Laser-induced Alignment Research Articles

Measurement of laser-induced alignment of molecules by cross defocusing

The field-free alignment of CO2 produced in response to the excitation of a molecule by a high-intensity femtosecond pump pulse is measured with a simple coronography-like technique. The technique is based on the defocusing of a time-delayed probe pulse produced by the spatial distribution of aligned molecules. In the intensity regime explored here, the technique is shown to give valuable information about dynamic alignment. With the help of simulations, the degree of alignment is extracted from the data.

Nonadiabatic laser-induced alignment of iodobenzene molecules

Nonadiabatic alignment of an asymmetric top molecule induced by a short, moderately intense laser pulse is studied theoretically and experimentally. Numerically, we solve nonperturbatively the time-dependent Schr\odinger equation for a general asymmetric top molecule subject to a moderately intense laser field, and analyze the dependence of the alignment dynamics on the field strength and on the rotational temperature. Experimentally, we use time-resolved photofragment imaging to measure the time-dependent angular distributions of the spatial orientation of the molecules. Our studies, using iodobenzene as a test molecule, focus on the short-time alignment dynamics, during and after the pulse.

Photoinduced birefringence in tellurite glasses

We report on photoinduced birefringence in TeO2-based glasses on exposure to linearly polarized subbandgap light. It is shown that the influence of glass-network modifiers, such as ZnO, Na2O, BaO, and of dopant Er2O3, on the photoinduced birefringence is not substantial indicating a dominating role of the TeO2 component. Photoinduced birefringence is optically reversible and its mechanism is proposed to be due to laser-induced alignment of intrinsic anisotropic defects of tellurite glass, such as distorted TeO4 bipyramids.

Alignment of molecules in pulsed resonant laser fields.

We investigate by numerical simulations the dynamics of alignment of linear molecules in resonant pulsed laser fields and its dependence on pulse length, field strength, and molecular parameters. We propose an analytical short-time approximation for the time-dependent wave packets. We provide a theoretical basis for the occurrence of saturation in the rotational pumping. We present a formula to predict the time at which the maximum alignment occurs. We discuss the magnitude of the laser-induced alignment and we relate it to a theoretical upper limit.

Femtosecond-laser-induced alignment in Coulomb explosion of N2

The dynamic process of laser-induced alignment in Coulomb explosion of N2 has been studied using linearly and circularly polarized, 800 nm, 40 fs Ti:sapphire laser pulses at intensities of 1014 to 1016 W cm−2. The fragment ions produced were observed for the different polarizations with an equal E-field along the detection axis. The intensity-dependent ratio of fragment ion signals has shown that the 40 fs pulses can never align the molecules during the ultrafast interaction at an intensity less than ∼1015 W cm−2, and the dissociative molecular ions start to be aligned at higher intensity. The degree of alignment is increased with an increase in the charge of the dissociative molecular ions.

Polarizability anisotropies of rare gas van der Waals dimers studied by laser-induced molecular alignment

The molecular alignment technique utilizing the interaction between the intense nonresonant laser field and the induced dipole moment is applied to the homonuclear rare gas dimers Rg2 (Rg=Ar, Kr, and Xe). The degree of alignment is investigated by Coulomb exploding Rg2 and by measuring the angular distributions of the fragment ions. At the same peak intensity of the laser field, the degree of alignment ≪cos2 θ ≫ becomes larger in order of Ar2, Kr2, and Xe2, reflecting the order of magnitudes of their polarizability anisotropy Δα. By taking I2 molecules as a reference, Δα of Ar2, Kr2, and Xe2 are estimated to be 0.5, 0.7, and 1.3 Å3, respectively.

Laser-induced nanoparticle ordering

Nanoparticles were produced on the surface of silicon upon pulsed-laser irradiation in the presence of an inert gas atmosphere at fluences close to the melting threshold. It was observed that nanoparticle formation required redeposition of ablated material. Redeposition took place in the form of a thin film intermixed with extremely small nanoparticles possibly formed in the gas phase. Through the use of nonpolarized laser light, it was shown that nanoparticles, fairly uniform in size, became grouped into curvilinear strings distributed with a short-range ordering. Microstructuring of part of the surface prior to the laser treatment had the remarkable effect of producing nanoparticles lying along straight and fairly long (approximately 1 mm) lines, whose spacing equaled the laser wavelength for normal beam incidence. In this work, it is shown that the use of polarized light eliminated the need of an aiding agent: nanoparticle alignment ensued under similar laser treatment conditions. The phenomenon of nanoparticle alignment bears a striking similarity with the phenomenon of laser-induced periodic surface structures (LIPSS), obeying the same dependence of line spacing upon light wavelength and beam angle of incidence as the grating spacing in LIPSS. The new results strongly support the proposition that the two phenomena, LIPSS and laser-induced nanoparticle alignment, evolve as a result of the same light interference mechanism.

Revival structures in picosecond laser-induced alignment of I2 molecules. II. Numerical modeling

We report quantum-mechanical calculations on the formation of aligned rotational wave packets in the ground state of I2, following the excitation of a rotationally cold sample of molecules with an intense picosecond laser pulse. Using these calculations, we have studied (1) the dynamic alignment during or shortly after the intense pump pulse, (2) the alignment at characteristic revival times following the laser excitation, and (3) the alignment between the revivals, as a function of the laser-pulse duration and peak intensity and the initial rotational temperature of the molecular beam. We conclude that the alignment at a revival is maximum for intermediate pulse durations (∼3 ps for I2) that are long enough to get efficient population transfer out of the initially populated state, yet short enough for the interaction to remain partially diabatic and partially adiabatic. The alignment at the revivals increases with laser intensity, although the time-dependent structure of the revivals becomes increasingly complicated as the laser intensity is increased, and the maximum degree of alignment that is obtained is eventually limited by the initial rotational temperature of the molecular beam. The findings in this paper qualitatively agree with experimental results on short-pulse laser-induced alignment that were presented in our experimental article (preceding paper).

Revival structures in picosecond laser-induced alignment of I2 molecules. I. Experimental results

We report experiments on the formation of wave packets consisting of coherently excited ground electronic state rotational levels of I2, using excitation with an intense nonresonant picosecond laser. As measured in a velocity-map-imaging experiment using Coulomb explosion for the determination of the angular distribution of the molecules, these wave packets display alignment of the internuclear axis along the laser polarization axis, both during the laser interaction and at well-defined time delays following the laser interaction, which correspond to rotational revival times of the molecule. The alignment is studied as a function of the intensity and the pulse duration of the pump laser, the rotational temperature of the molecular beam, the polarization geometry of the pump and probe lasers, and the fragment-ion-charge state used to probe the alignment. We observe experimentally that the alignment at revival times is maximal for intermediate pulse durations of a few picoseconds, where the laser–molecule interaction is neither diabatic nor adiabatic. The alignment increases with intensity, but reaches saturation once the intensity is raised sufficiently high. At this point the degree of alignment is limited by the initial rotational temperature of the molecular beam. Our conclusions are corroborated by model calculations, which are presented in detail in the following paper.

Experimental observation of revival structures in picosecond laser-induced alignment of I2.

We report the experimental observation of revival structures in the alignment of a ground-state rotational wave packet following nonresonant excitation of I2 molecules by an intense picosecond laser pulse. The revivals appear at characteristic time delays following the excitation by the pump laser pulse, and show a significant narrowing of the angular distribution during a few picoseconds. The interaction with the pump laser also leads to a steady-state alignment of the molecule, due to rotational pumping.

Enhancement of laser-induced molecular alignment by simultaneous photodissociation

From a three-dimensional time-dependent Schr\"odinger equation simulation for alignment of ${\mathrm{Cl}}_{2}$ by short laser pulses, we show that alignment of the ground state $(X{}^{1}{\ensuremath{\Sigma}}_{g}^{+})$ can be increased by photodissociation via a repulsive dissociative state ${(}^{1}{\ensuremath{\Pi}}_{u}).$ Initially, an intense nonresonant laser pulse ($2\ifmmode\times\else\texttimes\fi{}{10}^{13}{\mathrm{W}/\mathrm{c}\mathrm{m}}^{2},$ 1064 nm) with linear polarization aligns the molecule in the direction of the polarization as measured by $〈{\mathrm{cos}}^{2}\ensuremath{\theta}〉,$ which depends on the $J,$ ${M}_{J}$ initial conditions. Then, a second pulse with the same polarization is added in order to make perpendicular resonant transitions to the repulsive state $(1\ifmmode\times\else\texttimes\fi{}{10}^{13}{\mathrm{W}/\mathrm{c}\mathrm{m}}^{2},$ 375 nm). We show as an example for $J=4$ that the second pulse increases the alignment for each initial ${M}_{J}$ condition as measured by $〈{\mathrm{cos}}^{2}\ensuremath{\theta}{〉}_{{J=4,M}_{J}}$ and the average alignment $〈〈{\mathrm{cos}}^{2}\ensuremath{\theta}〉{〉}_{J=4}$ by reducing the proportion of the ${M}_{J}\ensuremath{\ne}0$ states. This improvement in the alignment of the molecule increases the amplitudes of the average rotational recurrences after the laser pulse and reduces the randomness in these recurrences by removing nonaligned ${M}_{J}\ensuremath{\ne}0$ states.

Laser-induced alignment dynamics of HCN: Roles of the permanent dipole moment and the polarizability

Laser-induced alignment dynamics of HCN: Roles of the permanent dipole moment and the polarizability

Laser-induced alignment dynamics in multiphoton dissociation of H2+

Laser-induced alignment dynamics in multiphoton dissociation of H2+

Effects of different population, orientation, and alignment relaxation rates in resonant four-wave mixing

We present a combined theoretical and experimental study on the effects of different population, orientation, and alignment relaxation rates in resonant four-wave mixing (RFWM). Signal generation in RFWM can be viewed as the formation of and scattering from laser-induced population, orientation, and alignment gratings. We show that the relative contributions from the upper-state and lower-state population, orientation, and alignment gratings to the observed output signal can be changed by varying the polarizations of the three input fields. A theory is developed to account for these changes in collisional environments where the three multipole moments of the total angular momentum distribution, i.e., the population, the orientation, and the alignment, relax unequally. This theory is applied to the OH radical in an atmospheric-pressure H2/O2/He flame for which we have measured the line profiles using high-resolution degenerate and nearly degenerate four-wave mixing. We find that orientation and alignment gratings relax more rapidly than population gratings for low rotational levels of OH in the presence of He but at essentially the same rate for high rotational levels. A discussion is presented of the importance of this effect in the interpretation of RFWM experiments.

Laser-induced molecular alignment in dissociation dynamics.

Fragment angular distributions resulting from photodissociation induced by intense, short, linearly polarized laser pulses are investigated using a two-dimensional time-dependent split operator wave-packet propagation technique. The rotational excitation of the molecule leads, in general, to an alignment of the photofragments with respect to the field polarization vector but, unexpectedly, the reverse situation may also happen for some specific wavelengths for which fragmentation mainly occurs in a plane nearly orthogonal to the polarization axis. A detailed and comprehensive interpretation is provided referring to two typical wavelength regimes corresponding either to the blue or to the red wing of the single photon absorption line shape. The relevant strong field mechanisms upon which this interpretation rests are, respectively, the bond softening and the vibrational trapping of the molecule by the radiation which induces substantial modifications on the internal force field, viewed in the radiatively dressed adiabatic frame. An illustrative example, concerning the photodissociation of an isotropic distribution of ortho-${\mathrm{H}}_{2}^{+}$ in the $^{2}$${\mathcal{J}}_{\mathit{g}}^{+}$(J=1/2, N=1,v=0) initial state, pertaining to Hund's (b) case, reveals that, at \ensuremath{\lambda}=1600 \AA{}, an efficient alignment is obtained with femtosecond pulses delivering an intensity in the range ${10}^{13}$--${10}^{14}$ W/${\mathrm{cm}}^{2}$, as a consequence of the bond softening mechanism. More surprising is the possibility of observing fragments in a near orthogonal direction to the laser polarization, predicted by the trapping mechanism and confirmed by quantum calculations at shorter wavelengths, such as \ensuremath{\lambda}=800 \AA{}.

Laser-induced nuclear motions in the Coulomb explosion of C2H2+ ions.

The laser-induced multifragmentation of C[sub 2]H[sub 2] into protons and multicharged carbon ions is shown to be a direct instantaneous explosion of the molecule. The evolution of the overall nuclear structure is studied through ion-ion correlation peak shapes. The ratios of the maxima of the kinetic-energy release distributions to the Coulomb repulsion energies calculated at the equilibrium internuclear distances are measured to be 45% for the protons and 53% for the C[sup [ital Z]+] ions for all the detected H[sup +]+C[sup [ital Z][prime]+]+C[sup [ital Z]+] +H[sup +] fragmentation channels. The time scale for electronic polarization and stripping compared with the intramolecular electronic and nuclear time evolutions does not allow using a frozen molecular ion structure for the description of the explosion, thus explaining in part the observed fragmentation pattern. During the laser-induced alignment and subsequent stabilization of the molecular frame around the laser polarization direction, the carbon-carbon axis undergoes small damped oscillations that remain larger than the corresponding oscillations of the hydrogen-hydrogen axis. This difference comes from the lower moment of inertia of the hydrogen atoms compared with that of the carbon atoms in the molecule. However, the subsequent deviation from the initial linear structure remains small and is observedmore » when the molecular ion is not completely aligned along the laser electric field.« less

Influence of polarization effects and magnetic fields on time-resolved laser-induced fluorescence measurements.

The theory of time-resolved laser-induced fluorescence (LIF) in a static magnetic field is reformulated by means of the graphical method of angular momentum theory. Invariant general geometrical relations of the time-resolved fluoresence signal are obtained in an explicit form for arbitrary polarization and propagation directions of the primary and secondary radiation and for any direction of the magnetic field. On this basis an extensive experimental study was carried out to find general conditions for lifetime experiments using laser-induced fluorescence, under which the direction of polarization in the excitation and the observation channel and the influence of external magnetic fields does not change the time evolution of the fluorescence signal. It is shown that besides the well-known ``magic-angle'' excitation also measurements at very low pressures (p\ensuremath{\approxeq}${10}^{\mathrm{\ensuremath{-}}4}$ mbar) provide conditions where laser-induced alignment is negligible and does not disturb the time evolution of the decay signal. Furthermore the experiments confirm that the magnitude of laser-induced alignment depends very sensitively on the chosen transition scheme, on the value of the cross section for alignment destroying collisions, and on the geometry of the experiment. Interferences on the time-resolved fluorescence signal by external magnetic fields are also discussed. It is shown that the presence of a strong magnetic field can lead to lifetime results systematically too short under conditions typical for LIF lifetime experiments.

Laser-Induced Pretransitional Ordering And Nonlinear Optical Properties Of Rigid-Rod Polymer Suspensions

ABSTRACTWe investigate the pretransitional behavior in laser-induced alignment of rigid rod-like polytetraflouroethylene (PTFE) suspensions. Using a laser-induced birefringence experiment, we measure both the orientational order parameter and the orientational relaxation time. We find that both increase as the volume fraction of colloidal particles approaches the critical value for the isotropic-nematic phase transition. Experimental results are compared with theory which takes into account the possibility of a first-order phase transition induced by a laser electric field.

Preparation and detection of alignment with high ‖m‖ selectivity by saturated laser optical pumping in molecular beams

We describe a technique for preparation of molecules in single ‖m‖ levels involving saturated laser optical pumping on molecular P, R, and Q transitions. This technique is not limited to small rotational quantum numbers j. It allows the determination of the populations of ‖m‖ levels for arbitrary distribution functions f ( j,m) or alternatively, the determination of all moments of f ( j,m). In principle, this method is able to completely determine the angular distribution of j vectors. Experimental verification of the high ‖m‖-state purity achieved in Na2 supersonic beams, as well as of inherent limitations due to hyperfine interaction is provided. For illustration, experimental data on laser-induced alignment, the dependence of the flow induced molecular alignment, and on Δm-propensity rules in differential rotationally inelastic scattering are presented.

Measurement of laser-induced alignment in a molecular beam

Quantitative results have been obtained for the alignment, due to optical pumping, in a beam of Na2 molecules. This laser-induced alignment offers the possibility of experiments with strongly aligned beams of diatomic of molecules in specific internal states.