Probing solvation dynamics with femtosecond vibrational spectroscopy

We explain why solvent reorganization can induce both red- and blue-shifting of vibrational transitions of a particular probe molecule upon excitation to the S₁ electronic state. We observe with femtosecond vibrational spectroscopy, after hydrogen-bond cleavage dynamics, an additional blue shift of the carbonyl stretch of coumarin 102 of 7 cm^-1 when dissolved in chloroform. Received: 28 June 2000 / Published online: 7 August 2000     

Fourier-transform sum-frequency surface vibrational spectroscopy with femtosecond pulses

We present a Fourier-transform spectroscopic technique for investigation of surfaces and interfaces based on IR-visible sum-frequency generation with femtosecond light pulses. The observed spectrum has a resolution that is independent of the input pulse characteristics.     

Lasing dynamics study by femtosecond time-resolved fluorescence non-collinear optical parametric amplification spectroscopy

Femtosecond time-resolved fluorescence non-collinear optical parametric amplification spectroscopy(FNOPAS) is a versatile technique with advantages of high sensitivity, broad detection bandwidth, and intrinsic spectrum correction function. These advantages should benefit the study of coherent emission, such as measurement of lasing dynamics. In this letter,the FNOPAS was used to trace the lasing process in Rhodamine 6G(R6G) solution and organic semiconductor nano-wires.High-quality transient emission spectra and lasing dynamic traces were acquired, which demonstrates the applicability of FNOPAS in the study of lasing dynamics. Our work extends the application scope of the FNOPAS technique.     

Non-perturbative wave-packet calculations of time-resolved four-wave-mixing signals

We extend a method proposed by Seidner et al. [J. Chem. Phys. 103, 3998 (1995)] to extract directional terms from a time-dependent molecular polarization to obtain time-resolved four-wave-mixing (FWM) signals. Instead of employing perturbation theory, the total polarization induced in the molecular sample by the interaction with several femtosecond laser pulses is determined by solving the time-dependent Schr?dinger equation for the nuclear dynamics in coupled electronic states numerically exact. Repeating the calculation for several combinations of relative phases of the involved fields leads to a linear system of equations for the contributions emitted in different directions. Gas-phase I₂ molecules serve as a numerical example. Received: 22 September 1999 / Published online: 30 June 2000     

The influence of phase-modulation on femtosecond time-resolved coherent Raman spectroscopy

The influence of phase-modulation on femtosecond time-resolved coherent Raman scattering is investigated theoretically and experimentally. The coherent Raman signal taken as a function of the spectral position shows unexpected temporal oscillations close to time zero. A theoretical analysis of the coherent Raman scattering process indicates that the femtosecond light pulses are amplitude and phase modulated. The pulses are asymmetric in time with more slowly decaying trailing wings. The phase of the pulse amplitude contains quadratic and higher-order contributions.     

Wave packet dynamics in different electronic states investigated by femtosecond time-resolved four-wave-mixing spectroscopy

This paper reviews results on wave packet dynamics investigated by means of femtosecond time-resolved four-wave-mixing (FWM) spectroscopy. First, it is shown that by making use of the various degrees of freedom which are offered by this technique information about molecular dynamics on different potential-energy surfaces can be accessed and separated from each other. By varying the timing, polarization, and wavelengths of the laser pulses as well as the wavelength of the detection window for the FWM signal, different dynamics are coherently excited and probed by the nonlinear spectroscopy. As a model system we use iodine in the gas phase. These techniques are then applied to more-complex molecules (gas phase: benzene, toluene, a binary mixture of benzene and toluene; solid state: polymers of diacetylene matrix-isolated in single crystals of monomer molecules). Here, ground-state dynamics are investigated first without any involvement of electronically excited states and then in electronic resonance to an absorption transition in the investigated molecules. Signal modulations result which are due to wave packet motion as well as polarization beats between modes in different molecules. Phase and intensity changes yield information about intramolecular vibrational energy redistribution, population decay (T₁), phase relaxation (T₂), and coherence times. Received: 12 October 1999 / Published online: 13 July 2000     

Extremely slow intramolecular vibrational redistribution: Direct observation by time-resolved raman spectroscopy in trifluoropropyne

We have studied the dynamics of intramolecular vibrational redistribution (IVR) from the initially excited mode v₁ ≈ 3330 cm⁻¹ (acetylene-type H-C bond) in H-C≡C-CF₃ molecules in the gaseous phase by means of anti-Stokes spontaneous Raman scattering. The time constant of this process is estimated as 2.3 ns—this is the slowest IVR time reported so far for the room-temperature gases. It is suggested that so long IVR time with respect to the other propyne derivatives can be explained by a larger defect, in this case, of the Fermi resonance of v₁ with v₂ + 2v₇—the most probable doorway state leading to IVR from v₁ to the bath of all vibrational-rotational states with the close energies. In addition, it is shown that the observed dynamics is in agreement with a theoretical model assuming strong vibrational-rotational mixing.     

Ultrafast dynamics of o-fluorophenol studied with femtosecond time-resolved photoelectron and photoion spectroscopy

The ultrafast dynamics of o-fluorophenol via the excited states has been studied by femtosecond time-resolved photoelectron imaging. The photoion and photoelectron spectra taken with a time delay between 267 nm pump laser and 800 nm probe laser provide a longer-lived S1 electronic state of about ns timescale. In comparison,the spectra obtained by exciting the S2 state with femtosecond laser pulses at 400 nm and ionizing with pulses at 800 nm suggest that the S2 state has an ultrashort lifetime about 102 fs and reflects the internal conversion dynamics of the S2 state to the S1 state.     

Molecular dynamics in low vibrational states investigated by fs time-resolved coherent anti-Stokes Raman spectroscopy technique

The molecular dynamics process is investigated in this paper using a broadband fs time-resolved coherent anti-Stokes Raman spectroscopy (CARS) technique. By varying the timing of laser pulses, low vibrational states are started and studied on both the electronically excited B(³Π_0u⁺) state and ground X(¹Σ_0g⁺) state of iodine in the gas phase at room temperature. According to change the pump wavelength or Stokes pulse as well as the wavelength of the detection window for the CARS signal, dynamics on different potential-energy surfaces can be accessed and detected by the CARS spectroscopy. Results show that the period of the oscillation is decreased for the excited B(³Π_0u⁺) state as the wavelength of the pump pulses is increased, while it is increased for the ground X(¹Σ_0g⁺) state with the increase of the Stokes wavelength.     

Coherent coupling between vibrational modes of C-H bonds at different positions studied by femtosecond time-resolved coherent anti-Stokes Raman scattering

We performed femtosecond time-resolved coherent anti-Stokes Raman scattering(fs-CARS) measurements on liquid toluene and PVK film.For both samples,we selectively excited the CH stretching vibrational modes and observed the expected quantum beat signals.The frequency of the well-defined beats is in good agreement with the energy difference between the two simultaneously excited modes,which demonstrates that a coherent coupling between the vibrational modes of the C-H chemical bonds exists at the different positions of the molecules.The dephasing times of the excited modes are obtained simultaneously.     

Temperature measurements in reacting flows by time-resolved femtosecond coherent anti-Stokes Raman scattering (fs-CARS) spectroscopy

Time-resolved femtosecond coherent anti-Stokes Raman scattering (fs-CARS) spectroscopy of the nitrogen molecule is used for the measurement of temperature in atmospheric-pressure, near-adiabatic, hydrogen-air diffusion flames. The initial frequency-spread dephasing rate of the Raman coherence induced by the ultrafast (∼85 fs) Stokes and pump beams is used as a measure of gas-phase temperature. This initial frequency-spread dephasing rate of the Raman coherence is completely independent of collisions and depends only on the frequency spread of the Raman transitions at different temperatures. A simple theoretical model based on the assumption of impulsive excitation of Raman coherence is used to extract temperatures from time-resolved fs-CARS experimental signals. The extracted temperatures from fs-CARS signals are in excellent agreement with the theoretical temperatures calculated from an adiabatic equilibrium calculation. The estimated absolute accuracy and the precision of the measurement technique are found to be ±40 K and ±50 K, respectively, over the temperature range 1500-2500 K.     

Post-scission dissipative motion and fission-fragment kinetic energy

A dynamic model developed earlier is used to describe the last stage of nuclear fission: the postscission motion of fission fragments. In this process, Coulomb repulsive energy turns into the kinetic energy of fission fragments measured by experimenters. It is shown that the dissipated energy can be as high as 10% of the average experimental kinetic energy.     

皮秒级时间分辨超快高能脉冲激光光谱

介绍了一种利用光电摄谱法和条纹管相结合测量ps级时间分辨超快高能脉冲激光光谱的方法。论述了条纹相机工作原理和平面衍射光栅的分光原理,分析指出利用介绍的装置,可以实现波长300 nm ～1 600 nm、脉宽＞2 ps超快高能脉冲激光的光谱测量。采用1 054 nm超快高能脉冲激光器,实验得到了条纹像,对条纹像进行数据处理后得到测量光谱曲线,通过能量标定后,得到了超快高能脉冲激光器实际光谱曲线,验证了ps级时间分辨超快高能脉冲激光光谱方法。讨论了系统中耦合透镜组对光谱测量和光纤色散角对条纹图像的影响,论述了ps级时间分辨超快高能脉冲激光光谱的作用。随着条纹管制造技术的飞速发展,该方法可用于fs级激光光谱的测量。     

Coherent coupling between vibrational modes of Cben H bonds at different positions studied by femtosecond time-resolved coherent anti-Stokes Raman scattering

We performed femtosecond time-resolved coherent anti-Stokes Raman scattering (fs-CARS) measurements on liquid toluene and PVK film. For both samples, we selectively excited the CH stretching vibrational modes and observed the expected quantum beat signals. The frequency of the well-defined beats is in good agreement with the energy difference between the two simultaneously excited modes, which demonstrates that a coherent coupling between the vibrational modes of the C-H chemical bonds exists at the different positions of the molecules. The dephasing times of the excited modes are obtained simultaneously.     

飞秒时间分辨质谱和光电子影像对分子激发态动力学的研究

分子量子态的研究,特别是分子激发态演化过程的研究不仅可以了解分子量子态的基本特性和量子态之间的相互作用,而且可以了解化学反应过程和反应通道间的相互作用.飞秒时间分辨质谱和光电子影像是将飞秒抽运-探测分别与飞行时间质谱和光电子影像相结合的超快谱学方法,为实现分子内部量子态探测,研究分子量子态相互作用及超快动力学过程提供了强有力的工具,可以在飞秒时间尺度下研究单分子反应过程中的光物理或光化学机理.本文详细介绍了飞秒时间分辨质谱和光电子影像的技术原理,并结合本课题组的工作,展示了这两种方法在量子态探测及相互作用研究领域,特别是激发态电子退相、波包演化、能量转移、分子光解动力学以及分子激发态结构动力学研究中的广泛应用.最后,对该技术的发展前景以及进一步的研究工作和方向进行了展望.     

Design and application of a femtosecond optical parametric oscillator for time-resolved spectroscopy of semiconductor heterostructures

Femtosecond pulses of a collinearly pumped Optical Parametric Oscillator (OPO) are applied for investigations of the carrier dynamics in ternary and quaternary semiconductor quantum wells. The design and the specifications of the OPO are given in detail. We show that no measurable jitter exists between the pump pulses and the output pulses of the OPO. Therefore, it is possible to use the OPO and its pump laser for two-color experiments with a time resolution limited by the pulse lengths. We present and discuss results of transient four-wave mixing experiments on (InGa) As/InP quantum wells, and find a new kind of polarization-dependent quantum beat phenomenon. In addition, non-degenerate experiments on quantum wells from the quaternary (InGaAl) As material system, using two pulses at different wavelengths (one from the OPO and one from the pump laser), are discussed as a novel experimental technique to study carrier trapping into quantum wells.