Xianming L. Han

Collision-induced superfluorescence

We have studied superfluorescence (SF) in Ca vapor evolving on the 3d4s ^3 D_J -4s4p ^3 P_J-1 transitions at 1.9 µm by exciting the 4s^21 S_0 -4s4p ^1 P_1 with a pulsed dye laser. SF is generated following population transfer by spin-changing collisions with an inert gas Ar from the 4s4p ^1 P_1 and 3d4s ^1 D_2 levels. We show for the first time to our knowledge that the time delay for SF evolution follows the 1/sqrt N dependence expected for the case of uniform excitation of the vapor column by collisional transfer. Here, N is the number of participating atoms that was measured directly from the photon yield. The measured photon yield for the signal as a function of Ar pressure was found to be consistent with rate equations that simulate the buildup of populations in the ^3 D_J levels based on known collisional rates. This suggests that collisional rates can be directly inferred on the basis of SF photon yields and the atomic level populations. The pulse shapes for SF show temporal oscillations that depend on two distinct factors. The first is the presence of a number of independently evolving regions in the gain medium, and the second is the presence of spatial modes. Temporal ringing is a well-known effect related to the exchange of energy between the atoms and the radiation field during pulse propagation. However, the temporal ringing observed in this experiment is far more pronounced than in previous SF experiments due to a particular choice of evolution parameters. This should make it feasible to compare our results with detailed numerical simulations that have been carried out previously.

Chromospheric activity on late-type star DM UMa using high-resolution spectroscopic observations

We present new 14 high-resolution echelle spectra to discuss the level of chromospheric activity of DM UMa in {He I} D3, {Na I} D1, D2, Halpha, and {Ca II} infrared triplet lines (IRT). It is the first time to discover the emissions above the continuum in the {He I} D3 lines on 2015 February 9 and 10. The emission on February 9 is the strongest one ever detected for DM UMa. We analysed these chromospheric active indicators by employing the spectral subtraction technique. The subtracted spectra reveal weak emissions in the {Na I} D1, D2 lines, strong emission in the Halpha line, and clear excess emissions in the {Ca II} IRT lines. Our values for the EW8542/EW8498 ratio are on the low side, in the range of 1.0-1.7. There are also clear phase variations of the level of chromospheric activity in equivalent width (EW) light curves in these chromospheric active lines (especially the Halpha line). These phenomena might be explained by flare events or rotational modulations of the level of chromospheric activity.

Diamond growth reactor chemistry and film nucleation enhancement using chlorinated hydrocarbons

Abstract The chemistry of diamond film growth from chlorinated hydrocarbons has been investigated using a hot filament reactor coupled to an orifice sampling mass spectrometer. The relative concentrations of the species present near the growth surface have been determined as a function of filament temperature for dilute mixtures of CH 4 , CH 3 Cl, CH 2 Cl 2 and CHCl 3 in H 2 . Mass spectral analysis indicated that chlorinated hydrocarbons are sequentially dechlorinated in the presence of hydrogen at moderate reactor temperatures. A dark film was deposited on all surfaces of the reactor during studies of this dechlorination of CHCl 3 . Raman analysis indicated that these deposits are small particle polycrystalline graphite. Pretreatment of a Si 〈111〉 substrate under conditions that create the graphite deposit is seen to produce a significant nucleation enhancement of diamond when followed by growth at a higher temperature. Chemical mechanisms for some of these processes are proposed.

Investigations of Superfluorescent Cascades

Abstract We report our studies of superfluorescent cascades in atomic calcium which result from two-photon excitation of several levels reasonably close to the ionization limit. We have observed significant conversion efficiencies for some of these transitions which result in subnanosecond pulses particularly in the visible wavelengths. We report the discovery of a novel two-photon scattering mechanism which could prove to be a useful method for determining collisional broadening rates. In addition, a hyper Raman transition near 17 μm is discovered which appears to be a promising candidate for a tunable source.

Cataclysmic variables based on the stellar spectral survey LAMOST DR3

1 College of Physics/Department of Physics and Astronomy & Guizhou Provincial Key Laboratory of Public Big Data, Guizhou University, Guiyang 550025, China; liy zhang@hotmail.com 2 Dept. of Physics and Astronomy, Butler University, Indianapolis, IN 46208, USA 3 Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, Kunming 650011, China 4 National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, China 5 Department of Astronomy, Beijing Normal University, Beijing 100875, China

Chromospheric activity on the late-type star V1355 Ori using Lijiang 1.8-m and 2.4-m telescopes

We obtained new high-resolution spectra using the Lijiang 1.8-m and 2.4-m telescopes to investigate the chromospheric activities of V1355 Ori as indicated in the behaviors of Ca ii H&K, Hdelta, Hgamma, Hbeta, Na i D1, D2, Halpha and Ca ii infrared triplet (IRT) lines. The observed spectra show obvious emissions above the continuum in Ca ii H&K lines, absorptions in the Hdelta, Hgamma, Hbeta and Na i D1, D2 lines, variable behavior (filled-in absorption, partial emission with a core absorption component or emission above the continuum) in the Halpha line, and weak self-reversal emissions in the strong filled-in absorptions of the Ca ii IRT lines. We used a spectral subtraction technique to analyze our data. The results show no excess emission in the Hdelta and Hgamma lines, very weak excess emissions in the Na i D1, D2 lines, excess emission in the Hbeta line, clear excess emission in the Halpha line, and excess emissions in the Ca ii IRT lines. The value of the ratio of EW8542/EW8498 is in the range 0.9 to 1.7, which implies that chromospheric activity might have been caused by plage events. The value of the ratio E Halpha/E Hbeta is above 3, indicating that the Balmer lines would arise from prominence-like material. We also found time variations in light curves associated with equivalent widths of chromospheric activity lines in the Na i D1, D2, Ca ii IRT and Halpha lines in particular. These phenomena can be explained by plage events, which are consistent with the behavior of chromospheric activity indicators.

Laser Optogalvanic Spectroscopy and Collisional State Dynamics Associated with Hollow Cathode Discharge Plasmas

In this chapter we will discuss the laser optogalvanic effect in a discharge plasma environment, specifically associated with an iron-neon (Fe–Ne) hollow cathode lamp. The history of the optogalvanic effect will serve as an introduction to the importance of the phenomena. The theoretical model behind the optogalvanic effect will provide insight into the importance of laser optogalvanic spectroscopy as a tool for spectral characterization of the plasma processes and enhanced understanding of the collisional state dynamics associated with the discharge species in hollow cathode lamps. The present chapter will focus on transition states of neon in the Fe–Ne hollow cathode lamp. The results presented here will use, for illustrative purposes, the waveforms associated with the laser-excited optogalvanic transitions of neon: 1s4–2p3 (607.4 nm), 1s5–2p7 (621.7 nm), 1s3–2p5 (626.6 nm), 1s5–2p8 (633.4 nm) and 1s5–2p9 (640.2 nm). A comparison between the experimentally recorded optogalvanic signal waveforms and the Monte Carlo fitting routine, along with a discussion related to the variation of the (ai and bj) fitting coefficients as a function of the discharge current, will illustrate the success of our theoretical model. We will also briefly touch upon the potential applications of the optogalvanic effect at the nanoscale in fields such as graphene-based nanoelectronics and nanoplasmonics.