Infrared laser-emission study in a resonantly excited sodium vapor

We report on a study of time dependence of ir laser emission at 3.41 μm in a dense Na vapor that is resonantly excited to the 3² P levels by a flashlamp, pumped dye laser. Energy-pooling collisions (excitation transfer) between excited atoms create a large population inversion among the 5² S _1/2 and 4² P _3/2 states which is destroyed by electron collisions as a result of subsequent plasma formation. The ir laser emission may serve as a new diagnostic tool to follow the time evolution of such media.     

Absorption spectrum of a resonantly driven degenerate V-type atom with a dc-field coupling between excited states

We study the absorption spectra of a degenerate V-type atom, where a resonant driving field and a probe field drive different branches of transitions and a dc field is applied to drive the transition between two excited states. The effects of vacuum induced coherence (VIC) on the absorption spectra are investigated. It is demonstrated that in some special cases the VIC can lead to the depression of absorption and narrow resonance. The origin of these features are discussed. When the pump field and the dc field have the same intensity, it is interesting to find that the whole absorption spectrum comes mainly from the absorptions induced by the interferences among different transitions between dressed states.     

Selective population and neutron decay of an excited state of 23O

We have observed a resonance in neutron-fragment coincidence measurements that is presumably the first excited state of 23O at 2.8(1) MeV excitation energy which decays into the ground state of 22O. This interpretation is consistent with theory. The reaction mechanism supports the assignment of the observed state as the 5/2+ hole state. This assignment and the recently observed 3/2+ particle state advance the understanding of 23O.     

Radiative lifetime of resonantly excited excitons in quantum dots

Il Nuovo Cimento D - We report on the intrinsic radiative lifetime of excitons in InP quantum dots embedded in an In0.48Ga0.52 P matrix. This lifetime is about 400 ps as measured by time-resolved...     

Alpha decay of the first excited state of the Th-229 nucleus

The α decay of the anomalously low-lying isomeric level 3/2⁺ (3.5±1.0 eV) of the ²²⁹Th nucleus is studied. The lifetime of the isomer with respect to a decay is predicted and the spectrum of the emitted a particles is calculated. It is noted that the complete α spectrum of the isomer and accelerated α decay of ²²⁹Th can be observed by exciting the nuclei with laser radiation. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 5, 319–323 (10 September 1996)     

Suppression of coherent population trapping upon spontaneous decay of an excited state

The interaction of a model three-level Λ system with electromagnetic radiation is considered under the conditions where a coherence (an off-diagonal matrix element) between lower sublevels arises upon spontaneous decay of an excited state. The cases of bichromatic and incoherent broadband fields are analyzed. It is shown that the occurrence of coherence may significantly change the character of evolution of the system under study with time. These changes are most pronounced for broadband fields. Some features of this phenomenon are considered for multilevel atomic systems.     

Pico-second transient luminescence of resonantly excited excitonic molecules in CuCl

Time-resolved luminescence spectra of excitonic molecules in CuCl created by the two-photon resonance excitation are studied at 4.2 K with a time resolution of ～ 60 psec. Two sharp emission lines observed are ascribed to the radiative decay of “cold” excitonic molecules with k ～ 2K₀. Stimulated emission process gives a considerable influence on luminescence properties.     

卟啉酞菁二元分子光谱及激发态弛豫的非简并四波混频研究

通过对卟啉酞菁二元分子TTPP-O-Pc和TTPP-O(CH_2)_5-O-Pc的发射光谱和激发光谱的系统研究,证实了(tetratolylphenyl porphyrin)TTPP到Pc的能量转移,区分了后一种材料中发射光谱的两组谱带的来源.利用非简并四波混频(NDFWM)方法研究了二元分子激发态弛豫过程,获得了这两种材料的激发态弛豫时间分别为3.6ps和4ps,以及电荷弛豫时间分别为18.5ps和82ps.     

Femtosecond time-resolved interferences of resonantly excited excitons in bulk GaN

It is shown that excitonic transients of bulk GaN can be accurately measured by an interferometric correlation technique used in reflection geometry. This is demonstrated for resonantly excited excitons in high quality GaN epilayers. The interferograms exhibit a clear modulation of the signal due the quantum beats between A and B excitons. The period of these beats is in good agreement with the A–B exciton energy splitting measured by means of frequency-resolved reflectivity. The decrease of the overall signal is mainly connected to the crystal quality and strongly dependent on the inhomogeneous broadening which can be extracted from the correlation spectra. These results are in agreement with numerical calculations based on the Fourier transform determination of the time-resolved reflection coefficient in the framework of linear semi-classical theory carried out in order to obtain the autocorrelation response.     

Fission and decay of excited nuclei

The statistical theory of the decay of excited nuclei including the competition between particle emission and fission is considered. Fission barriers are given by a phenomenological formula which is an analog of the known Cameron formula and approximates well experimental data on nuclei up to transuranic elements. The contribution of shell corrections and odd-even effects is discussed. The calculated ratios of evaporation and fission widths Γ_n/Γ_f agree with experiment even in the region of nuclei with Z > 90. The $\text{Γ}{}_{\mathrm{<mtext>n</mtext>}}\text{Γ}{}_{\mathrm{<mtext>f</mtext>}}$ values depend weakly on excitation energy for nuclei with Z ≈ 90, While in other regions this dependence proves rather strong. The last conclusion does not contradict known experimental data since in experiments one usually measures some effective value $\text{〈}\text{Γ}{}_{\mathrm{<mtext>n</mtext>}}\text{Γ}{}_{\mathrm{<mtext>f</mtext>}}\text{〉}$, which is only indirectly related to the real $\text{Γ}{}_{\mathrm{<mtext>n</mtext>}}\text{Γ}{}_{\mathrm{<mtext>f</mtext>}}$, values. It is shown that the calculated values of $\text{〈}\text{Γ}{}_{\mathrm{<mtext>n</mtext>}}\text{Γ}{}_{\mathrm{<mtext>f</mtext>}}\text{〉}$ depend on excitation energy either. The effects of angular momentum and uncertainty in the choice of level density parameter are discussed.     

Atomic chemisorption on simple metals: Chemical trends and core-hole relaxation effects

We have obtained essentially exact numerical solutions for a simple model of atomic chemisorption on simple metals. The approximations constituting the model are the semi-infinite jellium simulation of the metal substrate and the self-consistent local density theory of exchange and correlation. The solutions provide a detailed picture of the electronic charge making up the chemisorption bond. The variation of this picture with the valence of the adatom exhibits in a direct and microscopic way the roles of electronegativity, charge transfer, and covalency. Predicted bond energies, bond lengths, and dipole moments are consistent with measurements and independent theoretical considerations.     

Dynamical decay of nuclei at high temperature: Competition between particle emission and fission decay

A generalized diffusion equation is propounded to follow the time evolution of an excited nucleus towards fission including along the particle decay. This theoretical model is built in order to try to analyse the anomalous behaviour of particle emission observed in many experimental data for heavy-ion induced reactions. Some calculations for the systems¹⁹⁴Hg,¹⁷⁰Yb and²⁴⁸Cf are presented. A possible extension of this generalized formalism is suggested to deal more consistently with the experimental data.     

Carrier-envelope-offset phase control of ultrafast optical rectification in resonantly excited semiconductors

Ultrashort pulse light-matter interactions in a semiconductor are investigated within the regime of resonant optical rectification. Using pulse envelope areas of around 1.5-3.5 pi, a single-shot dependence on carrier-envelope-offset phase (CEP) is demonstrated for 5 fs pulse durations. A characteristic phase map is predicted for several different frequency regimes using parameters for thin-film GaAs. We subsequently suggest a possible technique to extract the CEP, in both sign and amplitude, using a solid state detector.