical emulations, encoding, and communications. Soliton interactions are generally related to breathing dynamics in nonlinear dissipative systems, maintaining potential applications in spectroscopy. In the present work, dichromatic breather molecules (DBMs) are created in a synchronized mode-locked fiber laser. Real-time delay-shifting interference spectra are measured to display the temporal evolution of the DBMs, that cannot be observed by means of the usual real-time…
We present a comprehensive study of enantioselective orientation of chiral molecules excited by a pair of delayed cross-polarized femtosecond laser pulses. We show that by optimizing the pulses' parameters, a significant (~ 10%) degree of enantioselective orientation can be achieved at zero and at five kelvin rotational temperatures. This study suggests a set of reasonable experimental conditions for inducing and measuring strong enantioselective orientation. The strong enantioselective orientation and the wide availability of the femtosecond laser systems required for the proposed experiments…
Steady-state plasmonic lasing is studied theoretically for a system consisting of many dye molecules arranged regularly around a gold nano-sphere. A three-level model with realistic molecular dissipation is employed to analyze the performance as function of the pump field amplitude and number of molecules. Few molecules and moderate pumping produce a single narrow emission peak because the excited molecules transfer energy to a single dipole plasmon mode by amplified spontaneous emission. Under strong pumping, the single peak splits into broader and weaker emission peaks because two molecular …
In this research, we employ accurate time-dependent density functional calculations for ultrashort laser spectroscopy of nitrogen molecule. Laser pulses with different frequencies, intensities, and durations are applied to the molecule and the resulting photoelectron spectra are analyzed. It is argued that relative orientation of the molecule in the laser pulse significantly influence the orbital character of the emitted photoelectrons. Moreover, the duration of the laser pulse is also found to be very effective in controlling the orbital resolution and intensity of photoelectrons. Angular res…
