Field-free Alignment Research Articles

Molecular influence on nuclear-quadrupole-coupling effects in laser induced alignment.

We computationally studied the effect of nuclear-quadrupole interactions on the field-free impulsive alignment of different asymmetric-top molecules. Our analysis is focused on the influence of the hyperfine- and rotational-energy-level structures. These depend on the number of nuclear spins, the rotational constants, and the symmetry of the tensors involved in the nuclear spin and external field interactions. Comparing the prototypical large-nuclear-spin molecules iodobenzene, 1,2-diiodobenzene, 1,3-diiodobenzene, and 2,5-diiodobenzonitrile, we demonstrate that the magnitude of the hyperfine splittings compared to the rotational-energy splittings plays a crucial role in the spin-rotational dynamics after the laser pulse. Moreover, we point out that the impact of the quadrupole coupling on the rotational dynamics decreases when highly excited rotational states dominate the dynamics.

Dynamics of molecular alignment steered by a few-cycle terahertz laser pulse

The field-free alignment of molecule ClCN is investigated by using a terahertz few-cycle pulse (THz FCP) based on the time-dependent density matrix theory. It is shown that a high degree of molecular alignment can be obtained by changing the matching number of the THz FCPs in the adiabatic regime and the non-adiabatic regime. The matching number can affect both the maximum value of the alignment and the time at which it is achieved. It is also found that a higher degree of alignment can be achieved by using the THz FCP at lower intensity and there exists an optimal threshold of molecular alignment with the increase of the field amplitude. Also found is the frequency sensitive region in which the degree of maximum alignment can be enhanced greatly by modulating the center frequencies of different THz FCPs. The investigation demonstrates that comparing with a THz single-cycle pulse, a better result of the field-free alignment can be created by a THz FCP at a constant rotational temperature of molecule.

Picosecond pulse-shaping for strong three-dimensional field-free alignment of generic asymmetric-top molecules

Fixing molecules in space is a crucial step for the imaging of molecular structure and dynamics. Here, we demonstrate three-dimensional (3D) field-free alignment of the prototypical asymmetric top molecule indole using elliptically polarized, shaped, off-resonant laser pulses. A truncated laser pulse is produced using a combination of extreme linear chirping and controlled phase and amplitude shaping using a spatial-light-modulator (SLM) based pulse shaper of a broadband laser pulse. The angular confinement is detected through velocity-map imaging of H+ and C2+ fragments resulting from strong-field ionization and Coulomb explosion of the aligned molecules by intense femtosecond laser pulses. The achieved three-dimensional alignment is characterized by comparing the result of ion-velocity-map measurements for different alignment directions and for different times during and after the alignment laser pulse to accurate computational results. The achieved strong three-dimensional field-free alignment of < {cos }^{2}delta > =0.89 demonstrates the feasibility of both, strong three-dimensional alignment of generic complex molecules and its quantitative characterization.

Investigation of photoassociation with full-dimensional thermal-random-phase wavefunctions

By taking the femtosecond two-photon photoassociation (PA) of magnesium atoms as an example, we propose a method to calculate the thermally averaged population, which is transferred from the ground X1Σg + state to the target (1)1Πg state, based on the solution of full-dimensional time-dependent Schrödinger equation. In this method, named as method A, we use thermal-random-phase wavefunctions with the random phases expanded in both the vibrational and rotational degrees of freedom to model the thermal ensemble of the initial eigenstates. This method is compared with the other two methods (B and C) at different temperatures. Method B is also based on thermal-random-phase wavefunctions, except that the random-phase expansion is merely used for the vibrational degree of freedom. Method C is based on the independent propagation of every initial eigenstate, instead of the thermal-random-phase wavefunctions. Taking the (1)1Πg state as the target state, it is found that although these three methods can present the same population on the (1)1Πg state, the computation efficiency of method A increases dramatically with the increase in temperature. With this efficient method A, we find that the PA process at 1000K can also induce rotational coherence, i.e., the molecular field-free alignment in the excited electronic states.

Field-free alignment of triatomic molecules controlled by a slow turn-on and rapid turn-off shaped laser pulse

The field-free alignment of XCN (X = F, Cl, Br, I) molecules steered by a slow turn-on and rapid turn-off shaped laser pulse is theoretically investigated by using the density matrix theory. Based on the interactions of permanent dipole, polarizability and hyperpolarizability, the results of relatively accurate molecular alignment are obtained in an non-adiabatic regime, and the alignment differences among the molecules are compared. It is shown that the maximum degree of molecular alignment can be obviously improved when the rising time of the shaped laser pulse is adjusted. It is found that the N = 1 shaped pulse has an advantage over the Gaussian or super-Gaussian shaped pulse in enhancing the maximum degree of field-free alignment. Within a certain range, the optimal alignment can be achieved when the pulse amplitude takes a smaller value for FCN, ClCN and BrCN, while the pulse amplitude must take a larger value for ICN. Finally, we analyse the variation of molecular alignment by changing the rotational temperature and find that the large permanent dipole moment and polarizability, and the small hyperpolarizability for those molecules, are more conducive to create a high degree of molecular alignment at the same temperature. Abbreviations: PACS 33.80.-b: Photon interactions with molecules; PACS 33.15.Vb: Correlation times in molecular dynamics; PACS 33.15.Kr: Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility

Time-resolved x-ray scattering from impulsively aligned or oriented molecules

Impulsive laser excitation of molecules can create rotational wave packets that lead to transient alignment or orientation. We present a theoretical analysis of the signatures of post-pulse field-free time-dependent alignment and orientation of diatomic molecules in time-resolved nonresonant x-ray scattering. This shows that alignment and its time dependence due to the interference terms of the rotational wave packet are visible in the x-ray scattering signal whereas signatures of orientation and its time dependence are absent. To that end, we discuss the time dependence of the coherent rotational motion associated with electronically resonant one-photon excitations. We illustrate our findings with calculated two-dimensional scattering signals for the sodium fluoride molecule (NaF) in the gas phase.

Bromine-isotope Selective Ionization Using Field-free Alignment of IBr Isotopologues with a Switched Nanosecond Laser Pulse

We demonstrate isotope selective ionization of I79Br and I81Br isotopologues using field-free alignment induced by a linearly-polarized nanosecond laser pulse with rapid turn off at the maximum. Following the “switched” nanosecond pulse irradiation, a 60-fs laser pulse ionizes one of the isotopologues preferentially. The ion yield ratio I(I81Br+)/I(I79Br+) varies in the range of 0.93–1.06, depending on time delay between the pulses. This is the first demonstration of laser isotope separation of heavy elements using field-free alignment and angular dependent ionization.

Intersystem crossing of 2-Methlypyrazine studied by femtosecond photoelectron imaging

The ultrafast nonadibatic relaxation dynamics of the excited state of 2-methylpyrazine has been studied by using femtosecond time-resolved photoelectron imaging and femtosecond time-resolved mass spectrometry. The first excited state S&lt;sub&gt;1&lt;/sub&gt; of 2-methylpyrazine was excited by 323 nm pump light, and the excited state deactivation process is detected by 400 nm probe light. The lifetime of S&lt;sub&gt;1&lt;/sub&gt; state 98 ps is obtained by time-resolved mass spectroscopy. The intersystem crossing from the S&lt;sub&gt;1&lt;/sub&gt; state to the T&lt;sub&gt;1&lt;/sub&gt; state is observed on real time. The relaxation dynamics of S&lt;sub&gt;1&lt;/sub&gt; state of 2-methlypyrazine is different from that of pyrazine, the results show that the intersystem crossing process between S&lt;sub&gt;1&lt;/sub&gt; and T&lt;sub&gt;1&lt;/sub&gt; is the main relaxation channel of S&lt;sub&gt;1&lt;/sub&gt; state of 2-methlypyrazine, but the internal conversion process between S&lt;sub&gt;1&lt;/sub&gt; and S&lt;sub&gt;0&lt;/sub&gt; is also a main relaxation channel of S&lt;sub&gt;1&lt;/sub&gt; state. By using the advantages of femtosecond time-resolved photoelectron imaging, the photoelectron angular distribution at different pump-probe time delay was obtained experimentally. From the photoelectron angle distribution combined with photoelectron kinetic energy distributions, we tried to observe the field-free nonadiabatic alignment. However, due to the fact that the molecular symmetry of 2-methylpyrazine is lower than that of pyrazine, it is more challenging to observe the phenomenon of molecular nonadiabatic alignment with lower symmetry. Therefore, it is fail to observe nonadiabatic alignment feature of 2-methylpyrazine in this experiment. This work provides a clearer physical picture for S&lt;sub&gt;1&lt;/sub&gt; state nonadibatic relaxation dynamics of 2-methylpyrazine.

Field-free molecular alignment of carbon dioxide molecules measured with above-threshold ionization yields

We present measurements of above-threshold ionization (ATI) of CO2 in a pump-probe experiment where the pump pulse creates a rotational wave packet and the linearly polarized probe pulse generates the ATI spectrum as a function of the pump-probe delay time, which sweeps over the revival time of field-free alignment and its quarter fractions. The observed alignment signals, which are the electron yields of ATI by a probe pulse as a function of a delay between the pump and probe pulse, are compared with the calculated alignment parameter [Formula: see text]. The results are explained by nuclear spin statistics and the wave packet evolution of the CO2 molecule in terms of the highest occupied molecular orbital.

Torsional control of the methyl group in methanol

Experimental and theoretical investigations of the field-free alignment of the nonrigid methanol molecule are reported. The molecule is subject to a $140\ensuremath{-}\mathrm{TW}/{\mathrm{cm}}^{2}$-intensity laser pulse with a 100-fs duration. The experimental signal displays a constant permanent alignment and a fast decaying transient alignment consistent with a prolatelike molecule with $(B+C)/2$ on the order of $0.808\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. The theoretical model takes into account the large-amplitude internal rotation of the methyl group with respect to the hydroxyl group. In the case of a continuous-wave laser field, a rotational alignment close to that of a rigid molecule is predicted. Torsional alignment also occurs even though there is no explicit dependence of the polarizability tensor on the angle of internal rotation. In the case of a strong short laser pulse, the theoretical approach shows that permanent and transient rotational alignment take place. The latter displays an exponential-like decay due to the high density of rotation-torsion levels. Torsional alignment also occurs and depends on the temperature. The theoretical model allows us to reproduce the experimental signal provided one component of the polarizability tensor is adjusted and dissipation effects due to molecular collisions are taken into account.

Molecular movie of ultrafast coherent rotational dynamics of OCS

Recording molecular movies on ultrafast timescales has been a longstanding goal for unravelling detailed information about molecular dynamics. Here we present the direct experimental recording of very-high-resolution and -fidelity molecular movies over more than one-and-a-half periods of the laser-induced rotational dynamics of carbonylsulfide (OCS) molecules. Utilising the combination of single quantum-state selection and an optimised two-pulse sequence to create a tailored rotational wavepacket, an unprecedented degree of field-free alignment, 〈cos2θ2D〉 = 0.96 (〈cos2θ〉 = 0.94) is achieved, exceeding the theoretical limit for single-pulse alignment. The very rich experimentally observed quantum dynamics is fully recovered by the angular probability distribution obtained from solutions of the time-dependent Schrödinger equation with parameters refined against the experiment. The populations and phases of rotational states in the retrieved time-dependent three-dimensional wavepacket rationalises the observed very high degree of alignment.

Field-free alignment dynamics of FCN molecule induced by a terahertz half-cycle pulse

The molecular alignment induced by external fields is of significance in manipulating a chemical reaction. In this paper, the field-free alignment of a FCN molecule steered by a terahertz (THz) half-cycle laser pulse (HCP) is investigated by solving the density matrix equation. It is shown that a high degree of molecular alignment can be achieved by adjusting the carrier envelope phase (CEP) of the THz HCP. The appearance of a saturation threshold for the molecular alignment by a THz HCP when the field amplitude is increased arises from the 1st-order dipole moment interaction. The effect of the central frequency on the molecular alignment, which is manipulated by a THz HCP, is discussed in detail. By changing the matching number of the THz laser pulse at different CEPs, it is found that under the same conditions, a THz HCP has a certain advantage over a few-cycle laser pulse in improving the FCN molecular alignment. Comparing with a double THz HCP, a better result of the molecular alignment can be obtained by a single THz HCP at the same rotational temperature of molecules.

Long-lasting field-free alignment of large molecules inside helium nanodroplets

Molecules with their axes sharply confined in space, available through laser-induced alignment methods, are essential for many current experiments, including ultrafast molecular imaging. For these applications the aligning laser field should ideally be turned-off, to avoid undesired perturbations, and the strong alignment should last long enough that reactions and dynamics can be mapped out. Presently, this is only possible for small, linear molecules and for times less than 1 picosecond. Here, we demonstrate strong, field-free alignment of large molecules inside helium nanodroplets, lasting >10 picoseconds. One-dimensional or three-dimensional alignment is created by a slowly switched-on laser pulse, made field-free through rapid pulse truncation, and retained thanks to the impeding effect of the helium environment on molecular rotation. The opportunities field-free aligned molecules open are illustrated by measuring the alignment-dependent strong-field ionization yield of dibromothiophene oligomers. Our technique will enable molecular frame experiments, including ultrafast excited state dynamics, on a variety of large molecules and complexes.

Two Color Pump-Probe Dichroism and Birefringence Measurements in Atmospheric Molecules

The experimental study of the laser induced alignment of linear atmospheric molecules naturally present into the air using an ultra-short (femtosecond temporal range) and ultra-intense (TW · cm–2 fluence range) linearly polarised laser pulse is described. The measurements are conducted under a two-color pump-probe configuration using an IR pump beam and a blue probe pulse (red-blue configuration), which interacts with the molecules after they have been exposed to the IR alignment pulse. The optical birefringence and dichroism polarimetric signals have been both strictly measured into the same experimental conditions into this two-color configuration. A balanced detection permits the heterodyne signal to be got directly. Under the short and intense laser pulse exposure, the molecules present into the atmospheric air (under standard room temperature conditions) aligne, and periodic transient revivals are observed (field free alignment approach into non-adiabatic conditions). The results obtained are in accordance with those obtained in the red-red configuration and confirm the approach proposed as relevant for atmospheric sensing.

Communication: Switched wave packets with spectrally truncated chirped pulses.

A new technique for obtaining switched wave packets using spectrally truncated chirped laser pulses is demonstrated experimentally and numerically by one-dimensional alignment of both linear and asymmetric top molecules. Using a simple long-pass transmission filter, a pulse with a slow turn-on and a rapid turn-off is produced. The degree of alignment, characterized by ⟨cos2 θ2D⟩, rises along with the pulse intensity and reaches a maximum at the peak of the pulse. After truncation, ⟨cos2 θ2D⟩ drops sharply but exhibits pronounced half and full revivals. The experimental alignment dynamics trace agrees very well with a numerically calculated trace based on the solution of the time-dependent Schrödinger equation. However, the extended periods of field-free alignment of asymmetric tops following pulse truncation reported previously are not reproduced in our work.

Field-free molecular alignment induced by a super-Gaussian laser pulse

In this paper, the field-free molecular alignment induced by a super-Gaussian laser pulse is studied by a quantum mechanical calculation. It is shown that the super-Gaussian pulse has an advantage over the standard Gaussian pulse in enhancing the maximal degree of field-free alignment at constant pulse energy in an impulsive regime. Furthermore, the maximal degree of the molecular alignment during the laser pulse depends on the laser intensity and the pulse duration, and that after the laser pulse is related to all the parameters including the intensity, duration, and shape of the laser pulse, as well as the molecular property in intermediate regimes. The oscillation frequency of the molecular alignment during the laser pulse is proportional to the amplitude of the laser pulse at the same duration. Additionally, for the post-pulse alignment, a specific laser intensity and a proper pulse energy can improve the molecular alignment and the large shape parameter N can be beneficial for enhancing the molecular alignment to some extent.

Dynamical analysis of the effect of elliptically polarized laser pulses on molecular alignment and orientation

In this Letter, we study the molecular alignment and orientation driven by two elliptically polarized laser pulses. It is shown that the field-free molecular alignment can be achieved in a three-dimensional (3D) case, while the field-free molecular orientation is only along the x and y directions, and that the field-free alignment and orientation along different axes are related to the populations of the rotational states. It is demonstrated that changing the elliptic parameter is efficient for controlling both in-pulse and post-pulse molecular alignment and orientation. The delay time also has an influence on the field-free molecular alignment and orientation.

Excited state dynamics of molecules studied with femtosecond time-resolved mass spectrometry and photoelectron imaging

Study of quantum states of molecules, especially the evolution of excited states can help to understand their basic features and the interactions among different states. Furthermore, the information about the chemical reaction process and the interactions among several reaction channels can be obtained. Femtosecond time-resolved mass spectrometry (TRMS) and time-resolved photoelectron imaging (TRPEI), which combine pump-probe technique with time of flight mass spectrometry and photoelectron imaging, are powerful tools for detecting the molecular quantum state and for studying the molecular quantum state interaction and molecular ultrafast dynamics. With these methods, the photochemistry and photophysics mechanism of isolated molecule reaction process can be investigated on a femtosecond time scale. The principles of TRMS and TRPEI are introduced here in detail. On the basis of substantial research achievements in our group, the applications of TRMS and TRPEI are presented in the study of ultrafast internal conversion and intersystem crossing, wavepacket evolution dynamics at excited states of polyatomic molecules, energy transfer process of polyatomic molecules, ultrafast photodissociation dynamics and structural evolution dynamics of molecular excited states. In the study of ultrafast internal conversion and intersystem crossing, the methyl substituted benzene derivatives and benzene halides are discussed as typical molecular systems. In the study of wavepacket evolution dynamics at excited states of polyatomic molecules, the real-time visualization of the dynamic evolution of CS2 4d and 6s Rydberg wave packet components, the vibrational wave packet dynamics in electronically excited pyrimidine, the rotational wave packet revivals and field-free alignment in excited o-dichlorobenzene are reported. In order to discuss the energy transfer process of polyatomic molecules, the intramolecular vibrational energy redisctribution between different vibrational states in p-difluorobenzene in the S1 low-energy regime and the intramolecular energy transfer between different electronic states in excited cyclopentanone are presented. For the study of ultrafast photodissociation dynamics, the dissociation constants and dynamics of the A band and even higher Rydberg states are investigated for the iodine alkanes and iodine cycloalkanes. Structural evolution dynamics of molecular excited states is the main focus of our recent research. The structural evolution dynamics can be extracted from the coherent superposition preparation of quantum states and the observation of quantum beat phenomenon, by taking 2, 4-difluorophenol and o-fluorophenol as examples. Time-dependent photoelectron peaks originating from the planar and nonplanar geometries in the first excited state in 2, 4-difluorophenol exhibit the clear beats with similar periodicities but a phase shift of π rad, offering an unambiguous picture of the oscillating nuclear motion between the planar geometry and the nonplanar minimum. Also, the structural evolution dynamics in o-fluorophenol via the butterfly vibration between planar geometry and nonplanar minimum is mapped directly. Finally, the potential developments and further possible research work and future directions of these techniques and researches are prospected.

Isotope-selective ionization utilizing field-free alignment of isotopologues using a switched nanosecond laser pulse

We propose and numerically simulate a method of laser isotope separation based on field-free alignment of isotopologues, utilizing an intense “switched” nanosecond (ns) laser field which is slowly turned on and rapidly turned off at the peak with the falling time of 200 fs. The femtosecond (fs) laser-induced alignment of isotopologues including heavy isotopes is severely disturbed by ionization because of their small ionization potential. Our simulations for I79Br and I81Br isotopologues demonstrate that the switched ns laser field can make isotopologues well-aligned with the reduced ionization probability at the laser intensity [I 0 = 2 TW/cm2 (= 2 × 1012 W/cm2)] which is an order-of-magnitude lower than a typical intensity (I 0 = 20 TW/cm2) for field-free alignment induced by a fs laser field. In our simulations, I i Br isotopologues (i = 79 or 81) aligned with the switched ns laser field are selectively ionized by an intense fs laser pulse at an appropriate time delay from the alignment pulse.

Directional quantum-controlled chemistry: Generating aligned ultracold molecules via photoassociation

Photoassociation of ultracold atoms is shown to lead to alignment of the product molecules along the excitation laser polarization axis. We theoretically investigate pulsed photoassociation of $^{87}Rb$ atoms into a specific weakly-bound level of the a $^3\Sigma_u^+$ metastable electronic state and find both stationary and time-dependent field-free alignment. Although a transform-limited pulse yields significant alignment, a frequency-chirped pulse dramatically enhances the molecular formation rate at the cost of a slight decrease in the alignment. Employing multiple pulses synchronized with the vibrational and rotational periods leads to coherent enhancement of both population and alignment of the target state.