Measurement of the field-free alignment of diatomic molecules

Tracing the Coherent Manipulation of Rotational Dynamics by Shaped Femtosecond Pulse-Induced Two-Dimensional Rotational Coherent Spectrum

The temporal delayed orthogonal pulse pairs generated by the phase shaping technique are used to study the coherent control of the rotational wave packet dynamics in air. By continuously changing the intrapulse delay of the pump pulse, we measured the corresponding revival signals and obtained a two-dimensional rotational coherent spectrum (2D RCS). An additive property of the rotational dynamics is observed from the revival signals. Moreover, combining with the coherent control model, we find that the 2D RCS can be used to demonstrate the control over the underlying Raman rotational excitation. A beat frequency-dependent oscillation of each rotational transition is obtained. The transition process is revealed from the Fourier transformation about the pump delay. The scheme of this work can be used for further control and detection of the rotational wave packet and can be extended to other molecular dynamic researches.

Enhanced Coherent Emission from Ionized Nitrogen Molecules by Femtosecond Laser Pulses

The forward emission spectra were experimentally measured for ionized nitrogen molecules by an 800 nm pump laser and a delayed seed laser. It was found that emission lines around both 428 and 391 nm are greatly enhanced upon use of a 391 or 428 nm seed laser. The emission lines around 391 and 428 nm can be assigned to the rotational transitions of N₂⁺ [B²Σ_u⁺(v' = 0) → X²Σ_g⁺(v = 0)] and N₂⁺ [B²Σ_u⁺(v' = 0) → X²Σ_g⁺(v = 1)], respectively. They originate from seed-induced superfluorescence and resonant stimulated Raman scattering. The genetic algorithm was utilized to simulate the experimental observations and determine the relative population of B²Σ_u⁺(v' = 0), X²Σ_g⁺(v = 1), and X²Σ_g⁺(v = 0). The result verifies that vibrational population inversion is achieved between B²Σ_u⁺(v' = 0) and X²Σ_g⁺(v = 0) by the 800 nm pump laser. Our finding provides new insights into controlling the coherent emission of ionized nitrogen molecules, which has promising application in filamentation-based remote atmospheric sensing.

Population inversion in the rotational levels of the superradiant N2 + pumped by femtosecond laser pulses

Nitrogen molecular ions (N₂ ⁺) in air plasma pumped by femtosecond laser pulses give rise to superradiant emission at 391.4 nm in the presence of an external seed pulse at proper wavelength. Due to the transient alignment of the nitrogen molecular ions, the superradiance signal presents a strong modulation as a function of the temporal delay between the pump and the seed pulses. Through Fourier transformation with high frequency resolution, we distinguished the contribution of the finely separated rotation levels of the upper and lower states. It was found that the population density of certain rotational levels in the upper state is higher than that in the lower one, indicating that population inversion of the rotation levels of the two involved states is a key enabling factor for this superradiant emission.

Rotational Dynamics of Quantum State-Selected Symmetric-Top Molecules in Nonresonant Femtosecond Laser Fields

Rotational dynamics of quantum state selected and unselected CH₃I molecules in intense femtosecond laser fields has been studied. The orientation and alignment evolutions are derived from a pump-probe measurement and in good agreement with the numerical results from the time-dependent Schrödinger equation (TDSE) calculation. The different rotational transitions through nonresonant Raman process have been assigned from the Fourier analysis of the orientation and alignment revivals. These revivals are derived from a pump-probe measurement and in good agreement with the numerical results from the TDSE calculation. For the molecules in rotational state |1, ±1, ∓1⟩, the transitions can be assigned to ΔJ = ±1, ±2, while for thermally populated molecules, the transitions are ΔJ = ±2. Our results illustrate that the orientation and alignment revivals of the rotational quantum-state-selected molecules give a deep insight into the rotational excitation pathways for the transition of different rotational states of molecules in ultrafast laser fields.

Direct measurement of field-free molecular alignment by spatial (de)focusing effects

We directly measure the field-free molecular alignment of various room-temperature molecular gases based on alignment-induced spatial focusing and defocusing effects. By imaging the spatial profile of a time-delayed probe pulse with increased and decreased local intensity at the beam center, the parallel and perpendicular molecular alignments are clearly characterized. Meanwhile, the electronic Kerr effect, weak plasma contribution and field-free molecular alignment impact could be distinguished from the measured signals.

Alignment structures of rotational wavepacket created by two strong femtosecond laser pulses

We experimentally and theoretically study the alignment structures of the rotational wavepacket created by linear molecules and two strong femtosecond laser pulses. In the experiment, we observe that the alignment structures depend on the time delay between the two laser pulses. In the theory, we find that the alignment structures are composed of the self-coupling term and the cross-coupling term. The contributions of these two terms are separately calculated. Their coherent superposition reproduces the alignment structures observed in the experiment.

Molecular rotational excitation by strong femtosecond laser pulses

We study the rotational wave packet created by nonadiabatic rotational excitation of molecules with strong femtosecond laser pulses. The applicable condition of the Delta-Kick method is obtained by comparing the laser intensity and pulse duration dependences of the wave packet calculated with different methods. The wave packet evolution is traced analytically with the Delta-Kick method. The calculations demonstrate that the rotational populations can be controlled for the rotational wave packet created by two femtosecond laser pulses. The evolution of the rotational wave packet with controlled populations produces interference patterns with exotic spatial symmetries. These calculations are validated by comparing the theoretical calculations with our experimental measurements for the rotational wave packet created by thermal ensemble CO(2) and two strong femtosecond laser pulses. Potential applications in molecular science are also discussed for the rotational wave packet with controlled populations and spatial symmetries.

Measurement of field-free molecular alignment by cross-defocusing assisted polarization spectroscopy

We demonstrate the measurement of field-free molecular alignment of nitrogen by a cross-(de)focusing assisted polarization spectroscopy technique, allowing us to distinguish the transient orientation of prealigned molecules on the basis of the relative ratio of the adjacent revival peaks of the measured signal. For impulsive excitation with high pump intensities, the involved orientation-dependent ionization of the neutral molecules contributes to the signal.