Laser-induced Alignment Research Articles

Cross sections for rotational decoherence of perturbed nitrogen measured via decay of laser-induced alignment

We quantitatively determine cross sections for rotational decoherence from the decay of nonadiabatic laser-induced alignment in nitrogen and nitrogen-foreign gas mixtures in a temperature range between 80 K and room temperature. The cross section for rotational decoherence in pure nitrogen decreases from 102 A(2) at 80 K to 48 A(2) at 295 K, leading to long-lived coherences even at high temperatures. Comparison with the broadening of the transition lines of the Raman Q-branch reported in the literature shows that the decay of rotational coherence proceeds at the same rate as rotational depopulation. This is verified also for mixtures of nitrogen with hydrogen, helium, argon, and krypton. We discuss limits posed by a possible J-dependence of the cross sections and strategies for state resolved determination from the time-dependent alignment signal.

Fabrication of Dot-like Nano-protrusions on Silicon Surfaces Using Nanosecond Pulse Nd:YAG Laser Irradiation

An array of Nano-protrusions (NPs) was fabricated on silicon surfaces in air and a low pressure environment using pulsed laser irradiation. The uniformly sized NPs were linearly aligned at a low laser energy density, which is typically a few kilojoules per square meter. The linear array of NPs appeared perpendicular to the laser polarization vector E, and the interval of the NPs lines was almost equal to the wavelength of the incident laser. The primary NPs grew epitaxially and were covered with an oxide film layer, which was determined by means of a structural and elemental analysis using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (EDS). We propose a nano-patterning fabrication method using the laser-induced NPs alignment based on a microscopic structural analysis.

Field-free alignment in repetitively kicked nitrogen gas

We demonstrate a high level of laser-induced transient alignment in room temperature and density ${\text{N}}_{2}$ with a technique that avoids laser field ionization. Our measured alignment shows an improvement over previous one-pulse or two-pulse experimental alignment results and approaches the theoretical maximum value. We employ eight equally spaced ultrafast laser pulses with a separation that takes advantage of the periodic revivals for the ensemble of quantum rotors. Each successive pulse increases the transient alignment $[⟨{\text{cos}}^{2}\text{ }\ensuremath{\theta}⟩(t)]$ and also moves the rotational population away from thermal equilibrium. These measurements are combined with simulations to determine the value of $⟨{\text{cos}}^{2}\text{ }\ensuremath{\theta}⟩$, the $J$-state distributions, and the functional dependencies of the alignment features.

Laser-induced Alignment and Coulomb Explosion of CO2

Dynamic processes of CO2 are experimentally studied in intense femtosecond laser fields with laser intensity varying from 1 1013 W/cm2 to 6 1014 W/cm2. When the laser intensity is below the ionization threshold, a coherent rotational wave-packet is formed for CO2 at room temperature through nonadiabatic rotational excitation. The evolution of the wave-packet leads to transient alignment. The field-free alignment revives periodically after the laser pulse is over. The revival structure can be modified by a second laser pulse for the rotational wave-packet through precisely adjusting the time delays between the two laser pulses. When the laser intensity excesses the ionization threshold, ionization and Coulomb explosion occur. The atomic ions Cm+ (m = 13) and On+ (n = 13) observed in the experiment exhibit highly anisotropic angular distributions relative to the laser polarization. Using two linearly polarized laser pulses with crossed polarization, we conclude that the anisotropic angular distribution results from dynamic alignment, in which the rising edge of the laser pulse aligns the neutral CO2 along the laser polarization direction prior to ionization.

Quantum-state selection, alignment, and orientation of large molecules using static electric and laser fields

Supersonic beams of polar molecules are deflected using inhomogeneous electric fields. The quantum-state selectivity of the deflection is used to spatially separate molecules according to their quantum state. A detailed analysis of the deflection and the obtained quantum-state selection is presented. The rotational temperatures of the molecular beams are determined from the spatial beam profiles and are all approximately 1 K. Unprecedented degrees of laser-induced alignment (<cos2 theta2D>=0.972) and orientation of iodobenzene molecules are demonstrated when the state-selected samples are used. Such state-selected and oriented molecules provide unique possibilities for many novel experiments in chemistry and physics.

Laser-Induced Alignment and Orientation of Quantum-State-Selected Large Molecules

A strong inhomogeneous static electric field is used to spatially disperse a supersonic beam of polar molecules, according to their quantum state. We show that the molecules residing in the lowest-lying rotational states can be selected and used as targets for further experiments. As an illustration, we demonstrate an unprecedented degree of laser-induced one-dimensional alignment (cos;(2)theta_(2D)=0.97) and strong orientation of state-selected iodobenzene molecules. This method should enable experiments on pure samples of polar molecules in their rotational ground state, offering new opportunities in molecular science.

Sympathetic cooling by collisions with ultracold rare gas atoms, and recent progress in optical Stark deceleration

We propose a general scheme for sympathetic cooling of molecules to microK temperatures on a timescale of seconds. Experimental parameters have been estimated from theory, which indicate the viability of the scheme. This method, which is particularly suited to optical Stark deceleration, utilises ultracold, laser cooled metastable rare gas atoms quenched to their ground state as collision partners to co-trapped molecular species within a deep optical trap (150 mK). We also describe the measurement of the role of laser-induced molecular alignment on the dipole force in optical Stark deceleration and outline progress towards the realisation of chirped optical Stark deceleration for producing slow molecular beams with mK energy spreads.

Laser-induced 3D alignment and orientation of quantum state-selected molecules

A strong inhomogeneous static electric field is used to spatially disperse a rotationally cold supersonic beam of 2,6-difluoroiodobenzene molecules according to their rotational quantum state. The molecules in the lowest-lying rotational states are selected and used as targets for 3-dimensional alignment and orientation. The alignment is induced in the adiabatic regime with an elliptically polarized, intense laser pulse and the orientation is induced by the combined action of the laser pulse and a weak static electric field. We show that the degree of 3-dimensional alignment and orientation is strongly enhanced when rotational state-selected molecules, rather than molecules in the original molecular beam, are used as targets.

Laser-induced selective alignment of water spin isomers

We consider laser alignment of ortho and para spin isomers of water molecules by using strong and short off-resonance laser pulses. A single pulse is found to create a distinct transient alignment and antialignment of the isomeric species. We suggest selective alignment of one isomeric species (leaving the other species randomly aligned) by a pair of two laser pulses.

Space- and time-resolved measurement of rotational wave packet revivals of linear gas molecules using single-shot supercontinuum spectral interferometry

Femtosecond laser-induced alignment and its periodic quantum revivals are measured in a range of linear gas molecules using an improved version of single-shot supercontinuum spectral interferometry. In particular, room temperature alignment revivals of hydrogen and deuterium molecules are measured for the first time, to the best of our knowledge, in a single shot. Wave packet collisional dephasing rates are also measured.

Polarization State of High-Order Harmonic Emission from Aligned Molecules

High harmonic emission in isotropic gases is polarized in the same direction as the incident laser polarization. Laser-induced molecular alignment allows us to break the symmetry of the gas medium. By using aligned molecules in high harmonic generation experiments, we show that the polarization of the extreme ultraviolet emission depends strongly on the molecular alignment and the orbital structure. Polarization measurements give insight into the molecular orbital symmetry. Furthermore, molecular alignment will allow us to produce attosecond pulses with time-dependent polarization.

Strong-field control of x-ray absorption

Strong optical laser fields modify the way x rays interact with matter. This allows us to use x rays to gain deeper insight into strong-field processes. Alternatively, optical lasers may be utilized to control the propagation of x rays through a medium. Gas-phase systems are particularly suitable for illustrating the basic principles underlying combined x-ray and laser interactions. Topics addressed include the impact of spin-orbit interaction on the alignment of atomic ions produced in a strong laser field, electromagnetically induced transparency in the x-ray regime, and laser-induced alignment of molecules.

Alignment effects of NO in femtosecond laser field

The laser-induced alignment effects of NO in linearly polarized femtosecond laser field within the intensity order of 10 14 W/cm 2 at 820 nm have been investigated experimentally by using velocity mapping technique. The angular and velocity distributions of fragment ions with different charge numbers are measured. The alignment degrees increased with the charge numbers, and were nearly independent of the laser intensities. Our results indicate the geometric alignment dominates before the ionization of NO into NO 2+, while the dynamic alignment of the molecular ions dominates after the ionization.

Laser-induced field-free alignment of the OCS molecule

We investigate the dynamical alignment of jet-cooled OCS molecules induced by a short laser pulse. The alignment is measured through the orientational contribution of the optical Kerr effect using a second weak laser pulse as a probe. Maximum alignment is observed at conditions close to saturation of ionization. The results are analysed with a quantum mechanical model solving for the rotational dynamics.

Investigation of OH X^2? collisional kinetics in a flame using picosecond two-color resonant four-wave-mixing spectroscopy

Time-resolved two-color resonant four-wave-mixing spectroscopy was used to investigate collisions affecting the ground electronic state of the hydroxyl radical. Picosecond laser pulses provided adequate time resolution for measurements in an atmospheric-pressure methane-air flame. The grating spectroscopy technique used a combination of double resonance, time-delayed probing, and independent control of the polarization of each of the four fields involved in the wave-mixing process to enable measurement of the decay of laser-induced population, alignment, and orientation, as well as state-to-state transfer of these three moments. Results are presented for individual rotational levels of OH in X(2)II(3/2)(v=1).

Single-shot, space- and time-resolved measurement of rotational wavepacket revivals in H_2, D_2, N_2, O_2, and N_2O

Femtosecond laser-induced alignment and periodic recurrences in hydrogen and deuterium are measured in a single shot for the first time, in a room temperature gas cell. Single-shot Supercontinuum Spectral Interferometry (SSSI) is employed, with measurements also performed in room temperature samples of nitrogen, oxygen, and nitrous oxide. Unlike previous optical techniques for probing molecular alignment in gases or liquids, SSSI quantitatively and directly measures the degree of molecular alignment without reliance on model fits, and it can do so with spatial resolution transverse to the pump beam. In addition, wavepacket collisional dephasing rates can be directly measured in gas samples at useful densities.

Numerical Simulations of femtosecond-laser-induced dynamic alignment of molecules in the high-frequency off-resonance regime

The motion equation for ϑ between the molecular axis and laser polarization direction in a high-frequency off-resonance femtosecond laser field is deduced while simultaneously examining the effects of a permanent dipole moment and field-induced polarizability and hyperpolarizability to molecular rotation. Femtosecond-laser-induced dynamic alignment of CO, N2, and Br2 molecules are investigated by numerically solving the obtained rotation equation for the angle ϑ. The effects of the molecular permanent dipole moment and the field-induced polarizability and hyperpolarizability on the degree of alignment are presented at different intensities. Our computational results show that the dynamic alignment of molecules is primarily determined by field-induced polarizability and the second hyperpolarizability for the laser intensity range from 5 × 1014 to 5 × 1016 W/cm2. The contributions of higher order correction terms to molecular alignment can usually be neglected. The polarizability-field interaction makes the angular distributions of a molecule have a maximum along the polarization axis and a minimum perpendicular to it. The role of the second hyperpolarizability keeps the molecular counts maximum along the laser polarization direction but minimum at an angle of 45° between the molecular axis and the polarization direction. There is also a second maximum of molecular counts perpendicular to the polarization axis. For CO, N2, and Br2 molecules, the dependences of laser-induced dynamic alignment on laser intensity exhibit completely different characteristics.

Field-free one-dimensional alignment of ethylene molecule

We report an experimental study of nonadiabatic laser-induced molecular alignment of ethylene $({\mathrm{C}}_{2}{\mathrm{H}}_{4})$ using a linearly polarized short laser pulse of moderate intensity. The information about the confinement of the $\mathrm{C}\mathrm{C}$ bond axis along the direction of the applied electric field is obtained by measuring the depolarization of a second short pulse of weak intensity interacting with the molecules after they have been exposed to the first pulse. The experimental data are compared with the numerical simulation of the Schr\"odinger equation written for the nonresonant interaction of an asymmetric top rigid rotor with a linearly polarized electric field. The field-free alignment is described by analyzing the time evolution of the angular distribution of the molecular axes together with the expectation value of the relevant squared direction cosine matrix elements.

Single-shot measurement of revival structures in femtosecond laser-induced alignment of molecules

We present a single-shot detection technique for field-free molecular alignment. The method is based on probing the time-varying birefringence of an aligned sample by use of a chirped probe pulse, thus encoding the dependence of the alignment on time onto the spectral domain. The technique is applied to alignment of O2. The recorded signals are well described by an analytical formula.

Dynamics of laser-induced molecular alignment in the impulsive and adiabatic regimes: A direct comparison

Quantum-mechanical calculations are performed of the dynamic alignment of linear molecules induced by a strong nonresonant laser field. Within this framework we have treated in a unified fashion the alignment with laser pulses of varying duration from the short pulse impulsive limit $({\ensuremath{\tau}}_{\text{pulse}}⪡{T}_{\mathrm{rot}})$ to the long pulse adiabatic limit $({\ensuremath{\tau}}_{\text{pulse}}g{T}_{\mathrm{rot}})$. The temporal behavior of the alignment in both these limits, and in the intermediate pulse duration regime, have been analyzed. For the impulsive limit the dependence of the degree of maximum alignment upon the laser pulse duration was examined and the intensity-dependent optimum pulse duration explained. A comparison between the degree of alignment under the same conditions of pulse intensity and rotational temperature was performed between the impulsive and adiabatic cases. The adiabatic case was found to always provide a better degree of alignment for a given intensity which we show is due to the zero relative phasing between the component states of the superposition that form the pendular states. We have explicitly calculated the angular distribution of an ensemble of linear molecules as it evolves through a rotational revival; a rich structure is found that may be useful in guiding future experiments that utilize the field free alignment in a revival.