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.     

Ultrafast dissociation processes in the NO dimer studied with time-resolved photoelectron imaging

Ultraviolet photodissociation of the NO dimer is studied with femtosecond time-resolved photoelectron imaging (TR-PEI) spectroscopy. Pump pulses in the range 200–235 nm are employed, while probe pulses are kept at 300 nm. The time dependencies of the observed photoelectron kinetic energies and photoelectron angular distributions support a picture in which valence state optically excited in the dimer evolves on a time scale of <1 ps to the dimer 3s Rydberg state. This dimer Rydberg state then undergoes fragmentation on a time scale of a few ps. In this study we focus on dissociation into an NO ground state fragment and an NO fragment in its 3s Rydberg A²Σ⁺ state. Every stage of this continuous process, viz. the dimer valence state, the dimer 3s Rydberg state, the separating NO(X) + NO(A) fragments, and the isolated NO(A) fragment is interrogated with TR-PEI.     

Ultrafast hot-electron dynamics observed in Pt( -)(3) using time-resolved photoelectron spectroscopy

Time-resolved two-photon photoelectron spectra have been measured for free Pt( -)(3) using femtosecond pulses of 1.5 eV photon energy in a pump-probe configuration. The time-dependent photoelectron distribution reveals a lifetime of optically excited states of less than 70 fs. Such an unexpected fast electron relaxation in Pt( -)(3) suggests the existence of inelastic electron-electron scattering processes in a triatomic cluster which result in a lifetime similar to those of bulk metals.     

Ultrafast structural dynamics studied by kilohertz time-resolved x-ray diffraction

Ultrashort multi-ke V x-ray pulses are generated by electron plasma produced by the irradiation of femtosecond pulses on metals. These sub-picosecond x-ray pulses have extended the field of x-ray spectroscopy into the femtosecond time domain. However, pulse-to-pulse instability and long data acquisition time restrict the application of ultrashort x-ray systems operating at low repetition rates. Here we report on the performance of a femtosecond laser plasma-induced hard x-ray source that operates at 1-k Hz repetition rate, and provides a flux of 2.0 × 1010 photons/s of Cu Kαradiation. Using this system for time-resolved x-ray diffraction experiments, we record in real time, the transient processes and structural changes induced by the interaction of 400-nm femtosecond pulse with the surface of a 200-nm thick Au(111) single crystal.     

Optical phonon dynamics of GaAs studied with time-resolved terahertz spectroscopy

We investigate the reflection near the reststrahlen band of the optical phonon in bulk GaAs in the time domain, using time-resolved terahertz spectroscopy. We find that the dynamics of the reflection measured for GaAs differs strongly from the reflection dynamics that would be expected for a TO phonon with a frequency-independent dephasing time.     

Ultrafast charge transfer and nuclear dynamics studied with resonant X-ray spectroscopy

In complex systems, composed from many atomic units, it is of equal importance to determine the time scales of dynamic processes and to have control at which atomic site a dynamic process was initiated. With resonant X-ray spectroscopic tools like autoionization and resonant inelastic X-ray scattering, an atomically localized excited state can be created and its dynamics monitored in comparison to the ultrafast time scale of the transient core-hole resonance. With this core-hole-clock method, charge transfer dynamics of only 320±90 attoseconds could be determined for Sulphur adsorbed on the Ru(0001) surface, extending the core-hole-clock method into the range of attoseconds. Exploiting the symmetry selection rules of resonant inelastic X-ray scattering, ultrafast atomic motion can also be investigated, which changes the molecular symmetry during a transient core-excited state. With this approach the vibrational dynamics of C₂H₄ on the time scale of a Carbon 1s core-hole resonance has been investigated. PACS 73.20.Jc; 32.80.Dz; 33.20.Rm; 33.20.Tp; 33.20.Fb     

Surface plasmon dynamics in silver nanoparticles studied by femtosecond time-resolved photoemission

Multiphoton photoelectron spectroscopy reveals the multiple excitation of the surface plasmon in silver nanoparticles on graphite. Resonant excitation of the surface plasmon with 400 nm femtosecond radiation allows one to distinguish between photoemission from the nanoparticles and the substrate. Two different previously unobserved decay channels of the collective excitation have been identified, namely, decay into one or several single-particle excitations.     

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.     

Ultrafast time-resolved coherent degenerate four-wave-mixing spectroscopy for investigating molecular dynamics in different states

In this paper, ultrafast time-resolved coherent degenerate four-wave-mixing (DFWM) spectroscopy is performed to investigate molecular dynamics in the gaseous phase. Laser pulses lasting for 40 fs are used to create and monitor different vibrational eigenstates of iodine at room temperature (corresponding to a low saturation pressure of about 35 Pa). Using an internal time delay in the DFWM process resonant with the transition between the ground X-state and the excited B-state, the vibrational states of both the electronically excited and the ground states are detected as oscillations in the DFWM transient signal. The dynamics of either the electronically excited or ground state of iodine molecules obtained are consistent with the previous high temperature studies on the femtosecond time-resolved DWFM spectroscopy.     

Energy-gap dynamics of superconducting NbN thin films studied by time-resolved terahertz spectroscopy

Using time-domain terahertz spectroscopy we performed direct studies of the photoinduced suppression and recovery of the superconducting gap in a conventional BCS superconductor NbN. Both processes are found to be strongly temperature and excitation density dependent. The analysis of the data with the established phenomenological Rothwarf-Taylor model enabled us to determine the bare quasiparticle recombination rate, the Cooper pair-breaking rate and the electron-phonon coupling constant, λ=1.1±0.1, which is in excellent agreement with theoretical estimates.     

Photoinduced reconfiguration cycle in a molecular adsorbate layer studied by femtosecond inner-shell photoelectron spectroscopy

A time-resolved study of core-level chemical shifts in a monolayer of aromatic molecules reveals complex photoinduced reaction dynamics. The combination of electron spectroscopy for chemical analysis and ultrashort pulse excitation in the extreme ultraviolet allows performing time-correlated 4d-core-level spectroscopy of iodine atoms that probe the local chemical environment in the adsorbate molecule. The selectivity of the method unveils metastable molecular configurations that appear about 50 ps after the excitation and are efficiently quenched back to the ground state.     

Ultrafast electron spin dynamics in bulk CdTe investigated by femtosecond pump-probe reflection spectroscopy

The transient spin polarization dynamics in bulk cadmium telluride (CdTe) at 70 K is investigated by the circularly polarized pump-probe reflection technique. A general expression is derived from the rate equations of a two-level system with small signal approximation to describe the light-helicity-dependent reflection spectrum. The initial degree of electron spin polarization in the excited state and the electron spin relaxation time in bulk CdTe at low temperature with different carrier density are analyzed according to this model. Our experimental results reveal that the D?yakonov–Perel? mechanism based on a fully microscopic kinetic spin Bloch equation (microscopic KSBE) approach dominates in the electron spin relaxation process in bulk CdTe crystal.     

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     

微纳光子学中超快光谱的若干应用

在光电器件的工作单元已经小到微米甚至纳米尺寸的今天,只有充分研究和了解单一元件及其复合系统的光物理性质,才能为进一步提高器件性能提出更好的可行性方案。将研究物质激发态过程的超快光谱技术引入微纳光子学,用来研究可能成为微纳光电器件中基本功能单元的纳米光电材料的光物理特性,探索微纳尺度下光与物质的相互作用基本规律,便能为更好地理解现有微纳光电器件的工作机理提供重要的研究手段,为以后新型微纳光电器件的制备和改进提供可靠的实验依据。文章简单介绍了超快光谱的工作原理,重点介绍了超快光谱技术在半导体纳米晶和荧光碳纳米材料的光物理性质研究中的应用及其在微纳光子学中的应用前景。最后,对同时具有高时间分辨与高空间分辨能力的四维时间分辨显微光谱进行了简要介绍。     

Ultrafast time-resolved imaging of femtosecond laser-induced periodic surface structures on GaAs

We report the formation dynamics of periodic ripples on Ga As induced by femtosecond laser pulses（800 nm, 50 fs） via a collinear time-resolved imaging technique with a temporal resolution of 1 ps and a spatial resolution of 440 nm. The onset of periodic ripples emerges in the initial tens of picoseconds in the timescale of material ejection. The periodic ripples appear after irradiation of at least two pump pulses at surface defects produced by the first pulse and the ripple positions kept stable until the formation processes complete. The formation mechanisms of laser-induced periodic ripples are also discussed.     

Ultrafast spin dynamics in magnetic semiconductor quantum wells studied by magnetization modulation spectroscopy

1-x Mn_xTe quantum well structures at room temperature using time-resolved magnetization modulation spectroscopy. Access to the different electron and hole spin dynamics is obtained by carefully measuring the spectroscopic changes of the magneto-optical response during the first hundreds of femtoseconds after excitation. Experimental results are discussed in the framework of a simple model for a two-dimensional band structure. The spectroscopy is shown to be intimately related to the spectral band width of the applied ultrashort laser pulses. The general potential of the method for fundamental studies on other materials and systems is addressed. Received: 20 September 1998