硅的间接跃迁双光子吸收系数谱

利用皮秒Nd:YAG脉冲激光器作为激发光源,测量出光子能量介于1.36 μm (0.912 eV)—1.80 μm (0.689 eV)之间的硅间接跃迁双光子吸收系数谱.尽管此波段范围内的激光光子能量小于硅间接带隙,但当激光辐照在硅基光电二极管受光面时,在二极管两电极端仍然探测到了显著的脉冲光伏信号.光伏信号峰值强度与入射光强呈二次幂函数关系,表明其是双光子吸收过程.采用pn结等效结电容充放电模型,将光伏响应信号峰值与入射光强相关联,从中提取出硅的间接跃迁双光子吸收系数,改变入射波长得到系数谱.研究表明:     

硅光电二极管的双光子响应

报道用连续光1.3μmInGaAsP半导体激光器探测硅光电二极管中的双光子响应(我们把倍频效应和双光子吸收统称为双光子响应).通过实验,证明在硅光电二极管中必然有倍频吸收引起的光电流存在.     

碱金属钾蒸气混合双光子跃迁的研究

利用脉冲染料可调谐激光激发原子密度为10~(14)～10~(18)cm~(-3)的稠密碱金属钾蒸气,在5780(?)～5920(?)波段内测量了激光共振电离光谱.结果表明,除了钾原子4s—nd(n=10～16)、4s—ns(n=12～18)双光子跃迁以外,还出现了钾4P—7s、4P—5d跃迁.这种跃迁的电离光谱信号是由于在钾蒸气中钾分子-原子混合双光子跃迁而产生的.本文对钾蒸气混合双光子跃迁进行了讨论.     

光泵远红外分子激光的多光子过程

本文在过去工作的基础上,研究六能级系统在10MW/cm~2功率泵浦下CH_3F分子气体的多光子过程.文中提出了多光子共振图,从而可预示系统中各类多光子过程以及在x-y平面远红外频率随光泵频率变化的轨迹轮廓.理论计算揭示了三光子共振过程对系统增益的贡献,表明在一般情况下,三光子过程具有很小的增益值,而在某些特定频率下,却具有可观的增益,但调谐范围很窄.此外,还讨论了多光子过程之问的相互作用,这些作用除将影响远红外激光的调谐特性外,还存在互相抑制和互相增强的效应.     

CO(A~1∏←X~1∑~+和e~3∑~-←X~1∑~+)双光子共振电离光谱的归属和双光子吸收截面的测量

我们得到了CO经过共振态A~1∏态和e~3∑~-态的转动分辨的紫外双光子共振四光子电离光谱,并测量了它们的双光子吸收截面,σ_(Xe)~(2)、σ_(A1)~(2)=1.4×10~(99)cm~8sec~(-1),σ_(A1)~(2)=2.6×10-~(50)cm~4see_(-1).用双色共振增强的方法明确地归属了受到扰动的CO A←X双光子吸收(0.0)带.     

碘原子共振多光子电离光谱研究

用可调谐的染料激光(4600～5000(?)),共振(3+2)和(4+1)多光子电离探测了I(5p~2P_(3/2)~0)和1~#(5p~2P_(1/2)~0).碘原子是由碘分子(I_2)经激光解离而产生的.在上述激光波段中共观察到12个原子跃迁,其中,(4+1)多光子电离跃迁是首次观察到的.     

四能级系统中的双光子过程

光泵远红外激光一般只能提供分立的频谱.这一特点使它的应用受到一定的限制。1977年以来,不少学者对如何进行光泵远红外激光的频率调谐做了许多工作。由于分子的远红外激射跃迁过程是在多能级系统中进行的,所以理论计算相当繁琐,不容易得到比较精确的结果。我们应用图论方法研究了四能级系统的双光子过程。结果表明,通过连续地改变泵的频率,有可能对远红外激光频率进行调谐。频率调谐范围约等于两对远红外跃迁频率之差。本文给出了调谐曲线以及增益系数与泵频率失谐量之间的关系曲线.     

稀土元素铕(Fu)的单光子共振和双光子共振激光增强电离光谱

本文首次报道稀土元素铕(Eu)的激光增强电离光谱.用紫外可调谐激光器检测到EuI 287.779um,EnI 287·887nm,EuI 289.254nm,Eul 289.303nm,EuI 289.383nm的单光子共振激光增强光谱.用可见区可调谐染料激光器检测到EuI的双光子共振激光增强电离光谱.对铕(Eu)原子激发和离化机理进行了讨论.稀土元素铕的检测限达10ng/ml.     

利用单光子和双光子共振电离增强He原子谐波强度

理论研究了He原子在吸收紫外光子后对高次谐波的强度的影响。结果表明：当紫外光光子能量满足He原子基态与激发态的单光子和双光子共振跃迁时,谐波强度有超过2个数量级的增强。当紫外光光子能量满足He原子基态与激发态的多光子共振跃迁时,谐波强度增强明显减弱。分析电离几率表明,单、双光子共振电离在紫外共振电离中起主要作用,因此导致谐波强度的明显增强。同时,在啁啾调频场的作用下谐波截止能量也明显延伸,可以获得一个强度较高的谐波光谱连续区。选取该谐波连续区可获得脉宽为40 as的孤立脉冲。     

双光子共振四波差频产生的真空紫外激光特性研究

介绍以惰性气体Xe为非线性介质、利用双光子共振四波混频差频方法产生的可调谐真空紫外激光(VUV)的特性研究 .实验中 ,传播方向和空间均相互重合的两束脉冲染料激光经透镜聚焦于Xe气混频池内 ,其中一束激光的波长 (2 4 9.6 2 6nm)对应于Xe原子 6P[1/2 ,0 ]←← 5P的双光子共振跃迁 ,通过调谐另一束激光的波长 ,产生在 15 1～ 171nm可连续调谐的VUV激光 .经估算 ,产生的VUV激光的强度约为每脉冲约 0 .2 μJ ,转换效率约为 0 .1% .通过测量超声射流冷却下CO分子的A1Π←X1Σ+ (0 ,0 )带的转动分辨激光诱导荧光 (LIF)光谱 ,确定出VUV激光的线宽为 0 .3cm-1.此外 ,还对VUV激光强度与惰性气体压力、染料激光强度等依赖关系进行了研究 .     

Excited-state laser spectroscopy of the impurities in III–V and II–VI semiconductors

At temperatures where the shallow donors become occupied, weak impurity lines appear on sweeping the field with laser wavelenghts where photon energy is insufficient to cause transitions from the ground state ti any of the excited states if the impurities. With all the materials investigated, the two most prominent of these lines, which are attributed to transitions between excited states, lie on the same two curves on a dimensionless diagram of excitation energy against cyclotron energy. One of these lines is identified as arising from the 2p⁻ to 2s transition. The other probably also originates from the 2p⁻ state. The photon energy for the 311μ laser line is just sufficient to cause transitions from the 1s to 2p⁻ state in n-GaAs. Because of this almost exact coincidence, fine structure due to the central cell corrections for individual impurities is particularly well resolved in magnetic field.     

Far-infrared spectroscopy of impurities in semiconductors

Far-infrared spectroscopy of the electronic transitions between bound states of impurities provides a very high resolution technique for studying chemical shifts and thereby identifying residual contaminants. The use of photoconductivity generated within the sample itself, usually by the photothermal mechanism (“photothermal ionisation spectroscopy”), enables very high sensitivity to be achieved even with very thin films or ultrahigh-purity material. The current knowledge about the identity of the residual shallow donors in GaAs, InP, InAs and InSb obtained with this technique is reviewed. With high-purity materials the magneto-optical spectrum of the shallow donors can be particularly rich and more than fifty lines can be observed with both GaAs and InP.

Hydrostatic pressure provides a valuable additional experimental parameter in studies of impurities. Not only does the pressure-induced increase in mass improve the resolution of the “fine structure” due to different chemical species but additional states can be introduced into the forbidden energy gap. Results with both InSb and GaAs have shown that generally donors in direct-gap III-V materials may be expected to have three types of state: the familiar gamma-associated donors, localised states with A₁ symmetry which are normally resonant within the conduction band and metastable DX states.

Negatively charged shallow donor states (D^- states) and “molecular” combinations where the electrons are shared between two or more donor sites have been studied by infrared spectroscopy of III-V materials. These states are important precursors of the metal-insulator transition.

Recently there have been a number of studies of impurities within quantum wells and heterostructures. The dependence of impurity energy on distance from the well edge has been established and it has been shown that high concentrations of D^- states can be formed by remote deping of the structures.     

Impact ionization induced negative far-infrared photoconductivity inn-GaAs

Far-infrared photoconductivity ofn-GaAs epitaxial layers showing impact ionization breakdown has been investigated by molecular lasers at photon energies below the 1s-2p shallow donor transition energy. Negative photoconductivity was observed if a magnetic field was applied to the crystals and if impact ionization of donors by the electric bias field was the dominant electron excitation mechanism. The experimental results are qualitatively explained on the basis of the generation-recombination kinetics of electrons bound to donors. Negative photoconductivity is attributed to optically induced free to bound transitions of electrons from theN=0 Landau band to donor levels shifted by the magnetic field above the low energy edge of the conduction band.     

NaYF_4微晶中Tm~(3+)-Er~(3+)耦合对间的能量传递

在NaYF4微晶中借助于Tm3+-Er3+耦合对间能量传递过程,能够将一个高能291 nm紫外光光子剪裁成近红外796 nm和蓝色476 nm两个光子。在291 nm(34 364 cm-1)紫外光激发下,Tm3+的1I6能级首先被布居,再经过一个交叉弛豫过程使得Er3+的4I9/2和Tm3+的1G4能级同时被布居,从而实现了Tm3+的1G4→3H6[476 nm(21 008 cm-1)]和Er3+的4I9/2→4I15/2[796 nm(12 563 cm-1)]辐射跃迁。估算了这种下转换过程的能量传递效率ηET和量子效率ηQE。通过这种量子剪裁可以解决光谱失配问题,提高GaAs太阳能电池中的转换效率。     

尼古丁分子的能级结构和光谱计算

研究尼古丁分子的能级结构与光谱特征,对更好地了解尼古丁分子的毒性和药性有理论指导作用.基于密度泛函理论(DFT),本文利用Gaussian 09软件在B3LYP/6-311G(d,p)基组水平上对尼古丁分子进行结构优化,再采用含时密度泛函理论(TD-DFT)在乙醇溶剂中计算尼古丁分子的15个激发态.使用Multiwfn波谱分析软件对分子前线轨道进行计算,并绘制出分子的红外谱图和紫外谱图.通过前线轨道分析可知,尼古丁分子的亲核位点是吡啶环上的C3和N6、亲电位点是吡咯环上的N22.吡啶环上的C-H、N-H键面内伸缩振动峰主要集中在3049～3079 cm~(-1),吡咯环上的甲基、亚甲基的伸缩振动峰主要集中在2796～3005 cm~(-1),其中在2816 cm~(-1)处甲基上C-H键振动峰最为明显,占比43.3%;吡咯环与吡啶环的摆动峰主要集中在1027～1455 cm~(-1),吡啶环的面内振动峰主要集中在1008～1027 cm~(-1),在800 cm~(-1)以下吸收峰都为吡啶环的面外摆动峰.紫外光谱的最大吸收峰位于173.46571 nm处,主要是由基态S0跃迁到激发态S5、S6、S10、S11、S12、S13、S14形成的,其中基态S0跃迁到激发态S11的贡献最大,其余激发态跃迁振子强度小于0.03,为禁阻跃迁.     

Detection of photoquenching in CuGaTe2

The photoconductivity spectrum of a single crystal of p type of CuGaTe₂, has been examined at 300K and 77K. Structural details, which appeared on the high energy side of The band gap transition, are attributed to the valance band structure. Pronounced photoquenching is also observed on the high energy side and it is due to the recombination of holes with the ejected electrons from the inner 3d level of copper atom. Such a photoquenching effect has not been reported so for.     

Multiphoton Ionization of Potassium Atoms in Femtosecond Laser Fields

We study the multiphoton ionization of potassium atoms in 800 nm and 400 nm femtosecond laser fields.In the 800 nm laser field,the potassium atom absorbs three photons and emits one electron via one photon resonance with the 4p intermediate state with the help of the ac-Stark shift.The resonance feature is clearly shown as an Autler-Townes(AT) splitting and is mapped out in the electron kinetic energy spectrum.In a 400 nm laser field,although one photon resonance is possible with the 5p state,no splitting is observed.The different transition amplitudes between 4s-4p and 4s-5p explain the observed results.Due to the AT effect,an unexpected peak in the photoelectron energy spectrum that violates the dipole transition rule is observed.A preliminary explanation involving the spin-orbit interaction in the p state is given to account for this component.The observed ATsplitting in the electron kinetic energy distribution can be used as an effective method to calibrate the intensity of a laser field.     

基于密度泛函的茶多酚分子EGCG和GCG的光谱计算

茶多酚是绿茶中主要生化活性成分之一。选取茶多酚中含量较高,同时也是性质较活泼、功效较明显的表没食子儿茶素没食子酸酯（EGCG）及其异构体没食子儿茶素没食子酸酯（GCG）分子做红外光谱和紫外光谱的计算和研究。使用Gaussian软件,采用B3LYP密度泛函理论（DFT）在6-311g(d,p)基组水平上优化其几何构型。频率计算得到红外光谱后,再进行振动特征分析,可以看到在EGCG和GCG的红外光谱图中每个振动模式下所有基团振动的权重,结合谱图做出相应的振动归属和对比分析。发现：两分子红外谱图相似,分别在1 711和1 717 cm-1处为羰基的伸缩振动吸收峰,苯环上酚羟基的伸缩振动吸收峰集中在3 500~3 800 cm-1,1 000～1 600 cm-1的多个峰都有苯环面内弯曲振动参与,在1 350和1 280 cm-1附近吸收峰是亚甲基次甲基面内弯曲振动引起的,在500 cm-1以下吸收峰都为原子的面外弯曲振动。采用固相粉末压片法,使用IRPRESTIGE-21红外光谱仪测量了EGCG分子的红外光谱（400～4 000 cm-1）,对比理论计算的EGCG分子红外光谱各吸收峰位值,发现在固相中实际测得的EGCG分子的红外光谱与气相下的理论计算值基本吻合,理论计算值略微有些红移,原因可能是理论计算在气相条件下采用的势函数存在误差,相比于无分子相互作用力的气相,实际测量固相光谱的分子键强度比气相条件下要略大些。使用Gaussian软件,采用含时密度泛函理论（TD-DFT）,选取乙醇作为溶剂,计算了EGCG分子的15个激发态,分析了激发态的组成和能级跃迁情况。计算所得的2个吸收峰分别位于229.3和276.4 nm处,主要对应p电子与苯环π键上电子形成的p-π共轭的电子跃迁及苯环、杂环上π→π*跃迁。从分析振子强度得知,基态跃迁到S4,S5,S6和S12激发态为产生紫外光谱的主要原因,另外的激发态可能为禁阻跃迁,振子强度均小于0.01。上述计算值与使用UV-6100S型紫外分光光度计所测得的EGCG分子在乙醇溶剂中235.1和278.7 nm的最大吸收峰吻合,计算值略有蓝移,可能是茶多酚提取时或本身就带有弱碱性所致。该研究可为研究EGCG分子和GCG分子的性质和生物活性及茶多酚的抗氧化性提供理论参考。     

Photoquenching effect and its consequence in p type gallium arsenide

In order to examine the origin of the occasionally observed shift in the photoconductivity peak with respect to the band gap value; optical and electro-optical investigation of a p type GaAs crystal was carried out. The absorption and photochonductivity spectra were recorded and by comparing both spectra, a model based on the transition of electrons from the oxygen “adsorbate surface states” is proposed to explain the strong photoquenching observed on the high energy side of the band gap value. This also explains the observed shift.     

On the interpretation of extrinsic photoconductivity spectrum

The interpretation of extrinsic photoconductivity spectrum has been reanalysed on the basis of crystal field splitting.It has been found out that the application of electric field reduces the symmetry environment from Td to D_2d and p states do split in two components B₂ and E.The electric field dependence of the separation has been experimentally observed and theoretically estimated.