Inherent spectral distortions in pump-probe Brillouin scattering spectroscopy

It is shown that the pump-probe technique in Brillouin scattering spectroscopy is accompanied by two specific distortions to a registered spectrum. The first appears due to an acoustic wave that unavoidably accompanies in a continuous medium the density wave driven by the pump and probe signals. The registered spectrum then splits to two spectral components, one corresponding to the propagation velocity of the driven density wave, and the other to the acoustic velocity. The other distortion is asymmetry of the spectrum which is shown to occur when the velocity of the driven density wave is greater than the acoustic velocity in the medium.     

The interplay of topography and energy dissipation in pentacene thin films

Nonlinear photoemission electron microscopy was used to study the morphology-dependent lifetime of electronic excitations in pentacene islands on Si(0 0 1) and (√3 × √3)R30°-Ag/Si(1 1 1). After an optical excitation of electrons by a λ = 400 nm femtosecond laser pulse the characteristic decay times were measured with spatial resolution in a pump-probe setup. For pentacene on Si(0 0 1), the observed lifetimes vary by a factor of two between the wetting layer and the fractal-shaped pentacene islands. The measured lifetime difference is explained by a difference in the electronic coupling of the pentacene islands and the wetting layer to the substrate. For pentacene on (√3 × √3)R30°-Ag/Si(1 1 1), similar lifetimes are found, although the orientation of the pentacene molecules in the compact islands is rotated. Our findings suggest that electronic excitations in higher layers of the pentacene islands do not diffuse to the interface before they decay.     

Microviscosity and wavelength effects on radical cage pair recombination

This study probed two aspects of the reactivity of geminate radical cage pairs formed by photolysis of (Cp′ = η⁵-C₅H₄CH₃). The first aspect studied examined whether the bulk viscosity has any predictive power in determining the magnitude of the cage recombination efficiency (F_cP). Although there is a clear relationship between the magnitude of F_cP and viscosity for systems where the bulk viscosity of the solution is altered by the addition of a non-macroscopic viscosity enhancer, the relationship is unclear for systems where the bulk viscosity is altered using polymeric viscosity enhancers. For this investigation, F_cP values were measured using femtosecond pump-probe transient absorption spectroscopy. The results clearly indicate that bulk viscosity can change drastically without affecting F_cP in systems containing small amounts of added polymers. The bulk viscosity is thus a poor parameter for predicting the cage effect in such systems. The second investigation looked at the effect of the photochemical excitation energy on F_cP for the [Cp′(CO)₃Mo, MoCp′(CO)₃] cage pair in hexane. The results showed that F_cP increased when the wavelength of irradiation was changed from 546 nm to 436 nm, and then remained constant as the wavelength of irradiation was changed from 436 nm to 404 nm to 366 nm. These results are somewhat surprising because the recombination efficiencies for diatomic and triatomic molecules have been shown to decrease monotonically with increasing excitation energy. Two explanations are offered for the reverse wavelength dependence observed in this study. The first explanation invokes the different dynamic behaviors of the two excited states involved at the selected wavelengths, and the second invokes the different speeds of radical separation following irradiation at the selected wavelengths.     

飞秒时间分辨的光电子影像对3-甲基吡啶分子超快动力学研究

利用飞秒时间分辨的光电子影像技术结合时间分辨的质谱技术,研究了3-甲基吡啶分子激发态的超快过程. 实时观察到了3-甲基吡啶分子S₂态向S₁态高振动能级的超快内转换过程,该内转换的时间大约为910fs. 二次布居的S₁态主要通过内转换衰减到基态S₀,该内转换的时间尺度为2.77 ps. 光电子能谱分布和光电子角分布显示,S₂态和S₁态在电离的过程中跟3p里德堡态发生偶然共振. 本次实验中还用400 nm两个光子吸收的方法布居了3-甲基吡啶的3s 里德堡态. 研究表明,3s 里德堡态的寿命为62 fs,并主要通过内转换快速衰减到基态.     

The photodissociation of SO2 between 200 and 197 nm

Pump-probe experiments on the C state of SO₂ have been performed to assess the details of the oxygen photodissociation process between 197 and 200 nm. The findings show that a prompt dissociation event occurs in less than 265, but more than 100 fs. The reorganization of internal energy occurs in 15 or 28 ps depending upon the excitation energy; the change in lifetime represents contributions from asymmetric vibrational states seen in previous experiments. In addition, a long-lived species (lifetime >150 ps) exists that does not decay on the timeframe of our experiment. These findings are discussed in comparison to previous studies, particularly in the context of an avoided crossing in this region of the potential energy surface of SO₂.     

Optical and gain properties of series of crystals LiF-YF3-LuF3 doped with Ce and Yb ions

Here, we present first results of systematic studies of host cation variation impact on spectral-kinetic, photochemical and gain properties of Ce³⁺-doped LiYF₄ (YLF), LiLuF₄ (LLF) and LiY_1−xLu_xF₄ family crystals. 5d-4f luminescence decay of Ce³⁺ ions studies, together with pump-probe experiments, indicate that previously reported twice higher luminescence quantum yield in LLF compared with that of YLF crystals is provided by more efficient upper lasing level feeding due to recombination and higher color center destruction rate in LLF against YLF crystals. Namely, it is responsible for higher energetic characteristics of laser based on Ce³⁺:LLF crystals. Strong and wide pump-induced absorption band centered at 310 nm is observed in Ce³⁺:YLF. This band is shifted to blue and its intensity goes down with Lu content. We have evaluated free charges recombination rate, excited state absorption cross-section for Ce³⁺ ions and some other photodynamic processes related microparameters. Fitting results indicate that pump-induced color centers lifetime decreases with Lu-content in LiYF₄-LiLuF₄ mixture and it can be associated with more efficient color center bleaching by Ce³⁺ ions 5d-4f fluorescence.     

Nascent Rotational Distributions of MgH in Reaction of Mg(4s 1S0) with H2 and HD

The nascent rotational distributions of MgH(X²σ⁺, v″ = 0, 1) resulting from the reaction of Mg(4s¹S₀) with H₂ and HD have been determined by using the laser pump-probe technique. Like the case of Mg(¹P₁), the distributions appear to be bimodal which peak mainly at high N(26) with minor peaks at low N(6). There are no apparent differences when the escaping mass is doubled. The similarity between product rotational distributions from the 4S state and those from the 3P state, and no evidence of a kinematic isotope effect, reveal that a harpoon-type mechanism may be operative. Our ab initio calculations reveal that in the entrance channel on the 3¹A₁ surface in C_2v symmetry or ¹σ⁺ surface in C_∞v symmetry for 4S state, the reaction may lake place via a series of non-adiabatic curve crossings with the ionic Mg⁺H₂^? potential surface.     

Limits in Enhancement Factor in Near-Brewster Angle Reflection Pump-Probe Two-Dimensional Infrared Spectroscopy

In this work, we simulated 2D infrared spectroscopy (IR) spectroscopy in both transmission geometry and Brewster-angle reflection geometry. Light dispersion and the leakage of s-polarized light are considered in simulating the enhancement factor of the reflection mode. Our simulation shows that the dispersion in reflection will only alter the 2D IR lineshape slightly and can be corrected. Leaking s-polarized light due to imperfectness of IR polarizers in the reflection geometry may limit the enhancement factor, but such limit is above what a typical experiment can reach. In the current experiment, the enhancement factor is mainly limited by the precision of incident angle, for which ordinary rotation stages are probably not adequate enough. Moreover, traditional energy ratio of pump and probe pulses, which is 9:1, may not be ideal and could be changed to 2:1 in the reflection geometry. Considering all the above factors, the enhancement on the order of 1000 is possible in the current experiment. Nevertheless, near-Brewster angle reflection will enhance both the signal and the noise caused by the signal itself, therefore this method only works if the noise is unrelated to the signal, particularly if the noise is caused by the fluctuation in the probe. It cannot improve the signal to noise ratio when the dominate noise is from the signal itself. The theoretical results here agree reasonably well with published experiment results and pave way for realizing even higher enhancement at nearer-Brewster angle.     

GaAs光生载流子动力学机制的超快光谱学分析

利用飞秒激光泵浦探测技术,通过改变光学参数,如中心波长、功率,分别对未故意掺杂高纯n型砷化镓的差分反射谱进行研究,进而分析室温下砷化镓光生载流子动力学过程.首先,当泵浦光功率恒为100mW,探测光功率恒为10mW时,随着中心波长的增大,差分反射率峰值随之增大,信噪比也随之增加.其次,通过拟合不同延迟时间下泵浦光功率和差分反射率的实验曲线,并和理论模型比较后发现,在一定范围内的泵浦功率和差分反射率呈线性相关,未故意掺杂高纯n型砷化镓的饱和载流子浓度为(3.590 1±0.310 3)×1017 cm~(-3).在此基础上,把光生载流子动力学过程分为3个过程:804±67fs的光激发过程、134~268fs的初始散射过程、1ps和3~6ps的复合过程.研究表明,差分反射率与探测功率不存在显著的依赖性,但差分反射谱的信噪比与探测功率存在相关性.